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## Simultaneous Observation of Phase-Stepped Images for Photoelasticity Using Diffraction Gratings

### Experimental Mechanics (2013-10-01) 53: 1343-1355 , October 01, 2013

Phase-stepped photoelasticity is a powerful method for full-field stress analysis, but sequential collection of the multiple required images limits the technique to static loading applications. We have developed a system that utilizes diffraction gratings to collect four phase-stepped images simultaneously with a single camera for transient loading applications. Two adjacent, perpendicularly oriented, 1D Ronchi rulings are placed after a transparent sample to split the light into equal intensity beams for each diffraction order. The four beams that are diffracted once in the x direction and once in the y direction transmit through arrays of analyzing polariscope elements, with different combinations of fast-axis orientations for four phase-stepped images. The mirrors and imaging lenses in the system work in concert to focus each beam onto separate quadrants of the same CCD. We demonstrate the system for stress analysis of compressive loading of a Homalite-100 disk and of a Homalite-100 plate with a central hole. This system has the potential for photoelastic analysis of time-dependent materials and of dynamic events, when equipped with a high-speed camera.

## Residual Stress Analysis of Orthotropic Materials Using Integrated Digital Image Correlation

### Experimental Mechanics (2014-09-01) 54: 1279-1292 , September 01, 2014

Measuring residual stress in an orthotropic material is a difficult task due to the complex behavior of the material. Recently, two different approaches based on Smith’s simplified real value formulation and the general solution developed by Lekhnitskii have been proposed. Both solutions assume the measurement of the displacement field via interferometric optical methods and estimate stress values through solving an inverse problem. However, the high sensitivity to vibrations of interferometric techniques makes their use difficult outside optical laboratories; standard Digital Image Correlation could be used, but its low sensitivity and relatively high standard deviation of displacements severely affect the reliability of estimates. In this work we propose to integrate the residual stress displacement functions related to orthotropic materials into the shape functions of Digital Image Correlation. This makes it possible overcome most of the problems related to low sensitivity and large standard deviation because a single large patch can be used for the measurement, thus providing an accurate and reliable algoritm for the measurement of residual stress.

## On Evaluation of Stress Intensity Factor from In-Plane and Transverse Surface Displacements

### Experimental Mechanics (2016-06-02): 1-9 , June 02, 2016

Experimental approaches to the evaluation of stress intensity factor (SIF) of through-cracked plate components are currently based on the classical plane stress (2D) asymptotic power series expansion of strains or displacements near the crack tip (Williams’ solution). Besides the plasticity effects, the quantitative evaluation of SIF has to take into account a finite domain of convergence of the series expansion as well as three-dimensional (3D) effects, which prevail in the close vicinity of the crack tip. In this paper we demonstrate and confirm that attempts to fit Williams’ solution to experimental data in the near crack tip region can provide misleading results. In addition, it is verified that the SIF can be linked to the transverse displacements in the region controlled by 3D effects, and in particular, by 3D corner singularity. Under mode I loading, the transverse displacement field in this region is uniform, and largely unaffected by the higher order terms of the asymptotic power series expansion, which make this way of the evaluation of SIF particularly attractive for experimental measurements.

## Stress Monitoring of Post-processed MEMS Silicon Microbridge Structures Using Raman Spectroscopy

### Experimental Mechanics (2012-11-01) 52: 1341-1353 , November 01, 2012

Inherent residual stresses during material deposition can have profound effects on the functionality and reliability of fabricated Micro-Electro-Mechanical Systems (MEMS) devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not well controlled during deposition. The residual stress; for example, tends to vary significantly for different deposition methods. Currently, few nondestructive techniques are available to measure residual stress in MEMS devices prior to the final release etch. In this research, micro-Raman spectroscopy is used to measure the residual stresses in polysilicon MEMS microbridge devices. This measurement technique was selected since it is nondestructive, fast, and provides the potential for *in-situ* stress monitoring. Raman spectroscopy residual stress profiles on unreleased and released MEMS microbridge beams are compared to analytical and FEM models to assess the viability of micro-Raman spectroscopy as an *in-situ* stress measurement technique. Raman spectroscopy was used during post-processing phosphorus ion implants on unreleased MEMS devices to investigate and monitor residual stress levels at key points during the post-processing sequences. As observed through Raman stress profiles and verified using on-chip test structures, the post-processing implants and accompanying anneals resulted in residual stress relaxation of over 90%.

## An apparatus for tensile testing of concrete

### Experimental Mechanics (1979-03-01) 19: 109-111 , March 01, 1979

This paper describes a test fixture and alignment jig used for testing small concrete or rock cylinders in uniaxial tension. The fixture may be easily modified to test cube specimens and is adaptable for use in any standard universal testing machine. A testing procedure is described and results presented for 3 in. × 6 in. (76 mm × 152 mm) concrete cylinders. The main virtues of the fixture are its ease of construction, simple alignment procedure for the test specimen, and accuracy comparable to that obtained from the standard compression test of concrete cylinders.

## On the measurement of Poisson's ratio for modeling clay

### Experimental Mechanics (1971-09-01) 11: 402-413 , September 01, 1971

Resonance testing of Plasticine clay indicates that, for small strains (≤10^{−5}) in the frequency range 100–3000 Hz, the material can be considered to be a linear viscoelastic solid with parameters which depend on temperature, frequency and prior large-strain history. In order to measure Poisson's ratio, it is necessary to take special precautions to eliminate large straining between small-strain tests of different tensorial character. A simple but effective test configuration for measuring Poisson's ratio is described and test results are displayed.

## Adjacent Equilibria in Highly Flexible Upright Loop on Rigid Foundation

### Experimental Mechanics (2015-07-01) 55: 1191-1197 , July 01, 2015

For *very* slender structural components, self-weight may compete with elastic flexural stiffness in determining equilibrium configurations. In cases where the inherent elastic stiffness is low (relative to self-weight) we observe a variety of types of highly nonlinear behavior in the equilibrium shapes, together with changes in the natural frequencies of small oscillations about these equilibrium configurations. This technical note describes a specific phenomenon observed in experiments on very slender polycarbonate loops. In addition to profound changes in equilibrium shapes as a function of weight-to-stiffness ratio, under some circumstances it is possible to have two adjacent, co-existing equilibrium configurations. This robust, highly nonlinear snap-through behavior is demonstrated by perturbing from one shape to the other.

## An automated biaxial sheet tester

### Experimental Mechanics (1972-03-01) 12: 155-157 , March 01, 1972

A mechanical system is described which can stretch a sheet of solid propellant in two directions as a function of time. The sheet displacements along each of the two directions are independently controlled from a computer tape. Two load cells record the instantaneous resulting loads. Biaxial strains are measured by means of a 70-mm camera, which also is operated by the tape. The instrument is designed to stretch a 10-cm square by 0.25-cm-thick sheet to 50-percent strains at cross-head rates from quasi-static to 150 cm/min.

## Application of X-ray diffraction, micromagnetic and hole drilling methods for residual stress determination in a ball bearing steel ring

### Experimental Mechanics (2005-08-01) 45: 344-350 , August 01, 2005

A basic understanding of distortion problems requires the analysis of a complete manufacturing process including an almost complete overview of residual stress states in the component during each production step. To reduce the measurement time in the future, three measurements methods (X-ray diffraction, micromagnetic and blind hole drilling methods) have been used to analyze residual stress states in machined AISI 52100 ball bearing rings. X-ray diffraction was used as a state-of-the-art method for machining induced residual stresses with pronounced gradients. The ring exhibited a complex residual stress state with high tensile residual stresses at the surface, a strong gradient in depth, and also showed some variation along the outer circumference due to a superimposition of machining induced residual stresses and effects from the clamping device process. Due to this surface state, micromagnetic signals depend on the analyzing frequency. A calibration of the signals was only possible with the X-ray diffraction data. The results of the three different measurement methods correlate reasonably well.

## Pulsatile flows in a lateral aneurysm anchored on a stented and curved parent vessel

### Experimental Mechanics (2004-06-01) 44: 253-260 , June 01, 2004

We present particle tracking velocimetry measurements and flow visualization of pulsatile flow fields in a stented cerebrovascular lateral aneurysm model with a wide ostium anchored on a curved parent vessel. Among the stent parameters, the blocking ratio*C*_{α} ranging from 0% to 75% was selected to study its effect on the changes of intra-aneurysmal hemodynamics for the reference of minimally invasive endovascular aneurysm treatment. The Womersley number was 3.9 and the mean, peak, and minimal Reynolds numbers based on the bulk average velocity and diameter of the parent vessel were 600, 850, and 300, respectively. The results are characterized in terms of velocity vector field, coded streak images, region averaged velocity, vorticity, and wall shear stress. A critical range of*C*_{α} related to the inflow location as well as the shape and number of intra-aneurysmal vortices is identified. The intra-aneurysmal flow activity, vortex strength, and wall shear stress are found to decrease with increasing*C*_{α}. Among the*C*_{α} examined,*C*_{α}=75% is the most favorable in attenuating the risk of aneurysmal rupture and promoting intra-aneurysmal thrombus.

## Determination of stress-intensity factors by the method of caustics in anisotropic materials

### Experimental Mechanics (1995-06-01) 35: 137-143 , June 01, 1995

This paper studied the applicability of the method of caustics to anisotropic materials under Mode I and mixed-mode static-loading conditions and introduced the procedure to obtain stress-intensity factors (SIF) in anisotropic materials by the method of caustics.

The mapping equations for initial and caustic curves in anisotropic materials were introduced and their computer graphical images were compared to the experimental ones to check the validity of the mapping equations proposed in this paper. The agreement between them was found to be satisfactory.

Two kinds of equations to determine SIF in anisotropic materials by the method of caustics are proposed in this paper. Corroborative experiments carried out by using the orthotropic materials under various loading conditions are presented. In the case of Mode I loading condition, the SIF's obtained by this paper's methods were found to be close to the results by another method, i.e., boundary-element method (BEM). And in the case of mixed-loading condition, the SIF's by this paper and BEM show little differences, (2.2–24.4 percent) with respect to the slanted angle of crack.

## Stresses in spheres with concentric spherical cavities under diametral compression by three-dimensional photoelasticity

### Experimental Mechanics (1966-05-01) 6: 244-250 , May 01, 1966

This paper presents the experimental solution of the problem of a sphere with a concentric spherical cavity under diametral compression using the three-dimensional photoelastic method and calculations of the complete stress distributions along lines of interest using Golecki's analytical series solution. The experimental procedure, the stress distributions along different lines as well as the comparison with the theoretical results and a discussion of Golecki's solution are included.

## Continuous monitoring of fatigue-crack growth by acoustic-emission techniques

### Experimental Mechanics (1974-02-01) 14: 71-81 , February 01, 1974

The application of acoustic emission to the detection of fatigue-crack propagation in 7075-T6 aluminum and 4140 steel is investigated. The relationship between crack-growth rate, cyclic stress-intensity factor, load-cycling rate and observed acoustic-emission behavior is presented. Crack-growth rates of less than 10^{−6} in./ cycle could be detected, and acoustic-emission counts per cycle were shown to be closely related to the energy released by crack extension per cycle. A quantitative relationship for the threshold conditions for detection of fatigue-crack growth is presented which agrees with experimental test results. The results also showed that fatigue-crack growth occurs in an accelerating and decelerating manner, even though the stress-intensity range remains uniform, and that the count rate posses through a peak that is believed to be associated with a plane strain-plane stress transition. The effects of instrumentation sensitivity and frequency bandpass are also investigated. The results obtained indicate that acoustic-emission techniques should be suitable for in-service monitoring of a variety of cyclically loaded structures, even in the presence of high background noises.

## Tensile testing of materials at high rates of strain

### Experimental Mechanics (1981-05-01) 21: 177-185 , May 01, 1981

A tension version of the split Hopkinson bar or Kolsky apparatus is developed for conducting tests in tension at high rates of strain up to 10^{3} s^{−1}. A number of aluminum, titanium, and steel alloys tested in tension show increasing degrees of rate sensitivity above 10 to 10^{2} s^{−1}. Tests on 6061-T651 and 7075-T6 aluminum show measurable strain-rate sensitivity in tension at the highest strain rates, although similar tests in compression in the literature show essentially no strain-rate sensitivity. Details of the apparatus and instrumentation and guidelines for its use are presented.

## High-temperature low-cycle fatigue: Survey of british work

### Experimental Mechanics (1968-07-01) 8: 309-318 , July 01, 1968

## Fatigue damage accumulation around a notch using a digital image measurement system

### Experimental Mechanics (2004-06-01) 44: 241-246 , June 01, 2004

A digital image measurement (DIM) system is used to study the plastic damage accumulation around a notch under conditions of low-cycle fatigue. This system incorporates a contrast correlation method to evaluation the level of plastic damage at each point of the studied area from two images acquired before and after the introduction of fatigue deformation. A compact tension specimen of 304 stainless steel with a notch radius of 1 mm is analyzed during the stages of fatigue crack initiation and growth. The results obtained using this measurement system are compared with those attained by means of a recrytallization technique.

## A Load-Based Multiple-Partial Unloading Micro-Indentation Technique for Mechanical Property Evaluation

### Experimental Mechanics (2010-07-01) 50: 737-743 , July 01, 2010

A load-based multiple-partial unloading micro-indentation technique has been developed for evaluating mechanical properties of materials. Comparing to the current prevailing nano/micro-indentation methods, which require precise measurements of the indentation depth and load, the proposed technique only measures indentation load and the overall indentation displacement (i.e. including displacement of the loading apparatus). Coupled with a multiple-partial unloading procedure during the indentation process, this technique results in a load-depth sensing indentation system capable of determining Young’s modulus of metallic alloys with flat, tubular, or curved architectures. Test results show consistent and correct elastic modulus values when performing indentation tests on standard alloys such as steel, aluminum, bronze, and single crystal superalloys. The proposed micro-indentation technique has led to the development of a portable load-depth sensing indentation system capable of on-site, *in-situ* material property measurement.

## A Piezoelectric Technique for Evaluation of Crack Closure

### Experimental Mechanics (2009-01-22) 49: 871-876 , January 22, 2009

The ability of the piezoelectric materials to work as sensors and actuators was employed in a technique for monitoring the degree of crack closure and to detect the crack opening load. The technique is demonstrated through experiments with a cracked beam. It consists in exciting the specimen with a piezoelectric actuator and recording the electromechanical response of piezoelectric sensors placed near the crack mouth, while applying a bending moment to open the crack. The sensors in the neighborhood of the crack present a reduction in the amplitude response signal due to the progressive decrease of the dynamic strains near the crack, as the bending load causes the crack to open, reducing the contact between the surfaces of the fatigue crack and the load transmission through the contact area. The results show that the method has a high sensitivity to the state of crack closure, allowing for the direct determination of the crack opening load.

## Enhanced sensitivity residual-stress measurements using taper-hole drilling

### Experimental Mechanics (1995-06-01) 35: 124-129 , June 01, 1995

A novel method for enhancing the strain sensitivity of the hole-drilling method for measuring uniform residual stresses is examined. Such enhanced strain sensitivity is important because it improves the accuracy of the residual-stress evaluation. The new method involves enlarging the effective hole size by drilling a reverse taper hole. A simple practical technique for drilling reverse taper holes is described. The strain sensitivity for this new method is compared with that of the conventional hole-drilling method. Experimental results show excellent correspondence with theoretical results. The reasons for the sensitivity improvement are explained.

## A Shear-Corrected Formulation for the Sandwich Twist Specimen

### Experimental Mechanics (2012-01-01) 52: 17-23 , January 01, 2012

The sandwich plate twist test method involves torsion loading of a panel by application of concentrated loads at two diagonally opposite corners and supporting the panel at the other two corners. Compliance measured in this test can be used to extract the shear moduli of monolithic, composite and sandwich plates, and it may also be employed for determination of the twist stiffness, *D*_{66}. Previous studies of the plate twist specimen have shown that classical laminated plate theory does not adequately predict the compliance of sandwich panels with a low density/modulus core, as a result of transverse shear deformation. This work proposes a “shear-corrected” model for accurate prediction of the plate twist compliance by incorporation of the transverse shear stiffnesses of the core. This model was used to extract the transverse shear modulus of a range of low density PVC foam cores from the measured panel twist compliance. Good agreement with published PVC foam core shear modulus values was obtained.

## Biomechanical study of the constitutive laws of vascular walls

### Experimental Mechanics (1974-11-01) 14: 440-444 , November 01, 1974

Static mechanical behaviors of three different arterial walls were examined through changes in external radius due to distending pressure. In order to examine the distensibility of these vessels, distension ratio was defined as the ratio of eternal radius at each pressure to that at zero pressure. Linear relations were observed between the logarithmic pressure and the distension ration, and they were described by on exponential function. Two parameters used in this equation were related quantitatively to the area fraction of elastin or collagen component occupied in the cross section of wall. Stress-strain relation was then determined from their pressure-diameter data by using finite-deformation theory. An exponential function was established between tangential stress and tangenital strain. These results can be used to study the resistance of arterial walls to cardio-vascular disease.

## Photoelastic analysis of the stress field surrounding a fatigue crack

### Experimental Mechanics (1974-02-01) 14: 64-68 , February 01, 1974

A photelastic analysis was carried out on plane polyester specimens containing a fatigue crack, in order to study the effect of plastic yielding around the crack tip on the elastic stress distribution in the vicinity of the crack. In general, results were in good agreement with values calculated for the case of a sharp-tipped crack. However, very near the crack tip, principal stresses obtained experimentally were slightly lower than calculated stresses, probably due to the bluntness of the fatigue crack. Also lines of constant stress tended to move behind the crack tip, in contrast with the calculated stresses, which occurred further forward over the field of investigation.

## Stress waves in pyramids by photoelasticity

### Experimental Mechanics (1968-07-01) 8: 289-299 , July 01, 1968

The propagation of stress waves in pyramids was studied photoelastically with the application of a laser-photomultiplier tube system and an internal polariscope for recording moving fringes. Dispersion and attenuation of stress waves were considered in a straight bar, a 5-deg pyramid, and a 20-deg pyramid made of Hysol 4290 epoxy plastic. In the straight bar and 5-deg pyramid, longitudinal waves propagate without any dispersion even though the waves attenuate as they progress down the models; in the 20-deg pyramid, however, the dispersion of the stress waves is quite significant. The distributions of the axial and radial stresses and the photoelastic fringe patterns obtained on the 20-deg pyramid show that the stress wave front is spherical with the maximum stress along the central axis of the pyramid. A one-dimensional theory of wave propagation without correction factors in a small-angle infinite cone compares well with the experimental results.

## An optical method to assess electromechanical coupling in ferroelectric ceramics

### Experimental Mechanics (2005-06-01) 45: 290-294 , June 01, 2005

A new technique is described, which allows the assessment of elastic and inelastic regions around a macroscopic defect in ferroelectric-ferroelastic ceramics. The accuracy and robustness of the method are demonstrated on a PZT plate with a centered hole subjected to uni-axial compressive stresses. From the electrical potential distribution on the sample surface, the mechanical response of the material is obtained at different load levels.

## Determining the contact force during the transverse impact of plates

### Experimental Mechanics (1987-03-01) 27: 68-72 , March 01, 1987

The contact force during the transverse impact of a plate is determined from dynamic strain-gage measurements made on the plate. Experimental results for the impact of an aluminum plate are presented, and comparisons are made with finite-element predictions and measurements from a force transducer.

## Determination of natural frequencies of vibration of a sandwich plate

### Experimental Mechanics (1967-11-01) 7: 490-493 , November 01, 1967

In this paper, an expression is derived for the natural frequencies of vibration of a simply supported sandwich plate. Experimental procedures and results for the subject problem are presented. The experimental data obtained are in good agreement with the theoretical results.

A literature survey^{1, 2} shows that no work has been done on the experimental determination of natural frequencies of vibration of sandwich plates. A general analysis of flexural vibrations of elastic sandwich plates was presented by Yu in Ref. 3. The vibration analysis based upon this theory is, in general, very complicated due to the high order of the equations. It was then simplified^{4} for low-frequency ranges and for ordinary sandwich plates.

## On the dynamic elastic limit

### Experimental Mechanics (1982-07-01) 22: 270-276 , July 01, 1982

The fact that the elastic limit of some solids increases with increasing stress rate has been qualitatively and semiquantitatively established for many decades. Well known experimental difficulties have impeded reliable quantitative measurements of the magnitude or, in some solids, even the existence of such an increase of the elastic limit with stress rate.

The present paper describes a simple method for accurately measuring the dynamic elastic limit in any solid which has a linear-elastic domain at small strain, including high-strength structural metal alloys. This method has the advantages of laboratory simplicity, a minimum of complex assumptions, and a close parallel with the manner in which the quasistatic elastic limit generally is determined.

Although it is subsidiary to the main focus of this paper, evidence is presented here that a knowledge of the dynamic elastic limit firmly established by experiment, can be of considerable value for subsequent research in the continuum mechanics of solids, particularly with respect to the existence and properties of two distinct yield surfaces during impact loading.

## Holographically determined isopachics and isochromatics in the neighborhood of a crack in a glass composite

### Experimental Mechanics (1980-02-01) 20: 53-56 , February 01, 1980

The isochromatic and isopachic fringes are obtained holographically in the neighborhood of a central crack in a tensile, orthotropic glass-composite plate. The general inability to separate the principal stresses or strains from such information alone under anisotropic conditions is discussed, as are the results relative to fracture-mechanics implications.

## An Energy Based Fatigue Life Prediction Framework for In-Service Structural Components

### Experimental Mechanics (2011-06-01) 51: 707-718 , June 01, 2011

An energy based fatigue life prediction framework has been developed for calculation of remaining fatigue life of in service gas turbine materials. The purpose of the life prediction framework is to account aging effect caused by cyclic loadings on fatigue strength of gas turbine engines structural components which are usually designed for very long life. Previous studies indicate the total strain energy dissipated during a monotonic fracture process and a cyclic process is a material property that can be determined by measuring the area underneath the monotonic true stress-strain curve and the sum of the area within each hysteresis loop in the cyclic process, respectively. The energy-based fatigue life prediction framework consists of the following entities: (1) development of a testing procedure to achieve plastic energy dissipation per life cycle and (2) incorporation of an energy-based fatigue life calculation scheme to determine the remaining fatigue life of in-service gas turbine materials. The accuracy of the remaining fatigue life prediction method was verified by comparison between model approximation and experimental results of Aluminum 6061-T6. The comparison shows promising agreement, thus validating the capability of the framework to produce accurate fatigue life prediction.

## Letter from the technical editor

### Experimental Mechanics (2005-12-01) 45: 477 , December 01, 2005

## Two-dimensional moiré method and grid method using Fourier transform

### Experimental Mechanics (1989-12-01) 29: 399-404 , December 01, 1989

Formerly, the authors presented one-dimensional strain analysis by a moiré method using a Fourier transform. In the present work, the method is extended to two-dimensional strain analysis. The analysis is completely automated by introducing digital image processing. All of the laborious and subjective procedures required in the classical and conventional moiré method, such as separation of the two gratings, fringe-sign determination, fringe ordering and fringe interpolation, are completely eliminated; and objective, fast and accurate analysis can be made.

## Stress concentrations around horizontal circular tunnels

### Experimental Mechanics (1968-10-01) 8: 474-478 , October 01, 1968

The problem of axial variation of stress concentrations at the periphery and normal to the axis of a circular tunnel is solved by means of the three-dimensional photoelasticity technique, under the following conditions: 1.

The center lines of two horizontal tunnels of equal diameter (2*r*) are separated by a distance*K* and include an angle α.

*K* and α assume values of 0, 3*r*, 7/2*r*, 4*r* and 30 deg, 60 deg, 90 deg, respectively.

The tunnels are located in a uniform, uniaxial stress field normal to the axes of the tunnels.

## Development of a measuring system for segmented ship models

### Experimental Mechanics (1989-06-01) 29: 101-108 , June 01, 1989

The use of segmented ship models to test and study various ship responses in model scale poses a challenge to instrumentation and test engineers. In recent years, the authors have developed a segmented ship model to study and trace ice loads acting on the ship hull. The model contains three segments at the bow. Each segment is supported at multiple points which enable the resolution of the location, magnitude, and direction of the external loads. The typical segment support system consists of four vertical, two transverse, and one longitudinal support points with uniaxial compression pression transducers. Stabilization of the segment is achieved by using three specially designed tension links acting in the vertical, transverse and longitudinal directions, respectively. The system is subjected to several levels of calibration which include individual transducer calibration, calibration using internal loading applied by the tension links, external calibration using a specially designed and built calibration rig capable of exerting normal and inclined loads, and calibration of the effects of buoyancy changes in a floating model. The results of calibration and ice-breaking tests indicate that position prediction of the external load can be made within 20 mm. The normal load of less than 100 Newtons can be determined within a few percentage points but the frictional load magnitude and direction are found to be subject to greater errors particularly for low friction factors of the order of 0.1.

## A simplified optical method for measuring residual stress by rapid cooling in thermosetting resin strip

### Experimental Mechanics (1986-06-01) 26: 185-192 , June 01, 1986

This paper presents a simplified optical method for measuring the residual stresses by rapid cooling in thermosetting resin strips. First, the fundamental equations for calculating the residual stress from the residual birefringence were obtained by the linear photoviscoelastic theory. The specimens were then subjected to rapid cooling. After rapid cooling, the residual stress was measured by two methods, the simplified optical method mentioned above and the well-known layer-removal method. The effectiveness of the simplified optical method was discussed by comparing results of the two methods.

## Aerodynamic Performance of a Gliding Swallowtail Butterfly Wing Model

### Experimental Mechanics (2010-11-01) 50: 1313-1321 , November 01, 2010

In the present study, we perform a wind-tunnel experiment to investigate the aerodynamic performance of a gliding swallowtail-butterfly wing model having a low aspect ratio. The drag, lift and pitching moment are directly measured using a 6-axis force/torque sensor. The lift coefficient increases rapidly at attack angles less than 10° and then slowly at larger attack angles. The lift coefficient does not fall off rapidly even at quite high angles of attack, showing the characteristics of low-aspect-ratio wings. On the other hand, the drag coefficient increases more rapidly at higher angles of attack due to the increase in the effective area responsible for the drag. The maximum lift-to-drag ratio of the present modeled swallowtail butterfly wing is larger than those of wings of fruitfly and bumblebee, and even comparable to those of wings of birds such as the petrel and starling. From the measurement of pitching moment, we show that the modeled swallowtail butterfly wing has a longitudinal static stability. Flow visualization shows that the flow separated from the leading edge reattaches on the wing surface at *α* < 15°, forming a small separation bubble, and full separation occurs at *α* ≥ 15°. On the other hand, strong wing-tip vortices are observed in the wake at *α* ≥ 5° and they are an important source of the lift as well as the main reason for broad stall. Finally, in the absence of long hind-wing tails, the lift and longitudinal static stability are reduced, indicating that the hind-wing tails play an important role in enhancing the aerodynamic performance.

## Optical strain determination at transient high temperatures in stainless steel

### Experimental Mechanics (1964-11-01) 4: 320-327 , November 01, 1964

Type 304 stainless-steel plates 0.24×2.25×4 in. in size were subjected to stagnation heating in an oxyacetylene-flame apparatus. Transient thermal strains at temperatures up to 2000°F were measured with a water-cooled Tuckerman optical strain gage shielded from heat transfer by a cooled radiant-heat shield. The general equations of thermoelasticity are used for a flat homogeneous plate which is subjected to a particular transient three-dimensional temperature distribution. The optically measured strains on the unheated surfaces of the specimens were in close agreement with the thermal strains from the analytical solution using experimentally obtained temperature distributions in the specimens.

## Photoelastic stress analysis of an elliptical hole in a thick plate subjected to uniform in-plane compressive Loading

### Experimental Mechanics (1985-06-01) 25: 105-114 , June 01, 1985

A three-dimensional photoelastic analysis was conducted to determine the stress distribution and concentration around the periphery of a centrally located elliptical hole in a plate of finite thickness. The edge of the plate was subjected to a uniformly distributed compressive uniaxial in-plane load. The principle of superposition was employed to study the effect of uniform biaxial loading.

Elliptical holes with five different major/minor axis ratios (β) ranging from 1.0 to 2.64 were investigated. Among the results of this study, it was established that the variation of the principal stresses at the edge of the hole is not linear across the plate thickness. It was also found that in loading the plate in a direction parallel to the major axis of the ellipse, the value of the maximum tangential principal stress (σ_{η}) occurs in a plane other than the middle plane of the plate. However, in loading the plate in a direction either parallel or perpendicular to the major axis, the maximum transverse stress (σ_{z}) occurs at the middle plane. In addition, the maximum value of (σ_{z}) was about 20 percent of the maximum value of the tangential stress for all models tested. Furthermore, the effect of the bixial loading has reduced the value of the maximum tangential stress at the periphery of the hole as compared with uniaxial loading.

As a three-dimensional theoretical solution does not exist for this problem, the present findings were correlated with the well established two-dimensional solutions.

## Recent research activities in experimental mechanics in China

### Experimental Mechanics (1982-05-01) 22: 171-173 , May 01, 1982

A brief review is presented of the recent activities in the field of experimental mechanics in the People's Republic of China.

The current research work covers the following subjects: (1) photoelastic phenomena, such as the classical three-dimensional photoelasticity, the scattered-light technique, birefringent coatings, birefringent materials; (2) holography, holographic interferometry, speckle interferometry and their applications; (3) moiré method; (4) strain-gage techniques and strain indicators.

## The dependence of yield stress on strain rate as determined from ball-indentation tests

### Experimental Mechanics (1966-02-01) 6: 87-92 , February 01, 1966

A ball-indentation experiment was conducted to explore the possibility of using this test for the investigation of strain-rate dependence of yield properties of materials. The initial velocities of indentation used were between 0.0002 and 3 ips. Force and depth of penetration were measured continuously during a test. Using these and the results given in a previous report, the dependence of yield pressure of the indentation test on penetration velocity is shown for a range of velocities from 0.0002 to 300 ips. Results are presented for annealed C1018 steel, annealed 1100 F and annealed 6061-T6 aluminums. For all materials tested, the dependence of yield stress on strain rate seems to be derivable from the ball-indentation results by using Tabor's empirical formulas and an equation relating strain rate and velocity of penetration. The increase of yield pressure over the range of indentation velocities is 100 percent for steel, 30 percent for 1100 aluminum and 20 percent for 6061 aluminum.

## Surface topography by multisource moiré patterns

### Experimental Mechanics (1968-02-01) 8: 82-87 , February 01, 1968

Moiré patterns produced by interference of a series of periodically arranged linear light sources with a line-specimen grating of similar pitch are located at a plane parallel to the sources and the specimen grating and at some distance away from these planes. A transparent or reflecting specimen located at a distance from the specimen grating distorts the image of the multisource projected on the specimen grating due to its surface irregularities and forms a moiré pattern. This pattern yields the partial-slope contours of the topography of the specimen along a direction normal to the lines of the grating. Two such contour patterns taken at mutually perpendicular directions are sufficient to yield the complete topographic picture of the surface.

The method was used for determining gradients of thickness variations in two-dimensional specimens due to lateral contraction. The technique is highly accurate in determining the values of thickness in such cases, since the integration of the slope of thickness variation along any traverse of the specimen is a steady and accurate process.

## On the Repeatability of Friction and Wear Tests for Polyimides in a Hertzian Line Contact

### Experimental Mechanics (2007-05-26) 48: 233-246 , May 26, 2007

Tribological properties for polymers are mainly experimentally determined, while few standards are available and variation in data is often large. Polyimide is slid against steel on a cylinder-on-plate tribotester at 50–200 N and 0.3 m/s. There is a statistical variation of ±7% on dynamic friction, ±10% on static friction, ±8% on wear rates from weight loss and ±12% from wear rates from dimensional measurements, or even higher at high normal loads. Different parameters influencing statistical repeatability are discussed. Friction for polymers shows higher variation and wear rates show lower variation than steel/steel pairs due to visco-elastic deformation that has a contribution to friction but reduces stress concentrations. It is experimentally verified that the visco-elastic deformation of polymers in line contact is higher than calculated from theoretical models. The role of recoverable and permanent deformation is verified and there is a difference in deformation at 50–100 N and 150–200 N revealed from static loading tests, creep tests and wear measurements. The limit between running-in and steady-state coincides with the stabilization in contact pressures after 250 μm diameter reduction. Besides test rig design, a variation in counterface roughness seems the second most important influence.

## On the measurement of residual-stress gradients in aluminum-alloy specimens

### Experimental Mechanics (1978-07-01) 18: 269-276 , July 01, 1978

The assessment or prediction of fatigue life or strength improvement due to residual stresses requires knowledge of their magnitude and distribution. This paper presents an extension of the modified hole-drilling technique (MHDT) to the measurement of stress gradients in a biaxial-residual-stress field. This is achieved by taking a series of ‘point’ measurements and evaluating the stress profile with due consideration to the effects of hole location, the interaction between holes and the redistribution of stress due to hole drilling.

An application to the measurement of residual stresses induced in 2024-T3 aluminum-alloy specimens by edge-dimpling technique is described and the method of compensation for the effect of redistribution of stress is explained. The experimental results are shown to be in good agreement with those obtained elsewhere by an analytic-numerical solution.

## Nondestructive Evaluation of Crack Depth in Concrete Using PCA-compressed Wave Transmission Function and Neural Networks

### Experimental Mechanics (2008-04-01) 48: 225-231 , April 01, 2008

Cracks in concrete are common defects that may enable rapid deterioration and failure of structures. Determination of a crack’s depth using surface wave transmission measurement and the cut-off frequency in the transmission function (TRF) is difficult, in part due to variability of the measurement data. In this study, use of complete TRF data as features for crack depth assessment is proposed. A principal component analysis (PCA) is employed to generate a basis for the measured TRFs for various simulated crack (notch) cases in concrete. The measured TRFs are represented by their projections onto the most significant PCs. Then neural networks (NN), using the PCA-compressed TRFs, are applied to estimate the crack depth. An experimental study is carried out for five different artificial crack (notch) cases to investigate the effectiveness of the proposed method. Results reveal that the proposed method can effectively estimate the artificial crack depth in concrete structures, even with incomplete NN training.

## Review of Experimental Investigations on Compressibility of Arteries and Introduction of a New Apparatus

### Experimental Mechanics (2012-09-01) 52: 895-902 , September 01, 2012

Arterial tissue incompressibility is a common notion used in numerical simulations and analytical studies. However, only a few experimental investigations have been performed to characterize arterial tissue incompressibility. Such studies have used various approaches, ranging from the initial purely mechanical measurements in 1954 to the more recent image-based analyses (2004). The results of these studies are rather diverse because different arteries have been tested (human/dog/mouse, carotid/pulmonary/iliac). This has therefore made accurate comparisons between studies challenging. In the first part of this report, a review of the experimental investigations on the compressibility of arteries is presented, with particular focus on the test rigs that have been used. In the second part of this report, a novel simple apparatus to test samples under physiological or supra-physiological conditions is described. Recommendations for a testing procedure are also provided. Finally, preliminary results on porcine renal arteries indicate significant levels of compressibility are possible (>10 %), thereby suggesting the need for further investigation.

## On residual-stress measurements in light truck wheels using the hole-drilling method

### Experimental Mechanics (1985-03-01) 25: 54-59 , March 01, 1985

An investigation of the effect of drilling speed, milling-cutter wear, drilling mode, and applied drilling force on residual-stress measurements in a light truck wheel using a milling guide manufactured by Measurements Group, Inc. is described. The milling variables chosen were used to minimize the residual stresses induced by the introduction of a hole into the wheel material. An improved hole-drilling procedure was developed and found to be successful in the residual-stress measurements for a light truck wheel.

## Study of three-dimensional deformation of a pallet using phase-shiff shadow moiré and finite-element analysis

### Experimental Mechanics (2005-02-01) 45: 9-17 , February 01, 2005

We present the results of a comparative study on the static deformation of a pallet, made from oil-palm fiberreinforced composite material, using the phase-shift shadow moiré method and finite-element analysis (FEA). The pallet was designed and analyzed using a commerical software package. The effect of various joint types on the deformation profile was studied to obtain a simplified model to represent the actual design. A one-fifth scale model of the pallet was fabricated and the deformation due to static loading was measured using the phase-shift shadow moiré method. The comparison between the measurement and FEA results on the deformed profile showed a maximum difference of 13.7% at the center of the some of the deck boards, but a smaller difference at other deck boards. The FEA results also produced a larger deformation gradient compared to the measurements.

## A photoelastic-model study of stress distribution and rock fracture around mining excavations

### Experimental Mechanics (1969-02-01) 9: 75-81 , February 01, 1969

A three-dimensional photoelastic-model study of two horizontal mining excavations with a central pillar is described. Details of experimental techniques used are given and stress distributions, rock-fracture initiation zones and the directions of subsequent fracture propagation around these excavations are determined. It is shown that the techniques adopted for this investigation offer realistic means for comparative design studies of best shape and layout of mining excavations.

## Experimental critical loads for perforated square plates

### Experimental Mechanics (1968-02-01) 8: 69-74 , February 01, 1968

Experimentally determined critical loads are presented for simply supported square plates with a circular central hole under the action of uniform edge displacements for hole diameters varying up to seventenths the dimension of the plate. The critical load is defined as the inflection point on the load-deflection curve as measured by a set of dial gages at the edge of the hole. Least-squares-curve fitting techniques are used for reducing all experimental data and the entire set of computations is carried out on the electronic digital computer. Finally, it is shown that the experimental results agree well with the theoretical critical loads as determined by the Ritz energy method.

## Experimental and analytical investigation of the modified wyoming shear-test fixture

### Experimental Mechanics (1994-03-01) 34: 10-15 , March 01, 1994

The interaction between the isopescu shear-test specimen and the modified Wyoming shear-test fixture was investigated. This involved three areas of study. First, strip strain gages were applied to the specimen contact surfaces to measure strains induced by the fixture. While it has generally been assumed that the fixture induces an asymmetric load into the specimen, the measured strains indicated a nonasymmetric loading. Second, the effect of an out-of-plane moment, externally applied to the fixture, on gage section shear strains was observed. The shear strains observed were small leading to the conclusion that the fixture is very resistant to out-of-plane moments. Finally, the effect of fixture misalignment on the observed shear modulus was determined. The results indicate that the observed shear modulus increased significantly for slight amounts of misalignment. Therefore, experimental techniques should be modified to include a check for misalignment. Beyond this, it is suggested that the fixture design be modified to prevent misalignment and to achieve an asymmetric loading of the specimen.

## An Energy-Based Torsional-Shear Fatigue Lifing Method

### Experimental Mechanics (2012-09-01) 52: 705-715 , September 01, 2012

An energy-based fatigue-life prediction framework for the determination of full-life, remaining-life, and critical-life of in-service structures subjected to torsional-shear loading has been developed. This framework is developed upon the existing foundation of energy-based fatigue models crafted for the axial, uniaxial bending, and transverse-shear loading cases, which state: the total strain energy density accumulated during both a monotonic event and a cumulative cyclic process is the same material property. The modified energy-based torsional-shear fatigue-life prediction framework is composed of the following entities: (1) the development of a torsional-shear fatigue testing procedure capable of assessing strain energy density per cycle in a pure shear stress state and (2) the determination of the remaining-life and critical-life of in-service aluminum (Al) 6061-T6 structures subjected to shear fatigue through the application of the energy-based prediction method. Experimental data was shown to be affected by load-frame misalignment which was estimated and successfully incorporated into the validation results. Close correlation between adjusted experimental results and the full-life and critical-life predictions stemmed from a 3-to-2 shear-to-axial biaxial loading assumption, which was supported by crack path comparisons. Results of the study effectively demonstrated the versatility of the energy-based lifing method.

## Experimental investigation of longitudinal wave propagation in an elastic rod with coulomb friction

### Experimental Mechanics (1970-04-01) 10: 137-144 , April 01, 1970

This paper presents the experimental results obtained from the propagation of a compression stress wave generated by the longitudinal impact of two cylindrical elastic rods. One of these rods is subjected to a uniformly distributed coulomb-friction force. In order to determine the stress-wave shape and the decay rate, the rod is subjected to longitudinal impact for different values of coulomb friction. As the stress wave propagates along the rod, it is measured at strain-gage stations located on the stationary rod.

In order to correlate the experimental results, the solution of the modified wave equation with coulomb friction is obtained for the longitudinal impact of two semi-infinite rods.

## A Simple Methodology to Measure the Dynamic Flexural Strength of Brittle Materials

### Experimental Mechanics (2011-10-01) 51: 1325-1334 , October 01, 2011

A simple methodology is proposed for measuring the dynamic flexural strength of brittle materials. The proposed technique is based on 1-point impact experimental setup with (unsupported) small beam specimens. All that is needed is a measurement of the prescribed velocity as a boundary condition and the fracture time for a failure criterion, both to be input in a numerical (FE) model to determine the flexural strength. The specimen was modeled numerically and observed to be essentially loaded in bending until its final inertial failure. The specimen’s geometry was optimized, noting that during the very first moments of the loading, the specimen length does not affect its overall response, so that it can be considered as infinite. The use of small beam specimens allow large scale testing of the flexural strength and comparison between static and dynamic loading configurations. Preliminary experiments are presented to illustrate the proposed approach.

## Mechanical properties of ultrananocrystalline diamond thin films relevant to MEMS/NEMS devices

### Experimental Mechanics (2003-09-01) 43: 256-268 , September 01, 2003

The mechanical properties of ultrananocrystalline diamond (UNCD) thin films were measured using microcantilever deflection and membrane deflection techniques. Bending tests on several free-standing UNCD cantilevers, 0.5 μm thick, 20 μm wide and 80 μm long, yielded elastic modulus values of 916–959 GPa. The tests showed good reproducibility by repeated testing on the same cantilever and by testing several cantilevers of different lengths. The largest source of error in the method was accurate measurement of film thickness. Elastic modulus measurements performed with the novel membrane deflection experiment (MDE), developed by Espinosa and co-workers, gave results similar to those from the microcantilever-based tests. Tests were performed on UNCD specimens grown by both micro and nano wafer-seeding techniques. The elastic modulus was measured to be between 930–970 GPa for the microseeding and between 945–963 GPa for the nanoseeding technique. The MDE test also provided the fracture strength, which for UNCD was found to vary from 0.89 to 2.42 GPa for the microseeded samples and from 3.95 to 5.03 for the nanoseeded samples. The narrowing of the elastic modulus variation and major increase in fracture strength is believed to result from a reduction in surface roughness, less stress concentration, when employing the nanoseeding technique. Although both methods yielded reliable values of elastic modulus, the MDE was found to be more versatile since it yielded additional information about the structure and material properties, such as strength and initial stress state.

## An experimental study of the elasto-plastic behavior of certain grid frameworks

### Experimental Mechanics (1965-04-01) 5: 120-125 , April 01, 1965

An experimental investigation of the elastoplastic behavior of certain torsionless grid frameworks is presented. The purpose of the test program was to determine the significance of the stiffness criterion in the evaluation of collapse loads of interconnected beam systems. The load-deflection relationships in the various members, the sequence of hinges and moment redistribution are studied. The test values are used to verify the theoretical values obtained by the application of the simple plastic theory.

## The Evolving Role of Experimental Mechanics in 1-D Nanostructure-Based Device Development

### Experimental Mechanics (2011-01-01) 51: 1-9 , January 01, 2011

Future generations of transistors, sensors, and other devices maybe revolutionized through the use of one-dimensional nanostructures such as nanowires, nanotubes, and nanorods. The unique properties of these nanostructures will set new benchmarks for speed, sensitivity, functionality, and integration. These devices may even be self-powered, harvesting energy directly from their surrounding environment. However, as their critical dimensions continue to decrease and performance demands grow, classical mechanics and associated experimental techniques no longer fully characterize the observed behavior. This perspective examines the evolving role of experimental mechanics in driving the development of these new devices. Emphasis is placed on advances in experimental techniques for comprehensive characterization of size effects and their coupling, as well as assessment of device-level response.

## Use of mixed-mode stress-intensity algorithms for photoelastic data

### Experimental Mechanics (1984-12-01) 24: 300-307 , December 01, 1984

A generalization of irwin's method and a new algorithm, called ‘quadratic,’ are proposed for the extraction of mixed-mode stress-intensity factors (SIF) in three-dimensional cracked-body problems. These new algorithms are evaluated along with the existing ones of Smith, Sanford-Dally and Theocaris-Gdoutos, and their SIF results compared.

It is shown that all these algorithms deliver SIF values in good agreement and that they all can be applied reliably to near-tip photoelastic data.

## Techniques for measuring stress-strain relations at high strain rates

### Experimental Mechanics (1966-08-01) 6: 395-402 , August 01, 1966

The qualitative dependence of the mechanical behavior of some materials on strain rate is now well known. But the quantitative relation between stress, strain and strain rate has been established for only a few materials and for only a limited range. This relation, the so-called constitutive equation, must be known before plasticity or plastic-wave-propagation theory can be used to predict the stress or strain distribution in parts subjected to impact stresses above the yield strength.

In this paper, a brief review of some of the experimental techniques for measuring the stress, strain, strain-rate relationship is given, and some of the difficulties and shortcomings pointed out. Ordinary creep or tensile tests can be used at plastic-strain rates from 10^{−8} to about 10^{−1}/sec. Special quasi-static tests, in which the stress- and strain-measuring devices as well as the specimen geometry and support have been optimized, are capable of giving accurate results to strain rates of about 10^{2}/sec. At higher strain rates, it is shown that wave-propagation effects must be included in the design and analysis of the experiments. Special testing machines for measuring stress, strain and strain-rate relationships in compression, tension and shear at strain rates up to 10^{5}/sec are described, and some of the results presented. With this type of testing machine, the analysis of the data requires certain assumptions whose validity depends upon proper design of the equipment. A critical evaluation of the accuracy of these types of tests is presented.

## Comparison of measurement of piston deformation using holographic interferometry and finite elements

### Experimental Mechanics (1993-12-01) 33: 336-342 , December 01, 1993

Holographic interferometry was used to measure the thermal expansion of a diesel-engine piston and the results are compared with those predicted by a finite-element model. Various assumptions were made in the generation of the model and the holographic results provided a means of assessing the effects of these assumptions on the deformed profile of the piston. A holographic rig has been developed that enables one to measure simultaneously the deformation on the front and rear surfaces of the piston using a single holographic plate and hence detect any body displacement. A simple ‘mirror’ concept is introduced that can be used to measure in-plane and out-of-plane displacements simultaneously. The holographic results showed expansion values of ten-percent greater than the predictions of the model at midskirt level of the piston. A closer agreement of the results were observed at other levels of the skirt.

## Holographic velocimetry for flow diagnostics

### Experimental Mechanics (1988-12-01) 28: 402-408 , December 01, 1988

Currently, there are a number of flow diagnostic tools available for the evaluation of fluid dynamic systems. In spite of its great potential, holographic velocimetry is one technique which has not been widely used. It does, however, have great potential in this area due to its inherent three-dimensionality. As demonstrated in this study of fully developed turbulent flow in a pipe, full three-dimensional mapping can be achieved at any instant in a flow cycle. Comparisons of holographic results with analytical predictions and laser-Doppler-anemometry (LDA) measurements demonstrate the accuracy of the technique as well as some of its advantages and disadvantages relative to LDA. Although relatively poor spatial resolution is obtained, the fact that holographic velocimetry is both an instantaneous and full volume measuring tool makes it useful for a range of complex and high-speed flow-measurement applications.

## On the history of some recent measurements described in Experimental Mechanics

### Experimental Mechanics (1977-09-01) 17: 359-360 , September 01, 1977

## Characterization of the Grain-Level Mechanical Behavior of Eglin Sand by Nanoindentation

### Experimental Mechanics (2014-06-01) 54: 871-884 , June 01, 2014

Characterization was made on the structure and grain-level mechanical behavior of Eglin sand (Quikrete #1961 sand quarried in Pensacola, FL). The as-received assorted sand was sorted to six grain sizes: 0.60 mm, 0.50 mm, 0.42 mm, 0.30 mm, 0.212 mm, and 0.15 mm. The sand chemical constituents and crystalline structures were determined using energy dispersive X-ray spectroscopy, X-ray diffraction and transmission electron microscopy. The Young’s modulus and hardness were determined using nanoindentation with a Berkovich tip, and the fracture toughness was measured using a cube-corner tip. The median Young’s modulus, hardness and fracture toughness were determined as 90.4 GPa, 12.8 GPa and 2.32 MPa⋅m^{0.5}, respectively. The mechanical properties were analyzed statistically and the parameters of the Weibull distribution were determined. The grains show highly ductile behavior under nanoindentation due to confinement by high pressure induced by Berkovich tip. An inverse problem solving approach using finite element method (FEM) with the consideration of the Ramberg-Osgood model was used to determine the stress–strain relationship for individual sand grains.

## Ultimate tests of thick shells

### Experimental Mechanics (1974-07-01) 14: 290-296 , July 01, 1974

Experimental investigation of the elastoplastic and ultimate load response of thick hemispheres with cylindrical skirts is presented. A shell with a hinged boundary condition, loaded symmetrically, and one having a fixed edge, with unsymmetric lateral load, were tested. Electrical-resistance bonded foil strain gages and photoelastic surface coatings were used in monitoring the strain response while spring-activated dial gages were used in recording displacements. Both panchromatic and color still photographs were utilized in observing the photoelastic patterns, while the failure mode of one model was recorded by means of high-speed motion pictures.

## A new application of differential interferometry for stress analysis

### Experimental Mechanics (1984-12-01) 24: 277-285 , December 01, 1984

In the past, differential interferometry has found interesting applications in gas dynamics. The gradients of density could be measured in gas flows. Now, a first trial is made to extend this method to the experimental treatment of stress problems. A Wollaston prism with polarizing elements is used in the optical arrangement. This prism combines two beams of light which have penetrated the model at locally separated points. A field of interference fringes can be produced behind the Wollaston prism. The deflections of the different conjugated light beams, which are caused by the deformed elements of the model, lead to a shifting of the interference fringes. A Stress Differential-interferometer Law is derived theoretically in order to interpret the optical data According to this theory, the optical effect caused by the deflection in this arrangement is proportional to the gradient of the sum of principal stresses. A calibration test is performed by using a circular disk, this method is applied to a circular ring for measuring the stress gradients. Under special conditions, interference fringes could be produced which represent the loci of equal stress gradient. Plexiglas plane models are loaded diametrically by single loads. The experimental results verify the statements of the developed theory.

## Strain-field investigations with plane diffraction gratings

### Experimental Mechanics (1965-07-01) 5: 233-238 , July 01, 1965

The development of means for producing small, separate, plane diffraction gratings as integral parts of the surfaces of flat metal specimens has led to modification of the method of Bell and to the investigation of strain fields by study of diffraction phenomena. Choice of a particular diffraction-strain relationship and the use of master gratings in conjunction with diffraction gratings to be used as strain gages permits use of simple optical instrumentation. The diffraction phenomena from multiple gages at the same orientation may be photographed simultaneously during loading for subsequent analysis, and instrumentation in the form employed permits resolving strain increments of magnitude 0.0008 in the range 0.0030 to 0.030.

## A sensitive thrust-measuring device

### Experimental Mechanics (1975-09-01) 15: 365-368 , September 01, 1975

A centilevered beam with mounted strain gages is used to indirectly measure the velocity of a stream of drops by measuring the thrust produced by this stream. A design procedure is explained and implemented to determine a suitable geometry for this device. Sensitivity, frequency response and fragility are the factors considered in the design. Laboratory-test results show the usefulness of this device.

## Strain distribution measurement in a coarse-grained titanium alloy

### Experimental Mechanics (1993-12-01) 33: 320-325 , December 01, 1993

Strain distributions in a coarse-grained titanium alloy under tensile loading were measured using a moiré-interferometry method. Analysis of the interferograms show that variations of more than 25 percent away from the mean level are present, most noticeably in the direction transverse to the loading axis.

## Poisson's ratio variations in concrete

### Experimental Mechanics (1976-04-01) 16: 154 , April 01, 1976

## Effects of thermal impact on a thin spherical cap

### Experimental Mechanics (1962-10-01) 2: 303-306 , October 01, 1962

Because of the importance of thin shells as structural elements in all aircraft and missile vehicles, the response of the shell to transient thermal loads is of extreme interest. At very high heating rates, as can be produced by the absorption of intense radiation energy, dynamic mechanical response of the structure may be produced. This paper describes a brief exploratory investigation of the effects of short-duration, high-intensity thermal radiation exposure of a thin-shell structure. Exposure of a spherical cap in a arc-image furnance failed to produce buckling under the test conditions. Rapid thermal rupture, however, did result in interesting failure surfaces.

## Phase-measuring profilometry using sinusoidal grating

### Experimental Mechanics (1993-06-01) 33: 117-122 , June 01, 1993

When a sinusoidal amplitude grating is projected on an object, the surface-height distribution of the object is translated to a phase distribution of the deformed grating image. In this paper, two algorithms developed for phase acquisition of such images are presented and compared. The phase-acquisition algorithms are sufficiently simple that high-resolution phase maps using a highresolution area detector array can be generated in a short time. The average detection error is within 30 mm, which can be reduced further by changing the period of the projected grating and the angle offset between the projection and the observation optics.

## Resonant-Based Identification of the Poisson’s Ratio of Orthotropic Materials

### Experimental Mechanics (2010-04-01) 50: 437-447 , April 01, 2010

The resonant-based identification of the in-plane elastic properties of orthotropic materials implies the estimation of four principal elastic parameters: *E*_{1}, *E*_{2}, *G*_{12}, and *ν*_{12}. The two elastic moduli and the shear modulus can easily be derived from the resonant frequencies of the flexural and torsional vibration modes, respectively. The identification of the Poisson’s ratio, however, is much more challenging, since most frequencies are not sufficiently sensitive to it. The present work addresses this problem by determining the test specimen specifications that create the optimal conditions for the identification of the Poisson’s ratio. Two methods are suggested for the determination of the Poisson’s ratio of orthotropic materials: the first employs the resonant frequencies of a plate-shaped specimen, while the second uses the resonant frequencies of a set of beam-shaped specimens. Both methods are experimentally validated using a stainless steel sheet.

## Quantitative Impact Testing of Energy Dissipation at Surfaces

### Experimental Mechanics (2009-08-01) 49: 511-522 , August 01, 2009

Impact testing with nanoscale spatial, force, and temporal resolution has been developed to address quantitatively the response of surfaces to impingement of local contact at elevated velocities. Here, an impact is generated by imparting energy to a pendulum carrying an indenter, which then swings towards a specimen surface. The pendulum displacement as a function of time *x*(*t*) is recorded, from which one can extract the maximum material penetration *x*_{ max }, residual deformation *x*_{r}, and indentation durations *t*_{in} and *t*_{out}. In an inverse application one can use the *x*(*t*) response to extract material constants characterizing the impact deformation and extent of energy absorption, including material specific resistance coefficient C_{in}, coefficient of restitution *e*, and dynamic hardness *H*_{imp}. This approach also enables direct access to the ratio *H*/*E*, or resilience of the deformed material volume, at impact velocities of interest. The impact response of aluminum was studied for different contact velocities, and the mechanical response was found to correlate well with our one-dimensional contact model. Further experiments on annealed and work hardened gold showed that dynamic hardness *H*_{imp} scales with contact velocity and highlighted the importance of rate-dependent energy absorption mechanisms that can be captured by the proposed experimental approach.

## Balanced Biaxial Testing of Advanced High Strength Steels in Warm Conditions

### Experimental Mechanics (2013-11-01) 53: 1681-1692 , November 01, 2013

The main purpose of the present work is to measure the stress–strain behavior under warm conditions (about 100 °C) of advanced high strength steel (AHSS) sheets up to large strains compared to uniaxial tension. The test equipment consists of two main parts, i.e., a hydraulic bulge tester and a heating device. A mechanical system is attached to the test equipment for measuring the membrane stress and thickness strain at the bulge pole. The stress–strain curves were measured for three kinds of AHSS sheets with the proposed test method for various initial temperatures (10, 50 and 100 °C). The proposed method does not provide isothermal stress–strain curves because the specimen temperature increases during the test due to the effect of deformation-induced heating. A numerical scheme using thermo-mechanical finite element (FE) simulations is suggested to deconvolute the isothermal stress–strain curves.

## Moiré interferometry at VPI & SU

### Experimental Mechanics (1983-06-01) 23: 203-210 , June 01, 1983

A relatively easy technique for producing high-frequency gratings on specimens extends moiré techniques into the high-sensitivity domain. Whole-field patterns of inplane displacement components are obtained with grating frequencies of 1200, 2400 and 4000 l/mm (30,480, 60,960 and 101,600 l/in.). Moiré interferometry is a case of two-beam interference, characterized by extensive range, excellent fringe contrast and fringe localization on the specimen surface. It is a reflection technique, compatible with opaque specimens and live observation of deformation.

## A new look at the bending strength of gear teeth

### Experimental Mechanics (1980-07-01) 20: 217-225 , July 01, 1980

This paper reports on an extensive two-dimensional photoelastic study of the basic deformation behavior of gear teeth under load. Limitations of existing bending-strength design procedures are used to formulate a test program which considers the magnitude of the critical root-fillet stresses, the effects of varying the load position on the flank, the effects of friction forces at contact and the relationship between bending and shearing deformations. Particular interest is devoted to a study of the observed movement of the maximum-stress position around the fillet as load moves up the tooth flank and to the associated variation in stress-concentration factors (around the fillets) due to radial, bending and shear loads. Note is also made of the problems associated with observed ‘proximity effects’ and of the procedures carried out in an attempt to qualify the nature of the effect.

The results of the program have been used to, explain certain anomalies observed in earlier three-dimensional photoelastic model tests^{1.7} and reference is made to the development of a new design procedure for the bending strength of helical gears.

## Experiments on tension measurement in manufacturing processes

### Experimental Mechanics (1982-03-01) 22: 106-110 , March 01, 1982

Today's demand to preserve energy, to optimally utilize limited resources and to still increase quality calls for more-precise measurements in virtually all manufacturing processes.

One such field is the measurement of tension in, for instance, paper machines, printing presses, machines for the manufacturing of rubber, plastics, fabrics, etc.

Newspaper-printing presses produce today an impressive number of paper breaks. Measurement of the web tension at critical positions could give an early warning when the tension in the paper is growing to dangerous values.

In a paper macnine, it is necessary to introduce webtension measurements at many positions to optimize quality, to prevent paper breaks and to reduce wear.

Measurements have been carried out with a particular force transducer that can be easily built into an existing, as well as a new machine. Several installations, mainly in paper machines and newspaper-printing presses, show that

- the number of paper breaks are drastically reduced

- a more-uniform quality of the paper can be achieved

- the lifetime of machine parts, such as wires and felts, can be increased.

## Separation of the isochromatics-isopachics patterns by use of retarders in holographic photoelasticity

### Experimental Mechanics (1976-04-01) 16: 146-150 , April 01, 1976

By double passing the object beam in photoelastic-holographic interferometry, separation of the isochromatic-isopachic patterns can be achieved. The object beam must then interact with a suitable polarizing element between the first and second pass.

This problem is analyzed using Jones' matrices. This analysis shows that, apart from active and passive rotators, half-wave and quarter-wave retarders can also be used as polarizing elements. Possible experimental arrangements of the method are reviewed and experimental verifications are presented.

## Annular Pulse Shaping Technique for Large-Diameter Kolsky Bar Experiments on Concrete

### Experimental Mechanics (2014-10-01) 54: 1343-1354 , October 01, 2014

The goal of this study is to design a novel annular pulse shaping technique for large-diameter Kolsky bars for investigating the dynamic compressive response of concretes. The purpose of implementing an annular pulse shaper design is to alleviate inertia-induced stresses in the pulse shaper material that would otherwise superpose unwanted oscillations on the incident wave. This newly developed pulse shaping technique led to well-controlled testing conditions enabling dynamic stress equilibrium, uniform deformation, and constant strain-rate in the testing of a chosen concrete material. The observed dynamic deformation rate of the concrete is highly consistent (8 % variation) with the stress in the specimen well equilibrated confirming the validity of this new technique. Experimental results at both quasi-static (10^{−4} s^{−1}) and dynamic (100 s^{−1}, 240 s^{−1}) strain rates showed that the failure strength of this concrete is rate-sensitive.

## Geometric moiré methods with enhanced sensitivity by optical/digital fringe multiplication

### Experimental Mechanics (1993-12-01) 33: 349 , December 01, 1993

## Measurement of residual stresses by the hole-drilling method: General stress-strain relationship and its solution

### Experimental Mechanics (1990-03-01) 30: 33 , March 01, 1990

## Stress analysis by combination of holographic interferometry and boundary-integral method

### Experimental Mechanics (1983-06-01) 23: 196-202 , June 01, 1983

The paper describes a hybrid experimental and numerical method analysis of bodies. It consists of the experimental method of double-aperture speckle interferometry and the boundary-integral method. The interference patterns allowing evaluation of the displacement vector are obtained by the speckle interferometry. The boundary displacements obtained experimentally are conveniently used for the calculation of stresses in the body by the boundary-integral method. Some examples bear witness of the effectiveness and accuracy of the hybrid technique.

## Service-simulation tests to determine the fatigue life of outside-diameter-notched thick-wall cylinders

### Experimental Mechanics (1982-03-01) 22: 96-100 , March 01, 1982

Simulation specimens were used to model the fatigue behavior of outside-diameter-notched internally pressurized cylinders of alloy steel. Results from continuum mechanics and finite-element analyses are described for use in selection of simulation-test conditions. The effects of notch depth and residual stress on fatigue life are determined from the simulation tests.

## The rapid deposition of moiré grids

### Experimental Mechanics (1971-05-01) 11: 375-377 , May 01, 1971

This technical note describes a simple method for using the nickel meshes with up to 2000 lines/in. (lpi) as masks for producing moiré grids. The main advantages, when compared with other methods, are the substantial increase in the speed of application and the precision of the reproduced grid.

## Strain-history effect on isotropic and anisotropic plastic behavior

### Experimental Mechanics (1972-06-01) 12: 264-271 , June 01, 1972

An experimental investigation was performed to evaluate the effect of strain history on an initially isotropic material. A hot-rolled 2.5-in.-diam bar of SAE 1045 steel provided all the test specimens. Axial and circumferential compression data indicated that the steel was isotropic. Additional tension and torsion data indicated that the steel was an isotropic-hardening von Mises material; this was also confirmed by proportionate loading of thin-walled cylinders such that the ratio of axial to circumferential stresses was either 0, 1/2, 1, 2 or ∞. Two additional sets of cylinders were preloaded either in simple axial tension or as closed-ended cylinders to an effective plastic strain of 0.006 before they were proportionately loaded. The preloading had a pronounced effect on yield surfaces for reloading if the effective plastic strain on reloading was only slightly greater than that for the preloading. The effect of preloading on the yield surfaces was small when the effective plastic strain was three to four times that for the preloading. Hill's anisotropic theory was used to predict stress-strain relations for several of the reloaded cylinders. Good agreement was obtained between theory and experiment.

## Photoelastic determination of stresses in the tube plate of a 70-mw steam generator

### Experimental Mechanics (1969-10-01) 9: 449-455 , October 01, 1969

This article describes the examination, by three-dimensional photoelasticity, of the tube plate of a sodium steam generator. The tube plate is flat on the side of the tube bundle (121 tubes) and spherical (concave) on the other side.

The photoelastic model was made by precision casting, there being no glued joints at the points which are important from the point of view of stresses, such as the tube-tube plate junctions.

Both the stress distribution along important sections and the stress concentrations in different types of tube-tube plate junctions due to the internal pressure were determined.

The investigation described in this article was carried out in the framework of the Association—Euratom TNO/RCN on Fast Reactors, on behalf of the TNO—Neratoom Sodium Technology Project.

## Dynamic-stress-data management for aeromechanical testing of turbomachinery

### Experimental Mechanics (1977-06-01) 17: 207-212 , June 01, 1977

Aeromechanical testing of turbine engines is an important benchmark in the development of today's modern aircraft propulsion systems, and detailed dynamic-strain gaging of the engine's compressor and turbine are required to provide insight to the integrity of these engine components. Comprehensive real-time visualization of the compressor/turbine dynamic stress is necessary for safe engine operation and rapid posttest data-editing/processing systems are essential for day-to-day test-program direction.

This paper reviews the evolution of on-line data-monitoring and posttest data-processing/analysis techniques that have been utilized at the Arnold Engineering Development Center to support dynamic strain-gage test programs. The transition from hardwire single-channel analog analysis equipment to the incorporation of digital computers for aiding on-line data monitoring, bulk processing of test data, and rapid editing/analysis of test results is discussed. The present on-line monitoring and posttest processing/analysis systems are presented, and refinements for improving the on-line data monitoring and posttest data-processing capabilities are discussed.

## Electronic techniques in isodyne stress analysis: Part 1. Basic relations

### Experimental Mechanics (1997-03-01) 37: 33-38 , March 01, 1997

The purpose of the study was to develop electronic techniques for collecting, processing and evaluating information in experimental isodyne stress analysis. The original technique involved chemical photography recording and manual evaluation of the normal and shear isodyne functions and their derivatives, which are proportional to the stress components. One objective of the reported study was to show that it is feasible to reliably reconstruct isodyne surfaces which contain information on the internal force intensities and the components of the stress tensor. It is shown that the new technique satisfies all the theoretical conditions and constraints imposed by the theory of the analytical and optical isodynes. Thus another objective of the reported study was to demonstrate that the isodyne stress analysis allows one to obtain reliable data on the actual three-dimensional stresses in a cost-effective manner. The procedure developed to date and presented in the paper is a hybrid electronic-manual procedure. It involves electronic recording of the isodyne fields, manual determination of the isodyne orders in chosen sections, and electronic determination of the indicated and load-induced isodyne functions and of the isodyne surfaces. It is shown that the developed techniques are more reliable, accurate and cost-efficient than the traditional techniques of photomechanics. Pertinent data are illustrated by examples presented in Part 2.

## A simple method for determining principal-stress directions in embedded-polariscope models

### Experimental Mechanics (1970-06-01) 10: 390-393 , June 01, 1970

A disadvantage of the embedded-polariscope method is the inability to observe isoclinics because the embedded Polaroid sheets are fixed within the model. Perforating the embedded active layer with an array of small-diameter holes allows observation of stress trajectories in the layer from the isochromatic patterns. Further, it is shown that the full-field fringes are little disturbed by the presence of the small perforations. The techniques of model fabrication are described, as well as an extension of the method to reveal bending-stress trajectories in plates loaded out of plane.

## Holding characteristics of fasteners in bone

### Experimental Mechanics (1971-05-01) 11: 363-369 , May 01, 1971

A satisfactory method was developed for evaluating the holding characteristics of fasteners in bone. Using this method in over 100 tests, the ultimate pull-out forces and shear stresses were determined for two sizes of sheet-metal type of screws with various interference fits, for a commercial orthopedic self-tapping screw, and for two sizes of machine screws in tapped bone, each at five sections of equine metacarpus. The ultimate pull-out force was maximum at the midlength of the bone, and minimum at the distal end. In general, the failure mechanisms were bone-thread shear for low pull-out forces, bone splitting at intermediate pull-out forces, and bone fragmentation at high pull-out forces. The failure mechanisms of the bone indicate that orthopedic fasteners should possibly not be designed for maximum holding force.

## An analogue technique for determining torsion and flexure functions of uniform beams

### Experimental Mechanics (1969-03-01) 9: 117-122 , March 01, 1969

This paper describes an extremely simple, and yet accurate technique, based on electric analogy, for determining torsion and flexure functions of beams of uniform section, acted upon by terminal loads. Equations governing the torsion and flexure of such beams have been expressed to state the problem as a Neumann-type boundary-value problem; thus, the problem reduces to finding a function (or functions), which is harmonic within the cross section of the beam, and whose normal derivatives at the boundary of the section are prescribed.

Prescribed current densities are introduced at the boundary, and consequent voltages are obtained as the analogue of required functions. Instrumentation is very simple and, once what has been described here as the “influence matrix” of voltages is obtained, both torsion and flexure functions can be obtained simply by multiplying the “influence matrix” by vectors of appropriate normal derivatives at the boundary. None of the usual drawbacks of membrane analogy arise here. Use and accuracy of the technique have been demonstrated by a number of examples.

## Automated measurement of birefringence: Development and experimental evaluation of the techniques

### Experimental Mechanics (1989-09-01) 29: 252-257 , September 01, 1989

Traditional photoelasticity has started to lose its appeal since it requires a well-trained specialist to acquire and interpret results. A spectral-contents-analysis approach may help to revive this old, but still useful technique. Light intensity of the beam passed through the stressed specimen contains all the information necessary to automatically extract the value of retardation. This is done by using a photodiode array to investigate the spectral contents of the light beam. Three different techniques to extract the value of retardation from the spectral contents of the light are discussed and evaluated. An experimental system was built which demonstrates the ability to evaluate retardation values in real time.

## An Experimental Investigation on the Influence of Annealed Microstructure on Wave Propagation

### Experimental Mechanics (2015-07-01) 55: 1023-1030 , July 01, 2015

In this work grain growth associated with isochronous annealing in polycrystalline pure copper is studied using nonlinear ultrasonic method. In isochronous annealing, holding time is constant but annealing temperatures vary. It is observed that, grain growth due to isochronous annealing significantly influences the ultrasonic nonlinearity parameter, *β*. A decrease in nonlinearity parameter with increase in grain size is noticed. Further, micro-hardness measurements as well as metallographic results are presented to underscore the utility of the nonlinear ultrasonic method in gauging the progress of annealing. As the time and effort involved in this method is less, with suitable calibration, this method may be gainfully employed for determination of grain size on annealing.

## Using Depth-of-Penetration Tests to Design Transparent Armor

### Experimental Mechanics (2013-01-01) 53: 47-51 , January 01, 2013

The depth-of-penetration (DOP) test has been modified for application to transparent armor materials. Armor materials to be tested are bonded to a polycarbonate substrate and penetration into the substrate measured. The tests can be used both for comparing armor materials and for evaluating laminate parameters. Materials tested were all armor-grade transparencies: borosilicate glass, soda-lime glass, glass ceramic, and spinel. For blunt projectiles, armor material performance is inversely correlated with density. For hard, sharp projectiles, material strength is most important, with spinel providing the best performance. The efficiency of spinel-glass laminates was only weakly dependent on the spinel thickness. Linear scaling can reconcile data obtained with different size projectiles. Relatively thick bond layers do not adversely affect the efficiency of laminates. DOP tests can be used to evaluate effects of damage from single cracks and multiple hits, and results also provide an estimate of optimal armor design against the threat used in the DOP experiments.

## Measurement of Orthogonal Stress Gradients Due to Impact Load on a Transparent Sheet using Digital Gradient Sensing Method

### Experimental Mechanics (2013-01-01) 53: 97-111 , January 01, 2013

A full-field optical method called Digital Gradient Sensing (DGS) for measuring stress gradients due to an impact load on a planar transparent sheet is presented. The technique is based on the elasto-optic effect exhibited by transparent solids due to an imposed stress field causing angular deflections of light rays quantified using 2D digital image correlation method. The measured angular deflections are proportional to the in-plane gradients of stresses under plane stress conditions. The method is relatively simple to implement and is capable of measuring stress gradients in two orthogonal directions simultaneously. The feasibility of this method to study material failure/damage is demonstrated on transparent planar sheets of PMMA subjected to both quasi-static and dynamic line load acting on an edge. In the latter case, ultra high-speed digital photography is used to perform time-resolved measurements. The quasi-static measurements are successfully compared with those based on the Flamant solution for a line-load acting on a half-space in regions where plane stress conditions prevail. The dynamic measurements, prior to material failure, are also successfully compared with finite element computations. The measured stress gradients near the impact point after damage initiation are also presented and failure behavior is discussed.

## Application of the diffraction-grating strain-gage technique for measuring strains and rotations during elastic impact of rods

### Experimental Mechanics (1974-12-01) 14: 509-512 , December 01, 1974

Experimental results are presented which establish the applicability of the diffraction-grating strain-gage technique to the measurement of dynamic elastic strains and rotations on the surface of axially impacting metallic rods. A strain resolution of 2.2×10^{−4} is achieved. The smallest detectable angle of surface rotation is 10^{−4}. The accuracy of the DGSG technique for elastics-strain measurements is confirmed by the agreement of the far-field DGSG-measured strains with the theoretical prediction of elementary rod theory.

## Using Optical Interferometry to Restart the Ring-Core Method

### Experimental Mechanics (2016-04-11): 1-12 , April 11, 2016

Residual stresses are a well-known technical problem because they add to the stress field induced by external loads, thus causing mechanical components to fail at a load level significantly lower than expected. Of the various techniques developed to measure them, the ring-core method is one of the few which in principle can be restarted (by removing the core and re-installing the strain gauge rosette). Thus, it is theoretically able to measure residual stress at significantly greater depth than other methods. Although the idea is interesting, its practical implementation is quite difficult: in particular, re-installing the rosette and re-wiring is almost impossible when depth becomes significant, thus the incremental measurement is more a theoretical possibility than a real experimental approach. In this work we propose to replace the strain gauge rosette with an optical (interferometric) technique. In this way the incremental approach becomes viable, although, depending on the optical technique used, some practical problems have to be addressed.

## Full-field separation of principal stresses by combined thermo- and photoelasticity

### Experimental Mechanics (1996-12-01) 36: 318-324 , December 01, 1996

The combined use of thermoelastic stress analysis and full-field reflection photoelasticity based on the phase-stepping technique has been developed for twodimensional problems. The first method determines the sum of the principal stresses, the latter evaluates the difference of the principal stresses. Thus the principal stresses were separated at each point in the field of view without reference to neighboring points. An evaluation of this approach has been performed using a tensile plate with a central circular hole. The results show that the analysis carried out combining thermo- and photoelasticity incurred errors no larger than those of each system working independently.

## Effect upon shock spectra of the dynamic reaction of structures

### Experimental Mechanics (1961-05-01) 1: 145-151 , May 01, 1961

Attempts to use fiducial limit curves of a set of classes of shock spectra as a basis for the design of structures have shown that the design spectra obtained by the combinatorial analysis of many shock spectra tend to be overconservative. This paper presents a possible explanation for this. It exhibits some experimental evidence to show that the values of interest in a shock spectrum plot tend to lie in the valleys of that plot and not upon the peaks, whereas fiducial limit curves are controlled by the peaks of the individual shock spectra.

## Delamination of a high-temperature sandwich plate

### Experimental Mechanics (2002-06-01) 42: 206-213 , June 01, 2002

This paper describes the development of a method for determining the fracture toughness of the core/faceplate bond in high-temperature sandwich plates. The tensile deformation behavior of a sandwich element was also determined. The results from the latter experiment were used in a beam on elastic foundation analysis of the fracture specimen. The faceplate/core toughness was determined at 23 and 180°C. The room temperature toughness was slightly higher and, in both cases, the toughness decreased with crack length. The higher toughness was associated with a greater degree of interlaminar failure in the faceplates, as opposed to core-pullout.

## Experiments in mechanical and optical coincidence in photoplasticity

### Experimental Mechanics (1961-02-01) 1: 43-47 , February 01, 1961

In the extension of the shear-difference method to the separation of interior principal stresses in the elasto-plastic state^{1}, a basic question arises whether the optical isoclinics give the directions of the principal stresses, i.e., whether optical coincidence exists. Experiments are described aiming to answer this question, and preliminary results are given for cellulose nitrate as a model material. Experiments are also described dealing with mechanical coincidence.