MODE II STABLE CRACK GROWTH

Abstract Mode II stable crack extension has been examined for an aircraft grade aluminium alloy D16AT. Both theoretical and experimental results are presented. The experimental observations include load displacement diagrams, plastic wake, crack front tunnelling and scanning electron micrographs of the fracture surfaces. The crack shows a tendency for in-plane extension, and the fracture surface is very flat, smooth and free of any dimples. The crack front advances with neghgible tunnelling at all stages of extension. The span of mode II stable crack growth (SCG) is longer than in the case of mode I SCG reported earlier for the same material and there is also more extensive plastic deformation. In the presence of a slight mode I load, the crack grows out-of-plane and the fractured surface facets resemble that of a mode I or mixed-mode dimpled fracture. The theoretical study is based on a finite element analysis using small deformation theory and incremental plasticity. Some of the experimental results have been theoretically predicted using the COA criterion as the governing criterion. The theoretical results include load-displacement diagrams, crack edge displacement curves, plastic zones and the J resistance curves. There is good agreement between the load-displacement diagrams. The initiation and maximum loads differ by less than 15%. The J resistance curve has a constant slope over the whole span of stable crack growth.     

A NEW METHOD FOR THE MEASUREMENT OF MODE II FATIGUE THRESHOLD STRESS INTENSITY FACTOR RANGE ΔKτth

Abstract— In order to evaluate the threshold value Δ K _τth for mode II fatigue crack growth, a new measurement method of mode II fatigue crack growth has been developed. This method uses a conventional closed-loop tension—compression fatigue testing machine without additional loading attachments. Mode II fatigue tests for structural steel and rail steel have been carried out. This method has proved successful and has reproduced mode II fatigue fracture surfaces similar to those found in the spalling of industrial steel-making rolls. The crack length during testing was measured by an AC potential method. The relationships between d a /d N and Δ K _τ and AK _τth for several materials have been obtained.