Fatigue and Fracture Mechanics research in the Solid Mechanics and Materials research group, Department of Engineering Science, University of Oxford.

Fatigue, in the sense of the quantification of the rate of propagation of cracks is a mature subject and well understood.  What is far from well understood are the processes by which cracks nucleate.  Some cracks ‘freely initiate’ from smooth surfaces, perhaps because of surface finish irregularities, or material imperfections, and this is very  hard to quantify rigorously, but many cracks star at the joints between engineering components fastened together, and notionally stationary with respect to each (not rolling or sliding, that is).  But even stationary (or fixed) contacts are not truly stationary, i.e. there is often some relative movement between the components caused by cyclic service loads, and this ‘worries’ or ‘frets’ the surfaces and so encourages cracks to start. 

A major research topic within the group is the study of fretting fatigue, and its quantification.  A lot of our effort goes into understanding the solid mechanics associated with fretting, both analytically, and experimentally.  The analytical and numerical work is targeted at understanding where the interfacial slip occurs and its amplitude, whereas our experimental efforts are designed, mainly, to measure the number of cycles of remote loading needed to start cracks off.  A particular theme of the research is to quantify the hinterland around contact edges by asymptotic solutions (analogous to crack tip  solutions employed in  fracture mechanics and used successfully to quantify the rate of crack propagation).  Here these solutions enable us to match the conditions at the contact edges in laboratory experiments, where we use servo-hydraulic test apparatus designed within the group, to complicated prototype geometries, such as the roots of fan blades in gas turbines and the locking segments used to secure risers to  wellhead fittings on the seabed.