SHORT CRACK PROPAGATION AND CLOSURE EFFECTS IN A508 STEEL

Abstract— Fatigue crack growth measurements are usually made on standard specimens containing long cracks (～10 mm) although in most practical situations, a large part of the fatigue life is spent with much shorter dimensions. The purpose of the present study is a comparison of crack growth behaviour for long cracks (～13–16 mm) in CT specimens and smaller ones (～0.3–0.5 mm) in four point bend specimens. Large effects are noticed indicating that, at a given stress intensity factor amplitude, the crack growth rate is significantly higher in specimens with short cracks. Mouth displacement measurements for both specimen configurations show that the crack closure phenomenon accounts for the observed effect. Crack closure is likely to be associated with fracture surface roughness as shown by partly machining the material left behind the crack tip in CT specimens.