Slip Band Analysis by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM)

Extract

Fatigue is a common cause of failure in components subjected to cyclic stresses. Repeated loads much smaller than the yield stress of the material can cause a fatigue failure. One such component subject to cyclic stresses is a titanium heart valve housing. The valve is subjected to a low level of stress with each heart beat as a disc pivots and closes against the housing. Interest in the initiation of fatigue in such components was an impetus to examine surface plasticity and accumulated damage which could lead to fatigue initiation. Grade 4 titanium used in heart valves and pure titanium have been studied.

The three phases of fatigue are initiation of a crack, propagation, and catastrophic failure. Minimum defect size for crack propagation and failure are fairly well understood by scientists and engineers. However, the initiation portion of fatigue as a function of accumulated surface microplasticty is less well defined. SEM and AFM have been be used to evaluate microplasticity on the surface of titanium and correlate damage accumulation to the initiation of a crack.

Fully reverse loaded four point bend testing was conducted on a an electropolished pure titanium sample 15 mm by 5 mm by 1 mm thick. The sample was oxidized at 300°C for one hour prior to fatigue testing. The sample was fatigued for 25,000 cycles using a ±45N load.