Standard Test Method for Measurement of Initiation Toughness in Surface Cracks Under Tension and Bending
5.1 Surface cracks are among the most common defects found in structural components. An accurate characterization and understanding of crack-front behavior is necessary to ensure successful operation of a structure containing surface cracks. The testing of laboratory specimens with surface cracks provides a means to understand and quantify surface crack behavior, but the test results must be interpreted correctly to ensure transferability between the laboratory specimen and the structure.
5.2 Transferability refers to the capacity of a fracture mechanics methodology to correlate the crack-tip stress and strain fields of different cracked bodies. Traditionally, the correlation has been based on the presence at fracture of a dominant, asymptotically singular, crack-tip field with amplitude set by the value of a single parameter, such as the stress intensity factor, KI, or the J-integral. For components and specimens with high crack-tip constraint, the singular crack-tip field dominates over microstructurally significant size scales for loads ranging from globally linear-elastic conditions to moderately large-scale plasticity. For specimens with low crack-tip constraint, a dominant single-parameter crack-tip field exists only at low levels of plasticity. At higher levels of plasticity, the opening mode stress of the low constraint specimen is lower than predicted by the single-parameter, asymptotically singular fields. Therefore, low constraint specimens often exhibit larger fracture toughness than do high constraint specimens. If feasible, users are strongly encouraged to generate high constraint fracture toughness data using methods such as Test Methods E399 or E1820 prior to testing the surface crack geometry.
5.2.1 To address this phenomenon, two-parameter fracture criteria are used to include the influence of crack-tip constraint. Crack-tip constraint has been quantified using various scalar parameters including the T-stress (6, 7, 8), Q (9, 10), stress triaxiality (11, 12), and αh (13, 14). Fracture toughness in a two-parameter methodology is not a single value, but rather is a curve that defines a critical locus of fracture toughness and constraint values (2). Fig. 2 illustrates a toughness-constraint locus for application of two-parameter fracture mechanics to structures. A structural analysis provides the driving force curve for the configuration of interest, and is plotted with the toughness-constraint locus obtained from specimen test data. Crack extension is predicted when the driving force curve passes through the toughness-constraint locus.
5.3 Tests conducted with this method provide data to assist in the prediction of structural capability in the presence of a surface crack by including a measure of crack-tip constraint in the interpretation of fracture toughness values. This improves the correlation of test specimen and structural conditions. To achieve the most accurate comparison, the conditions tested in accordance with this test method should match the structure as closely as possible. For conservative structural assessment, the user should ensure that conditions in the test specimen produce higher levels of constraint relative to the structure in application of the data. Factors that influence test specimen conditions include, but are not limited to, specimen geometry, a/c, a/B, loading conditions, as well as the amount and type of crack extension that occurred during the test.
Note 2: The use of a constraint-based framework for the analysis of surface cracks permits a more realistic assessment of structural capability. This approach generally leads to a less conservative assessment than would be achieved, for example, by using a measure of high-constraint fracture toughness obtained from testing standard C(T) and SE(B) specimens of the material following Test Method E1820. It is essential that constraint effects measured in surface crack tests with this method be applied to any structural assessment with the requisite understanding to maintain appropriate levels of conservatism.
5.4 This test method does not address environmental effects or loading rate effects that may be significant in assessing service integrity.
1.1 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.
1.2 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.
1.3 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.
1.4 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.
1.5 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 to 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.
1.6 This test method provides a quantitative framework to categorize test specimen conditions into one of three regimes: (I) a linear-elastic regime, (II) an elastic-plastic regime, or (III) a field-collapse regime. Based on this categorization, analysis techniques and guidelines are provided to determine an applicable crack-tip parameter for the linear-elastic regime (K or J) or the elastic-plastic regime (J), and an associated constraint parameter. Recommendations are provided to assess the test data in the context of a toughness-constraint locus (2). The user is directed to other resources for evaluation of the test specimen in the field-collapse regime when extensive plastic deformation in the specimen eliminates the identifiable crack-front fields of fracture mechanics.
1.7 The specimen design and test procedures described in this test method may be applied to evaluation of surface cracks in welds; however, the methods described in this test method to analyze test measurements may not be applicable. Weld fracture tests generally have complicating features beyond the scope of data analysis in this test method, including the effects of residual stress, microstructural variability, and non-uniform strength. These effects will influence test results and must be considered in the interpretation of measured quantities.
1.8 This test method is not intended for testing surface cracks in steel in the cleavage regime. Such tests are outside the scope of this test method. A methodology for evaluation of cleavage fracture toughness in ferritic steels over the ductile-to-brittle region using C(T) and SE(B) specimens can be found in Test Method E1921.
1.9 Units—The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.10 This practice may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the users of this standard to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use.
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