FCP Report No. 3

Characterization of Ductile Fracture in Wrought Steels and Weldments

by

E. P. Cox
F. V. Lawrence, Jr.
G. M. Sinclair

Abstract

Collapse stresses of flawed structural steels were predicted by a two failure criterion model based on plasticity limit load theory and linear elastic fracture mechanics. The fracture behavior of wrought steel specimens could be characterized by either the limit load (LL) or fracture toughness (FT) failure criterion. The controlling failure criterion depended on the tensile strength (Su), fracture toughness (Kc), and section size (W); or more explicitly, on the magnitude of Kc/Su W1/2 .

Low values of Kc/Su W1/2 resulted in FT controlled fracture; whereas, larger Kc/Su W1/2 resulted in LL controlled fracture. The boundary separating the two failure criteria was dependent on geometry and crack length. The two failure criteria model was extended to characterize the fracture behavior of A514/E110 structural steel weldments containing incomplete joint penetration discontinuities.

A514/E110 weldments had large Kc/Su W1/2 values; consequently, the LL failure criterion characterized the fracture behavior of those weldments containing flaw widths greater than a certain width, 2a". Flaw widths smaller than 2a" resulted in collapse stresses approximately equal to the tensile strength of the base metal, and flaw widths less than another size, 2a’ resulted in base metal fractures. The flaw widths 2a’ and 2a" depended on the degree of weld metal overmatching.

Deformations at collapse were predicted for flaw widths greater than 2a" by an expression which included the J-integral fracture toughness and the LL failure criterion. The proposed two failure criteria model did not include corrections for subcritical crack extension or crack-tip plasticity. When subcritical crack extension was considered, one of three possibilities resulted depending on the tearing modulus (T) and the body’s dimensions: (1) there was no effect, (2) the controlling failure criterion changed from LL to FT, or (3) fracture occurred by stable tearing. Incorporating small-scale plastic zone corrections into the model resulted in a slight increase in the Kc /Su W1/2 boundary separating LL from FT controlled fracture behavior.

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