Incipient Spallation Fracture In Light Metals From 3d X Ray Tomography 2d Microscopy And Molecular Dynamics Simulations

Author : Alfred Schwartz
Publisher :
ISBN 13 :
Total Pages : 23 pages
Book Rating : 4.:/5 (316 download)

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Download or read book Microstructural Origins of Dynamic Fracture in Ductile Metals written by Alfred Schwartz and published by . This book was released on 2004 with total page 23 pages. Available in PDF, EPUB and Kindle. Book excerpt: Dynamic fracture is a material failure process at high strain-rates. Here, we limit our discussion to spallation fracture during shock wave loading. When a compressive shock wave reflects from a free surface, internal states of tension are created. If this tension exceeds the rupture strength of the material, it fails by nucleating and growing microscopic voids in ductile metals and cracks in brittle solids. This effect, known as spallation, was reported by Hopkinson in 1872 [1]. Rinehart and Pierson [2] give an historical introduction to spallation and other aspects of high strain-rate deformation. This phenomenology of the nucleation and growth of microscopic voids is common to all fracture in ductile metals, including dynamic fracture. The importance of pulse duration was not fully appreciated until the 1960's through the work of Butcher and colleagues [3, 4], leading to the concept of cumulative damage. This concept of damage accumulation was put on a strong experimental foundation by Barbee et al. [5], who performed gas gun recovery experiments and tediously measured the size and distribution of microscopic flaws using 2D microscopy. The resulting continuum material model of dynamic fracture is known as the SRI-NAG model [6, 7], for Nucleation And Growth. In a critical assessment of dynamic fracture, Meyers and Aimone [8] reviewed the abundant continuum spallation models and found the model of nucleation, growth, and coelescense of voids to qualitatively reproduce the observed extent of damage from spall recovery experiments. As in the work of Barbee et al. [5], Meyers and Aimone do not find any voids smaller than about 1 {micro}m, suggesting that the voids either nucleated at that scale or when they are nucleated they rapidly grow to the micron size scale. The continuum models are phenomenological in nature with parameters fit to experiment. The advent of the Advanced Strategic Computing program has enabled direct numerical simulation of the nucleation and growth of microscopic voids. However, there exists no experimental data to both guide the modeling to the essential physical phenomena and to validate meso-scale modeling of dynamic fracture in ductile metals. The goal of this project is to obtain this new experimental data for dynamic fracture in ductile metals by characterizing and analyzing the microscopic processes using modern experimental facilities not previously available. In particular, we measured with increased precision the damage produced during spallation fracture in ductile metals. Dynamic gas-gun recovery experiments were used to generate incipient spallation damage while concurrently measuring the free surface velocity wave profile. This wave profile has been used extensively to characterize spallation fracture. The concurrent measurement enables us to correlate the observed incipient damage with features in the wave profile, such as the velocity pull back. The incipient spallation damage was characterized using both advanced Synchrotron-based 3D X-Ray tomography and traditional 2D microscopy. A direct correlation was made between the traditional 2D microscopy and the 3D distribution of voids on the same sample. 2D microscopy, including optical, EBS/OIM, and TEM, was used to quantify, for the first time, the plastically deformed zone in the metal surrounding an incipiently grown void. Nanoindentation hardness experiments were used to quantify the increased strength in the plastically deformed zone from the greatly increased dislocation density.