The effect of constraint on fracture of carbon and low alloy steel

The constraint at a crack tip has an effect on the fracture of all but the most brittle materials unless the specimen size is very large. Standard fracture toughness tests are designed so that constraint is high and the fracture parameters obtained from such tests can be conservative when applied to shallow cracks of low constraint in carbon and low alloy steels. Low constraint has two effects. It can greatly increase the critical CTOD or J Ic for ductile tearing or cleavage fracture and also, because low constraint limits the stress, it can cause a change in the mode of fracture from cleavage to ductile tearing. For ductile tearing it is suggested that the effects of constraint can be predicted if the stress-strain relationship for the fracture process zone is known. For cleavage fracture and fracture transition it is shown that the critical stress over a characteristic distance is applicable.

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