Showing papers in "Experimental Mechanics in 1991"
TL;DR: In this paper, a detailed evaluation of the feasibility of determining displacements and displacement gradients from measured surface displacement fields is presented, and an improved methodology for both the estimation and elimination of noise is proposed.
Abstract: A detailed evaluation of the feasibility of determining displacements and displacement gradients from measured surface displacement fields is presented. An improved methodology for both the estimation and elimination of noise is proposed. The methodology is used to analyze the gradients for three tests: (1) uniform rotation, (2) uniform strain, and (3) crack-tip displacement fields. Results of the study indicate that the proposed methodology can be used to extract the underlying two-dimensional displacements and their corresponding gradients from the noisy data with reasonable accuracy. Specifically, it is shown that (a) the digital correlation method for acquiring displacement fields has an error in strain of approximately 150 μ strain at each point, (b) the average strain in a region of uniform strain has much less error, typically on the order of 20 μ strain, (c) the displacement ‘nolse’ present in digital correlation is very small, approximately 0.01 pixels, (d) the proposed methodology for reducing noise in the data is essential to the accurate evaluation of displacement gradients and (e) the successful evaluation of displacement and displacement gradients for all three cases indicates that the proposed methodology can be used both to quantify the displacement fields and to reasonably estimate the overall gradient trends.
301 citations
TL;DR: A direct-tension split-Hopkinson-bar apparatus for experiments with test durations of up to 500 μs was introduced in this article. But the results were only consistent when the ratio is larger than about 1.60.
Abstract: A direct-tension split-Hopkinson-bar apparatus is introduced. In this apparatus the specimen is loaded by a tensile wave that is generated by the release of a stored load in a section of the input bar. The system can be used for experiments with test durations of up to 500 μs. The effect of specimen geometry (length to diameter ratio) is investigated. Consistent results are obtained when the ratio is larger than about 1.60. Results from tests with 6061-T651 aluminum are in agreement with published data.
214 citations
TL;DR: In this article, a three-point-bend bimaterial specimen with a relatively large stiffness mismatch between the two materials is studied and a recently developed optical method of coherent gradient sensing (CGS) is used to map crack-tip deformation fields.
Abstract: The paper presents a preliminary experimental investigation of crack-tip deformation fields near quasistatically and dynamically growing cracks in bimaterial interfaces. A three-point-bend bimaterial specimen with a relatively large stiffness mismatch between the two materials is studied. A recently developed optical method of coherent gradient sensing (CGS) is used to map crack-tip deformation fields.
124 citations
TL;DR: In this article, an image processing-based automated grid method is investigated to determine the method's displacement and strain accuracy limits, and how these limits are influenced by the choice of camera-calibration models.
Abstract: An image-processing-based automated grid method is investigated to determine the method's displacement and strain accuracy limits, and how these limits are influenced by the choice of camera-calibration models A CCD camera and a PC-based frame grabber are used to record grid spot motion, then ordering and centroiding are used to identify each spot and calculate their individual displacements The displacements are fitted with a moving biquadratic surface, and the strains are obtained by analytical differentiation of that surface Camera-calibration models which are considered include various combinations of image-perspective transformation, image stretching, and elliptical-lens distortion The strain and displacement accuracy are explored through rigid-body motion and uniaxial tension tests In the process, sensitivity to in-plane and out-of-plane rigid-body translation, and extreme sensitivity to in-plane rigid-body rotation (for non-synchronized frame grabbers) are confirmed It is found that under the best conditions the displacement accuracy is 015 pixels and that the strain accuracy is 120 microstrain Finally, the automated grid method is used to investigate the strains developed in an aluminum perforated strip subjected to uniaxial tension
84 citations
TL;DR: In this article, the full-field technique of high-sensitivity moire interferometry in conjunction with a multiple-hole-drilling procedure is applied to residual-stress measurements in the presence of gradients.
Abstract: The full-field technique of high-sensitivity moire interferometry in conjunction with a multiple-hole-drilling procedure is applied to residual-stress measurements in the presence of gradients. The method arrives at residual-stress estimates starting from in-plane displacement components. Successful applications of the method to problems simulating the nonuniform transverse residual stresses of welded joints are reported.
71 citations
TL;DR: Moire interferometry has been widely used for measuring inplane deformations of complex materials and structures as discussed by the authors, including micromechanics of composites, thermal strain measurements, carrier fringes, and advances in methods for producing specimen gratings.
Abstract: Recent applications of moire interferometry at VPI & SU and related developments of laboratory techniques are reviewed. The applications are studies of composite bodies, including micromechanics and macromechanics of composites, thermal strains and residual strains. The techniques section discusses steady-state thermal strain measurements, carrier fringes, and advances in methods for producing specimen gratings. With its high sensitivity, high spatial resolution and extensive displacement range, moire interferometry has matured into a powerful technique for measuring inplane deformations of complex materials and structures.
53 citations
TL;DR: In this article, the experimental and analytical correlation of the mechanical properties of sandwich-beam specimens was investigated and four-point and three-point load tests were conducted on a large number of sandwichbeam specimens, fabricated by using fiberglass reinforced plastics (both unidirectional and woven-glass cloth) and DTD 685 aluminum alloy for the facings with aluminum honeycomb core and polyurethane foam cores and the indigenously available Araldite as the bonding medium between the core and the facade.
Abstract: This investigation is concerned with the experimental versus analytical correlation of the mechanical properties of sandwich-beam specimens. Such sandwich structures are commonly employed in the aircraft industry. Four-point and three-point load tests were conducted on a large number of sandwich-beam specimens, fabricated by using fiber-glass reinforced plastics (both unidirectional and woven-glass cloth) and DTD 685 aluminum alloy for the facings with aluminum honeycomb core and polyurethane foam cores and the indigenously available Araldite as the bonding medium between the core and the facings.
51 citations
TL;DR: In this paper, a traditional moire effect is observed using the AND operator while an enhanced moire is observed by using the XOR operation. And the average step method, resulting from the logical operations with values averaged over the pitch of the reference grating can also be used for fractional fringe-order determination.
Abstract: Moire fringes are generated using logical operations on one-bit binary gratings. A traditional moire effect is observed using the AND operator while an enhanced moire is observed using the XOR operation. Since the reference grating in this type of moire is computer generated, fringe shifting to increase sensitivity can be easily accomplished. An alternative scheme, called the average step method, resulting from the logical operations with values averaged over the pitch of the reference grating, can also be used for fractional fringe-order determination. These principles with some applications are demonstrated in this paper.
43 citations
TL;DR: In this article, a detailed experimental evaluation of the Iosipescu specimen tested in the modified Wyoming fixture is presented, where Moire interferometry is employed to determine the deformation of unidirectional and cross-ply graphite-epoxy specimens.
Abstract: A detailed experimental evaluation of the Iosipescu specimen tested in the modified Wyoming fixture is presented Moire interferometry is employed to determine the deformation of unidirectional and cross-ply graphite-epoxy specimens The results of the moire experiments are compared to those from the traditional strain-gage method It is shown that the strain-gage readings from one surface of a specimen together with corresponding data from moire interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to twisting A localized hybrid analysis is introduced to perform efficient reduction of moire data, producing whole-field strain distributions in the specimen test sections
39 citations
TL;DR: In this paper, the Schapery nonlinear viscoelastic model was used to describe the response of a fiber-reinforced epoxy composite to uniaxial applied loads.
Abstract: Viscoelastic response of a unidirectional aramid fiber-reinforced epoxy was measured. Procedures to measure all five time-dependent material properties necessary to describe behavior of a transversely isotropic continuous-fiber unidirectional lamina were implemented. The Iosipescu shear method was used to measure in-plane and interlaminar shear viscoelastic response. Applicability of the Schapery single-integral nonlinear viscoelastic constitutive model to describe time-dependent mechanical behavior of a laminated composite material containing two viscoelastic phases was explored. Linear and nonlinear viscoelastic parameters for this two-viscoelastic constituent composite were measured and data summaries are presented in the paper. The time-dependent behavior of this two-viscoelastic constituent composite material was found to be complex, but the Schapery nonlinear viscoelastic model did adequately fit the response of such a composite to uniaxial applied loads.
35 citations
TL;DR: In this paper, the authors proposed a new technique for the prediction of buckling loads of structural elements with postbuckling unstable characteristics such as cylindrical shells, which is based on the cubic parametric curve defined by the Hermite form.
Abstract: The paper presents a new technique for the prediction of buckling loads of structural elements with postbuckling unstable characteristics such as cylindrical shells. The technique introduced is based on the cubic parametric curve defined by the Hermite form,1 which is described in the paper. The technique requires as input the data obtained from vibration tests carried out on structural elements under compressive loading, specifically, the values of the applied load and the square of the measured corresponding natural frequency of vibration. The proposed technique is applied to a simplified cylindrical shell model2 and two cylindrical shells.3 A comparison between the predicted and the exact values of the buckling loads is presented for each case discussed in the paper, highlighting the accuracy of the proposed technique.
TL;DR: In this article, optical interference moire methods are analyzed using Fraunhoffer diffraction theory to relate general large surface deformations to the fringes observed, and an experimental scheme that allows the virtual reference grating to be varied is presented.
Abstract: Optical interference moire methods are analyzed using Fraunhoffer diffraction theory to relate general large surface deformations to the fringes observed. This analysis determines the Almansl strain in the current configuration from the gradients of the fringe number function. The analysis shows the advantages of an experimental scheme that allows the virtual reference grating to be varied. The ability to vary the virtual reference grating results in a larger dynamic range and the ability to maintain a fringe spacing for maximum accuracy. A moire microscope has been constructed which has this ability. Digital image processing coupled with optical filtering and phase control is used to enhance the accuracy of the fringe measurements. The variable virtual-reference-grating capability is demonstrated by using it to highlight several aspects of the deformation field near a crack tip in a single crystal of iron-silicon.
TL;DR: In this article, photoelastic data are combined with the finite-element method for stress solutions over regions partially bounded by free surfaces and axes of symmetry, and the least-squares solutions are obtained without presumed values of applied forces at element nodes and without isoclinic data.
Abstract: Photoelastic data are combined with the finite-element method for stress solutions over regions partially bounded by free surfaces and axes of symmetry. Least-squares solutions are obtained without presumed values of applied forces at element nodes and without isoclinic data. Varied example problems are used to compare the results to independent photoelastic and finite-element solutions and to theoretical stress values.
TL;DR: In this paper, various compression test methods for composite materials have been developed during the past few years incorporating specimens of different gage lengths and modes of load introduction, and the question arises as to whether these various test methods produce similar results.
Abstract: Various compression test methods for composite materials have been developed during the past few years incorporating specimens of different gage lengths and modes of load introduction. Most of these test methods are not recognized as standards at the present time. Thus the question arises as to whether these various test methods produce similar results.
TL;DR: In this article, the structure itself and a set of strain gages are used to determine each of the applied loads (forces and moments) responsible for the measured strains, in which the structure becomes the transducer.
Abstract: Many design and analysis situations require the determination of loads being transmitted to a structure. Since it is not always possible to insert a load cell, it is proposed that the structure itself and a set of strain gages be used to determine each of the applied loads (forces and moments) responsible for the measured strains. In essence, the structure becomes the transducer.
TL;DR: In this paper, the use of electrical-resistance metallic foil strain gages for measuring large plastic strain in dynamic experiments was studied, and the maximum nominal strains obtained in this investigation are 35 percent in compression, 25 percent in tension.
Abstract: The use of electrical-resistance metallic foil strain gages for measuring large plastic strain in dynamic experiments in studied. The maximum nominal strains obtained in this investigation are 35 percent in compression, 25 percent in tension. A linear variation of gage factor with strain is found in this range. The corrected maximum strains are in excellent agreement with permanent strains measured after the tests. Thus foil strain gages can be effectively used to measure the large dynamic plastic strains.
TL;DR: In this paper, a curved surface coated with light-sensitive material is exposed twice in a volume of virtual gratings formed by the interference of two coherent light beams split from a laser.
Abstract: A double-exposure moire-interferometry technique for topographic contour measurement of an arbitrarily curved object is presented. A curved surface coated with light-sensitive material is exposed twice in a volume of virtual gratings formed by the interference of two coherent light beams split from a laser. An adequate rotation of the curved surface relative to the virtual grating between the two exposures produces moire fringes which reveal topographic contour, or contour under some conditions, of the surface. The advantage of the present method in comparison with others is that it offers both reasonably good fringe quality and easily adjustable high sensitivity. The sensitivity of the technique is shown to be from the order of micrometer to that of millimeter depending on the frequency of the virtual grating and the amount of the relative rotation. This technique was successfully applied to the topographic contour measurement of a cylindrical shell with and without a diametrical point loading. The principle of this paper and some early results were presented at the SPIE conference held at Dearborn, MI on June 27–30, 1988 and appeared in its proceeding.1
TL;DR: In this article, the authors presented a localized hybrid method of stress analysis for three-dimensional problems in the mechanics of solids, which combines an experimental technique, moire interferometry, and a numerical method, finite-element analysis.
Abstract: Developments of the localized hybrid method which combines an experimental technique, moire interferometry, and a numerical method, finite-element analysis, are presented. In this localized hybrid method, the displacement fields which the moire experiments provide in some local regions of interest are used as input data for finite-element stress analyses. Based on finite-element theory, several variations on this localized hybrid method, associated with different displacement boundary conditions, are developed. Applications and limitations of the localized hybrid method are discussed in detail. In particular, applications of the localized hybrid method of stress analysis are presented for three-dimensional problems in the mechanics of solids. It is shown that this localized hybrid analysis not only provides a powerful and efficient technique for the reduction of moire experimental data, but also gives a good insight into the mechanics of the experimental observations.
TL;DR: In this paper, the effect of local geometric defects on axially compressed thin circular cylindrical shells is investigated experimentally, in the form of diamond-shaped local dimples, similar to the buckles of the Yoshimura pattern.
Abstract: The effect of local geometric defects on the buckling load of axially compressed thin circular cylindrical shells is investigated experimentally. Defects, in the form of diamond-shaped local dimples, similar to the buckles of the Yoshimura pattern, were introduced in otherwise near perfect isotropic epoxy shells by locally heating the shell wall. The behavior of the defects under load was monitored optically using a special whole-field grid-reflection technique. The effects of variations in shell geometry and defect size are also investigated. In general, the results indicate that the effect of local diamond-shaped defects on the stability of the axially loaded cylinder is not as detrimental as that of the global initial imperfections hitherto investigated.
TL;DR: In this paper, a series of experiments on plastic buckling of cylindrical aluminum alloy shells under biaxial loading (external pressure and axial tension), with well-defined loading and boundary conditions, was carried out to provide experimental data for evaluation of the suitability of different, plasticity models.
Abstract: The predictions for plastic buckling of shells are significantly affected by the plasticity model employed, in particular in the case of nonproportional loading. A series of experiments on plastic buckling of cylindrical aluminum alloy shells under biaxial loading (external pressure and axial tension), with well-defined loading and boundary conditions, was therefore carried out to provide experimental data for evaluation of the suitability of different, plasticity models. In the experiments, initial imperfections and their growth under load were measured and special attention was paid to buckling detection and load path control. The Southwell plot was applied with success to smooth the results. The results show that axial tension decreases resistance to buckling under external pressure in the plastic region due to ‘softening’ of the material behavior. Comparison with numerical calculations usingJ
2 deformation and incremental theories indicate that both theories do not predict correctly plastic buckling under nonproportional loading.
TL;DR: In this article, two computer-aided methods have been developed for analysis of in-plane and out-of-plane surface displacements of structures under load, which combine phase-stepped geometric moire with video and computer technologies.
Abstract: Two computer-aided methods have been developed for analysis of in-plane and out-of-plane surface displacements of structures under load. Both methods are whole-field techniques which combine phase-stepped geometric moire with video and computer technologies. With these methods, a displacement field of interest is determined by computer-processing phase stepped, geometric moire image data with fringe ordering done automatically within the software. The theory of the techniques is described and results of accuracy tests and application problems are given. It is shown that very good agreement is obtained between theory and experiment for in-plane strain determinations. For out-of-plane displacement determinations errors are only a few percent over the entire field of view. The application problems discussed are: (1) the measurement of composite column buckling, and (2) the determination of the shape of a slightly distorted, thin aluminum plate.
TL;DR: In this article, a laser-speckle pattern correlation is used for the determination of elastic-plastic boundary on the surface of well polished metal specimens, which employs a converging laser beam to illuminate the test object.
Abstract: Laser-speckle-pattern correlation is used for the determination of elastic-plastic boundary on the surface of well polished metal specimens. The method employs a converging laser beam to illuminate the test object. Diffracted verging laser beam to illuminate the test object. Diffracted images are recorded and processed on a computer-based vision system. Subsequent image-correlation analysis yields information on the surface deformation of the test objects. The technique is applied to a flat tensile specimen with two circular notches at the central portion. Measured plastic zone is compared with results obtained by the finite-element method.
TL;DR: In this paper, the dynamic moire interferometry was used to measure separately the u-and v-displacement fields surrouding a rapidly propagating crack tip in Homalite-100 and 7075-T6 aluminum-alloy plates.
Abstract: Dynamic moire interferometry was used to measure separately theu- andv-displacement fields surrouding a rapidly propagating crack tip in Homalite-100 and 7075-T6 aluminum-alloy plates. These transient crack-tip-displacement data were then used to compute the dynamic stress-intensity factor and the remote stress component.J-integral values were also estimated using the static approximate procedure of Kanget al. This static analysis provided the correctJ when the contour integral was taken within 3 mm of the crack tip.
TL;DR: In this article, the theoretical and experimental stress values obtained are reasonably close, and the load distribution is validated by strain-gage measurements, starting from the theoretical load distribution the stresses on the outer surface of the female member of a threaded connection are calculated.
Abstract: The thread load distribution has been examined, as is known, in literature both theoretically and experimentally. in the present paper the load distribution is validated by strain-gage measurements. Starting from the theoretical load distribution the stresses on the outer surface of the female member of a threaded connection are calculated. The theoretical and experimental stress values obtained are reasonably close.
TL;DR: In this paper, the residual stresses due to curing in a spin-coated polyimide film can be determined using the solution to the problem of a vibrating membrane, where the membrane is biaxially constrained and supported on a metal washer or copper substrate.
Abstract: It is proposed that the residual stresses due to curing in a spin-coated polyimide film can be determined using the solution to the problem of a vibrating membrane. The membrane is biaxially constrained and supported on a metal washer or copper substrate. A piezoelectric transducer is used to excite the sample. The membrane vibrates uniformly in response to its resonant frequencies. The vibration pattern is recorded using time-average holographic interferometry. The pattern produced is indicative of the mode of vibration. The biaxial stress in the film can be calculated from measuring the characteristic frequency, the density of the material, and the mode of vibration. The effect of mass loading of air on the apparent stress in these membranes is also investigated. Measurements made in vacuum appear to resolve this problem. The stresses calculated are on the order of 10 MPa.
TL;DR: In this article, the normal and sliding loads associated with multiple-point contact are measured using a steel die with a profile that simulates a rough surface and a very large scale factor is used in modeling this surface.
Abstract: A new method of measuring the normal and sliding loads associated with multiple-point contact is introduced. A multiple-point contact is modeled with a steel die with a profile that simulates a rough surface. A very large scale factor is used in modeling this surface. The steel die is placed in contact with a photoelastic model of a half plane and is subjected to a normal load. This normal load is partitioned over the multiple points of contact producing an isochromatic fringe pattern that describes the stress distribution in the local neighborhood of the contact points. A sliding load is then imposed on the model which destroys the symmetry of this fringe pattern. The fringe data in this pattern are sufficient to determine the local loadsPi andQi and the local coefficient of frictionfi=Qi/Pi at each contact point. An overdeterministic method is introduced which gives the solution forPi,Qi andfi using many data points taken from the isochromatic pattern in the local neighborhood of the contacts.
TL;DR: In this article, a scanning micro-interferometer is used to image surface displacements of samples containing a single-pitch-based carbon fiber embedded in an epoxy matrix, which gives the necessary resolution to detect small changes in thermal displacements in the fiber/matrix interface region.
Abstract: A micro-interferometric technique for measuring out-of-plane thermal displacements on a scale commensurate with the dimensions of the fiber/matrix unit cell is described. A scanning micro-interferometer is used to image surface displacements of samples containing a single-pitch-based carbon fiber embedded in an epoxy matrix. The interferometer design gives the necessary resolution to detect small changes in thermal displacements in the fiber/matrix interface region. The samples were heated electrically through the fiber to create radially symmetric temperature and displacement fields. Repeatable displacement measurements were obtained on a radial line across the interface region with an accuracy of ±25 A. A sharp expansion of the matrix surrounding the fiber was observed with each heating. Overall, the experiments demonstrate the utility of micro-interferometry for measuring submicron displacements.
TL;DR: In this paper, the applicability of embedded fiber-optic sensors for the determination of fracture parameters is demonstrated using a Mach-Zehnder interferometric setup and mode-1 stress-intensity factors are obtained by embedding singlemode fibers in single-edge-notched specimens fabricated from Plexiglas.
Abstract: The applicability of embedded fiber-optic sensors for the determination of fracture parameters is demonstrated. A Mach-Zehnder interferometric setup is used and mode-1 stress-intensity factors are obtained by embedding single-mode fibers in single-edge-notched specimens fabricated from Plexiglas. Optical fibers are embedded in-plane to measure axial strains at various depths and also in the transverse direction to measure the transverse strains, from which stress-intensity factors are determined. In both cases the experimental results compare well with the theoretical predictions.
TL;DR: In this paper, the mass of the load string, usually the grip, between the specimen and the load cell was found to differ from that experienced by the specimen when a materials testing machine was used dynamically.
Abstract: When a materials testing machine is used dynamically, the force recorded by the load cell may differ from that experienced by the specimen. This error is caused by the mass of the load string, usually the grip, between the specimen and the load cell.
TL;DR: In this paper, the authors developed a system for measuring the force on the chest of an anthropomorphic test device (dummy) as it penetrates into an inflated-airbag system during an impact.
Abstract: The goal of this research was to develop a system for measuring the force on the chest of an anthropomorphic test device (dummy) as it penetrates into an inflated-air-bag system during an impact. A spherical geometric analysis (SGA) method was developed to track dummy motion when film documentation was not reasonable, such as during air-bag deployment. The method uses three linear accelero-meter triaxial clusters for assessment of angular velocity and linear acceleration in a three-dimensional sense. From these measurements, angular acceleration and position, and linear velocity and position are derived.