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Showing papers in "Experimental Mechanics in 2002"


Journal ArticleDOI
TL;DR: In this article, a thin disk of annealed or hard C11000 copper is placed on the impact surface of the incident bar in order to shape the incident pulse, and after impact by the striker bar, the copper disk deforms plastically and spreads the pulse in the basin.
Abstract: We present pulse shaping techniques to obtain compressive stress-strain data for brittle materials with the split Hopkinson pressure bar apparatus. The conventional split Hopkinson pressure bar apparatus is modified by shaping the incident pulse such that the samples are in dynamic stress equilibrium and have nearly constant strain rate over most of the test duration. A thin disk of annealed or hard C11000 copper is placed on the impact surface of the incident bar in order to shape the incident pulse. After impact by the striker bar, the copper disk deforms plastically and spreads the pulse in the incident bar. We present an analytical model and data that show a wide variety of incident strain pulses can be produced by varying the geometry of the copper disks and the length and striking velocity of the striker bar. Model predictions are in good agreement with measurements. In addition, we present data for a machineable glass ceramic material, Macor, that shows pulse shaping is required to obtain dynamic stress equilibrium and a nearly constant strain rate over most of the test duration.

607 citations


Journal ArticleDOI
TL;DR: In this paper, a self-healing polymeric composite material that can recover as much as 90 percent of its virgin fracture toughness has been developed, based on biological systems in which damage triggers an autonomic healing response.
Abstract: Inspired by biological systems in which damage triggers an autonomic healing response, a polymer composite material that can heal itself when cracked has been developed. In this paper we summarize the self-healing concept for polymeric composite materials and we investigate fracture mechanics issues consequential to the development and optimization of this new class of material. The self-healing material under investigation is an epoxy matrix composite, which incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is triggered by contact with an embedded catalyst. The effects of size and concentration of the catalyst and microcapsules on fracture toughness and healing efficiency are investigated. In all cases, the addition of microcapsules significantly toughens the neat epoxy. Once healed, the self-healing polymer exhibits the ability to recover as much as 90 percent of its virgin fracture toughness.

549 citations


Journal ArticleDOI
TL;DR: In this paper, the systematic errors that arise from the use of undermatched shape functions, i.e., shape functions of lower order than the actual displacement field, are analyzed, under certain conditions, the shape functions used can be approximated by a Savitzky-Golay low-pass filter applied to the displacement functions, permitting a convenient error analysis.
Abstract: Digital image correlation techniques are commonly used to measure specimen displacements by finding correspondences between an image of the specimen in an undeformed or reference configuration and a second image under load. To establish correspondences between the two images, numerical techniques are used to locate an initially square image subset in a reference image within an image taken under load. During this process, shape functions of varying order can be applied to the initially square subset. Zero order shape functions permit the subset to translate rigidly, while first-order shape functions represent an affine transform of the subset that permits a combination of translation, rotation, shear and normal strains. In this article, the systematic errors that arise from the use of undermatched shape function, i.e., shape functions of lower order than the actual displacement field, are analyzed. It is shown that, under certain conditions, the shape functions used can be approximated by a Savitzky-Golay low-pass filter applied to the displacement functions, permitting a convenient error analysis. Furthermore, this analysis is not limited to the displacements, but naturally extends to the higher-order terms included in the shape functions. This permits a direct analysis of the systematic strain errors associated with an undermatched shape function. Detailed numerical studies are presented for the case of a second-order displacement field and first- and second-order shape functions. Finally, the relation of this work to previously published studies is discussed.

488 citations


Journal ArticleDOI
TL;DR: In this paper, a method for the characterization of freestanding thin films with thickness on the order of nanometers to micrometers is presented, which allows in-situ SEM and TEM observation of materials response under uniaxial tension, with measurements of both stresses and strains under a wide variety of environmental conditions such as temperature and humidity.
Abstract: We present a new experimental method for the mechanical characterization of freestanding thin films with thickness on the order of nanometers to micrometers. The method allows, for the first time, in-situ SEM and TEM observation of materials response under uniaxial tension, with measurements of both stresses and strains under a wide variety of environmental conditions such as temperature and humidity. The materials that can be tested include metals, dielectrics, and multi-layer composites that can be deposited/grown on a silicon substrate. The method involves lithography and bulk micromachining techniques to pattern the specimen of desired geometry, release the specimen from the substrate, and co-fabricate a force sensor with the specimen. Co-fabrication provides perfect alignment and gripping. The tensile testing fits an existing TEM straining stage, and a SEM stage. We demonstrate the proposed methodology by fabricating a 200 nm thick, 23.5 μm wide, and 185 μm long freestanding sputter deposited aluminum specimen. The testing was done in-situ inside an environmental SEM chamber. The stress-strain diagram of the specimen shows a linear elastic regime up to the yield stress σy MPa, with an elastic modulusE=74.6 GPa.

254 citations


Journal ArticleDOI
TL;DR: In this article, an apparatus has been designed and implemented to measure the elastic tensile properties (Young's modulus and tensile strength) of surface micromachined polysilicon specimens.
Abstract: An apparatus has been designed and implemented to measure the elastic tensile properties (Young's modulus and tensile strength) of surface micromachined polysilicon specimens. The tensile specimens are “dog-bone” shaped ending in a large “paddle” for convenient electrostatic or, in the improved apparatus, ultraviolet (UV) light curable adhesive gripping deposited with electrostatically controlled manipulation. The typical test section of the specimens is 400 μm long with 2 μm×50 μm cross section. The new device supports a nanomechanics method developed in our laboratory to acquire surface topologies of deforming specimens by means of Atomic Force Microscopy (AFM) to determine (fields of) strains via Digital Image Correlation (DIC). With this tool, high strength or non-linearly behaving materials can be tested under different environmental conditions by measuring the strains directly on the surface of the film with nanometer resolution.

248 citations


Journal ArticleDOI
TL;DR: In this paper, a B-spline function is used to represent the object deformation field throughout the entire image area, which is an improvement over subset-based image correlation methods by implicitly maintaining position and derivative continuity constraints among subsets up to a specified order.
Abstract: A full-field speckle pattern image correlation method is presented that will determine directly the complete, two-dimensional deformation field during the image correlation process on digital images obtained using computer vision systems. In this work, a B-Spline function is used to represent the object deformation field throughout the entire image area. This is an improvement over subset-based image correlation methods by implicitly maintaining position and derivative continuity constraints among subsets up to a specified order. The control point variables within the B-Spline deformation function are optimized iteratively with the Levenberg-Marquardt method to achieve minimum disparity between the predicted and actual deformed images. Results have shown that the proposed method is computationally efficient, accurate and robust. The general framework of this method can be applied ton-dimensional image correlation systems that solve for multi-dimension vector fields.

212 citations


Journal ArticleDOI
TL;DR: In this paper, a new specimen geometry, the shear-compression specimen (SCS), has been developed for large strain testing of materials, where two diametrically opposed slots are machined at 45° with respect to the longitudinal axis, thus forming the test gage section.
Abstract: A new specimen geometry, the shear-compression specimen (SCS), has been developed for large strain testing of materials. The specimen consists of a cylinder in which two diametrically opposed slots are machined at 45° with respect to the longitudinal axis, thus forming the test gage section. The specimen was analyzed numerically for two representative material models, and various gage geometries. This study shows that the stress (strain) state in the gage, is three-dimensional rather than simple shear as would be commonly assumed. Yet, the dominant deformation mode in the gage section is shear, and the stresses and strains are rather uniform. Simple relations were developed and assessed to relate the equivalent true stress and equivalent true plastic strain to the applied loads and displacements. The specimen was further validated through experiments carried out on OFHC copper, by comparing results obtained with the SCS to those obtained with compression cylinders. The SCS allows to investigate a large range of strain rates, from the quasi-static regime, through intermediate strain rates (1–100 s−1), up to very high strain rates (2×104s−1 in the present case).

178 citations


Journal ArticleDOI
TL;DR: The objective of this paper is to demonstrate mechanical aspects of some of these hard tissues, to discuss their structure-function relationships, and to reveal their potential utility in materials science and engineering applications.
Abstract: Biological hard tissues are composites of inorganics and biopolymers, and, therefore, represent hybrid systems. The inorganic components may be oxides (e.g., SiO2, Fe3O4), carbonates (e.g., CaCO3) sulfides (e.g., FeS, CdS), or others, mostly in crystalline forms but also occasionally in glassy forms. The biopolymer is often proteinaceous, but can also involve lipids and especially polysaccharides (e.g., chitin). These hybrid materials can be found in single celled organisms (such as bacteria and protozoa), invertebrates (such as mollusks), insects (such as beetles), and vertebrates (such as mammals). A common denominator of all hard tissues is that they are hierarchically structured from the nanometer scale to the microscale and the macroscale. It is these controlled structures that give biological hard tissues their unique and highly evolved functional properties. The engineering properties include mechanical, piezoelectric, optical, and magnetic. The hard tissues can be in the form of nanoparticles, spines, spicules, skeletons, and shells. The objective of this paper is to demonstrate mechanical aspects of some of these hard tissues, to discuss their structure-function relationships (with examples from the literature as well as from our research), and to reveal their potential utility in materials science and engineering applications.

171 citations


Journal ArticleDOI
TL;DR: In this paper, the authors describe the addition of rotational degrees of freedom into the minimization problem for digital volume correlation in order to improve the overall performance of the strain measurement.
Abstract: Digital volume correlation is a new experimental technique that allows the measurement of the full-field strain tensor in three dimensions We describe the addition of rotational degrees of freedom into the minimization problem for digital volume correlation in order to improve the overall performance of the strain measurement A parameterization of rotations that is particularly suited to the minimization problem is presented, based on the angle-axis representation of finite rotations The partial derivative of both a normalized cross-correlation coefficient and the sum-of-squares correlation coefficient are derived for use with gradient-based minimization algorithms The addition of rotation is shown to greatly reduce the measurement error when even small amounts of rigid body rotation are present in an artificially rotated test volume In an aluminum foam sample loaded in compression, including rotational degrees of freedom produced smoother contours of minimum principal strain Renderings of the aluminum foam architecture in areas of low, medium and high rotation showed material deformation pattern in detail

169 citations


Journal ArticleDOI
TL;DR: In this article, the authors used a single CCD camera to acquire the surface patterns of a zone of a specimen in the underformed and deformed states to determine in-plane displacement and strain fields.
Abstract: The “planar” digital image correlation technique needs a single CCD camera to acquire the surface patterns of a zone of a specimen in the underformed and deformed states. With these two images, one can determine in-plane displacement and strain fields. The digital image correlation technique used herein is based on Fast Fourier Transforms, which are very effective in reducing the computation cost. Its performance is assessed and discussed on artificial signals and in a real experimental situation. The technique is utilized to analyze experimental results of a plane shear experiment and validate a damage meso-model describing different degradations in a C/C composite material.

150 citations


Journal ArticleDOI
TL;DR: In this paper, the dynamic response of sheet metals at high strain rate was investigated with a tensile split Hopkinson bar test using plate type specimens, and the optimum geometry of the specimen was determined to minimize the loading equilibrium error.
Abstract: The dynamic response of sheet metals at high strain rate is investigated with a tensile split Hopkinson bar test using plate type specimens. The tension split Hopkinson bar inevitably causes some errors in the strain at grips with the plate type specimens, since the grip and specimens disturb the one-dimensional wave propagation in bars. To validate the experiment, the level of error induced from the grips is estimated by comparing the waves acquired from experiments with the Pochhammer-Chree solution. The optimum geometry of the specimen is determined to minimize the loading equilibrium error. High strain rate tensile tests are then performed with auto-body sheet metals in order to construct their appropriate constitutive models for use in crash-worthiness evaluation.

Journal ArticleDOI
TL;DR: In this article, the dynamic compressive stress-strain behavior of a rigid polyurethane foam with four values of density (78, 154, 299, and 445 kg/m3) has been determined in the strain-rate range of 1000−5000 s−1.
Abstract: The dynamic compressive stress-strain behavior of a rigid polyurethane foam with four values of density (78, 154, 299, and 445 kg/m3) has been determined in the strain-rate range of 1000–5000 s−1. A pulse shaping technique was used with a split Hopkinson pressure bar to ensure homogeneous deformation in the foam specimens under dynamic compression. Dynamic stress equilibrium in the specimen was monitored during each experiment using piezoelectric force transducers mounted close to the specimen end-faces. Quasi-static experiments were also performed to demonstrate rate effects. Experimental results show that both the quasistatic and the dynamic stress-strain curves of the foam exhibit linear elasticity at small strains until a peak is reached. After the peak, the stress-strain curves have a plateau region followed by a densification region. The peak stress is strain-rate sensitive and depends on the square of the foam density.

Journal ArticleDOI
TL;DR: In this paper, a parametric study of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses was performed on two different substrate materials, single crystal Si (100) and fused silica, are considered.
Abstract: We report parametric studies of elastic wave generation by a pulsed laser and associated spalling of thin surface films by the corresponding high stresses. Two different substrate materials, single crystal Si (100) and fused silica, are considered. Spallation behavior of Al thin films is investigated as a function of substrate thickness, film thickness, laser energy, and various parameters governing the source. Surface displacement due to the stress wave is measured by Michaelson interferometry and used to infer the stresses on the film interface. Consistent with previous studies, the maximum stress in the substrate and at the film/substrate interface increases with increasing laser fluence. For many of the conditions tested, the substrate stress is large enough to damage the Si. Moreover, the maximum interface stress is found to increase with increasing film thickness, but decrease with increasing substrate thickness due to geometric attenuation. Of particular significance is the development of a decompression shock in the fused sillica substrates, which results in very high tensile stresses at the interface. This shock enhances the failure of thin film interfaces, especially in thicker samples.

Journal ArticleDOI
TL;DR: In this paper, the authors used digital image correlation (DIC) to examine the mechanical behavior of arterial tissue from bovine aorta and found that arterial specimens exhibited a nonlinear hyperelastic stress-strain response and the stiffness increased with percent elongation.
Abstract: In this study, digital image correlation (DIC) was adopted to examine the mechanical behavior of arterial tissue from bovine aorta. Rectangular sections comprised of the intimal and medial layers were excised from the descending aorta and loaded in displacement control uniaxial tension up to 40 percent elongation. Specimens of silicon rubber sheet were also prepared and served as a benchmark material in the application of DIC for the evaluation of large strains; the elastomer was loaded to 50 percent elongation. The arterial specimens exhibited a non-linear hyperelastic stress-strain response and the stiffness increased with percent elongation. Using a bilinear model to describe the uniaxial behavior, the average minor and major elastic modulii were 192±8 KPa and 912±40 KPa, respectively. Poisson's ratio of the arterial sections increased with the magnitude of axial strain; the average Poisson's ratio was 0.17±0.02. Although the correlation coefficient obtained from image correlation decreased with the percent elongation, a correlation coefficient greater than 0.8 was achieved for the tissue experiments and exceeded that obtained in the evaluation of the elastomer. Based on results from this study, DIC may serve as a valuable method for the determination of mechanical properties of arteries and other soft tissues.

Journal ArticleDOI
TL;DR: In this paper, the role of mechanics in the development of biologically inspired materials has been clarified, including the helical structure of fibrous tissue, the multi-scale structure of wood, and the biologically inspired optimal structure of functionally graded materials.
Abstract: In the development of new materials, researchers have recently turned to nature for inspiration and assistance. A special emphasis has been placed on understanding the development of biological materials from the traditional correlation of structure to property, as well as correlating structure to functionality. The natural evolution of structure in biological materials is guided by the interaction between these materials and their environment. What is most notable about natural materials is the way in which the structure is able to adapt at a wide range of length scales. Much of the interaction that biological materials experience occurs through mechanical contact. Therefore, to develop biologically inspired materials it is necessary to quantify the mechanical behavior of and mechanical influences on biological structures with the intention of defining the natural structure-property-functionality relationship for these materials. In particular, the role mechanics has assumed in understanding biological materials, and the biologically inspired materials developed from this knowledge, will be clarified. The following will serve to elucidate on this role: the helical structure of fibrous tissue, the multi-scale structure of wood, and the biologically inspired optimal structure of functionally graded materials.

Journal ArticleDOI
TL;DR: In this paper, a new type of electroactive polymer gel is described, which is based on triepoxides cross-linked with polyfunctional amines, and the response time is in rough agreement with what would be expected for a diffusional process through a gel of this size (about 100 μm).
Abstract: We describe a new type of electroactive polymer gel. In contrast to cross-linked polyacrylates, this system is based on triepoxides cross-linked with polyfunctional amines. The stoichiometry is selected to form a cross-linked gel with excess amino hydrogens. In acid solutions these become cationic and the ionic interactions cause the gel to swell. We describe the mechanical properties of the gels, their response to changes in pH and to electrical activation. The response time is in rough agreement with what would be expected for a diffusional process through a gel of this size (about 100 μm). Samples of smaller dimensions would be expected to respond more rapidly.

Journal ArticleDOI
TL;DR: In this paper, a recursive reformulation of the governing equations in conjunction with a general finite element program is presented for determining force histories using experimentally measured responses, which can determine multiple isolated (uncorrelated) force histories as well as distributed pressures and tractions.
Abstract: This paper presents a method for determining force histories using experimentally measured responses. It is based on a recursive reformulation of the governing equations in conjunction with a general finite element program, this latter aspect making it applicable to complex structures. It can determine multiple isolated (uncorrelated) force histories as well as distributed pressures and tractions and allows for the data collected to be of dissimilar type. As a demonstration of the method and of its scalability, force reconstructions for an impacted shell and an impacted plate are determined using accelerometer and strain gage data.

Journal ArticleDOI
TL;DR: In this article, a lattice approach was used to model the structural properties of spruce spruce fiber bundles, and the model was found to be good at predicting the load-deformation response of both notched and unnotched specimens.
Abstract: In this paper, we explore ways to couple experimental measurements with the numerical simulations of the mechanical properties of wood. For our numerical simulations, we have adopted a lattice approach, where wood fibers or bundles of wood fibers are modeled as discrete structural elements connected by a lattice of spring elements. Element strength and stiffness properties are determined from bulk material properties. Damage is represented by broken lattice elements, which cause both stiffness and strength degradation. The modeling approach was applied to small specimens of spruce subjected to transverse uniaxial tension, and mode I transverse splitting. The model was found to be good at predicting the load-deformation response of both notched and unnotched specimens, including the post-peak softening response. In addition, the damage patterns predicted by the model are consistent with those observed in the experiments.

Journal ArticleDOI
TL;DR: In this article, the authors used HgCdTe infrared detectors to experimentally measure the temperature distribution at the surface of a workpiece during orthogonal cutting of aluminum as a function of depth of cut.
Abstract: During the machining of metals, plastic deformation and friction lead to the generation of heat in the workpiece, which results in thermomechanically coupled deformation. Recently, several numerical models of this highly coupled process have been produced in response to increased interest in high speed machining. It is important to characterize the thermal field in the cutting zone in order to completely verify these models of high speed machining and to direct further advancement in this area. In this work, HgCdTe infrared detectors are used to experimentally measure the temperature distribution at the surface of a workpiece during orthogonal cutting. From these temperature measurements, the heat generated in the primary shear zone and the friction zone can be examined and characterized. A modified Hopkinson bar technique has been developed to perform orthogonal machining at speeds ranging from 10 to 100 m/s. In the present work, a cutting velocity of 15 m/s is employed in all the tests in order to demonstrate the capability of the apparatus and characterize thermal fields during low speed machining. Temperature fields are obtained during the orthogonal cutting of aluminum as a function of depth of cut. It is seen that depth of cut can vary both the maximum temperature as well as the distribution of the temperature field in the aluminum workpiece. the maximum temperature increased with depth of cut (238°C for 1.5 mm cut, 207°C for 1.0 mm cut and 138°C for 0.5 mm cut) and the temperature field extended further beneath the cut surface with decreasing depth of cut.

Journal ArticleDOI
TL;DR: In this article, an automated photoelastic method based on the phase stepping technique is described, which provides full-field maps of the isoclinic parameter and the relative retardation.
Abstract: In this paper an automated photoelastic method based on the phase stepping technique is described. It provides full-field maps of the isoclinic parameter and the relative retardation. The technique is based on processing six images of a photoelastic specimen acquired using plane and circularly polarized light. The number of acquisitions and the type of polariscope used in this approach have been chosen with the aim at reducing the influence of quarter wave plate errors and obtaining raw photoelastic data in a periodic form suitable for easy applications of automatic unwrapping routines.

Journal ArticleDOI
TL;DR: In this paper, a combined analytical and experimental investigation of the indentation failure of a composite sandwich panel has been undertaken, where two cases have been studied: a sandwich panel with carbon/epoxy facing and a PVC foam layer supported on a rigid base and indented at the center with a cylindrical indentor.
Abstract: A combined analytical and experimental investigation of the indentation failure of a composite sandwich panel has been undertaken. Two cases have been studied: a sandwich panel with carbon/epoxy facing and a PVC foam layer supported on a rigid base and indented at the center with a cylindrical indentor; and a sandwich beam with symmetrical facing and core materials as in the sandwich panels. The load-deflection behavior of the loaded facing was monitored during the test. Strains were also measured near the load on both surfaces of the facing using embedded strain gages. A full-field analysis of the in-plane displacements in the foam was conducted using the moire method. The problem was modeled as an elastic beam resting on an elastic-plastic foundation. Initiation of indentation failure occurs when the foundation yields, while catastrophic failure takes place when the compression facing fractures. The experimental results are in good agreement with the results of the analytical modeling based on the Winkler foundation.

Journal ArticleDOI
TL;DR: In this article, the fatigue and fracture properties of bovine dentin are evaluated usingin vitro experimental analyses using double cantilever beam (DCB) specimens were prepared from Bovine maxillary molars and subjected to zeroto-tension cyclic loads.
Abstract: In this paper, the fatigue and fracture properties of bovine dentin are evaluated usingin vitro experimental analyses. Double cantilever beam (DCB) specimens were prepared from bovine maxillary molars and subjected to zeroto-tension cyclic loads. The fatigue crack growth rate was evaluated as a function of the dentin tubule orientation using the Paris law. Wedge-loaded DCB specimens were also prepared and subjected to monotonic opening loads. Moire interferometry was used to acquire the in-plane displacement field during stable crack growth, and the instantaneous wedge load and crack length were acquired to evaluate the crack growth resistance and crack tip opening displacement (CTOD) with crack extension. The rate of fatigue crack growth was generally larger for crack propagation occurring perpendicular to the dentin tubules. The Moire fringe fields documented during monotonic crack growth exhibited non-linear deformation occurring within a confined region adjacent to the crack tip. Both the wedge load and CTOD response provided evidence that a fracture process zone contributes to energy dissipation during crack extension and that dentin exhibits a risingR-curve behavior. Results from this preliminary investigation are being used as a guide for an evaluation of the fatigue and fracture properties of human dentin.

Journal ArticleDOI
TL;DR: In this paper, the fracture energy of notched Charpy A508 steel specimens was determined from the incident, reflected and single wire fracture gage signals, and it was shown that fracture occurs relatively early and prior to the taking off of the bar by rigid body motion.
Abstract: This paper reports our methodology and results for the assessment of the dynamic fracture energy of notched Charpy A508 steel specimens. The fracture tests consist of one-point bend impact applied to the specimen in contact with an instrumented bar. Fracture is caused by the inertia of the unsupported specimen only. The fracture energy is determined from the incident, reflected and single wire fracture gage signals. High-speed photographic recordings show that for all the specimens investigated in the “lower shelf” temperature regime, fracture occurs relatively early and prior to “taking off” of the bar by rigid body motion. It also confirms that the fracture gage readings indeed coincide with the formation of a crack from the notch tip.

Journal ArticleDOI
TL;DR: In this paper, the photoelastic effect of PMMA is exploited to examine the stress state within a two-dimensional particle bed and stress chain development within the bed is recorded and is shown to increase the stress states within some particles while leaving others unloaded.
Abstract: Cure cast plastic bonded explosives (PBXs) consist of relatively hard particles in a soft binder. Under compressive loading, the explosive cyrstals come into contact that causes high stress concentrations. The lines along which the crystals are loaded are called stress chains. Damage done to these particle beds during compressive loading can lead to reaction. The photoelastic effect of PMMA is exploited to examine the stress state within a two-dimensional particle bed. Stress chain development within the bed is recorded and is shown to increase the stress state within some particles while leaving others unloaded. These concentrations form early in the loading process, leading to fracture along the stress bridges and generating likely reaction initiation sites. Through material point method simulations, contact friction is shown to have a large effect on the stress distribution within the particle bed.

Journal ArticleDOI
TL;DR: In this article, the shrinkage characteristics of acrylic-based and epoxy-based stereolithography (SL) photopolymer resin systems after they have been laser cured and post-cured under ultraviolet (UV), and thermal exposure were determined by the shadow moire and hole-drilling strain-gage methods.
Abstract: An experimental study has been undertaken to investigate the shrinkage characteristics of acrylic-based and epoxy-based stereolithography (SL) photopolymer resin systems after they have been laser cured and post-cured under ultraviolet (UV), and thermal exposure. The induced residual stresses and strains were determined by the shadow moire and the hole-drilling strain-gage methods. Out-of-plane displacements (warpage) of acrylic-based post-cured resin plates were recorded by means of the shadow moire method and correlated to the shrinkage strains by theoretical analysis. The induced residual stresses in the epoxy-based cylindrical resin specimens were determined from strains of three-element strain-gage rosettes of the blind-hole drilling method. Results are presented for the shrinkage stresses and strains for both material systems as a function of the post-curing process (UV, thermal). It was found that the shrinkage strains in the acrylic-based photopolymer resin were of considerable magnitude, while thermal post-curing resulted in higher shrinkage stresses for both material systems. The values of the shrinkage stresses compare well with those of the existing literature.

Journal ArticleDOI
TL;DR: In this article, a modified split Hopkinson pressure bar (SHPB) system is used to perform dynamic fiber push-out experiments on model single fiber composite systems, where a tapered punch and a support connect a monofilament composite with the incident and transmitted bars of the SHPB.
Abstract: In the present work a modified Split Hopkinson Pressure Bar (SHPB) system is adopted to perform dynamic fiber push-out experiments on model single fiber composite systems. A tapered punch and a support connect a monofilament composite with the incident and transmitted bars of the SHPB. The tapered punch is used to apply compressive loading to a single fiber (either steel or aluminum) embedded in a surrounding matrix material (EPON 862). The SHPB allows real time measurement of relative fiber/matrix displacement and push-out force, as the debonding and push-out event progresses. Using this technique we have studied the effect of loading rate, material mismatch, fiber length, and surface roughness on the push-out event. It was seen that maximum push-out force increases with increasing loading rate. In addition dynamic interfacial strength and toughness is highly dependent on fiber surface roughness. Results from a finite element analysis incorporating a cohesive failure model were used to extract interface strength and toughness values. It was found that the particular aluminum/EPON interface used is characterized by a dynamic shear failure strength of 48±8 MPa, a mode II fracture toughness of 160±40 N/m, and a friction coefficient of 0.2 at a sliding rate of 6 m/s. For the rates tested here these quantities were found to be approximately constant.

Journal ArticleDOI
TL;DR: In this paper, an automated phase-stepping method was developed to enable the determination of the three characteristic parameters for three-dimensional or integrated photoelasticity, which have been described as the characteristic retardation, δ, and the primary and secondary characteristic directions, θ and θ+χ.
Abstract: A three-dimensional photoelastic body can be represented by an optically equivalent model, which consists of a linear retarder, δ, at a certain angle, θ, and a pure rotator, χ. These have been described as the characteristic retardation, δ, and the primary and secondary characteristic directions, θ and θ+χ. Until now these characteristic parameters have only been determined using manual, point-by-point collection methods which are involved and time consuming. Therefore an automated phase-stepping method has been developed to enable the determination of the three characteristic parameters for three-dimensional or integrated photoelasticity. Expressions have been derived to obtain δ, θ and θ+χ from six phase-stepped images. These images are collected using a CCD camera and the full-field data is processed using a standard personal computer. This novel method allows accurate, full-field maps of all three characteristic parameters to be obtained in a relatively short time, which makes full-field tomographic reconstruction of photoelastic data a real possibility.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the design, construction and testing of a load cell to measure axial force, shear force, and bending moment at the end of a structural beam.
Abstract: This paper describes the design, construction and testing of a load cell to measure the axial force, shear force, and bending moment at the end of a structural beam. The capacities of the load cell are 780 kN in axial load, 350 kN in shear, and 200 kNm in bending. These magnitudes, together with the requirement that the load cell should be kept as slim as possible, necessitated a novel design comprising three steel double-spring elements machined with semicircular channels to provide localized strain amplification. The load cell was designed with the aid of detailed finite element analysis and was machined from grade 55 steel. After strain gaging, it was subjected to an extensive series of calibration tests. Results from these tests are reported, together with those from some early experiments in which two load cells were used to measure the behavior of structural steel knee elements.

Journal ArticleDOI
TL;DR: In this paper, an experimental investigation of the standard shear test ASTM C273 carried out on sandwich structures is presented, highlighting and quantifying some parasitic effects that occur during this test.
Abstract: The present work is an experimental investigation of the standard shear test ASTM C273 carried out on sandwich structures. The goal is to highlight and to quantify some parasitic effects that occur during this test. A suitable optical method providing whole-field measurements has been used to capture the displacement and strain fields during the test. Some parasitic effects have been detected: the steel plates bend during the test, the shear strain reaches zero near the free edges and compressive/tensile strains occur in this zone.

Journal ArticleDOI
TL;DR: In this article, the Fourier transform and phase shifting techniques were applied to the use of the interferometric Strain/Displacement Gage (ISDG) during micro-sample tensile testing.
Abstract: The laser based interferometric Strain/Displacement Gage (ISDG) measures the in-plane surface deformation between two small reflecting surface markers. When illuminated with a coherent beam of light, the reflected beams from the two markers form an interference pattern, and monitoring the shift of the fringe pattern allows strain in the gage section of a specimen to be directly measured. A minimum on the fringe pattern can be isolated and tracked as the test proceeds, but this technique utilizes only a small part of the optical signal and often requires a complex programming scheme. This paper presents the application of Fourier transform and phase shifting techniques to the use of the ISDG during microsample tensile testing. The Fourier transform samples the entire fringe pattern and greatly improves the optical signal to noise ratio, and the phase shifting fringe pattern analysis has proven to be more robust and less affected by speckle or optical noise than fringe pattern minimum tracking. This results in the ability to measure larger deformations with a system resolution of ∼5 microstrain and an uncertainty of ±15.5 microstrain. An example involving the microsample tensile testing of a MEMS related LIGA Ni specimen is included to demonstrate the utility of these new techniques.