Benjamin L Hansen

Effects of grain size and boundary structure on the dynamic tensile response of copper

Plate impact experiments have been carried out to examine the influence of grain boundary characteristics on the dynamic tensile response of Cu samples with grain sizes of 30, 60, 100, and 200 μm. The peak compressive stress is ∼1.50 GPa for all experiments, low enough to cause an early stage of incipient spall damage that is correlated to the surrounding microstructure in metallographic analysis. The experimental configuration used in this work permits real-time measurements of the sample free surface velocity histories, soft-recovery, and postimpact examination of the damaged microstructure. The resulting tensile damage in the recovered samples is examined using optical and electron microscopy along with micro x-ray tomography. The free surface velocity measurements are used to calculate spall strength values and show no significant effect of the grain size. However, differences are observed in the free surface velocity behavior after the pull-back minima, when reacceleration occurs. The magnitude of th...

Controlled shock loading conditions for micrstructural correlation of dynamic damage behavior

Materials performance is recognized as being central to many emergent technologies. Future technologies will place increasing demands on materials performance with respect to extremes in stress, strain, temperature, and pressure. In this study, the dynamic ductile damage evolution of OFHC Cu is explored as a test bed to understand the role of spatial effects due to loading profile and defect density. Well-characterized OFHC Cu samples of 30 μm, 60 μm, 100 μm, and 200 μm grain sizes were subjected to plate impact uniaxial strain loading at 1.5 GPa. This spall geometry produced early stage (incipient) damage in the Cu samples that could be correlated to microstructural features in metallographic analysis. The recovered damaged microstructure was examined using traditional 2D metallographic techniques (optical and electron microscopy) as well as 3D x-ray microtomography. Calculated spall strength from the free surface velocimetry (VISAR) showed no change with respect to changes in grain size, however, the ma...

Characterization of Dynamic Damage of Copper Targets using X-ray Micro Tomography

A study is currently underway to examine damage and failure characteristics of polycrystalline materials such as copper. Damage and failure for these materials is ductile in nature which means that there may be large deformation plasticity, shear localization, shear banding or void formation. Typical failure for copper under high hydrostatic tensile loading involves void formation. Void formation due to damage processes contains elements of initiation, growth, and coalescence which all lead to material failure. Nucleated voids will grow until they begin to overlap. In an attempt to better understand the incipient stages of these processes, we are using X-ray micro tomography, and classical metallographic and serial section techniques. X-ray micro tomography is being used as a first screen of the shocked samples to find voids, quantify their size and numbers, and determine volumes for further examination. The use of real 3D information will also be important for designing follow on experiments as well as providing statistically relevant damage data to advance current damage models. These models are used to predict material response under extreme conditions and with this data, we can validate predictions.

The influence of shock loading on material properties and dynamic damage evolution.

Many commercial and defense applications require structural metals for extreme environments. Specifically, automotive, aerospace, and infrastructure applications need materials with damage tolerance during dynamic loading. To this end, many studies have examined dynamic deformation and damage evolution. These studies have shown that kinetics of loading are critically important to damage evolution of bulk metals. Particularly, in dynamic loading environments in which a shock wave is imparted to the metal, kinetic and spatial effects based on shock wave shape play important roles in damage. These studies also show that depending on crystal structure, shock loading can alter the subsequent properties of a material significantly. However, while these phenomena are gaining acceptance in the dynamic damage community, the ability to predict these phenomena is limited. Here, the influence of dynamic loading across strain rates 103–106/s will be discussed. The role of test platforms and crystallography to examine ...