The essential work of plane stress ductile fracture of a strain-aged steel

The paper reports the results of an experimental investigation on the essential work of fracture of a strain-aged low carbon (∼0.1% C) temper-rolled 16-gauge sheet steel which has been subjected to pre-strain levels of 2 to 12% and ageing temperatures of 80 and 100δC. Deep edge-notched tension specimens were used to determine the specific essential work by extrapolating the straight-line relationship between the specific work of fracture (W_f) and ligament length (I) to zero ligament length. The strain-aged steels at 80 and 100δ give approximately the same specific essential fracture work of 0.18 to ∼0.20 J mm^-2 which is independent of the amount of prestrain. Advancing crack opening displacements (C.O.D.) have also been analysed, which give 0.60 to 0.63 mm for the strain-aged steels. For comparison, the prestrained but unaged steels have a higher essential work of fracture of 0.275 J mm^-2 and a larger C.O.D. of 0.73 mm. It is concluded, therefore, that the causes of strain-ageing embrittlement are primarily due to the reduction of both the essential work of fracture and the advancing C.O.D. at the crack tip end region.