Publications

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This paper compares measurements made by Raman and infrared thermometry on a SOI (silicon on insulator) bent-beam thermal microactuator. Both techniques are noncontact and used to experimentally measure temperatures along the legs and on the shuttle of the thermal microactuators. Raman thermometry offers micron spatial resolution and measurement uncertainties of {+-}10 K; however, typical data collection times are a minute per location leading to measurement times on the order of hours for a complete temperature profile. Infrared thermometry obtains a full-field measurement so the data collection time is much shorter; however, the spatial resolution is lower and calibrating the system for quantitative measurements is challenging. By obtaining thermal profiles on the same SOI thermal microactuator, the relative strengths and weaknesses of the two techniques are assessed.

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Many ballistic fibers have been developed and utilized in soft body armors for military and law enforcement personnel. However, it is complex and challenging to evaluate the performance of ballistic resistance for the ballistic fibers. In applications, the fibers are subjected to high speed transverse impact by external objects. It is thus desirable to understand the dynamic response of the fibers under transverse impact. Transverse wave speed has been recognized a critical parameter for ballistic-resistant performance because a faster transverse wave speed dissipates the external impact energy more quickly. In this study, we employed split Hopkinson pressure bar (SHPB) and gas gun to conduct high-speed impact on a Kevlar fiber bundle in the transverse direction at different velocities. The deformation of the fiber bundle was photographed with high-speed digital cameras. Additional sensitive transducers were employed to provide more quantitative information of the fiber response during such a transverse impact. The experimental results were used for quantitative verification of current analytical models.

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This report describes a Laboratory Directed Research and Development (LDRD) project to use of synchrotron-radiation computed tomography (SRCT) data to determine the conditions and mechanisms that lead to void nucleation in rolled alloys. The Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory (LBNL) has provided SRCT data of a few specimens of 7075-T7351 aluminum plate (widely used for aerospace applications) stretched to failure, loaded in directions perpendicular and parallel to the rolling direction. The resolution of SRCT data is 900nm, which allows elucidation of the mechanisms governing void growth and coalescence. This resolution is not fine enough, however, for nucleation. We propose the use statistics and image processing techniques to obtain sub-resolution scale information from these data, and thus determine where in the specimen and when during the loading program nucleation occurs and the mechanisms that lead to it. Quantitative analysis of the tomography data, however, leads to the conclusion that the reconstruction process compromises the information obtained from the scans. Alternate, more powerful reconstruction algorithms are needed to address this problem, but those fall beyond the scope of this project.

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Composite materials, particularly fiber reinforced plastic composites, have been extensively utilized in many military and industrial applications. As an important structural component in these applications, the composites are often subjected to external impact loading. It is desirable to understand the mechanical response of the composites under impact loading for performance evaluation in the applications. Even though many material models for the composites have been developed, experimental investigation is still needed to validate and verify the models. It is essential to investigate the intrinsic material response. However, it becomes more applicable to determine the structural response of composites, such as a composite beam. The composites are usually subjected to out-of-plane loading in applications. When a composite beam is subjected to a sudden transverse impact, two different kinds of stress waves, longitudinal and transverse waves, are generated and propagate in the beam. The longitudinal stress wave propagates through the thickness direction; whereas, the propagation of the transverse stress wave is in-plane directions. The longitudinal stress wave speed is usually considered as a material constant determined by the material density and Young's modulus, regardless of the loading rate. By contrast, the transverse wave speed is related to structural parameters. In ballistic mechanics, the transverse wave plays a key role to absorb external impact energy [1]. The faster the transverse wave speed, the more impact energy dissipated. Since the transverse wave speed is not a material constant, it is not possible to be calculated from stress-wave theory. One can place several transducers to track the transverse wave propagation. An alternative but more efficient method is to apply digital image correlation (DIC) to visualize the transverse wave propagation. In this study, we applied three-pointbending (TPB) technique to Kolsky compression bar to facilitate dynamic transverse loading on a glass fiber/epoxy composite beam. The high-speed DIC technique was employed to study the transverse wave propagation.

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The ductile failure in metals has long been associated with void nucleation, growth and coalescence. Many micromechanics-based damage models were developed to study the effects of the voids sizes, shape and orientation to the nucleation, growth and coalescence of voids. However, the experimental methods to quantitatively validate these models were lacking. This paper is aimed to experimentally investigate at the microscale and nanoscale the effects of the shapes, sizes, orientation and density to the nucleation, growth and coalescence of voids and their relation to the ductility of the metal. In this work, notched tensile specimens with various radii were designed along different orientations. These specimens were tensile loaded up to different percentage of ultimate failure strain. The deformed specimens were then sectioned both along and perpendicular to the loading direction to microscopically study the voids size, shape and density. On the other hand, microtensile specimens were made out of these already deformed specimens. Using the advanced imaging capabilities of AFM and SEM combined with in-situ loading, the growth and coalescence of voids were in-situ studied at the microscale and nanoscale.

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Mode-I and Mode-ll fracture experiments of composites under high loading rates are presented. In the standard double cantilever beam (DCB) configuration, specimens are loaded with constant speed of 2.5 m/s (100 in/s) on a customized high-rate MTS system. Alternative high rate experiments are also performed on a modified split Hopkinson pressure bar (SHPB). One of the configurations for the characterization of dynamic Mode-I interfacial delamination is to place a wedge-loaded compact-tension (WLCT) specimen in the test section. Pulse-shaping techniques are employed to control the profiles of the loading pulses such that the crack tip is loaded at constant loading rates. Pulse shaping also avoids the excitation of resonance, thus avoiding inertia induced forces mixed with material strength in the data. To create Mode-ll fracture conditions, an (ENF) three-point bending specimen is employed in the gage section of the modified SHPB. © 2008 Society for Experimental Mechanics Inc.

Tensile deformation and fracture behavior of a closed-cell rigid polyurethane foam, called TufFoam, were investigated. During uniaxial tension tests and fracture mechanics tests, full-field deformation measurements were conducted by using digital image correlation technique. Uniform deformation fields obtained from the tension tests showed that both deviatoric and dilatational yielding contributed to the nonlinear deformation of the foam under tension. Fracture mechanics tests were performed with single-edge-notched specimens under three-point bending and uniaxial tension. A moderate specimen-size and loading-geometry dependence was observed in the measured fracture toughness values based on linear elastic fracture mechanics. Full-field deformation data near the crack-tip were used to investigate stable crack-growth in the foam until unstable fracture occurs. The path-independent J-integral and M-integral were calculated from elastic far-fields of the experimental data, and used to obtain crack-tip field parameters, such as crack-tip energy release rates and effective crack-tip positions. The combination of the full-field deformation measurement technique and the path-independent integrals was proven to be a useful approach to measure the initiation toughness of the foam that is independent of the specimen size and loading geometry. © 2008 Society for Experimental Mechanics Inc.

The mechanical properties of some materials (Cu, Ni, Ag, etc.) have been shown to develop strong dependence on the geometric dimensions, resulting in a size effect. Several theories have been proposed to model size effects, but have been based on very few experiments conducted at appropriate scales. Some experimental results implied that size effects are caused by increasing strain gradients and have been used to confirm many strain gradient theories. On the other hand, some recent experiments show that a size effect exists in the absence of strain gradients. This report describes a brief analytical and experimental study trying to clarify the material and experimental issues surrounding the most influential size-effect experiments by Fleck et al (1994). This effort is to understand size effects intended to further develop predictive models.

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Accurate material models are fundamental to predictive structural finite element models. Because potting foams are routinely used to mitigate shock and vibration of encapsulated components in electro/mechanical systems, accurate material models for foams are needed. A viscoplastic foam constitutive model has been developed to represent the large nonlinear and rate dependent crush of a polyurethane foam throughout an application space defined by temperature, strain rate and strain levels. Validation of this viscoplastic model, which is implemented in the transient dynamic Presto finite element code, is being achieved by modeling and testing a series of structural geometries of increasing complexity that have been designed to ensure sensitivity to material parameters. Both experimental and analytical uncertainties are being quantified to ensure fair assessment of model validity. Quantitative model validation metrics are being developed to provide a means of comparing analytical model predictions with experimental observations. This paper focuses on model validation of foam/component behavior over a wide temperature, strain rate, and strain level range using a Presto viscoplastic finite element model. Experiments include simple foam/component test articles crushed in a series of drop table tests. Material variations of density have been included. A double blind validation process is described that brings together test data with model predictions.

The foam material of interest in this investigation is a rigid closed-cell polyurethane foam PMDI with a nominal density of 20 pcf (320 kg/m3). Three separate types of compression experiments were conducted on foam specimens. The heterogeneous deformation of foam specimens and strain concentration at the foam-steel interface were obtained using the 3-dimensional digital image correlation (3D-DIC) technique. These experiments demonstrated that the 3D-DIC technique is able to obtain accurate and full-field large deformation of foam specimens, including strain concentrations. The experiments also showed the effects of loading configurations on deformation and strain concentration in foam specimens. These DIC results provided experimental data to validate the previously developed viscoplastic foam model (VFM). In the first experiment, cubic foam specimens were compressed uniaxially up to 60%. The full-field surface displacement and strain distributions obtained using the 3D-DIC technique provided detailed information about the inhomogeneous deformation over the area of interest during compression. In the second experiment, compression tests were conducted for cubic foam specimens with a steel cylinder inclusion, which imitate the deformation of foam components in a package under crush conditions. The strain concentration at the interface between the steel cylinder and the foam specimen was studied in detail. In the third experiment, the foam specimens were loaded by a steel cylinder passing through the center of the specimens rather than from its end surface, which created a loading condition of the foam components similar to a package that has been dropped. To study the effects of confinement, the strain concentration and displacement distribution over the defined sections were compared for cases with and without a confinement fixture.

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