Understanding and Predicting Dynamic Behavior of Materials : Spall Fracture in Metals -- Modeling and Experiments
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Saryu Fensin, Los Alamos National Laboratory; Avinash Dongare, University of Connecticut; Benjamin Morrow, Los Alamos National Laboratory; Marc Meyers, University of California, San Diego; George Gray, Los Alamos National Laboratory
Monday 8:00 AM
February 24, 2020
Room: 5A
Location: San Diego Convention Ctr
Session Chair: Saryu Fensin, Los Alamos National Laboratory
8:00 AM Introductory Comments
8:05 AM Invited
Influence of Grain Boundary Crystallography on Dynamic Failure (Spall): Mukul Kumar1; Roger Minich1; 1Lawrence Livermore National Laboratory
The scaling of mechanical properties with a microstructural length scale such as grain size is well known. However, the role of grain boundary crystallography is only recently starting to emerge. In this paper, we shall report on the scaling recently observed in the case of dynamic failure or spall under shock deformation conditions for different microstructures in high purity copper. The spall strength is observed to increase as the length scales coarsen, which is counter to the Hall-Petch relationship, eventually leveling off for single crystals. It is also observed that the spall strength decreases with increasing yield strength. These apparent contradictions will be explored in the context of nucleation site density and grain boundary character distribution in the context of the scaling laws that emerge from this data. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
8:45 AM
Experimental Measurements and Modeling of LatticeTotation around Inter and Transgranular Spall Voids in Shocked Copper Bicrystals: Elizabeth Fortin1; Benjamin Shaffer1; Saul Opie1; Pedro Peralta1; 1Arizona State University
Understanding the evolution of damage and deformation due to spall at grain boundaries (GBs) can provide a basis for connecting micro- to macroscale failure behavior in metals undergoing shocks Bicrystal samples were shock loaded using flyer-plate impacts with pressures ~ 3 to 5 GPa. Pulse duration as well as crystal orientation along the shock direction were varied for a fixed boundary misorientation to determine thresholds for void nucleation and coalescence as functions of these parameters. Samples were soft recovered and cross-sectioned to perform damage characterization using electron backscattering diffraction (EBSD) and Scanning Electron Microscopy (SEM) to gather information on damage characteristics at and around the GBs, with emphasis on void interactions and lattice rotation around boundary and bulk voids. The EBSD and SEM experimental data collected were compared to results from simulations of individual and interacting voids performed using finite element crystal plasticity models, to understand local plasticity effects.
9:05 AM
Understanding and Predicting Damage and Failure at Grain Boundaries in BCC Ta: Jie Chen1; Eric Hahn2; Avinash Dongare1; Saryu Fensin2; 1University of Connecticut; 2Los Alamos National Laboratory
Understanding how grain boundaries (GB) affects the deformation and spall behavior is critical to engineering materials with tailored fracture resistance under dynamic loading conditions. This understanding is hampered by a lack of a systematic data set, especially for BCC metals. To fill in this gap, molecular dynamics (MD) simulations are performed on a set of 74 Ta bi-crystals along [110] tilt axis to investigate the role of GB structure and properties (GB misorientation angle, energy, excess volume, etc.) on the deformation mechanism and the resultant spall strength. The spall strength is found to correlate directly with the capability of the GB to plastically deform by emitting dislocations and twinning. As the misorientation angle increases, a transition of dislocation-meditated to twinning-dominated plasticity is observed. Moreover, the local structure of the GB significantly affects the deformation behavior of Ta bi-crystals, resulting in significantly different spall strengths at the same misorientation angle.
9:25 AM
Application of X-ray Phase Contrast Imaging to Spall in Magnesium Alloy AZ31B: David Chapman1; Lukasz Farbaniec1; John Jonsson1; Michael Rutherford1; Liam Smith1; Emilio Escauriza1; Daniel Eakins1; 1University of Oxford
Our understanding of the underlying mechanisms leading to failure in the spallation process has typically been built-up from a combination of continuum-scale measurements, i.e. free-surface velocity, and exquisite shock recovery experiments facilitating interrogation of the recovered microstructure. Experimentally, time-resolved study of the failure process in-situ has been frustrated by the spatio-temporal scales involved. We present the novel use of synchrotron hard X-ray phase contrast imaging (PCI) to investigate spall in AZ31B, shocked parallel or perpendicular to the extrusion direction. The PCI measurements were augmented with optical-velocimetry and dedicated shock recovery experiments to provide a more complete understanding of the micro-structurally dependent failure in this textured alloy. We discuss the ability of the PCI to: quantify the growth and coalescence of voids leading to material failure, and explain features manifest in the velocimetry, showcasing the potential for exploiting PCI to probe the incipient stages of material failure under extreme loading conditions.
9:45 AM Break
10:05 AM Invited
A Grain Level Investigation of Ductile Failure using High-energy X-ray Characterization: Diwakar Naragani1; Jun-Sang Park2; Peter Kenesei2; Michael Sangid1; 1Purdue University; 2Argonne National Laboratory
Ductile failure through the growth and coalescence of voids is of particular relevance for many engineering materials. Yet, the lack of experimental measures of the mechanical state of the material at the appropriate length scale has limited further understanding of ductile failure. In this study, we explore local grain-scale measures for their relevance as a physical basis for ductile failure. The heterogeneous micromechanical state created around voids due to grain interactions is captured via far-field high-energy diffraction microscopy in IN718 samples manufactured via selective laser melting. These experiments determine narrow bands of (i) low stress triaxiality, at the onset of failure, which highlight the path of coalescence through inter-void shearing and (ii) intragranular plasticity, strain heterogeneity, and high triaxiality to form a criteria for coalescence through inter-void necking.
10:45 AM
Role of Shock Loading Orientation and Shock Velocity on the Shock Compression and Spall Behavior of Iron at Atomic Scales: Ke Ma1; Avinash Dongare1; 1University of Connecticut
Pressure induced bcc-to-hcp phase transformation in iron has been reproduced by molecular dynamics simulations, yet few researches have been focused on its role on the spall failure of Fe. In this work, large-scale molecular dynamics (MD) simulations are carried out to study the spall failure induced by shock deformation of single-crystal Fe microstructures. The focus is to understand the role of shock loading orientation and velocity on the wave propagation, phase transformation, defect nucleation and evolution, and failure behavior. The results suggest that phase transformation and deformation twinning behavior varies with loading orientations and hence has an impact on the predicted spall strengths. The spall strength is observed to be higher for a higher fraction of phase transformed regions and lower for systems with largest deformation twinned microstructures during shock compression. The interplay between dislocation slip, deformation twinning and phase transformation on the spall response will be presented.
11:05 AM
Shock Recompaction of Existing Spall Damage in Copper: David Jones1; Saryu Fensin1; Robert Hixson1; 1Los Alamos National Laboratory
Materials subjected to shock or impulsive loading can undergo spall fracture, where the rapid unloading after the shock event drives a state of high strain-rate tension. In ductile metals, damage accrues through void nucleation, growth, coalescence, and then possibly full fracture of a spall scab. Often, these shock events are not isolated and the material is expected to survive multiple events – armor, for example. Here, we systematically generate samples containing increasing amounts of incipient (i.e. before full scab formation) spall damage, characterizing the damage distribution in terms of void size, shape, and location. The samples are then subjected to a second shock event to examine how the damaged microstructures respond as a function of initial damage quantity, in particular whether the voids and cracks are merely pushed back together or if some sort of bonding process such as recrystallization is activated.