Fractographic investigation of fretting fatigue cracks in Ti-6Al-4V

Failure analysis with specific emphasis on fractography was implemented for this project with the intent of detecting crack features associated with the contact mechanics phenomena known as fretting and fretting fatigue. The primary objective of the research was to analyze and characterize the fracture surfaces associated with Ti-6Al-4V specimens obtained from laboratory controlled fretting fatigue experiments in order to provide evidence regarding the physical nature of fretting and fretting fatigue cracks. The specimens for this research came from fretting fatigue experiments that used dog-bone specimens and contact pads. Each fracture surface was analyzed with optical and scanning electron microscopes with the goal of locating and recording the fretting cracks. Crack profile data were collected and comparison data of heat tint markings and fractography were generated. The results indicate that the fretting cracks are physically small and shallow in depth when compared to the contact region, but with the combination of an external load, will propagate as a fatigue crack. Fretting cracks are localized around the contacting interface perimeter and propagate into the material on elliptic fronts which are dictated by the stress gradient, geometry, and material properties. The fracture appearance of fretting cracks can be described as somewhat smooth and featureless, and in the presence of a contrasting fracture surface like fatigue fracture becomes easily identifiable. The analysis and descriptions discussed are intended to provide physical characteristics of fretting cracks and highlight methods of locating them.