Effect of Surface Roughness and Residual Stress on Fatigue Performance of Ti-6Al-4V

Currently, manufacture of surgical implants has become the largest application of Ti-6Al-4V with aerospace, nuclear and automotive applications. One significant concern is the risk of in-vivo fatigue failure, which can lead to severe medical complications and the need for revision surgery. Due to these premature failures, implant manufacturers have been required to provide substantial financial compensation to affected patients. Examining in-vivo failures of medical implants confirm the influence of surface defects and residual stresses. The surface roughness may act as defects which influence the crack initiation, then the propagation catalyst by residual stresses. Due to manufacturing practices, the surface defects and residual stress states change over. This study focuses on the effect of the manufacturing practices on the surface roughness and the residual stresses, thereby to the life of hip prosthesis made of Ti-6Al-4V. Surface roughness and residual stress of manufacturing stages were investigated. Hourglass shaped samples were prepared adhering to the exact similar conditions to the actual implant manufacturing and rotating-bending fatigue tests were conducted. A clear variation of surface roughness and stress states were identified due to manufacturing practices and considerable variations of fatigue performance were observed. A mathematical equation is derived to assess the fatigue limit of a component with known surface roughness and residual stress. Understanding the effects of manufacturing practices on fatigue performance enables recommendations for good manufacturing practices and to eliminate in-vivo fatigue failures. Ultimately the knowledge will help to predict the life of Ti-6Al-4V products.