Mechanical heart valves (MHVs) make up approxmiately 60% of prosthetic heart valve (PHV) implants in the United States. Due to the high risk of cardioembolic stroke, patents receiving MHV implants are administered mandatory anticoagulation medication. However, this increases the risk of hemorrhage and does not eliminate risk of stroke. This study is based on the blood flow conditions present in MHVs. Computational fluid dynamics (CFD) tools are used to study flow through an MHV from St. Jude Medical (SJM) after implantation in physiologic 3D geometry. In this study, blood was taken to be a non-Newtonian two-phase fluid experiencing intermittent turbulence. The simulation is conducted using the Unsteady Reynolds Averaged Navier-Stokes (URANS) approach that employs the Wilcox k-ω turbulent model, which is able to account for intermittency of the turbulence. The results of the flow simulation are used to develop a new platelet damage model incorporating damage history (senescence and cumulative damage of repeated passages). This model is used to determine platelet activation resulting from combined stress and exposure time.