Spall Characteristics of Additively Manufactured Stainless Steel

Additive manufacturing (AM) has rapidly transformed from a novelty prototyping technology into a growing sector of production across a wide range of industries. Much work has been documented in literature to demonstrate the behavior of AM products under static and quasi-static loading conditions. However, the behavior of AM materials under high strain rate loading is not as well understood. This research attempts to advance the fundamental knowledge of the relationship between the unique aspects of AM and the mechanical performance under high velocity impact loading conditions.

This project examines the behavior of AM 316L stainless steel (SS) exposed to high velocity impact, the associated shock wave propagation, and the resistance to fracture as a function of orientation and internal engineered features (a design tool unique to AM). This research involves fabrication, characterization, plate impact spallation testing, experiment modeling, and post-mortem analysis of 316L SS samples fabricated using laser powder bed fusion (LPBF). Connections and correlations were established using a variety of data sets.

A build-impact orientation study and two engineered porosity studies (one with random distribution across the bulk and the other with single voids strategically placed) were conducted to develop a better understanding of shock wave propagation and spall fracture related to the unique aspects and capabilities of LPBF fabrication. This research demonstrated that impact orientation with respect to build direction influences the extent and location of spall damage due to the relative microstructural anisotropy and collections of powder filled voids slow and weaken the progressing shock front by presenting disturbances in portions of the wave front.

An engineering design study based on the findings of the earlier studies further utilized purposeful engineering design to control the propagation of the shock wave (and associated pressure front) through the material. The use of internal features, a capability unique to LPBF, was the primary goal of the study. This study successfully demonstrated that a large, powder-filled void space placed within a solid sample provides damping qualities that both slow the progression of the shock front and reduce the magnitude of the pressure stress realized at the rear free surface (opposite of impact).

Overall, the results of this research demonstrate that the anisotropic properties and unique capabilities of LPBF can be leveraged to control shock wave propagation and resultant damage in stainless steel materials. Unique aspects of this research include (1) comparing the spall response of LPBF fabricated samples to shock loading conditions applied at varying orientation relative to the build direction, (2) examining the use of powder-filled engineered void spaces to reduce the magnitude and velocity of the progressing shock front, along with the resulting damage, and (3) in both cases coupling the results of plate impact experiments with as-built and post-mortem sample characterization.