Fracture surface roughness and roughness-induced fatigue crack closure in Ti-2.5 wt% Cu

Fatigue crack closure and fracture surfaces in the alloy Ti-2.5 wt% Cu at low ΔK (da/dN<10−8 m cycle−1) were examined. For a solution-annealed equiaxed microstructure as well as for a fine and a coarse lamellar microstructure, roughness-induced crack closure occurs due to serrated crack path, which influences the crack growth rates significantly. The fracture surface roughness is found to be a measure of roughness-induced crack closure in lamellar microstructures where planar slip prevails. The height distribution and tilt angle distribution of the crack path facets are the decisive factors for roughness-induced crack closure. In the solution-annealed equiaxed microstructure, ductile fracture areas by the side of crystallographic facets can act as obstacles to crack tip sliding along planar slip planes, causing less mode II displacement and thus resulting in a much lower level of crack closure.