Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials

Importancia y uso:

5.1 The property K_Ic determined by this test method characterizes the resistance of a material to fracture in a neutral environment in the presence of a sharp crack under essentially linear-elastic stress and severe tensile constraint, such that (1) the state of stress near the crack front approaches tritensile plane strain, and (2) the crack-tip plastic zone is small compared to the crack size, specimen thickness, and ligament ahead of the crack.

5.1.1 Variation in the value of K_Ic can be expected within the allowable range of specimen proportions, a/W and W/B. K_Ic may also be expected to rise with increasing ligament size. Notwithstanding these variations, however, K_Ic is believed to represent a lower limiting value of fracture toughness (for 2 % apparent crack extension) in the environment and at the speed and temperature of the test.

5.1.2 Lower and more highly variable values of fracture toughness can be obtained from specimens that fail by cleavage fracture; for example, specimens of ferritic steels tested at temperatures in the ductile-to-brittle transition region or below. Specimens failing by cleavage are also more likely to exhibit warm prestressing effects, where precracking at a temperature higher than the test temperature can artificially increase the fracture toughness measured (2). The present test method is not intended for cleavage fracture. Instead, the user is referred to Test Method E1921 and E1820 which are applicable to cleavage fracture and contain safeguards against warm prestressing. Likewise this test method should not be used when specimen failure is accompanied by appreciable plastic deformation even after the specimen size has been maximized within product dimensional constraints. Guidance on testing elastic-plastic materials is given in Test Method E1820.

5.1.3 The value of K_Ic obtained by this test method may be used to estimate the relation between failure stress and crack size for a material in service wherein the conditions of high constraint described above would be expected. Background information concerning the basis for development of this test method in terms of linear elastic fracture mechanics may be found in Refs (1) and (3).

5.1.4 Cyclic forces can cause crack extension at K_I values less than K_Ic. Crack extension under cyclic or sustained forces (as by stress corrosion cracking or creep crack growth) can be influenced by temperature and environment. Therefore, when K_Ic is applied to the design of service components, differences between laboratory test and field conditions shall be considered.

5.1.5 Plane-strain fracture toughness testing is unusual in that there can be no advance assurance that a valid K_Ic will be determined in a particular test. Therefore, compliance with the specified validity criteria of this test method is essential.

5.1.6 Residual stresses can introduce bias into the indicated K_Q and K_Ic value determinations. The effect can be especially significant for specimens removed from as-heat treated or otherwise non-stress relieved stock, from weldments, from complex wrought products, rapidly-solidified castings, additively-manufactured products or from products with intentionally induced residual stresses. In addition, residual stresses will redistribute when the specimen is extracted from the host product and machined. The magnitude of residual stress influence on K_Q and K_Ic in the test specimen may be quite different from that in the original or finish machined product. In addition, the behavior of cracks in the full-sized product may not be predictable from the fracture toughness measured on the specimen because of the influence of the different residual stresses in each. Indications of residual stress include distortion during specimen machining, results that are specimen configuration dependent, and irregular fatigue precrack growth (either excessive crack front curvature or out-of-plane growth). Guide B909 provides supplementary guidelines for plane strain fracture toughness testing of aluminum alloy products for which complete stress relief is not practicable. Guide B909 includes additional guidelines for recognizing when residual stresses may be significantly biasing test results, and methods for minimizing the effects of residual stress during testing.

5.2 This test method can serve the following purposes:

5.2.1 In research and development, to establish in quantitative terms significant to service performance, the effects of metallurgical variables such as composition or heat treatment, or of fabricating operations such as welding or forming, on the fracture toughness of new or existing materials.

5.2.2 In service evaluation, to establish the suitability of a material for a specific application for which the stress conditions are prescribed and for which maximum flaw sizes can be established with confidence.

5.2.3 For specifications of acceptance and manufacturing quality control, but only when there is a sound basis for specifying minimum K_Ic values, and then only if the dimensions of the product are sufficient to provide specimens of the size required for valid K_Ic determination. The specification of K_Ic values in relation to a particular application should signify that a fracture control study has been conducted for the component in relation to the expected loading and environment, and in relation to the sensitivity and reliability of the crack detection procedures that are to be applied prior to service and subsequently during the anticipated life.

Subcomité:

E08.07

Referida por:

E2208-02R18E01, E0636-20, D6068-10R18, B0646-19, A0504_A0504M-18R23, F3055-14AR21, F1940-07AR24, F3056-14R21, G0168-17, F1624-12R18, G0107-95R20E01, F3184-16R23, D3433-99R20, G0129-21, B0771-11R21, F3055-14AR21, D8044-23, D8255-19, F3318-18, F2924-14R21, E1823-24C, G0129-21, G0058-85R23, F3213-17, F2924-14R21, F3302-18, F3439-22, E0740_E0740M-23, F3056-14R21, B0645-21, B0909-21A, F3607-22, E0604-18, E1681-23E01, E1221-23, E1457-23E01, D5045-14R22, E0647-24, D7779-20, C1576-05R17, E2760-19E03, E1304-97R20, E0606_E0606M-21, E0561-23, D7313-20, E2472-12R18E04, C1834-16R24, E2899-24E01, E1922_E1922M-22, A0788_A0788M-24A, E1820-24

Volúmen:

03.01

Número ICS:

77.040.10 (Mechanical testing of metals)