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Showing papers in "Experimental Mechanics in 2004"


Journal ArticleDOI
TL;DR: In this paper, an integrated experimental/analytical study has been conducted to examine the process of dynamic stress equilibrium in a soft rubber specimen, and an analytical model was employed to analyze the equilibrating processes observed in experiments.
Abstract: The condition of dynamic stress equilibrium is not satisfied automatically when a split Hopkinson pressure bar (SHPB) is employed to determine the dynamic properties of soft materials. In order to develop guidelines for the proper design of SHPB experiments under valid testing conditions, an integrated experimental/analytical study has been conducted to examine the process of dynamic stress equilibrium in a soft rubber specimen. Dynamic compressive experiments on a RTV 630 and an ethylene-propylene-diene monomer rubber with a SHPB modified for soft material testing were conducted to determine the effects of specimen thickness and loading rate on the stress equilibrating process. An analytical model was employed to analyze the equilibrating processes observed in experiments. It is found that the incident loading rate dominates the initial non-equilibrium stress state, and the specimen thickness mainly affects the dynamic stress equilibrium after the initial stage.

185 citations


Journal ArticleDOI
TL;DR: In this article, the authors illustrate the problems involved in calibrating a stereo microscope using traditional techniques and present a novel methodology for acquiring accurate, three-dimensional surface shape and deformation data on small-scale specimens.
Abstract: The increasing research focus on small-scale mechanical systems has generated a need for deformation and strain measurement systems for microscale applications. Optical measurement systems, such as digital image correlation, present an obvious choice due to their non-contacting nature. However, the transfer of measurement technology developed for macroscale applications to the microscale presents unique challenges due to the differences in the required highmagnification optics. In this paper we illustrate the problems involved in calibrating a stereo microscope using traditional techniques and present a novel methodology for acquiring accurate, three-dimensional surface shape and deformation data on small-scale specimens.

184 citations


Journal ArticleDOI
TL;DR: In this paper, a contour method using laser scanning is proposed to measure surface contours for application to residual stress measurement, where a part is cut in two with a flat cut, and the part deforms by relaxation of the residual stresses.
Abstract: We describe non-contact scanning with a confocal laser probe to measure surface contours for application to residual stress measurement. (In the recently introduced contour method, a part is cut in two with a flat cut, and the part deforms by relaxation of the residual stresses. A cross-sectional map of residual stresses is then determined from measurement of the contours of the cut surfaces.) The contour method using laser scanning is validated by comparing measurements on a ferritic steel (BS 4360 grade 50D) weldment with neutron diffraction measurements on an identical specimen. Compared to lower resolution touch probe techniques, laser surface-contouring allows more accurate measurement of residual stresses and/or measurement of smaller parts or parts with lower stress levels. Furthermore, to take full advantage of improved spatial resolution of the laser measurements, a method to smooth the surface contour data using bivariate splines is developed. In contrast to previous methods, the spline method objectively selects the amount of smoothing and estimates the uncertainties in the calculated residual stress map.

147 citations


Journal ArticleDOI
TL;DR: In this article, the authors compared the Young's modulus and bulge test for thin-film, low-pressure chemical vapor deposited silicon nitride, and concluded that the bulge-based method is a valid wafer-level test method.
Abstract: The mechanical properties of thin-film, low-pressure chemical vapor deposited silicon nitride were measured in uniaxial tension and by a bulge test method suitable for wafer-level testing. This research compares the two approaches and presents additional data on silicon nitride. The common property from the two test methods is the Young's modulus. Tensile tests performed at the Johns Hopkins University provided a value of 257±5 GPa. Bulge tests conducted by Exponent, Inc., an engineering and scientific consulting firm, yielded a value of 258±1 GPa. It is concluded that this bulge test is a valid wafer-level test method. These tensile results, when added to earlier results, yield the following properties for low-stress silicon nitride: Young's modulus =255±5 GPa, Poisson ratio=0.23±0.02, and fracture strength=5.87±0.62 GPa.

132 citations


Journal ArticleDOI
TL;DR: In this article, the authors developed a method to estimate the bridge deflection using fiber optic Bragg-grating (FBG) strain sensors, which are electromagnetic, noise-free and multipoint measurable.
Abstract: In this paper, we develop a method to estimate the bridge deflection using fiber optic Bragg-grating (FBG) strain sensors. For most structural evaluation of bridge integrity, it is very important to measure the geometric profile, which is a major factor representing the global behavior of civil structures, especially bridges. In the past, because of the lack of appropriate methods to measure the deflection curve of bridges on site, the measurement of deflection has been restricted to just a few discrete points along the bridge, and the measuring points have been limited to the locations installed with displacement transducers. However, by applying classical beam theory, a formula is rearranged to estimate the continuously deflected profile by using strains measured directly from several points. In addition, FBG strain sensors, which are electromagnetic, noise-free and multipoint measurable, are employed to obtain flexural strains more accurately and stably. The regression analysis is performed to obtain a strain function from the measured strain data. Finally, the deflection curve can be estimated by applying the strain function to the formula. An experimental test has also been carried out to verify the developed method.

123 citations


Journal ArticleDOI
TL;DR: In this article, a split Hopkinson pressure bar (SHPB) experiment was used to obtain dynamic stress-strain loops for engineering materials, including polymethyl methacrylate and nickel-titanium shape memory alloy.
Abstract: Pulse-shaping techniques are developed for both the loading and unloading paths of a split Hopkinson pressure bar (SHPB) experiment to obtain valid dynamic stress-strain loops for engineering materials. Front and rear pulse-shapers, in association with a momentum trap, are used to precisely control the profiles of the loading and unloading portions of the incident pulse. The modifications, ensure that the specimen deforms at the same constant strain rate under dynamic stress equilibrium during both loading and unloading stages of an experiment so that dynamic stress-strain loops can be accurately determined. Dynamic stress-strain loops with a constant strain rate for a nickel-titanium shape memory alloy and polymethyl methacrylate are determined using the modified SHPB. The modified momentum trap prevents repeated loading on a specimen without affecting the amplitude of the desired loading pulse and without damaging the bar at high stress levels.

122 citations


Journal ArticleDOI
TL;DR: In this paper, a miniaturized Kolsky bar system that includes the input bar is developed, together with the use of the laser occlusive radius detector to obtain local measurements of specimen strain during the very high rate deformations.
Abstract: A rigorous experimental and numerical assessment is made of the benefits and limits of miniaturization in the Kolsky bar system. The primary issues that arise in very high strain rate testing (stress equilibration, inertial effects, wave dispersion, friction, and controllability of deformations) are addressed through experiments coupled with explicit finite element analyses. A miniaturized Kolsky bar system that includes the input bar is developed, together with the use of the laser occlusive radius detector to obtain local measurements of specimen strain during the very high rate deformations. It is demonstrated that this miniaturized Kolsky bar system can be used to provide fully validated results, including the explicit determination of equilibration, over a very wide range of strain rates (1×103 to 5×104 s−1). The desired high strain rate can be achieved even at low accumulated strains, and the total strain developed can be controlled very effectively. Specific conditions are developed for determining the range of utility of the technique for a given material. The technique is applied to the characterization of 6061-T651 aluminum, and the results are compared with the results obtained using a conventional Kolsky bar.

99 citations


Journal ArticleDOI
TL;DR: In this article, the influence of strain rate, moisture content and loading direction on the stress-strain relationships for spruce wood has been investigated, and it was found that the strain rate has a large influence on the behavior of the wood, especially under the condition of full saturation, where water transport in the deforming specimen is of major importance.
Abstract: The influence of strain rate, moisture content and loading direction on the stress-strain relationships for spruce wood has been investigated. The strain rates were approximately 8×10−3 s−1, 17s−1 and 1000 s−1, and the states of moisture content were those corresponding to oven dry, fiber saturated and fully saturated. Compressive loads were applied along the principal directions of the stem of the tree, i.e., radially, tangentially and axially. The low and medium strain-rate tests were performed with the aid of a servohydraulic testing machine, while the high strain-rate tests were carried out using the split Hopkinson pressure bar (SHPB) technique. Magnesium or steel bars were used in the different SHPB tests in order to reduce impedance mismatch for the different directions of the wood specimens. The strain rate was found to have large influence on the behavior of the wood, especially under the condition of full saturation, where water transport in the deforming specimen is of major importance.

81 citations


Journal ArticleDOI
TL;DR: In this article, a broad-band, laser ultrasonic setup and a time-frequency wavelet transform processing was used to characterize longitudinal and flexural waves in terms of dispersive velocity and frequency-dependent attenuation.
Abstract: Methods based on guided ultrasonic waves are gaining increasing attention for the non-destructive inspection and condition monitoring of multi-wire strands used in civil structures such as prestressing tendons and cable stays. In this paper we examine the wave propagation problem in seven-wire strands at the level of the individual wires comprising the strand. Through a broad-band, laser ultrasonic setup and a time—frequency wavelet transform processing, longitudinal and flexural waves are characterized in terms of dispersive velocity and frequency-dependent attenuation. These vibrating frequencies propagating with minimal losses are identified as they are suitable for long-range inspection of the strands. In addition, the wave transmission spectra are found to be sensitive to the load level, suggesting the potential for continuous load monitoring in the field.

76 citations


Journal ArticleDOI
TL;DR: In this article, a tensile test system for ultra-high temperature alloys is presented, where strain is measured with a video extensometer by means of the software SuperCreep.
Abstract: Specially designed facilities for tensile testing of ultra-high temperature alloys are presented. Ohmic heating is chosen for easy access to the sample, fast attainable heating and cooling rates, simplicity in design and operation. Strain is measured with a video extensometer by means of the software SuperCreep. The algorithm for the strain measurements is described. Stability and accuracy of the test system were determined by testing an oxide dispersion strengthened Pt alloy. Performance of the video extensometer was checked by thermal expansion tests on pure Pt. Tensile tests of the oxide dispersion strengthened alloy Pt-10 wt% Rh DPH at 1600°C have proven the reliability of the equipment.

59 citations


Journal ArticleDOI
TL;DR: In this article, a novel experimental technique for measuring crack tipT-stress, and hence in-plane crack tip constraint, in elastic materials has been developed, which exploits optimal positioning of stacked strain gage rosette near a mode I crack tip such that the influence of dominant singular strains is negated in order to determine T-stress accurately.
Abstract: A novel experimental technique for measuring crack tipT-stress, and hence in-plane crack tip constraint, in elastic materials has been developed. The method exploits optimal positioning of stacked strain gage rosette near a mode I crack tip such that the influence of dominant singular strains is negated in order to determineT-stress accurately. The method is demonstrated for quasi-static and low-velocity impact loading conditions and two values of crack length to plate width ratios (a/W). By coupling this new method with the Dally-Sanford single strain gage method for measuring the mode I stress intensity factorK I , the crack tip biaxiality parameter\(\beta = T\sqrt {{{\pi a} \mathord{\left/ {\vphantom {{\pi a} K}} \right. \kern- ulldelimiterspace} K}} \) is also measured experimentally. Complementary small strain, static and dynamic finite element simulations are carried out under plane stress conditions. Time histories ofK I andT-stress are computed by regression analysis of the displacement and stress fields, respectively. The experimental results are in good agreement with those obtained from numerical simulations. Preliminary data for critical values ofK I and β for dynamic experiments involving epoxy specimens are reported. Dynamic crack initiation toughness shows an increasing trend as β becomes more negative at higher impact velocities.

Journal ArticleDOI
TL;DR: In this article, a convex interface/joint design, inspired by the shape and mechanics of trees, will result in reduced stress singularities at bimaterial corners for most engineering material combinations.
Abstract: An integrated experimental and numerical investigation was conducted for removing the free-adge stress singularities in dissimilar material joints. A convex inter-face/joint design, inspired by the shape and mechanics of trees, will result in reduced stress singularities at bimaterial corners for most engineering material combinations.In situ photoelasticity experiments on convex polycarbonate-aluminum joints showed that the free-edge stress singularity was successfully removed. As a result, the new design not only improves the static load transfer capacity of dissimilar meterial joints, but also yields more reasonable interfacial tensile strength evaluation. For convex polycarbonate-aluminum and poly(methyl methacrylate)-aluminum joint specimens, the ultimate tensile load increased up to 81% while the total material volume was reduced by at least 15% over that of traditional butt-joint specimens with severe free-edge stress singularities.

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a method to determine the signal-dependent shape of the Gabor wavelet transform for the best time-frequency localization, based on the Shannon entropy, which measures the extent of signal energy concentration in the timefrequency plane.
Abstract: The continuous Gabor wavelet transform (GWT) has been utilized as an effective and powerful time-frequency analysis tool for identifying the rapidly-varying characteristics of some dispersive wave signals. The effectiveness of the GWT is strongly influenced by the wavelet shape that controls the time-frequency localization property. Therefore, it is very important to choose the right Gabor wavelet shape for given signals. Because the characteristics of signals are rarely known in advance, the determination of the optimal shape is usually difficult. Based on this observation, we aim at developing a systematic method to determine the signal-dependent shape of the Gabor wavelet for the best time-frequency localization. To find the optimal Gabor wavelet shape, we employ the notion of the Shannon entropy that measures the extent of signal energy concentration in the time-frequency plane. To verify the validity of the present approach, we analyze a set of elastic bending wave signals generated by an impact in a solid cylinder.

Journal ArticleDOI
TL;DR: In this article, the authors present an application of the grid method to the assessment of crack initiation and growth in brittle materials, where a grid deposited on the surface of a structure can be utilized to characterize the crack location and opening.
Abstract: In this paper we present an application of the grid method to the assessment of crack initiation and growth in brittle materials. It is shown that a grid deposited on the surface of a structure can be utilized to characterize the crack location and opening. Furthermore, one single picture only is required to carry out a control over a predefined area. First, the principle of the method is introduced, showing that a periodical pattern encoding a cracked surface can be analyzed by signal processing. The phase modulation of the grid can be calculated very accurately for any small deformation of the structure. Discontinuities due to cracks are easily detected on the phase field and the crack width is proportional to the amplitude of the discontinuity. Secondly, the performances of the method are presented, mainly depending on the spatial frequency of both the pattern and the sampling. It is shown that good results can generally be obtained with few setting difficulties and equipment investment. Finally, we sum up some basic recommendations for the practical use of the technique.

Journal ArticleDOI
TL;DR: In this paper, a split Hopkinson pressure bar was used to dynamically load an intact ceramic materiaal by two consecutive stress pulses, and the second pulse determined the dynamic compressive constitutive behavior of the ceramic rubble.
Abstract: A novel dynamic compressive experimental technique has been developed based on a split Hopkinson pressure bar. This new method dynamically loads the ceramic specimen by two consecutive stress pulses. The first pulse determines the dynamic response of the intact ceramic materiaal and then crushes the specimen, and the second pulse determines the dynamic compressive constitutive behavior of the ceramic rubble. Precise pulse shaping ensures that the specimen deforms at nearly constant strain rates under dynamic stress equilibrium during the loading by both stress pulses. Pulse shaping also controls the amplitudes of loading pulses, the values of strain rates, the maximum strains in the rubble specimens, and the proper separation time between the two loading pulses. The feasibility of the new technique is demonstrated by the experimental results obtained on an AD995 alumina.

Journal ArticleDOI
TL;DR: In this article, three experimental techniques for determining the uniaxial tensile properties and functional swelling properties of microscale hydrogel structures have been developed, including particle image velocimetry, a method originally developed to measure velocity fields in fluid flows, was adapted to investigate the deformation rates at various times within hydrogels structures during volumetric swelling.
Abstract: Hydrogel actuators in microfluidic devices must endure the forces of aqueous flow, the constraint of device walls, and the restoring force of elastic membranes. In order to assess the capabilities of hydrogels, three experimental techniques for determining the uniaxial tensile properties and functional swelling properties of microscale hydrogel structures have been developed. Tensile tests were conducted to determine Young's modulus and Poisson's ratio at varying degrees of swelling equilibrium. Force response tests were performed to determine the force exerted by cylindrical hydrogel structures on compression platens held at fixed displacement. Particle image velocimetry, a method originally developed to measure velocity fields in fluid flows, was adapted to investigate the deformation rates at various times within hydrogel structures during volumetric swelling. The techniques and sample fabrication methods outlined are applicable to a variety of hydrogel chemistries.

Journal ArticleDOI
TL;DR: In this article, a polariscope that allows simultaneous capture of multiple images is described, and a prototype instrument is evaluated using both transmission and reflection photoelasticity, offering the potential for real-time data acquisition and processing of high-speed events.
Abstract: The goal of recent research in digital photoelasticity has been fast, reliable, and accurate full-field photoelastic data that will allow the technique to play a valued role in assessing material and structural integrity. A novel design for a polariscope that allows simultaneous capture of multiple images is described, and a prototype instrument is evaluated using both transmission and reflection photoelasticity. The design offers the potential for real-time data acquisition and processing of high-speed events, using a number of different approaches to digital photoelasticity. The evaluation of the instrument arranged for the phase-stepping method demonstrated that it was capable of providing results of comparable quality and accuracy to manual analysis and more conventional methods of acquiring phase-stepped images.

Journal ArticleDOI
TL;DR: In this paper, an experimental investigation of the oblique impact between a tennis ball and head clamped tennis racket was conducted, and it was found that the ball rebound spin was not a function of the material, gage or tension of the string used in the tennis racket.
Abstract: In this paper, we describe an experimental investigation of the oblique impact between a tennis ball and head clamped tennis racket. It was found that the magnitude of the ball rebound spin was not a function of the material, gage or tension of the string used in the tennis racket. Furthermore, it was concluded that all strings exhibit a sufficiently large friction coefficient that the ball begins to roll during impact. There is anecdotal evidence from tennis players that suggests that a high string tension or a rough string surface enable them to impart more spin to the ball. For example, players have been quoted as saying that a high string tension makes the strings “bite” into the ball, giving more spin. The data reported in this study do not support these observations. Analysis of the experimental data has shown that the balls are rebounding from the surface with more spin than would typically be associated with rolling. A second experiment showed that the balls commenced rolling at the mid-point of the impact. This information was used in a theoretical model to show that the spin that acts on the ball during the impact can be higher than the value of the rolling spin at the end of the impact.

Journal ArticleDOI
TL;DR: In this paper, preliminary results from an analytical/experimental study of the energy absorption characteristics of grid-stiffened composite structures under transverse loading were presented. But, the results of the tests and simulations show that these types of structures have excellent damage tolerance and that most of energy absorption occurs beyond initial failure.
Abstract: In this paper we summarize preliminary results from an analytical/experimental study of the energy absorption characteristics of grid-stiffened composite structures under transverse loading. Tests and quasi-static finite element analysis simulations were carried out for isogrid E-glass/polypropylene panels in three-point bending. The results of the tests and simulations show that these types of structures have excellent damage tolerance and that most of the energy absorption occurs beyond initial failure. It is also observed that even though the peak load is greater for loading on the skin side, the specific energy absorption and the range of displacements over which energy is absorbed are significantly better when the load is applied on the rib side.

Journal ArticleDOI
TL;DR: In this article, a hot forging apparatus has been developed and applied for the experimental determination of continuum mechanical sintering parameters and theoretical modeling of thin-film Sintering, and a precise automatic loading system and high-resolution laser measurement system enable accurate measurement of radial and axial strains of cylindrical specimens under constant or intermittent uniaxial loads of 0-5000 N during high-temperature densification.
Abstract: A new hot forging apparatus has been sucessfully developed and applied for the experimental determination of continuum mechanical sintering parameters and theoretical modeling of thin-film sintering. A precise automatic loading system and high-resolution laser measurement system enable accurate measurement of radial and axial strains of cylindrical specimens under constant or intermittent uniaxial loads of 0-5000 N during high-temperature densification. Preliminary experimental results are demonstrated and the key features of technological significance in the design introduced in detail.

Journal ArticleDOI
TL;DR: In this article, numerical investigations were conducted on plane and axisymmetric convex joints of polycarbonate and poly(methyl methacrylate) interfaces to gain a better understanding of the stress state along the interface and also to aid an experimental study conducted on the same issue.
Abstract: Numerical investigations were conducted on plane and axisymmetric convex joints of polycarbonate—aluminum and poly(methyl methacrylate)—aluminum interfaces to gain a better understanding of the stress state along the interface and also to aid an experimental study conducted on the same issue. Two-dimensional plane—stress investigations of convex joints revealed the successful elimination of the free-edge stress singulatities along the specimen width although stress singularities, along the thickness direction, persisted. A convex axisymmetric design, with the same material combination and joining angles, proves to be a better design in order to achieve an overall elimination of free-edge stress singularities of dissimilar materials and structures.

Journal ArticleDOI
TL;DR: The intra-aneurysmal flow activity, vortex strength, and wall shear stress are found to decrease with increasing stent parameters, and a critical range ofCα related to the inflow location as well as the shape and number of intra-aneurysmal vortices is identified.
Abstract: We present particle tracking velocimetry measurements and flow visualization of pulsatile flow fields in a stented cerebrovascular lateral aneurysm model with a wide ostium anchored on a curved parent vessel. Among the stent parameters, the blocking ratioC α ranging from 0% to 75% was selected to study its effect on the changes of intra-aneurysmal hemodynamics for the reference of minimally invasive endovascular aneurysm treatment. The Womersley number was 3.9 and the mean, peak, and minimal Reynolds numbers based on the bulk average velocity and diameter of the parent vessel were 600, 850, and 300, respectively. The results are characterized in terms of velocity vector field, coded streak images, region averaged velocity, vorticity, and wall shear stress. A critical range ofC α related to the inflow location as well as the shape and number of intra-aneurysmal vortices is identified. The intra-aneurysmal flow activity, vortex strength, and wall shear stress are found to decrease with increasingC α. Among theC α examined,C α=75% is the most favorable in attenuating the risk of aneurysmal rupture and promoting intra-aneurysmal thrombus.

Journal ArticleDOI
TL;DR: In this article, a set of test fixtures were designed and used to obtain failure loads of mild steel spot weld specimens under combined opening and shear loading conditions, and three different impact speeds were applied to examine the effects of separation speed on failure loads.
Abstract: Failure loads of spot weld specimens are investigated under impact combined loading conditions. A set of test fixtures was designed and used to obtain failure loads of mild steel spot weld specimens under combined opening and shear loading conditions. Three different impact speeds were applied to examine the effects of separation speed on failure loads. Micrographs of the cross-sections of failed spot welds were obtained to understand the failure processes in mild steel specimens under different impact combined loads. The experimental results indicate that the failure mechanisms of spot welds are very similar for mild steel specimens at various impact speeds. These micrographs show that the sheet thickness can affect the failure mechanisms. For 1.0 mm specimens, the failure occurs near the base metal in a necking/shear failure mode. For 1.5 mm specimens, the failure occurs near the heat-affected zone in a shear failure mode. Based on the experimental results, the effects of the inertia force, the separation speed, and the loading angle on the failure loads of spot welds are investigated. Failure criteria are proposed to characterize the failure loads of spot welds under impact combined opening and shear loads for engineering applications. The failure load can be expressed as a function of the tensile strength of the base metal, the nugget size, the sheet thickness, the maximum separation speed, the loading angle, and empirical coefficients for a given welding schedule.

Journal ArticleDOI
TL;DR: In this paper, a new multi-stage, multi-piece molding process is used to create heterogeneous polymer structures with geometrically complex interfaces consisting of rectangular and circular interlocking features.
Abstract: Interfaces in heterogeneous structures are typically engineered for optimal strength through the control of surface roughness and the choice of adhesives. Advances in manufacturing technologies are now making it possible to also tailor the geometries of interfaces from the nanoscale to the macroscale to create geometrically complex interfaces that exhibit enhanced performance characteristics. However, the impact of geometric complexity on the mechanical behavior of interfaces has not yet been ascertained. In this investigation, the first step is taken towards understanding the effects of geometric complexity on interfacial strength. A new multi-stage, multi-piece molding process is used to create heterogeneous polymer structures with geometrically complex interfaces consisting of rectangular and circular interlocking features. The structural integrity of these heterogeneous structures is characterized through interfacial tension testing. The full-field deformation measurement technique known as digital image correlation is also used during the testing to visualize the deformation fields around the geometrically complex features. Through this characterization, it is determined that the complex geometries increase the interfacial strength by approximately 20–25%, while reducing the statistical variation by 50%. These effects are attributed to a transition in the failure mechanism from interfacial fracture to homogeneous ligament failure. Results also indicate that geometrically complexity can be used on completely debonded interfaces to increase the strength to at least 25–35% of the bonded interface. Based on these results, some simple design rules have been proposed that enable geometrically complex interfaces to be engineered with enhanced strengths approaching the weaker of the two base materials. These design rules can also be used in the engineering of interfaces to facilitate the development of heterogeneous structures using new design paradigms, such as design for recyclability and the design of products based on bio-inspired concepts.

Journal ArticleDOI
TL;DR: In this article, a reliable drop/impact simulation for a cellular phone is carried out using the explicit code LS-DYNA, and globallocal experimental verification is accomplished by means of high-speed photography and impact response measurement.
Abstract: Conducting drop tests to investigate impact behavior and identify failure mechanisms of small-size electronic products is generally expensive and time-consuming. Nevertheless, strict drop/impact performance criteria for hand-held electronic products such as cellular phones play a decisive role in the design because they must withstand unexpected shocks. The design of product durability on impact has heavily relied on the designer's intuition and experience. In this study, a reliable drop/impact simulation for a cellular phone is carried out using the explicit code LS-DYNA. Subsequently globallocal experimental verification is accomplished by means of high-speed photography and impact response measurement. Using this methodology, we predict potential damage locations in a cellular phone and compare them with real statistical data. It is envisaged that development of a reliable methodology of drop/impact simulation will provide us with a powerful and efficient vehicle for improvement of the design quality and reduction of the product development cycle.

Journal ArticleDOI
TL;DR: In this paper, a test rig of asymmetric sandwich structures subjected to compression/shear loads was designed, validated, and set up, which conforms to the standard certification procedure for composite aeronautical structures set out in the test pyramid.
Abstract: Asymmetric sandwich technology can be applied in the design of lightweight, non-pressurized aeronautical structures such as those of helicopters. A test rig of asymmetric sandwich structures subjected to compression/shear loads was designed, validated, and set up. It conforms to the standard certification procedure for composite aeronautical structures set out in the “test pyramid”, a multiscale approach. The static tests until failure showed asymmetric sandwich structures to be extremely resistant, which, in the case of the tested specimen shape, were characterized by the absence of buckling and failure compressive strains up to 10,000 μ strains. Specimens impacted with perforation damage were also tested, enabling the original phenomenon of crack propagation to be observed step-by-step. The results of the completed tests thus enable the concept to be validated, and justify the possibility of creating a much larger machine to overcome the drawbacks linked to the use of small specimens.

Journal ArticleDOI
TL;DR: In this article, the authors describe the design and manufacture of an axial-torsion test specimen, and provide relationships needed when conducting stress-strain characterization experiments with the specimen.
Abstract: In this paper we describe the design and manufacture of an axial-torsion test specimen, and provide relationships needed when conducting stress-strain characterization experiments with the specimen. The specimen is a short hollow cylinder of rubber bonded between two steel mounting rings, in which simultaneous axial and shear strains are produced via independently controlled axial and twist displacements. We present calculations for the strain-displacement and stress-load relationships, and strain energy density. These relationships have been established and validated via a combination of analytical and experimental techniques, and finite element analysis. We have investigated the extent and effects of strain and stress field non-uniformity in the test specimen. The specimen design is sufficiently simple that a closed-form expression for the strain-displacement relationship has been successfully developed.

Journal ArticleDOI
TL;DR: In this article, a hybrid procedure for the in-plane mechanical characterization of orthotropic materials is presented, where the material identification reverse engineering problem is solved by combining speckle interferometry and numerical optimization.
Abstract: In this paper we present a novel hybrid procedure for the in-plane mechanical characterization of orthotropic materials. The material identification reverse engineering problem is solved by combining speckle interferometry and numerical optimization. The rationale behind the entire process is the following: for any specimen to be characterized and which has been subjected to some loading condition, it is possible to express the difference between experimental data and analytical/numerical predictions by means of an error function ψ, which depends on the elastic constants of the material. The ψ error will decrease as the elastic constants come close to their target values. Here, we build the ψ function as the difference between the displacement field measured with speckle interferometry and its counterpart computed by means of finite element analysis. Since the ψ function is highly non-linear, it has to be optimized with a global optimization algorithm, which perform a random search in the elastic constants design space. The hybrid material identification process finally allows us to determine values of the elastic constants. In order to prove the feasibility of the present approach, we have determined the in-plane elastic properties of an eight-ply composite laminate (woven fiberglass-epoxy) used as a substrate for printed circuit boards. The results indicate that the procedure proposed in this paper was able to accurately characterize the material under investigation. Remarkably, the elastic constants found by the identification procedure were less than 0.7% different from their target values, while the residual error between the displacements measured by speckle interferometry and those computed at the end of the optimization process was less than 3%.

Journal ArticleDOI
TL;DR: In this article, an analytical model is presented for determining surface residual stress using continuous indentation, where the elastic residual stress is assumed to have no influence on contact area or hardness and to be uniform over a volume that is several times larger than the indentation mark.
Abstract: An analytical model is presented for determining surface residual stress using continuous indentation. The elastic residual stress is assumed to have no influence on contact area or hardness and to be uniform over a volume that is several times larger than the indentation mark. A step-by-step analysis for the residual-stress-induced load difference at a given depth is outlined here and such concepts as stress interaction, stress-sensitive contact morphology, and reversible contact recoveries during a stress relaxation are described. Finally, the proposed method is applied to the interpretation of the continuous indentation results obtained from an SS400 steel beam in which controlled bending stresses are generated. The stress estimated, however, showed a high scatter due to plastic pile-up deformation. When the optically measured contact area is used as an alternative of the contact area calculated from the unloading curve, the re-evaluated stress agrees well with the already known applied stress.

Journal ArticleDOI
TL;DR: In this article, a methodology is proposed for estimating the local yield stress in work-hardened surface layers based on the concept of in-depth normalized variation of hardness and x-ray diffraction peak width, both of which measure the strain-hardening attained by the materials' surface-treated layers due to shot-peening.
Abstract: A methodology is proposed for estimating the local yield stress in work-hardened surface layers. It is based on the concept of in-depth normalized variation of hardness and x-ray diffraction peak width, both of which measure the strain-hardening attained by the materials' surface-treated layers due to, for example, shot-peening. Its principle is directly founded on the classical hardness theory. To study the evolution of those values with plastic deformation, specimens of five steels with different mechanical properties were subjected to interrupted tensile tests. The tests were performed at successive increments of plastic strain, until fracture occurred. The specimens were loaded and unloaded in increments of about 2% true strain. After each plastic strain increment, hardness and diffraction peak width were measured. It was observed that the variations of diffraction peak width and hardness are related to the material's strain-hardening, and their normalized variations can be considered proportional to the normalized variation of the material's yield stress. Thus, where the yield stress of the bulk material, its hardness or a characteristic diffraction peak width value, and their relative variations along the hardened layers, are known, an empirical expression could be used to estimate the local yield stress as a function of the treated depth.