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**Source: **STP34707S

New procedures for characterizing the fracture extension resistance of ductile metals have been established. The fracture extension resistance curve (R-curve), which delineates the increasing rate of plastic work to cause crack propagation in nonbrittle metals, is determined by using dynamic tear (DT) test procedures. The resistance parameter is the slope of the R-curve.

The effect of thickness on the R-curve slope was investigated for three high-strength steels of high, intermediate, and low resistance to ductile fracture. R-curves were determined for each steel in the full thickness (1 in.) and for thicknesses of 0.625 and 0.325 in. The R-curve slopes showed good agreement for each section size, and a transition in the fracture mode from flat fracture at short crack extensions to the metal's characteristic degree of shear fracture for long extensions was observed for each steel. The data can be described by an exponential equation involving fracture energy, specimen cross-section dimensions, and a constant, *Rp*, which is proportional to the R-curve slope. For each steel, *Rp* is unaffected by changing specimen geometry, thus indicating it to be a material property.

**Keywords:**

crack propagation, fracture toughness, stresses, fracture properties, high strength steels, shear strength, shear stress, tests, evaluation

**Author Information:**

Judy, RW *Head, Structural Mechanics Section and head, Strength of Metals Branch, Naval Research Laboratory, Washington, D.C.,*

Goode, RJ *Head, Structural Mechanics Section and head, Strength of Metals Branch, Naval Research Laboratory, Washington, D.C.,*

**Committee/Subcommittee:** E08.08

**DOI:** 10.1520/STP34707S