Crack propagation analysis of surface enhanced titanium alloys with fretting induced damage
Abstract
The objectives of this research project were to analyze, characterize, and predict the influences that surface treatments have on crack propagation in the presence of fretting fatigue damage. The titanium alloys, Ti-6Al-4V and Ti-17, were implemented for this research, and the surface enhancement methods consisted of shot peening and laser shock peening. The approach was to incorporate methods of contact mechanics, fractography, and fracture mechanics so that the influence of surface enhancements on fretting fatigue could be better understood. The specimens were obtained from prior fretting fatigue experiments that consisted of dog-bones and contact pads with both surface enhanced and bare conditions. The dog-bone specimens had fretting fatigue damage, which is a combination of a cyclic bulk load and the fretting induced damage. These specimens were incorporated in life prediction analyses in which a procedure for calculating fretting fatigue life by correlating nucleation and propagation through a non-arbitrary crack initiation criterion was introduced. The life prediction results show that the fretting fatigue life can be determined with knowledge of the fretting stress field and nature of the fretting cracks. The results also show that surface enhancements do not stop fretting fatigue cracks from forming, do slow the propagation and increase the fretting fatigue life. The contact pads had what is known as ‘pure fretting’ damage, which consists of the damage from the contact stresses but no cyclic bulk load. The contact pads are the basis for the development of the C-specimen experiment. The contact pads were machined into C-specimens that help measure the threshold stress intensity factor. The objective of the C-specimen experiment is to increase the cyclically applied load of the specimen through step testing until a fatigue crack propagates from the existing fretting induced crack. The testing technique provides for the threshold stress intensity factor to be measured while exposing the original fretting crack for fractographic observation. The results show that C-specimen type experiments are an effective method of measuring the threshold stress intensity factor, and that surface enhanced C-specimens will have larger threshold loads than the untreated specimens.
Degree
Ph.D.
Advisors
Grandt, Purdue University.
Subject Area
Aerospace materials
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