Showing papers in "Experimental Mechanics in 1971"
TL;DR: From experimental measurements of ground-to-foot force action and limb configurations, resultant load actions at junctions of leg segments can be calculated from a knowledge of the phasic activity of muscles and their anatomical location, the tension in relevant muscles and ligaments may be inferred, and the joint forces obtained as mentioned in this paper.
Abstract: From experimental measurements of groundto-foot force action and limb configurations, resultant load actions at junctions of leg segments can be calculated. From a knowledge of the phasic activity of muscles and their anatomical location, the tension in relevant muscles and ligaments may be inferred, and the joint forces obtained. From the measured geometry of the femur calculations are made of the stresses on the basis of simplifying assoumptions of material disposition and behavior.
72 citations
TL;DR: In this paper, a transparent model material fabricated from glass fibers and a modified polyester matrix exhibits continuous relatively smooth fringe patterns which are linearly related to the state of stress, which can be treated as a homogenous medium with orthotropic properties.
Abstract: A method of producing transparent model materials for photo-orthotropic-elastic studies is presented. This material fabricated from glass fibers and a modified polyester matrix exhibits continuous relatively smooth fringe patterns which are linearly related to the state of stress. As such, the heterogenous material can be treated as a homogenous medium with orthotropic properties.
69 citations
TL;DR: In this paper, a sharp crack is simulated by a relatively narrow notch terminating in a root radius of 0.010 in or less, and the results are used to determine the stress-intensity factor.
Abstract: The increasing number of analytical and numerical solutions for the crack-tip stress-intensity factor has greatly widened the scope of application of linear elastic fracture-mechanics technology. Experimental verification of a particular solution by elastic stress analysis is often a necessary supplement to provide the criteria for proper application to actual design problems. In this paper, it is shown that the photoelastic technique can be used to obtain rather good estimates of the stress-intensity factor for various specimen geometries and loading conditions. Treated are the following cases: wedge-opening load specimen, several notched rotating-disk configurations, and a notched pressure vessel. A sharp crack is simulated by a relatively narrow notch terminating in a root radius of 0.010 in or less. Stress distributions along the section of symmetry ahead of the notch tip are obtained using three-dimensional frozen-stress photoelasticity. The results are used to determine the stress-intensity factor, cK
I
, by three methods. Two of these are based on Irwin's expressions for the elastic stress field at the tip cf a crack, and the other is a result of Neuber's hyperbolic-notch analysis. Agreement, with available analytical solutions is good.
64 citations
TL;DR: In this paper, a linear elastic fracture mechanics is used to interpret observations of through-thickness fatigue-crack growth in sheet specimens of polymethylmethacrylate.
Abstract: Linear-elastic fracture mechanics is used to interpret observations of through-thickness fatigue-crack growth in sheet specimens of polymethylmethacrylate. The paper shows how statistical techniques have been used to design and interpret experiments to determine the interactions and main effects of frequency, mean and range of stress-intensity factor on fatigue-crack-growth rate.
63 citations
TL;DR: A machine for testing thin-walled tubes in torsion at shear-strain rates up to 25/sec is described in this paper, and constant and variable-rate tests are presented for 1100-0 aluminum, AISI 1020 steel, and 50-A titanium.
Abstract: A machine for testing thin-walled tubes in torsion at shear-strain rates up to 25/sec is described Results of constant and variable-strain-rate tests are presented for 1100-0 aluminum, AISI 1020 steel, and 50-A titanium Results indicate that 1100-0 aluminum is very slightly strain-rate sensitive, but steel and titanium are noticeably sensitive to both strain rate and strain-rate history Variable-rate tests show that subsequent dynamic loading on a statically prestrained specimen causes an increase in the flow stress in steel and a decrease in the flow stress in titanium
58 citations
TL;DR: In this article, the authors investigated elastic buckling under aial compression of finite, oval cylindrical shells with clamped boundaries and found that the discrepancy between theoretical and experimental initial buckling loads for the ovals is similar to that of the circular cylinders.
Abstract: Elastic buckling under aial compression of finite, oval cylindrical shells with clamped boundaries was investigated experimentally. The determination of the buckling strength was made on a series of oval shells made of Mylar A. The test results indicated that the discrepancy between theoretical and experimental initial buckling loads for the ovals is similar to that of the circular cylindrical shells. However, in contrast to the circular case, a collapse load significantly exceeding the initial buckling load is observed in the case of ovals with moderate-to-large eccentricity.
39 citations
TL;DR: In this paper, a new system has been developed which readily permits simultaneous acquisition of these fringe patterns without their undesirable mutual interaction, as well as providing increased sensitivity, and the magnitudes of the principal stresses can readily be determined throughout the field of observation.
Abstract: In holographic interferometry with a photoelastic model, two families of fringes are generated simultaneously when the model is stressed. One family represents the isochromatic-fringe pattern normally associated with photoelasticity which yields the difference between the principal stresses. The other family represents the isopachic-fringe pattern associated with interferometry which yields the sum of the principal stresses. From these complementary patterns, the magnitudes of the principal stresses can readily be determined throughout the field of observation. Unfortunately, these fringe patterns are not completely independent but interact in such a way as to make interpretation difficult in critical regions of the model. A new system has been developed which readily permits simultaneous acquisition of these fringe patterns without their undesirable mutual interaction, as well as providing increased sensitivity.
39 citations
TL;DR: In this article, a magnetic method for the measurement of residual longitudinal stress in the outer portions of cylindrical bars is developed and applied to nickel and steel, which involves measurement of the reversible effective permeability over a range of frequency of the applied alternating field.
Abstract: A magnetic method for the measurement of residual longitudinal stress in the outer portions of cylindrical bars is developed and applied to nickel and steel. It involves measurement of the reversible effective permeability over a range of frequency of the applied alternating field. Special composites specimens, in which any desired level of residual stress can be produced, serve as idealized test specimens. Magnetic stress measurements made on cold-drawn, machined and quenched rods are compared with measurements by X-ray diffraction and mechanical relaxation (slitting). A combination of magnetic and X-ray measurements yields qualitative information about the stress gradient in the outer portion of a bar.
36 citations
TL;DR: In this article, it was shown that the position and even the existence of the fringes are affected by the interaction of the isopachics and isochromatics, and that the independent superposition interpretation is not always valid and can result in serious errors in some cases.
Abstract: Previous studies had concluded that stressholo-interferometry patterns consist of the independent superposition of the isopachic family (with half-order fringe shifts) and the isochromatic family. It is shown here that this interpretation is not always valid and can result in serious errors in some cases. In particular, it is demonstrated that the position, and even the existence of the fringes, are affected by the interaction of the isopachics and isochromatics. This effect is most pronounced when the two families of fringes are nearly parallel and of approximately the same spatial frequency. The independent superposition interpretation is most accurate when the two families of fringes are orthogonal, whatever the ratio of spatial frequencies might be. These properties are illustrated using computer-generated holographic interference patterns.
35 citations
TL;DR: In this article, double-exposure holographic interferometry was used to record the propagation of transverse waves in a plate and plots of the transverse displacement vs. distance from the center of impact were obtained.
Abstract: The technique of double-exposure holographic interferometry was used to record the propagation of transverse waves in a plate. The waves were initiated by striking the plate in the center with a ballistic pendulum; this technique produced axisymmetric response of the plate. Several pulsed-laser interferograms were obtained, each of which showed the bending wave at a different time after the initiation of impact. From the interferograms, plots of the transverse displacement vs. distance from the center of impact were obtained. The experimental data compared favorably with analytical results for the problem.
34 citations
TL;DR: In this paper, an experimental method has been developed for the detailed, real-time study of the strain field near a crack tip in any transparent material, combining holography and interferometry to measure the field of averaged transverse strain in a sheet specimen under inplane loading.
Abstract: An experimental method has been developed for the detailed, real-time study of the strain field near a crack tip in any transparent material. This method combines holography and interferometry to measure the field of averaged transverse strain in a sheet specimen under inplane loading. It is shown that the solution for an edge crack in a semi-infinite sheet based on two-dimensional linear elastic theory can be applied to the finite-width polymethylmethacrylate specimen. This solution is observed to become less valid as the crack tip is approached due to the breakdown of the plane-stress assumption. This effect is evaluated for a range of thicknesses from much less to much more than the crack length.
TL;DR: In this article, a streak camera and moire technique were developed to measure the deflection as a function of time for the first few hundred microseconds after impact, and the measured and predicted displacements agree within a few percent, except near the boundary between the plastic and the elastic region, for all times up to when elastic unloading commences.
Abstract: The problem of plastic deformation of thin plates struck by blunt projectiles but not perforated is considered. A streak camera and moire technique were developed to measure the deflection as a function of time for the first few hundred microseconds after impact. The maximum radial strain is about three times the yield strain, the maximum tangential strain is very small. From these and other considerations it was found that there are three main regions of deformation. Since this is a traveling-wave problem, there is a radius beyond which there is no deformation. Inside of this is an annular region traversed by elastic bending waves of the Boussinesq type. Inside of this is an annular region whose dynamic behavior is governed by a membrane stress equal to the tensile yield stress and whose outer boundary travels at a wave velocity governed by the membrane equation. The inner region is a circular region of the diameter of the projectile and traveling with it. The projectile slows and comes to a stop after about 200 or 300 μsec. After this, the material unloads elastically; this elastic vibration behavior was not investigated. Both the time-displacement curve for the projectile and the displacement as a function of radius for specific times can be calculated numerically for dimensionless radius and dimensionless time for one parameter which is governed primarily by the projectile relative mass. The measured and predicted displacements agree within a few percent, except near the boundary between the plastic and the elastic region, for all times up to when elastic unloading commences.
TL;DR: In this paper, the effect of an intensity difference between the light used during the first and the second exposure has been investigated and it is shown that as the ratio of these two intensities is changed both the position and the intensity of the photoelastic-fringe pattern is altered.
Abstract: The success of double-exposure holography as an interferometric technique for experimental stress analysis has lead to several recent publications dealing with the theoretical expression which describes the resulting photoelastic patterns. This paper describes the extension of current theory to include the effect of an intensity difference between the light used during the first exposure and that used during the second exposure. It is shown that as the ratio of these two intensities is changed both the position and the intensity of the photoelastic-fringe pattern is altered. An interpretation of the photoelastic pattern as a simple combination of isochromatic and isopachic-fringe patterns is shown to be possible only under certain conditions.
TL;DR: In this article, a double-exposure holographic interferometry was used to record transverse waves propagating in a long beam, initiated by striking the beam in the center with a ballistic pendulum.
Abstract: The technique of double-exposure holographic interferometry was used to record transverse waves propagating in a long beam. The wave was initiated by striking the beam in the center with a ballistic pendulum. Pulsedlaser interferograms were obtained which showed the bending waves at three different times after the initiation of impact for three different experiments. From the interferograms, plots of the transverse displacement vs. distance along the beam were obtained. The experimental data compared favorably with analytical results for the problems. The experimental technique, differential interferometry, was also introduced to facilitate measurement of largeamplitude motions using holography.
TL;DR: In this paper, the modes of failure of a pressure vessel which are affected by the choice of material and wall thickness are given, and the factors which should be considered in choosing an allowable stress low enough to prevent these failure modes are discussed.
Abstract: The modes of failure of a pressure vessel which are affected by the choice of material and wall thick-ness are given. The factors which should be considered in the choice of an allowable stress low enough to prevent these failure modes are discussed.
TL;DR: In this paper, the problems associated with the use of true and distorted models are illustrated by means of selected examples, and the techniques and the problems involved with the design and use of models are covered.
Abstract: The techniques and the problems associated with the design and use of models are covered. Particular emphasis is placed on the development of modeling laws by means of dimensional analysis. Special problems associated with the use of true and distorted models are illustrated by means of selected examples.
TL;DR: The Fe−3Si steel etching technique for measuring localized plastic deformation is described in this paper, which is well suited for revealing plastic strain in the vicinity of notches, cracks and other stress concentrations, both on the micro- and macroscale.
Abstract: The Fe−3Si steel etching technique for measuring localized plastic deformation is described. The technique is well suited for revealing plastic strain in the vicinity of notches, cracks and other stress concentrations, both on the micro- and macroscale. The method is capable of yielding quantitative information about the strain distribution on the surface and in the interior of test specimens whose plastic-flow properties model the behavior of engineering alloys, in general, and low- and medium-strength steels, in particular. Methods of melting and fabricating the steel for this purpose, specimen preparation, and metallographic procedures are explained. Mechanical properties of the steel and its special etching response are described. In addition, results of some plastic-zone determinations are presented to illustrate the technique's capabilities, and other applications are mentioned.
TL;DR: In this paper, the relationship between plastic thickness change and plastic isochromatics occurring beyond the plostic tensile instability point of a uniaxial tensile specimen is investigated.
Abstract: The relationship between plastic thickness change and plastic isochromatics occurring beyond the plostic tensile instability point of a uniaxial tensile specimen is investigated. Mechanical thickness-change measurements and a hologram of thickness change are shown to be in close qualitative agreement with isochromatics in a region of gross plastic yielding. Analytical observations are discussed to aid the interpretation of the experimental results obtained. Finally, possible extensions and applications are discussed.
TL;DR: In this paper, a method for measuring strain using diffraction of light from a single aperture is described, and results of a comparison tensile test with an electrical-resistance strain gage are presented.
Abstract: A method for measuring strain using diffraction of light from a single aperture is described, and results of a comparison tensile test with an electrical-resistance strain gage are presented. The “diffractographic strain gage” is shown to have high sensitivity, linearity, accuracy and temperature compensation and the ability to operate in a variety of hostile environments. It is furthermore simple, inexpensive, and the data can be collected by eye without assistance from further instrumentation.
TL;DR: In this article, the cross-sectional shapes for the strongest, end-loaded column and the radially loaded arch whose neutral axis is circular were predicted and the predicted buckling loads were measured on metal models of tapered, pinned-end columns using strain gages.
Abstract: Recent theories predict the cross-sectional shapes for the strongest, end-loaded column and the radially loaded arch whose neutral axis is circular. These tapered shapes have higher predicted load-carrying capacities than uniform columns or arches of equal weight and length or span. The purpose of the present experimental investigation was to study the stability of these configurations, modified so that no experimental model had the predicted cross-sectional area of zero where the bending moments vanished. Precise buckling loads were measured on metal models of tapered, pinned-end columns using strain gages and a unique modification of Southwell's method. The dynamic stability of these end-loaded columns under transverse vibrations was observed. Static stability tests were also performed on Plexiglas models of the strongest shaped circular arch and on the uniform circular arch of equal weight and span. The predicted buckling loads for the strongest shapes agreed reasonably well with measurements. The shaped circular arch, however, was found to be an inefficient design for loading applied uniformly across the span rather than in the radial direction.
TL;DR: In this paper, it was shown that negative critical-speed variations reduce the disk stability and positive critical speed variations increase stability, and such shifts are directly related to variation in the critical rotation speed at which standing-wave-resonance instability occurs.
Abstract: Membrane stresses alter the dynamic behavior and stability of circular-disk elements, such as circular saws and grinding wheels, by shifting the disk's natural frequency spectrum. Such shifts are directly related to variation in the critical rotation speed at which standing-wave-resonance instability occurs. Negative critical-speed variations reduce the disk stability, and positive critical-speed variations increase stability.
TL;DR: The linear-elastic fracture-mechanics approach to the design against high-cycle-fatigue failure is described in this article, where the basic concepts underlying the technology are presented and the techniques used to establish realistic design criteria are demonstrated.
Abstract: The linear-elastic fracture-mechanics approach to the design against high-cycle-fatigue failure is described. The basic concepts underlying the technology are presented and the techniques used to establish realistic design criteria are demonstrated. An example problem is also included and the associated considerations and computations described in detoil.
TL;DR: In this article, a set of photoelastic experiments was conducted on plates, each containing a single through internal crack in a remote cylindrical bending field, where each crack suffered precatastrophic extension on the tensile side of the plate as well as closure on the compression side.
Abstract: Using the technique of stress freezing and slicing, a set of photoelastic experiments was conducted on plates, each containing a single through internal crack in a remote cylindrical bending field, where each crack suffered precatastrophic extension on the tensile side of the plate as well as closure on the compression side. Resulting stresses were compared with the Sih-Hartranft theory. Results indicate substantial stress relaxation near the tensile side of the plate due to precatastrophic extension. It is concluded that: (1) the Sih-Hartranft theory may be used to estimate local elastic stresses on the tensile side of the plate even when closure and precatastrophic extension occur; (2) complex coupling of closure and precatastrophic extension effects preclude the use of simple correction factors for existing mathematical models except for relatively small precatastrophic extension.
TL;DR: In this paper, the relative opening displacement between the crack surfaces and between the upper and lower boundaries of the strip necking region were measured by the moire method, and the strain ahead of the region and the thickness reduction were also measured.
Abstract: When a tensile stress is applied to a thin cracked plate, a strip necking region results ahead of a crack tip. The relative opening displacement between the crack surfaces and between the upper and lower boundaries of the strip necking region were measured by the moire method. The strains ahead of the strip necking region and the thickness reduction (therein) were also measured. The measured relative opening displacements were compared with the calculated values using the Dugdale strip necking model. The thickness reduction in the strip necking region is equal to the relative opening displacement.
TL;DR: In this paper, two techniques of moire-pattern differentiation are described and implemented, one of which is extremely simple since the grid printed in the model provides both the displacement information and the shearing mechanism required to display the displacement derivatives.
Abstract: Two techniques of moire-pattern differentiation are described and implemented. One of the techniques is extremely simple since the grid printed in the model provides both the displacement information and the shearing mechanism required to display the displacement derivatives. This technique can only be applied to the derivatives in the direction parallel to the projected displacement. To find the derivativs in the direction perpendicular to the projected displacement, a double-exposure technique is utilized.
TL;DR: In this article, an improved embedded-strain-gage technique for the measurement of strains at internal points in models of machine components is described, which allows an accurate embedment of gages at 0.043 in. below the surface of a flat model of epoxy resin.
Abstract: Improvements in the embedded-strain-gage technique for the measurement of strains at internal points in models of machine components are described. A New measurement module permits an experimental determination of the strain in models of epoxy resin. By its construction, the centers of the rosettes may be accurately located in the model mold, before casting. A large number of gages may be embedded with accuracy in a systematic manner. Application to the study of rolling contact is discussed. The improved technique has permitted an accurate embedment of gages at 0.043 in. below the surface of a flat model of epoxy resin which is then pressed against a spherical model having a radsius of curvature machined to 75 in. A testing machine is described and preliminary test results are presented.
TL;DR: In this paper, a simple but effective test configuration for measuring Poisson's ratio is described and test results are displayed, which is necessary to take special precautions to eliminate large straining between small-strain tests of different tensorial character.
Abstract: 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.
TL;DR: In this article, the authors generalized the moire gap equation to include linear and rotational mismatches and applied it to the measurement of deflection of beams and plates in optical whole field.
Abstract: The moire gap equation originally derived by Sciammarella and Chiang has been generalized to include linear and rotational mismatches. The equation is then applied to the measurement of deflection of beams and plates. The method has the advantages of being simple in comparison with other optical whole-field methods. The application of moire gap effect to the study of dynamic problems is also discussed.
TL;DR: In this article, the transformations that the light ellipse undergoes as it traverses through several optical elements give a better physical insight than the conventional trigonometrical transformations that are generally applied to the light vectors.
Abstract: In dealing with various types of polariscopes the transformations that the light ellipse undergoes as it traverses through several optical elements give a better physical insight than the conventional trigonometrical transformations that are generally applied to the light vectors. Further, an understanding of the transformations of the light ellipse suggests several interesting methods for the determination of the isoclinic parameter.
TL;DR: In this paper, a new method for determining the strain distribution at the surface of solid bodies is described, which uses the diffraction phenomena generated by a copy of a grating that is applied to the specimen.
Abstract: This paper describes a new method for determining the strain distribution at the surface of solid bodies. The method is purely optical; it uses the diffraction phenomena generated by a copy of a grating that is applied to the specimen. A suitable mask performs filtering of the diffraction pattern; the image that is reconstructed from this filtered pattern shows light and dark areas; it is shown that the boundary line of those areas is the locus of points exhibiting the same value of strain, measured along a certain direction.The magnitude of the strain can be easily calculated; it can be adjusted by a simple translation of the filter. A theoretical description of the system and some experimental results are presented.