Determination of constitutive response of plastically graded materials

Plastically graded materials (PGMs) are frequently used in high-performance tribological components such as case-hardened gears and bearings for jet and rocket engines due to their superior resistance to rolling contact fatigue. These components can have a surface hardness as high as 9 GPa and a decreasing hardness gradient over a depth of approximately 2 mm. This hardness gradient is achieved by a gradient in carbon content. Proper design of such components requires the knowledge of the constitutive response with depth. In this manuscript, a coordinated experimental and numerical method is presented to extract the constitutive response of commercially available case-hardened Pyrowear 675 (P675) stainless steel. Utilizing the variation in micro-Vickers hardness with depth for both virgin and plastically deformed PGMs, representative plastic strain, and well established hardness-yield strength relationships, the constitutive response of the PGM is uniquely determined. It is shown that this specific PGM has a linear variation in yield strength, but a constant strain hardening exponent. The procedure developed here can assist in surface engineering a PGM with optimized hardness profiles to maximize tribological performance.