The mechanisms of fracture in blunt-notched specimens of a low alloy steel and the effect on failure assessment

Abstract

Blunt-notched three-point bend bars (55 mm × 10 mm × 10 mm) of a low alloy steel with root radii ϱ in the range 0.51 – 4.07 mm have been tested to failure. As ϱ increased, the mode of failure changed from stress-controlled cleavage to strain-controlled microvoid coalescence. For specimens with ϱ ⩽ 1.02 mm the elastic outer fibre stress σ f at failure increased with ϱ, consistent with stress-controlled mechanisms. For ϱ > 1.02 mm the stress-controlled fracture requirements could not be met and failure became strain controlled. The effect of ϱ on fracture behaviour has been analysed in terms of the dependence of σ f , the plane strain fracture toughness K 1 C and the apparent fracture toughness K A ( ϱ) on 1/K t where K t is the elastic stress concentration factor at the root of the notch. It is concluded that K 1 C may in some circumstances be the failure parameter controlling the initiation of fast fracture from blunt notches. The incorporation of root radius effects in a failure assessment procedure is described, and predicted failure stresses are shown to be in good agreement with observed values.