| The phenomenon of spallation, which is the ejection of material from a solid subjected to mechanical shock, has been known for the last 100 years. Studies of the micromechanics of this failure mechanism have been performed with a wide variety of materials and shock sources. Much of this micromechanical research focuses on the initiation behavior of spall, and its propagation mechanisms. By contrast, very scant data is available concerning the effects of the residual stress wave traveling through a material after spall has taken place.;This project sought to examine the microstructural behavior of an A36 steel target following a spall event driven by the high explosive PBX-9501. The microscopy was not limited to the immediate vicinity of the spall fracture, but rather encompassed the entire cross section of material from the spall zone to the crater floor left by the explosive. The objective was to locate and characterize regions of damage inside the target. This damage was expected to consist of void formation, and localized or widely distributed regions exhibiting yielding and/or plastic deformation indicative of an impending fracture.;Over the course of this project, three samples of A36 steel were analyzed. These samples were taken from the same piece of 3/4 inch plate stock, so the initial properties were identical for all three. One sample was left as received, while the other two were shocked with 2.9g and 9.5g charges of PBX-9501, with production of spall as the end objective. The samples were then sectioned, and the microstructure of each was analyzed. The analysis included locations near the spall surface, along with locations within the sample interior. Sub-surface void formation was successfully observed within the material at distances up to 1 millimeter below the spall site, both in front of and behind the fracture. This behavior was documented in a sample which underwent incipient spall, with all target material remaining captive, and was also seen in a sample following complete spall where material was ejected from the sample. However no localized plastic deformation or yielding was seen anywhere inside either of the shocked samples. It is possible that the entire sample underwent permanent deformation, as in a rolling mill, but this kind of behavior was not the objective of this study. |
