New Measurement Method of Mode II Threshold Stress Intensity Factor ΔK_τ Range and Its Application

In order to measure the Mode II fatigue crack growth resistance of SS41 steel and a rail steel, a particular double-cantilever type specimen with side grooves was designed. Mode II fatigue crack growth tests by using the newly designed specimens were conducted by a conventional closed-loop type tension-compression fatigue testing machine under a constant load amplitude which corresponds to ΔK_II-decreasing test with increase in crack length. The crack length was measured by the AC potential method. Mode II crack growth rate gradually decreased as the crack grew. The typical fracture surface of Mode II fatigue crack growth which was quite different from Mode I fatigue fracture surface was observed. But the trace of local branching of crack growth mode from Mode II to I was observed on the overall fracture surface of Mode II. The relationship between Mode II crack growth rate (da/dN) and ΔK_II was obtained.