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Two independent optical methods are used to analyze the dynamic material behavior of Mylar and Homalite-100 subjected to hypervelocity impact. Birefringent targets are loaded in tension inside a two-stage light-gas gun vacuum chamber, and are impacted with a 5 mg nylon slug at velocities between 3 and 6 km/s. Caustics and photoelasticity combined with high-speed photography are used to determine dynamic stress intensity behavior around the crack tip during and after impact. Homalite-100 lower crack tip speeds are subjected to reflecting boundary shear waves from the nylon impact, and thereby the crack path exhibits distinct kinks; whereas Mylar higher crack tip speeds provides distinguishable isochromatic patterns and an unadulterated fracture surface. Shear wave patterns in the target from photoelastic effects are compared to results from numerical simulations using the Overture Suite, which solves linear elasticity equations on overlapping curvilinear grids by means of adaptive mesh refinement.

The quasi-static micromechanical damage mechanics originally formulated by Ashby and Sammis (PAGEOPH, 1990) has been expanded in three important ways: (1) An energy density function has been derived that allows a self-consistent inclusion of the effects of dynamic damage evolution on the elastic and anelastic response. (2) whereas the Ashby/Sammis model was only applicable to the case where the initial cracks are all parallel and the same size, we can now include a specified distribution of initial crack sizes with random azimuthal orientation about the loading axis, and (3) we allow for yielding of the weaker minerals in granite at relatively low stress levels which produce strong nonlinearity in the failure envelope and stress-strain curve. This new damage mechanics is validated using published experimental data for Westerly granite. In collaboration with the mechanics group at U.C. Santa Barbara, we have built our new damage mechanics into the commercial finite element program ABAQUS and used it to simulate an explosion in damaged rock. We find that an explosion in a pre-stressed medium or one that contains anisotropic initial damage generates strong S-wave radiation in agreement with previous simpler calculations by Johnson and Sammis (PAGEOPH. 2001) and experimental observations in damaged photoelastic plates., Presented at 2009 Monitoring Research Review - Ground-Based Nuclear Explosion Monitoring Technologies held in Tucson, AZ on 21-23 Sep 2009. Published in the Proceedings of 2009 Monitoring Research Review - Ground-Based Nuclear Explosion Monitoring Technologies, v3 p557-565, 2009. The original document contains color images. All DTIC reproductions will be in black and white.

High-speed digital photography was used to observe the generation of S waves by explosions in fracture-damaged photoelastic Homalite plates. We found that either a preferred orientation of the pre-existing fractures or an anisotropic initial stress field produced significant S wave radiation in the far field. These experimental observations support theoretical predictions by Johnson and Sammis (2001)., The original document contains color images. All DTIC reproductions will be in black and white.

A novel integrated approach is developed for a systematic validation of large-scale finite element simulations on dynamic crack propagations along a weak plane [1]. A set of well-controlled experimental scheme is specifically designed to provide accurate input data for the numerical simulations as well as to provide metrics for quantitative comparisons between experimental and numerical results. Dynamic photoelasticity with high-speed photography is used to capture experimental records of dynamic crack propagations along a weak plane and to provide the crack propagation history. In the dynamic experiments, a modified Hopkinson bar setup with a notch-face loading configuration is used to obtain controlled loading conditions for the dynamic fracture problem. Also an inverse-problem approach of cohesive zone model is employed to obtain a realistic cohesive law, i.e. a traction-separation law, of the weak plane, from independently measured crack-tip deformation fields using speckle interferometry technique. The experimentally collected data, the loading conditions and the cohesive law, are considered as input for the finite element simulations [2]. We employ finite-deformation cohesive elements to account for crack initiation and growth in bulk finite-element discretizations of the experimental sample. As it is well know, the cohesive elements introduce an additional material-dependent length-scale into the finite element model. The demand of accurately resolving this length-scale by the finite-element discretization, as required for truly mesh-independent results, may often lead to discretizations containing several millions of elements. We therefore resort to massively parallel computing. A comparison of the metrics from the numerical simulations with those from the experimental measurements is performed to validate the large-scale simulations. The numerical results show good agreements with the experimental results, leading to a successful validation of the large sca

Heat generation in metals during high-strain-rate plastic deformation was investigated. Experiments were designed to measure the partition of plastic work into heat and stored energy during dynamic deformations under adiabatic conditions. A Kolsky pressure bar was used to determine mechanical properties at high strain rates while a servo-hydraulic material testing system was used at low strain rates. For dynamic loading, in-situ temperature changes were measured using a high-speed infrared detector. The dependence of the fraction of plastic work converted to heat on strain and strain rate was determined for an aluminum 2024-T3 alloy and alpha-titanium. The flow stress and the fraction of plastic work converted to heat for 2024-T3 aluminum alloy were found to be a function of strain but not of the strain rate while they were found to be strongly dependent on strain rate for alpha-titanium.