Instrumented Indentation of M855 Cartridge, Core, and Jacket Materials

Abstract

Abstract : Instrumented indentation was used to estimate the mechanical properties of as-processed materials used in 5.56-mm M855 projectiles. Three indentation tip geometries were used: a Berkovich tip, a 1- m-radius conical tip with a 90 cone angle, and a 25- m-radius conical tip also with a 90 cone angle. Because the Berkovich tip geometry is close to that of an ideal pyramid, the resulting measurements of elastic modulus and hardness for homogeneous, isotropic materials exhibited the least amount of scatter compared to the other two tips. For nonisotropic materials, however, the measurements of elastic modulus and hardness using the Berkovich and 1- m-radius conical tip exhibited large amounts of scatter. These two probes were found to test similar volumes of material that were on the order of the size scale of the material heterogeneities. Thus, the results were a function of the local microstructure rather than being representative of the bulk material. The 25- m-radius tip was found to provide results for the nonisotropic materials that had small amounts of scatter and that were similar to bulk measurements, indicating that the volume of material being probed was sufficiently large to average out local microstructural effects. Initial efforts were made to provide full stress-strain data from indentation measurements of the brass cartridge case.