Showing papers in "Experimental Mechanics in 1987"

Strain gage methods for measuring the opening mode stress intensity factor, KI

Abstract: Measurements of strain near a crak tip with electrical-resistance strain gages do not usually provide a reliable measure ofK I because of local yielding, three-dimensional effects and limited regions for strain-gage placement. This paper develops expressions for the strains in a valid region removed from the crack tip, and indicates procedures for locating and orienting the gages to accurately determineK I from one or more strain-gage readings.

An experimental and analytical treatment of matrix cracking in cross-ply laminates

Abstract: The development of damage in cross-ply Hercules AS4/3502 graphite/epoxy laminates has been investigated. Specific endeavors were to identify the mechanisms for initiation and growth of matrix cracks and to determine the effect of matrix cracking on the stiffness loss in cross-ply laminates. Two types of matrix cracks were identified. These include both straight and curved cracks. The experimental study of matrix crack damage revealed that the curved cracks formed after the straight cracks and followed a repeatable pattern of location and orientation relative to the straight cracks. Therefore, it was postulated that the growth mechanism for curved cracks is driven by the stress state resulting from the formation of the straight cracks. This phenomenon was analytically investigated by a finite-element model of straight cracks in a cross-ply laminate. The finite-element results provide supporting evidence for the postulated growth mechanism. The experimental study also revealed that the number of curved cracks increased with the number of consecutive 90-deg plies. Finally, experimental results show as much as 10-percent degradation in axial stiffness due to matrix cracking in cross-ply graphite/epoxy laminates.

Further development of the losipescu shear test method

Abstract: Recent activities by the present investigators to further develop the losipescu-shear-test method for use with composite materials are summarized Finite-element analyses used to predict the stress states in the specimen are described, with particular emphasis on how they are influenced by the specific test-fixture configuration used These same analytical tools were also used to predict the influence of specimen notch depth, notch angle, and notch-root radius The result was a redesign of the original Wyoming version of the losipescu-shear-test fixture, and the establishment of guidelines for preparing specimens These are discussed in some detail Many references to available literature are included An attempt has been made to put the work performed to date into perspective, to aid the potential user of the losipescu-shear-test method in establishing proper test procedures

Determining the contact force during the transverse impact of plates

Abstract: 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.

Dependence of crack acceleration on the dynamic stress-intensity factor in polymers

Abstract: The caustics method in combination with high-speed photography was employed to study velocity effect on the dynamic-stress-intensity factor of fast cracks in polymethyl methacrylate and in Araldite D. The specimen geometry was so determined that both the accelerating and decelerating crack propagation occurred noticeably in one fracture event. Instantaneous crack velocity as well as its acceleration were expressed as a function of the crack length by using polynomials of the ninth order which were given on the basis of the least-square method. The results show that the dynamic-stress-intensity factor depends not only on the crack velocity but also on crack acceleration, and that the accelerating crack has a smaller value stress-intensity factor than the decelerating crack at the same velocity.

Experimental investigation of wave velocity and dynamic contact stresses in an assembly of disks

Abstract: Dynamic photoelasticity and strain-gage techniques are employed to study wave propagation and dynamic load transfer in granular media. The granular medium is modeled as one- and two-dimensional deterministic and random arrays of circular disks of polyester material Homalite 100. The dynamic loading is achieved by explosive excitation. The experimental data are analyzed to determine the wave velocities, to identify the characteristic load transfer paths, and to quantitatively obtain the dynamic contact forces in the granular assembly. It is observed that the wave-propagation and dynamic-load-transfer phenomenon depends on the disk diameter and the obliqueness and flexibility of the load-transfer paths. The wave speed drops significantly in the first few granules after which the decay is more gradual. The load transter is characterized by the contact length and the friction between the contacting granules. The peak loads drop as the distance of the contact points from the point of explosive loading increases. For two-dimensional wave propagation, the load-transfer paths and the magnitude of contacting forces depend on the angles made by the normals of the contacting disks at the contact point.

Quantitation of pressure-sensitive film using digital image scanning

Abstract: An experimental technique is presented for measurement of contact stress distribution using Fuji Pressensor film. The development of packets of Pressensor disks for use in small areas or surfaces of complex curvature is explained. A digital-image-scanning procedure has been developed to substantially increase the spatial resolution with which stress distributions can be constructed from stained Fuji Pressensor film. The technique is developed for small, discrete disks as well as for larger, continuous sheets of Pressensor. As an illustration, sealed packets of small Pressensor disks are used to measure the stress distribution across a cadaveric juvenile femoral head under load.

Review and evaluation of the double-torsion technique for fracture toughness and fatigue testing of brittle materials

Abstract: This paper reviews the double-torsion (DT) test as an experimental technique for the measurement of fracture toughness and slow-crack-growth behavior in brittle materials based on the authors' experiences and an evaluation of current literature. The DT technique has numerous advantages due primarily to the fact that the stress intensity is independent of crack length, at least for the central half of the specimen. Although the technique was first proposed about 20 years ago, and has been used extensively since then, there are a number of important unresolved questions concerning the methodology. To date there has been no standardization of test procedure or specimen geometry. A review of specimen geometries in use indicates that the proportions that are most commonly employed (based on the literature and experience) are width:W, length: 3W, thickness:W/6–W/15. Grooves on both the top and the bottom surfaces have been used to guide the crack, however it has been found that ungrooved, but very accurately aligned, specimens give the best results. Theoretical thickness-correction factors which account for both relatively thick specimens (with respect to width) and the effect of the size of the loading points, have been considered. The effect of crack-front profile on measured values of crack velocity and stress intensity is contentious. Althogh the stress intensity,K, varies along the crack front, the front merely translates axially. For the presentation of crackvelocity stress-intensity (V-K) data, the consensus seems to be that the only rational velocity to use is that based on the crack's intersection with the tensile surface. Despite some of the shortcomings mentioned above, the DT technique is widely accepted and gaining in popularity. It is particularly useful under cyclic fatigue conditions for investigating the effect of a change in a single parameter on crack-growth rate, using the very effective and elegant ‘changeover’ method.

D-c electric-potential method applied to thermal/mechanical fatigue crack growth

Abstract: The paper describes a d-c electric potential system for measuring crack length under thermal/mechanical fatigue-crack-growth (TMFCG) test conditions. A programmable d-c current supply and precision multimeter produce reliable electric-potential readings. H.H. Johnson's formula is used to calculate crack length from electric potential for the center-crack-tensionM(T) geometry. Calibration constants for the formula are determined from an initial optical crack-length measurement. The resolution of the system is 1.0 microvolt which corresponds to a crack extension of approximately 0.002 mm for the center-crack-tension geometry using a current of 10.00 amps. Good crack-length accuracy and low data scatter are achieved by taking special precautions to minimize or eliminate errors in potential measurement due to thermal effects. Material resistivity changes are identified as the cause of short and long term changes in the measured electric potential for uncracked specimens. Crack-length accuracy is discussed in terms of short-term scatter and longterm drift.

Dynamic crack curving and branching under biaxial loading

Abstract: A 16-spark-gap camera was used to record the dynamic photoelastic patterns of ten centrally cracked, Homalite-100 specimens which fractured under ten initial biaxial-stress ratios ranging from 3.7 to 0, some of which do not exist in normal fracture specimens. The dynamic photoelastic patterns of curved cracks were used to verify the previously developed dynamic-crack-curving criterion. Cracks which immediately curved upon propagation in three specimens under abnormally high inital biaxial loading were used to verify the static counterpart of the dynamic-crack-curving criterion under these extreme loading conditions. A previously developed dynamic-crack-branching criterion was also verified by four dynamic photoelastic results involving cracks which eventually branched under the lower initial biaxial loading.

An experimental method for determining the location and time of initiation of an unknown dispersing pulse

Abstract: It is shown that the phase information obtained from a spectral analysis of dispersive waves can be used to locate the source of the waves. Experimental results for dispersive flexural waves in beams are presented.

Stable crack growth in aluminum tensile specimens

Abstract: Post's white-light moire interferometry was used to obtain sequential records of the transientUy-displacement fields associated with stable crack growth in 7075-T6 and 2024-0, single-edge-notched (SEN) specimens with fatigued cracks. TheUy-displacement fields are used to evaluate the crack-tip opening displacement (CTOD), far- and near-fieldJ-integral values, Dugdale-strip-yield model, William's polynomial function and the HRR field.

Residual stresses and warpage in woven-glass/epoxy laminates

Abstract: Residual stresses are introduced in composite laminates during curing as a result of differential thermal expansion of the various plies. Residual stresses coupled with asymmetries in the laminate produce warpage. To study these phenomena, symmetric and asymmetric glass-fabric-reinforced laminates were fabricated. The laminate material was fully characterized by determining its physical and mechanical properties at room and elevated temperatures. Thermal strains during curing and subsequent thermal cycling were measured by means of embedded strain gages. Residual stresses were then calculated using the mechanical properties determined before. Warpage for known types of asymmetry was calculated by means of lamination theory and compared with experimental measurements using a projection moire technique. The residual stresses in the studied laminates were very low, owing to the balancing effect of the woven-fabric reinforcement. A crossplied antisymmetric laminate showed saddle-shaped warpage in agreement with the analytical prediction. Unexpected warpage found in symmetric laminates may be due to imperfections in fiber orientations and/or temperature nonuniformities during laminations.

Residual stresses in turned AISI 4340 steel

Abstract: The residual-stress distribution in the surface region of workpieces of annealed AISI 4340 steel that is turned under unlubricated conditions is determined using a deflection etching technique. The absolute value of the residual stresses at the machined surface are low and increase with an increase in depth beneath the machined surface to a maximum. They then decrease with a further increase in depth eventually becoming vanishingly small. Peak residual stresses are tensile at cutting speeds of 0.5 and 1.0 ms−1 and are compressive at a cutting speed of 1.5 ms−1 for all feed rates and depths of cut. Peak residual stresses and depth of the stressed region increase with an increase in feed rate and depth of cut, but decrease with an increase in cutting speed. The results of this investigation can be interpreted in terms of the variation of tool forces with cutting conditions.

Basic theory and experimental techniques of the strain-gradient method

Abstract: The theories of presently used experimental methods of stress and deformation analysis which employ radiant energy as a detector are based on the assumption that light propagates rectilinearly within both undeformed and deformed bodies which are initially homogeneous and isotropic when diffraction phenomena are negligible. This assumption is not correct: light propagation within deformed bodies is nonrectilinear in a general case. Although this has already been observed and applied practically by some researchers in photoelasticity, it has not so far been generally acknowledged and accepted in experimental mechanics. On the basis of empirical data produced by the authors in the period 1948–1983, we present theories and foundations of the techniques of a new experimental method which is based on the relations between stress/strain gradients and curvatures of light beams. This method is called the strain-gradient method or, less rigorously, gradient photoelasticity.

Determination of plastic stress-strain behavior by digital-image-processing techniques

Abstract: This work explores the application of digitalimage-processing techniques to the measurement of large plastic strains. Two sample problems have been selected, namely the uniform tensile deformation of aluminum-sheet metal strips and the post-necking deformation of copper circular rods. Images of these gridded metallic test pleces were captured, digitized and analyzed in a fully computerized way to evaluate strain distributions, anisotropic parameters and plastic stress-strain flow curves. For post-necked test pieces, Bridgman stress correction has been easily applied by defining the neck profile contour from the automated processing of digitized images. Results compare satisfactorily with those based on displacements measured by conventional microscopy. The presented technique, with added improvements, can consititute a viable one for accurate and fully computerized measurement of large deformations.

Axial compression buckling of conical and cylindrical shells

Abstract: Experiments on the axial compression buckling of high-quality epoxy cylindrical shells with imposed dimpletype defects are described. Additionally, a technique for the manufacture of high-quality epoxy conical shells which buckle at loads approaching the classical critical load is presented. For both types of shells, prebuckling deformations have been monitored optically. The sizes of defects determined from the optical examination when applied in the space-frame approach to shell buckling have led to predicted knock-down factors which are remarkably consistent with measured knock-down factors (i.e., the ratio of actual collapse to classical critical load).

Residual-Stress Determination of Concentric Layers of Cylindrically Orthotropic Materials

Abstract: Equations have been obtained for determining residual stresses in the wall of a hollow, axially symmetric body consisting of concentric layers of elastically dissimilar materials, all having cylindrical elastic orthotropy. These equations permit residual normal stresses in the radial, circumferential, and axial directions and residual shear stresses on planes normal to the axis of the body to be calculated from measurements of the strains developed on the inner or outer cylindrical surface of the body as thin layers of stressed material are serially removed from the outer or inner surfaces, respectively. The equations are applied to a parametric study of stresses in an elastically isotropic, two-component body to determine the nature of the differences in stresses between the composite body and a homogeneous body as a function of the difference in elastic constants.

Use of photoelasticity to determine orthotropicK I stress-intensity factor

Abstract: A new experimental method of obtaining orthotropic stress-intensity factor,K I , is presented. The orthotropic photoelasticity and orthotropic linear-elastic fracture-mechanics laws are combined. The combined set of equations is used along with half-fringe photoelasticity to determineK I in a compact-tension specimen made of a transparent unidirectional fiberglass-epoxy material. The results are compared with finite-element-method solutions.

An investigation of the losipescu and asymmetrical four-point bend tests

Abstract: The Iosipescu shear test, utilizing a notched specimen in bending and a modification—the asymmetrical four-point bend (AFPB) test—were evaluated as shear tests for composites. This paper summarizes the results of an extensive numerical and experimental investigation of the Iosipescu and AFPB test methods. Finite-element analyses were conducted to assess the influence of notch parameters and load locations on the stress state in the specimen. The shear moduli and the shear strengths were experimentally measured for a quasiisotropic graphite/epoxy laminate using both the Iosipescu and the AFPB test methods. The tests were conducted for various combinations of notch parameters and load locations. The test results indicate that changes in the notch geometry and load locations aimed at improving the stress distribution in the test section resulted in unexpected changes in the failure mode.

Is load control suitable for fracture-energy measurements for concrete?

Abstract: Sixteen notched concrete beams with\(\frac{{a_o }}{w} = 0.3\), 0.5, 0.7 were tested in three-point bending—eight in strain control and eight in load control. The time to peak load was kept approximately the same for both cases for a given notch depth. The load-displacement traces were very similar and equivalent energy values agreed well in most cases. However the fracture-energy values based on the RILEM proposed specification were affected by the type of response control used.

Hybrid stress analysis of vibrating plates using holographic interferometry and finite elements

Abstract: The work reported herein is concerned with an investigation of the potential of mating time-average holography and static-finite-element analysis (a ‘hybrid’ stress-analysis technique) in solving dynamic problems. The model studied is a vibrating thin rectangular cantilever plate. Realtime holography was used to locate natural vibrating modes of the plate. Experimental data were smoothed to remove scatter using a spline-like procedure in one or two dimensions, as applicable. Two methods of smoothing the experimental displacement data used in the hybrid procedure were considered. The first involved preprocessing (smoothing) the data before they were incorporated into the numerical model, while the second involved smoothing the data as part of the solution process. In both instances the amount of smoothing was specified by the user. The first and fourth vibrational modes were investigated. The results were compared to a NASTRAN dynamic solution and to a Ritz method series solution with very good results.

The effect of a symmetric discontinuity on adjacent material in a longitudinally vibrating uniform beam

Abstract: A three-element analytical model for a free-free longitudinally vibrating uniform beam containing a symmetric discontinuity is presented. Based on this model, an analytical expression is developed which relates the length of the section containing the damage to the depth of the discontinuity. It thereby fixes the length of the element used to model this section of the beam. This expression was verified experimentally and shown to be material independent.

Behavior of adhesive joints in corrosive environment

Abstract: An experimental study was undertaken to determine the effect of corrosive environments on the strength of adhesive joints between pairs of glass-fiber-reinforced-plastic (GRP) plates and plates of GRP bonded to aluminum (Al). Two corrosive agents, each in three concentrations, were used: sulphuric acid (30, 50 and 60 percent) and sodium chloride (15, 20 and 25 percent). The results indicate a loss of strength with immersion time that levels off after a certain time. This loss of strength was more severe for the sulphuric acid than for sodium chloride. Ultrasonic attenuation of the adhesive joints was also monitored and it was found to increase with the immersion time, suggesting that the corrosion created inhomogenieties in the bond. The loss of strength and the increase in ultrasonic attenuation were both greater for greater concentrations of each of the corrosive agents.

Mixed-mode crack growth in plates under three-point bending

Abstract: A combined theoretical and experimental study of the problem of crack growth in a plate subjected to unsymmetrical three-point bending was undertaken. The opening-modeKI and sliding-modeKII stress-intensity factors describing the local stress field around the crack tip were determined by a finite-element computer program. The crack growth was analyzed by the maximum circumferential stress and the minimum strain-energy density criteria. The critical loads for crack growth and the crack trajectories were determined both by theory and experiment. The experimental results corroborated the theoretical predictions.

Measurement of favorable residual stresses in polycarbonate

Abstract: Research in this laboratory has been directed toward the production and effects of beneficial residual stresses in plastics. Such stresses have been shown to have a dramatic effect on the impact strength and fatigue life of polycarbonate samples. For example, thermal quenching, in water or liquid nitrogen, of samples heated above their glass-transition temperature, resulted in an increase in the mean fatigue life of the material by as much as 20 times over that of annealed material. This increase is attributed in large part to the introduction of compressive stresses on the surfaces of the samples. This paper concentrates on methods used to measure residual stresses in the surface of the material and on the variation of these stresses with time after treatment. Three measurement techniques are described: (1) material slicing, (2) photoelastic fringe displacement and (3) the ASTM hole-drilling method. The advantages, limitations and comparative results of these three methods are described and analyzed.

Stress measurement by copper electroplating aided by a personal computer

Abstract: An image-processing system using a personal computer was applied to stress measurement by copper electroplating. The microscopic TV image of the plating structure of an electroplated specimen after cyclic loading was directly analyzed by the image-processing system. The density of grains grown in the plated layer was measured automatically. The surface stress of the specimen can be estimated based on the grain density. The stress-concentration factors of grooved shafts under torsion were obtained as an example of application. The result was in good agreement with the numerical values published previously.

Optoelectronic-strain-measurement system for rotating disks

Abstract: A recently developed optical technique is presented in the following paper for measuring in-plane deformations and strains of rotating hardware. The technique is fully described. Data are presented which were obtained in preliminary tests using the system on a rotating disk. The experiments show that the system is capable of making radial-deformation measurements to within 9.5 μm over a 10-mm range.

Transmission photoelasticity of centrally loaded generally and specially orthotropic beams

Abstract: Transmission photoelastic patterns for generally and specially orthotropic beams and an isotropic beam are presented. Theoretical isochromatic and isoclinic results for these beams, calculated from classical elasticity stress equations and stress-optic laws, are also presented and compared with experiment. The agreement between the theory and experiment is excellent. For the orthotropic beams, a stressoptic law which accounts for the effects of residual birefringence was used. The residual birefringence observed for the composite used in this study is greater than one fringe order and results from a matrix residual tension which is about one fourth of the resin's ultimate tensile strength. Finally, the influence of such a large residual birefringence on beam and calibration photoelastic data is discussed in detail.

Shear strains in a graphite/PEEK beam by moiré interferometry with carrier fringes

Abstract: A multispan quasi-isotropic graphite-PEEK beam exhibited dramatic shear strains in the interlaminar region between plies. Shear strains in the plies themselves varied in basic accord with fiber direction. The large anomalous shear strains were developed near the center of the beam height, where shear stresses were large. High-sensitivity moire interferometry with 2400l/mm (60,960l/in.) was used. A new technique of data extraction was developed, using carrier fringes to transform the pattern to one in which fringe slopes are proportional to derivatives of displacement. This technique enhanced detection and measurement of highly localized shear-strain gradients.