WEB Mechanical Behaviour and Microstructural Alterations of Ti-6Al-4V under Combined Pressure and Torsion Fatigue
During the last century, the use of endoprostheses in the human body has increased. In addition to the absolutely necessary biocompatibility, materials must possess good wear and corrosion resistance as well as adequate fatigue strength, depending on the function of the implant. Due to the cyclic and partially complex stresses that act on the material, fatigue has a significant influence on long-term use of e.g. hip joint endoprostheses. Under multiaxial loading materials often display unexpected behaviour and failure modes, which in many cases have not been explained to date.
In order to extend the understanding of fatigue behaviour under complex conditions, in the presented work cyclic torsion is combined with a constant axial pressure in the elastic range. Experimental investigations under such circumstances are very complex, so that the mechanisms of plastic deformation are not well understood. The aim of this study is to investigate the mechanical behaviour and microstructural alterations of alloy Ti-6Al-4V, which is a common biomedical alloy widely used in surgical intervention.
The mechanical tests were carried out on cylindrical double-cone samples with 10 mm diameter in the test section. The test parameters include a constant compressive stress of 350 MPa, combined with different torsional angles between 5° and 20°. The microstructure of the tested samples is characterized by scanning- and transmission-electron-microscopy. For all applied torsional angles failure occurred due to axial plastic flow and longitudinal cracking. An increase in the torsional angle results in an increase in axial strain and pronounced decrease in cycles to crack initiation. At the same time, depending on the torsional angle, a fast increase in temperature within few initial cycles is observed, reaching up between 183°C and 249°C. The combined action of axial plastic strain and increased temperature are accompanied by a significant change in the grain structure. Highly stressed samples show precipitation initiation at grain boundaries, grain reorientation and grain-clustering. Samples subjected to lower loads display no significant changes. A certain threshold seems to exist, in the present experiments at a torsional angle in the range of 18,5° and 20°, beyond which various microstructural processes are activated, which strongly accelerates the damage initiation.