MSE Colloquium February 17, David Bahr, Why does it seem like everything’s harder these days?

Abstract 

Measuring the hardness of materials is one of the easiest ways to assess the relative mechanical performance of materials, but also one of the most difficult test techniques to interpret the properties of these materials: hardness isn’t a unique, global, and scale independent property. But when your material isn’t conducive to machining into an ASTM approved tensile or compression specimen, or your material isn’t uniform in structure or properties, hardness testing provides a way to make relatively localized measurements that can show both quantitative and qualitative information about mechanical behavior. Nanoindentation has an added benefit of being able to measure elastic properties, plastic properties, and even residual stresses in a material can be inferred from the localized indentation.

For this presentation, I’ll show three different material systems where we use hardness measurements to explore the mechanical response of materials with spatial resolutions that you can’t get with ye olde tensile bar. First, surface modifications of metals via shot peening are common in industry, and indentation testing is useful to explore how much harder a surface can get without modifying chemistry when that surface modification is on the order of microns thick. Comparing Ti alloys with differing alpha/beta contents, and how their hardness and residual stresses change with peening (and how that alters subsequent phase transformations) allowed us to determine that compressive residual stresses from peening promoted the beta to alpha transition. Secondly, in nanostructured ultra-low density (relative density ≈ 1%) Cu metal foams, we used indentation testing to estimate the relative importance of solid solution strengthening versus precipitation hardening. Here in materials with free ligaments on the order of 100 nm, precipitation strengthening is important, but adding Zn to Cu (i.e. brass) isn’t as impactful as in bulk brass alloys. Finally, some materials you don’t want to machine; I’ll use indentation testing to show how we look for mechanical equivalent mock materials for explosives.

Bio

Prof. David Bahr is currently the Head of Materials Engineering at Purdue University. Prior to this position he was Director and Professor of Mechanical and Materials Engineering at Washington State University. He received his BS and MS in MSE at Purdue University, and a PhD in Materials Science at the University of Minnesota in 1997. He worked for a short time at Sandia National Laboratories during his PhD before starting as a faculty member in the School of MME at WSU in 1997. He has supervised 4 Post-docs, 29 PhD students, 30 MS students, and over 50 undergraduate researchers in the general area of mechanical behavior. In 2000 he won the Presidential Early Career Award for Scientists and Engineers for his work with Sandia on DOE stockpile stewardship, in 2003 he received the Bradley Stoughton Award from ASM International, and has received the Robert Lansing Hardy award and Brimicombe medal from TMS (where he served as a member of the board of directors from 2012-2015). He has published over 200 papers in the literature and given over 60 invited talks and seminars world-wide. He’s a fellow of ASM International and AAAS.