Publications

Abstract not provided.

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.

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.

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.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

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.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Honeycomb is a structure that consists of two-dimensional regular arrays of open cells. High-density aluminum honeycomb has been used in weapon assemblies to mitigate shock and protect payload because of its excellent crush properties. In order to use honeycomb efficiently and to certify the payload is protected by the honeycomb under various loading conditions, a validated honeycomb crush model is required and the mechanical properties of the honeycombs need to be fully characterized. Volume I of this report documents an experimental study of the crush behavior of high-density honeycombs. Two sets of honeycombs were included in this investigation: commercial grade for initial exploratory experiments, and weapon grade, which satisfied B61 specifications. This investigation also includes developing proper experimental methods for crush characterization, conducting discovery experiments to explore crush behaviors for model improvement, and identifying experimental and material uncertainties.