Showing papers in "Experimental Mechanics in 1972"
TL;DR: In this article, the effect of dynamic loading on the stress-strain and failure characteristics of nugget sandstone was investigated at strain rates from 102 to 103 per second and confining pressures to 30 ksi.
Abstract: The effect of dynamic loading on the stress-strain and failure characteristics of nugget sandstone was investigated at strain rates from 102 to 103 per second and confining pressures to 30 ksi. The apparatus developed for these experiments consists of a conventional split Hopkinson bar enclosed in a pressure vessel. The apparatus permitted determination and recording of all principal stresses and strains from the onset of loading to failure. A description of the experimental techniques used in obtaining data including the method of determining stress and strain, the method of reducing data, and use of various shaped projectiles to tailor the input-stress wave are reported together with experimental results for nugget sandstone. All rocks tested exhibited an increase in strength as the loading rate increased. The dynamic stress-strain curves were similar in shape to curves from quasi-static testing of the same material.
159 citations
TL;DR: In this paper, various laser-interferometer instrumentation techniques have been applied to the study of plane-stress-wave propagation in solids and the capabilities and limitations of present systems are described.
Abstract: The various laser-interferometer instrumentation techniques which have been applied to the study of plane-stress-wave propagation in solids are reviewed and discussed. The capabilities and limitations of present systems are described.
99 citations
TL;DR: In this article, a simple and reliable technique is proposed for predicting the cartesian components of a general displacement field from the usual double-exposure holographic fringes, where an overdetermined set of simultaneous equations is developed at each point of interest and a least-squares solution provides the three displacement components.
Abstract: A simple and reliable technique is proposed for predicting the cartesian components of a general-displacement field from the usual double-exposure holographic fringes. An overdetermined set of simultaneous equations is developed at each point of interest and a least-squares solution provides the three displacement components. The reliability of this technique was tested by varying the degree of overdeterminacy of the set of equations. The three-dimensional displacement field of a beam under pure bending was determined holographically and compared with that from a closed-form theoretical solution. Finally, a highly skewed marine-propeller-blade model under uniform air pressure was analyzed holographically and the results were correlated with those from a finite-element analysis.
63 citations
TL;DR: In this paper, the use of orthogonal bicolored moire fringes with the unique capability to selectively display either set of fringes while obscuring the other set is discussed.
Abstract: This paper presents the results of an investigation to develop new techniques to enhance the moire method of mechanical interferometry. The program demonstrated the practicality of developing the use of orthogonal bicolored moire fringes with the unique capability to selectively display either set of fringes while obscuring the other set. An interpolation method for the determination of fractional fringe orders is also discussed.
52 citations
TL;DR: In this paper, a technique is described by means of which torsional waves of large, essentially constant amplitude can be generated in an elastic bar, with rise times of order 25 μs and maximum angular velocities of order 103 rads−1 have been used to test tubular specimens at shear-strain rates up to 15×103 s−1.
Abstract: A technique is described by means of which torsional waves of large, essentially constant amplitude can be generated in an elastic bar Waves with rise times of order 25 μs and maximum angular velocities of order 103 rads−1 have been achieved and used to test tubular specimens at shear-strain rates up to 15×103 s−1 Results are presented for mild steel tested at 2×103 s−1, and it is shown that the flow stress correlates well with the trend found at lower rates using conventional methods The measured drop of stress at yield, however, was considerably smaller in the present tests than in earlier work; this is attributed to the generation of flexural waves which reach the specimen at the same time as the torsional wave
52 citations
TL;DR: In this paper, the dynamic stress-strain characteristics of beef-femur bone as a function of elapsed post-mortem time have been determined directly using the split-Hopkinson-bar technique.
Abstract: The dynamic stress-strain characteristics of beef-femur bone as a function of elapsed post-mortem time have been determined directly using the split-Hopkinson-bar technique. Specimens were fabricated from dense cortical material of the posterior part of the femoral midshaft and and subjected to dynamic compressive loading (1∼6) × 108 psi/sec covering a wide range of strain rates (10∼450 sec−1). Eighty-two test runs were conducted using 43 bone specimens for a range of post-mortem ages (1∼240 days). A linear-viscoelastic model describing the mechanical behavior of bone was obtained, including an estimate of the parameters immediately after death.
44 citations
TL;DR: In this paper, the yield locus for polycarbonate was determined and an analytical expression in the form of a hypotrochoid was developed, which indicated that although poly carbonate is not a Mises material, it may be approximated by a circle over a substantial range of Lode's variable.
Abstract: A controlled-strain apparatus capable of performing the tests essential to establishing the yield locus of polycarbonate was designed and built. Material characterization tests were performed in the form of uniaxial strain rate, creep, isotropy, isoclinic and reloading tests. The yield locus for polycarbonate was determined and an analytical expression in the form of a hypotrochoid was developed. Yield locus results indicate that, although polycarbonate is not a Mises material, the yield locus may be approximated by a circle over a substantial range of Lode’s variable. Tests were run on large plates with holes. Good correlation with two-dimensional theory was obtained in the elastic zone and in the plastic zone where plastic strains were small.
36 citations
TL;DR: In this article, a mixture of flexible and rigid polyester resins was found suitable, i.e., the mixture exhibited large strains and good optical response, and the unload birefringence (fringe order immediately upon removal of load) could be used to determine strain for a uniaxial-stress field.
Abstract: The object of this investigation was to develop a technique or method for elastoplastic stress analysis using the optical effects of transparent materials. Of paramount importance was the selection and characterization of a suitable model material. In particular, it was desirable that the material be able to undergo large plastic strains while, at the same time, exhibiting a suitable level of optical response. A mixture of flexible and rigid polyester resins was found suitable, i.e., the mixture exhibited large strains and good optical response. It was found that unload birefringence (fringe order immediately upon removal of load) could be used to determine strain for a uniaxial-stress field. In particular, it provided a means for evaluating stress- and strain-concentration factors. Comparisons with other methods showed that the proposed method was reliable and gave results that are similar to those by other means. The usefulness of the material and method for two- and three-dimensional problems awaits further study.
29 citations
TL;DR: In this paper, the speckle effect of coherent light is used to produce fringes due to displacements, and these fringes have a unique interpretation in relating to displacement and they localize on the surface.
Abstract: A method of full-field measurement of displacements as well as strain on arbitrarily curved surfaces is introduced The speckle effect of coherent light is utilized to produce fringes due to displacements Unlike the fringes produced by holographic interferometry, these fringes have a unique interpretation in relating to displacements and they localize on the surface
29 citations
TL;DR: In this article, the effect of the fiber to matrix modulus of elasticity ratio varying from 10 to 200 was investigated for a two-dimensional plane-stress composite configuration having a simulated fiber volume fraction of 045 and containing a discontinuous fiber Uniaxial loading parallel to the fibers was considered.
Abstract: The effect of the fiber to matrix modulus of elasticity ratio varying from 10 to 200 was investigated for a two-dimensional plane-stress composite configuration having a simulated fiber volume fraction of 045 and containing a discontinuous fiber Uniaxial loading parallel to the fibers was considered Two independent techniques were used: moire strain analysis and finite-element analysis Displacements were measured from four experimental models by utilizing optical fringe-multiplication techniques The finite-element method yielded stresses which agreed closely with those obtained from the experimental analysis Matrix stress-concentration factor near the discontinuous fiber was found to increase rapidly with increasing modulus ratio, reaching a value of 20 for a modulus ratio of 200 The finite-element method was shown to be a valuable tool for micromechanical stress analysis of composite materials, and the accuracy of strain analysis by moire-fringemultiplication techniques was demonstrated for problems containing sever strain gradients
25 citations
TL;DR: In this article, a 16-spark gap Cranz-Schardin camera system was used to determine certain conditions leading to fracture arrest by a circular hole ahead of a propagating crack.
Abstract: Dynamic photoelasticity employing a 16-spark gap Cranz-Schardin camera system was used to determine certain conditions leading to fracture arrest by a circular hole ahead of a propagating crack. Photoelastic models of 3/8×10×10-in. Homalite-100 plates with a 1/2-in. edge crack were loaded in a fixed-grip configuration and crack arrest was made possible by central holes of 1/2, 1/4, and 0.15-in. diameters. In one test of a uniformly loaded plate with a central hole of 0.15-in. diameter, the propagating crack continued through this hole. Changes in dynamic-stress-intensity factors, as the crack tip approaches the hole, as well as changes in the dynamic-stress-concentration factors at the far side of the hole were studied, and these results were compared with the corresponding static results determined by finite-element analysis. This comparison shows that the static analysis can be used to qualitatively assess the arrest capability of the hole using the maximum static-stress concept or the proposed concept of strain energy released as the crack penetrates the hole.
TL;DR: In this paper, a finite element-based experimental technique is presented for determining the anisotropic behavior of composite materials from a broad class of experimental specimens, including both flat sheets in plane stress and thin shells subjected to combined membrane and bending deformations.
Abstract: A finite-element-based experimental technique is presented for determining the anisotropic behavior of composite materials from a broad class of experimental specimens. A brief review of the mathematical formulation precedes the discussion of three experiments which constitute a laboratory verification of the experimental technique. The class of specimens under consideration in this paper includes both flat sheets in plane stress and thin shells subjected to combined membrane and bending deformations.
TL;DR: In this article, the authors give an interpretation to the fringes observed in holographic interferometry when plane-polarized light or circularly polarized light is utilized, and the obtained patterns are formed by the superposition of three families of fringes: the two families of absolute optical retardation and the family of relative retardation.
Abstract: The purpose of this paper is to give an interpretation to the fringes observed in holographic interferometry when plane-polarized light or circularly polarized light is utilized. It is shown that, when plane-polarized light is utilized and both the loaded and the unloaded states are considered, the obtained patterns are formed by the superposition of three families of fringes: the two families of absolute optical retardation and the family of relative retardation. The intensity distribution is also a function of the orientation of the plane of polarization, and along the points where the plane of polarization is parallel to one of the principal directions, only one of the families of absolute retardation is observed. By utilizing circularly polarized light, the dependence on the orientation of the principal axis is eliminated and patterns consisting of the superposition of the three above-mentioned families are obtained. If only the loaded state is considered, the holographic interferometer behaves as an ordinary polariscope with the reference beam playing the role of the analyzer. The relationships between the observed families are discussed. Examples of application to the disk and ring under diametral compression are also given.
TL;DR: In this paper, a holographic interferometer is used to obtain separate but simultaneous isochromatic and isopachic-fringe patterns for photoelastic models in states of plane stress.
Abstract: A shortcoming of the conventional holographic interferometer used in photoelasticity is that, for the double-exposure cases, the resulting fringe patterns are a complex combination of conventional isochromatic and isopachic fringes. This paper describes a holographic interferometer that may be used to obtain separate but simultaneous isochromatic- and isopachic-fringe patterns for photoelastic models in states of plane stress. The method requires a model with a partially reflecting front surface. Isopachics, which are proportional to the thickness change, are recorded using holographic interferometry from the transmitted light. The isochromatics are obtained from the transmitted light by conventional means. General equations relating the surface displacement of the specimen to the observed fringe patterns are developed, and examples of static and dynamic loadings are shown.
TL;DR: In this article, a model of a single-rooted tooth together with its support structure, the periodontium, is the subject of a plane-stress study with the objective to correlate crown loading, support reaction and initial tooth-displacement direction.
Abstract: The orthodontic appliance (“braces”) loads a number of individual teeth; in the case of each tooth, the displacement which results is related to the form of the active force system transmitted from the appliance to the tooth crown. In this paper, a model of a single-rooted tooth together with its support structure, the periodontium, is the subject of a plane-stress study with the objective to correlate crown loading, support reaction and initial tooth-displacement direction.
TL;DR: In this article, the application of holography to stress analysis of opaque, anisotropic composite plates subjected to static or dynamic transverse and in-plane loads is presented, and the equations of motion show that the complete state of stress at a generic point in any lamina, plus the moments and shear forces, are related to the temporal and spatial partial derivatives of the transverse displacement of the middle surface.
Abstract: The application of holography to stress analysis of opaque, anisotropic composite plates subjected to static or dynamic transverse and in-plane loads is presented. The equations of motion show that, for flexed, anisotropic, laminated composite plates, the complete state of stress at a generic point in any lamina, plus the moments and shear forces, are related to the temporal and spatial partial derivatives of the transverse displacement of the middle surface. Interferometric holography is shown to be well suited for obtaining the transverse displacement. Methods of determining the required spatial and temporal partial derivatives are discussed, an example being demonstrated. The holographic determination of anisotropic-material properties, stress and strain concentrations and the nondestructive evaluation of critical buckling loads for composite structures, plus the use of holographically obtained isopachics to supplement photoelastic analyses of composites, are also discussed. Experimental results for statically and dynamically loaded composite plates and beams with and without geometric discontinuities are presented to illustrate the concepts and techniques.
TL;DR: In this article, a physical explanation of photoelastic interferometry and holography is presented and reasons for special properties and specific restrictions are argued in relation to this physical picture.
Abstract: A physical explanation of photoelastic interferometry and holography is presented. Reasons for special properties and specific restrictions are argued in relation to this physical picture. The controlling intensity relationships for interferometry and holography are derived by simple physical analysis. The relationship between absolute retardation, isochromatic- and isopachic-fringe systems is clarified. Model material requirements for photoelastic interferometry and holography are shown to be identical.
TL;DR: In this article, the strength characteristics of S-glass fiber bundles and composites subjected to quasi-static loading were investigated and the experimental mean composite strength is compatible with that obtained on the basis of rule of mixtures and Gucer-Gurland models.
Abstract: The primary aim of this investigation was to establish the strength characteristics of S-glass fiber bundles and composites subjected to quasi-static loading. Ten glass-bundle specimens and glass-fiber unilayer specimens, each containing thirty-one approximately equally spaced fibers (S-glass, 0.00485-in. diameter) were prepared and tested in an Instron machine at three strain rates (0.0265 in./in./min, 0.66 in./in./min and 26.5 in./in./ min). Grid lines were placed on composite specimen producing interference moire fringes with a reference master grid placed in front of the specimen. The specimens were observed photographically during deformation. The experimental bundle strength compares well with that obtained on the basis of Daniels' theory. The experimental standard deviation is, however, much larger than that predicted theoretically. The experimental mean composite strength is compatible with that obtained on the basis of rule of mixtures and Gucer-Gurland models. The Zweben crack-propagation criterion [E2(fC)=1] gives too low a value for the composite strength. A new criterion [E3(fC)=1] is suggested for the present test series. Due to the rather large standard deviation and the small number of test samples it was not possible to quantitatively evaluate the effect of rate of straining. However, it is observed that, within the range of strain rates employed, the effect of strain rate on bundle and composite strengths does not exceed 20 percent and 10 percent, respectively.
TL;DR: In this article, the effect of material inhomogeneity on the distribution of stress in annulus fibrosus is studied, and the results of the theoretical analysis show that inhomogeneous influences the distribution for tangential stress considerably, whereas only heavy lateral pressure gradients disturb this same stress distribution to any significant extent.
Abstract: The intervertebral disk is a heavily loaded component of the human body. In the lumbar region, in vivo measurements by Nachemson (1966, 1970)4–5 show compressive loads in the range of 1000–3000 Newton under normal living conditions. Determination of stresses in intervertebral disks is, thus, a relevant biomechanical problem that has been studied experimentally and from a clinical point of view by Nachemson (1960, 1963, 1966, 1970).2–5 Galante (1967)1 investigated tensile properties of the lumbar annulus fibrosus. Nachemson found the state of stress in the nucleus pulposus to be hydrostatic, and Galante's investigation revealed a considerable inhomogeneity of the material in the annulus fibrosus. In this paper, the effect of material inhomogeneity on the distribution of stress in annulus fibrosus is studied. On the basis of previous experimental findings and complementary measurements of the lateral pressure distribution through the disk, the tangenital and radial stress distributions are determined theoretically. The results of the theoretical analysis show that inhomogeneity influences the distribution of tangential stress considerably, whereas only heavy lateral pressure gradients disturb this same stress distribution to any significant extent. Experimental results show that high pressure gradients may well be present in annulus. Only pure compression of the disk is considered.
TL;DR: In this article, a new method for determining the strain distribution at the surface of solid bodies is described, which is purely optical; it 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.
TL;DR: In this article, the authors describe the utilization of a fatigue-damage summation method which incorporates two of these variables, mean stress and overloads, for complex load histories in mild steel specimens in the intermediate to long-life range.
Abstract: Successful estimation of the service life of a structure or component which is subjected to a complex history of loading, depends on a suitable cumulative-damage summation technique. A general technique must be capable of predicting the effects on fatigue life of geometry, mean stress or strain, occasional overloads or overstrains, frequency of cycling and environment. As a contribution towards the general solution, this paper describes the utilization of a fatigue-damage summation method which incorporates two of these variables, mean stress and overloads. The method is tested for complex load histories in mild-steel specimens in the intermediate to long-life range.
TL;DR: In this paper, it is shown how the whole field of displacement components and their time and space derivatives (isothetics, isotachics and isoparagogics) can be obtained individually, as separate patterns in a simple and precise manner using spatial filtering techniques.
Abstract: It is shown in this paper how the whole field of displacement components and of their time and space derivatives (isothetics, isotachics and isoparagogics) can be obtained individually, as separate patterns in a simple and precise manner using spatial-filtering techniques. This result can be obtained even when crossed gratings are used on the deformed body. A method for achieving fringe multiplication in moire patterns produced by superposed, crossed gratings is also demonstrated. It is also shown that displacement components and their time and space derivatives in directions diagonal to the crossed-grating lines can be obtained by proper handling of grating transparencies and spatial-filtering techniques. Hence, the moire equivalent of a whole field of rosette-strain-gage measurements is obtained from a single photograph of a deformed crossed grating. A disk compressed between two wedges is used as an example. Important applications will be found in the fields of dynamics, nonlinear elasticity and plasticity.
TL;DR: In this paper, a general relation based upon laminated-plate theory is developed which relates the state of stress throughout the thickness of the fiber-reinforced composite laminate to experimentally determined surface strains.
Abstract: A general relation based upon laminated-plate theory is developed which relates the state of stress throughout the thickness of the fiber-reinforced composite laminate to experimentally determined surface strains. The usefulness of the developed relation is discussed through an example given for a typical graphite-epoxy structural laminate. Birefringent coatings employed as a method to determine laminate surface strains are investigated. Isochromatic disturbances in the vicinity of a laminate free-edge, due to a Poisson's ratio mismatch between coating and laminate and interlaminar stress transfer between discrete lamina of the laminate, are investigated through exact elasticity solutions.
TL;DR: In this article, the authors determined the stress concentrations in tensile strips having large circular holes where the ratio of the hole diameter to strip width is greater than 0.5 and showed that the stress-concentration factor based on net area approaches a value of one as the ratio between hole diameter and strip width approaches the value of 1.
Abstract: The purpose of this investigation was to determine more accurately the stress concentrations in tensile strips having large circular holes where the ratio of the hole diameter to strip width is greater than 0.5 Results of numerical elasticity analyses are presented for ratios of hole diameter to bar width ranging from 0.10 to 0.99, and photoelastic data are presented for the range from 0.40 to 0.94. Both numerical results and photoelastic data indicate that the stress-concentration factor based on net area approaches a value of one as the ratio of hole diameter to bar width approaches a value of one.
TL;DR: The past decade has witnessed a widespread resurgence of interest in micromechanics of continuous media which lay dormant for over half a century as discussed by the authors, with emphasis placed on their applicability to actual engineering problems, particularly to problems of stress and vibration analysis.
Abstract: The past decade has witnessed a widespread resurgence of interest in micromechanics of continuous media which lay dormant for over half a century. Some of the ideas underlying these nonclassical theories are reviewed, with emphasis placed on their applicability to actual engineering problems, particularly to problems of stress and vibration analysis.
TL;DR: In this article, the authors examined the validity of split-Hopkinson-pressurebar technique for measuring dynamic strain and stress by comparing the average strain computed by use of that technique with the strain at the midpoint of the sample.
Abstract: The validity of the split-Hopkinson-pressurebar technique for measuring dynamic strain and stress is examined by comparing the average strain computed by use of that technique with the strain at the midpoint of the sample. The midpoint strain is measured by use of the inter-ferometric-strain-gage method. It is found that the midpoint surface strain and the average strain agree quite well after a strain of approximately 1 percent. It is concluded that the pressure-bar technique is valid for large strains.
TL;DR: In this article, an idealized cantilever beam under end-loading and uniformloading conditions is simulated on a computer and the shear strain and bending moment results are compared with the known analytic results.
Abstract: Holographic measurements of an idealized cantilever beam under end-loading and uniform-loading conditions are simulated on a computer. Cubic splines are fitted to the simulated data and then used to evaluate the shear strain and bending moment. Controlled data errors are introduced into the simulated measurements and the strain and bending-moment results are compared with the known analytic results. When the data are not resolution limited, the accuracy of the results increases as more cubic splines are taken. However, when the data are resolution limited, the accuracy of the results is a maximum for an optimum number of cubic splines and the use of more splines actually decreases the accuracy.
TL;DR: In this article, the results of an experimental investigation of the postbuckling behavior of a ferromagnetic cantilever beam in a transverse magnetic field are presented, and the experimental apparatus and the technique used to cancel the effect of initial curvature and other invevitable imperfections by systematically varying the misalignment of the field and the normal to the beam are discussed.
Abstract: The results of an experimental investigation of the postbuckling behavior of a ferromagnetic cantilever beam in a transverse magnetic field are presented. A description of the experimental apparatus and the technique used to cancel the effect of initial curvature and other invevitable imperfections by systematically varying the misalignment of the field and the normal to the beam are discussed. For each misalignment, the buckling field is measured. A plot of buckling field vs. misalignment has the usual cusp and rapid decline of the buckling field with increasing imperfection that has come to be associated with imperfection-sensitive structures. Previous theoretical predictions are found to be in good qualitative agreement with the experimental results. Possible explanations for quantitative differences between experimental and theoretical results and sources of experimental error are discussed.
TL;DR: In this paper, finite-incremental and total-strain theories were derived for solid circular section torsion-tension members subjected to non-proportionate loading, based on the assumption that the material was isotropic and even.
Abstract: Based on the assumption that the material was isotropic and even, and satisfied the condition of isotropic hardening for a von Mises material, finite-incremental and total-strain theories were derived for solid circularsection torsion-tension members subjected to nonproportionate loading. Torsion-tension members made of SAE 1045 steel and aluminum alloy 7075-T6 were subjected to proportionate and nonproportionate loading. During the nonproportionate loading, either the axial loal P or torque T was held constant while the other was increased. Excellent agreement was found between the incremental theory and experimental data indicating that the assumption of isotropic hardening is valid for this type of loading. For some of the nonproportionate loading paths, incremental and total-strain theories gave nearly identical results.
TL;DR: In this paper, a conducting polymer consisting of graphite particles in on epoxy matrix has been employed as a transducer, and the conductor undergoes a resistance change which is due to the variation in contact pressure between the particles.
Abstract: A conducting polymer consisting of graphite particles in on epoxy matrix has been employed as a transducer. When subjected to strain, the conductor undergoes a resistance change which is due to the variation in contact pressure between the particles. Of more importance is the permanent resistance change produced in the conductor as it is cycled. This resistance change is due to a wear mechanism which improves the contact area between adjacent particles.