ASTM International

Standard Test Method for Measurement of Initiation Toughness in Surface Cracks Under Tension and Bending
 N E2899

Annotation

This test method describes the method for testing fatigue-sharpened, semi-elliptically shaped surface cracks in rectangular flat panels subjected to monotonically increasing tension or bending. Tests quantify the crack-tip conditions at initiation of stable crack extension or immediate unstable crack extension.

This test method applies to the testing of metallic materials not limited by strength, thickness, or toughness. Materials are assumed to be essentially homogeneous and free of residual stress. Tests may be conducted at any appropriate temperature. The effects of environmental factors and sustained or cyclic loads are not addressed in this test method.

This test method describes all necessary details for the user to test for the initiation of crack extension in surface crack test specimens. Specific requirements and recommendations are provided for test equipment, instrumentation, test specimen design, and test procedures.

Tests of surface cracked, laboratory-scale specimens as described in this test method may provide a more accurate understanding of full-scale structural performance in the presence of surface cracks. The provided recommendations help to assure test methods and data are applicable to the intended purpose.

This test method prescribes a consistent methodology for test and analysis of surface cracks for research purposes and to assist in structural assessments. The methods described here utilize a constraint-based framework (1, 2)2 to evaluate the fracture behavior of surface cracks.

NOTE 1—Constraint-based framework. In the context of this test method, constraint is used as a descriptor of the three-dimensional stress and strain fields in the near vicinity of the crack tip, where material contractions due the Poisson effect may be suppressed and therefore produce an elevated, tensile stress state (3, 4). (See further discussions in Terminology and Significance and Use.) When a parameter describing this stress state, or constraint, is used with the standard measure of crack-tip stress amplitude (K or J), the resulting two-parameter characterization broadens the ability of fracture mechanics to accurately predict the response of a crack under a wider range of loading. The two-parameter methodology produces a more complete description of the crack-tip conditions at the initiation of crack extension. The effects of constraint on measured fracture toughness are material dependent and are governed by the effects of the crack-tip stress-strain state on the micromechanical failure processes specific to the material. Surface crack tests conducted with this test method can help to quantify the material sensitivity to constraint effects and to establish the degree to which the material toughness correlates with a constraint-based fracture characterization.