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

Importancia y uso:

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, K_I, 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 (10, 11, 12), Q (13, 14), stress triaxiality (15, 16), and α_h (17, 18). 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 3: 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.

Subcomité:

E08.07

Referida por:

E1823-24C, F3439-22

Volúmen:

03.01

Número ICS:

19.040 (Environmental testing)

Palabras clave:

CMOD; constraint; crack initiation; crack mouth opening displacement; deformation limit; elastic-plastic regime; field-collapse regime; J-dominance; J-integral; K-dominance; length scale; linear-elastic regime; one-parameter fracture; stable crack extension; stress intensity factor; T-stress; two-parameter fracture; unstable crack extension;