Taylor Sloop
Advisors: Josh Kacher and Naresh Thadhani
will defend a doctoral thesis entitled,
The Effects of Controlled Porosity on the CDynamic Compression and Tensile Failure Strength of Additively Manufactured 316L Stainless Steel
On
Wednesday, October 9 at 3:00 pm.
J Erskine Love Building, Room 295
and
Virtually via MS Teams
Committee
Prof. Josh Kacher – School of Materials Science and Engineering (Co-advisor)
Prof. Naresh Thadhani – School of Materials Science and Engineering (Co-advisor)
Prof. Aaron Stebner – School of Materials Science and Engineering
Prof. David McDowell – School of Materials Science and Engineering
Dr. Saryu Fensin – Los Alamos National Laboratory
Abstract
Process-inherent microstructural heterogeneities in materials can have marked effects on the shock wave propagation and the resulting tension-induced spall failure. Incorporating intentional heterogeneities in the form of micro-scale pores of controlled size and distribution can allow for a deeper investigation of their effects on shock wave motion and interactions. Additive manufacturing (AM) techniques provide control over the micro- and macro-scale structure, including the ability to incorporate pre-existing pores of controlled size and location. The proposed research focuses on understanding the shock compression and eventual dynamic tensile (or spall) failure of AM fabricated steels with presence of intentional pre-existing pores using plate impact-on-plate impact gas gun experiments.
The research was conducted on 316L SS printed using Powder Bed Fusion (PBF) to generate a better understanding of the effects of size, fraction, and location of pre-existing pores on dynamic mechanical properties. A high-throughput experimental method involving multiple samples simultaneously impacted in each experiment employing the 80-mm diameter single-stage gas gun and multiplexed PDV diagnostics was utilized. Multiple PDV probes mounted off of the back surface of disk-shaped samples were used to measure the free-surface particle velocity profiles. Signatures associated with the peak state and spall pull-back were used to determine the spall strength. Additionally, the impacted samples were soft recovered, and their cross-sections and fracture surfaces were analyzed using electron microscopy to identify stress and strain accommodation as well as spall-induced void nucleation and growth processes affected by variations in microstructure caused by the collapse of the pre-existing pores. The combined PDV and microstructure analysis were correlated to investigate the effects of changes in shock wave propagation due to their interactions with pores.
The resulting analysis revealed shock-wave mitigation caused by the collapse of pre-existing pores that varied dependent on the pore size and location within the sample leading to a decrease in damage in the spall plane. When many pores are present at a lower peak stress, a higher number of pores and a larger pore size led to an increase in the dissipation of the shock wave as well as a dispersion of the shock wave. An investigation into isolated pores revealed that a single pore most effectively disrupts the shock wave and limits the spall damage experienced by the material, with larger pores having a more exaggerated effect. However, the presence of multiple pores at a higher peak stress, both in the plane of the impact direction as well as perpendicular to it, does not dissipate the shock wave as effectively, and more damage is observed. This work provides insights into the shock mitigation mechanisms of alloys containing small volume fractions of pores, and furthers our understanding of the resulting microstructural deformation processes dependent on the pore size and location.