ASTM E399 – Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
Significance and Use
5.1 The property KIc 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 KIc can be expected within the allowable range of specimen proportions, a/W and W/B. KIc may also be expected to rise with increasing ligament size. Notwithstanding these variations, however, KIc 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 KIc 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 KI values less than KIc. 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 KIc 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 KIc 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 KQ and KIc 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 KQ and KIc 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.
FIG. 2 Double–Cantilever Clip-In Displacement Gage Showing Mounting by Means of Integral Knife Edges
(Gage Design Details are Given in Annex A1)
5.2.3 For specifications of acceptance and manufacturing quality control, but only when there is a sound basis for specifying minimum KIc values, and then only if the dimensions of the product are sufficient to provide specimens of the size required for valid KIc determination. The specification of KIc 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.
1.1 This test method covers the determination of fracture toughness (KIc and optionally KIsi) of metallic materials under predominantly linear-elastic, plane-strain conditions using fatigue precracked specimens having a thickness of 1.6 mm (0.063 in.) or greater2 subjected to slowly, or in special (elective) cases rapidly, increasing crack-displacement force. Details of test apparatus, specimen configuration, and experimental procedure are given in the annexes. Two procedures are outlined for using the experimental data to calculate fracture toughness values:
1.1.1 The KIc test procedure is described in the main body of this test standard and is a mandatory part of the testing and results reporting procedure for this test method. The KIc test procedure is based on crack growth of up to 2 % percent of the specimen width. This can lead to a specimen size dependent rising fracture toughness resistance curve, with larger specimens producing higher fracture toughness results.
1.1.2 The KIsi test procedure is described in Appendix X1 and is an optional part of this test method. The KIsi test procedure is based on a fixed amount of crack extension of 0.5 mm, and as a result, KIsi is less sensitive to specimen size than KIc. This less size-sensitive fracture toughness, KIsi, is called size-insensitive throughout this test method. Appendix X1 contains an optional procedure for reinterpreting the force-displacement test record recorded as part of this test method to calculate the additional fracture toughness value, KIsi.
NOTE 1: Plane-strain fracture toughness tests of materials thinner than 1.6 mm (0.063 in.) that are sufficiently brittle (see 7.1) can be made using other types of specimens (1).3 There is no standard test method for such thin materials.
1.2 This test method is divided into two parts. The first part gives general recommendations and requirements for testing and includes specific requirements for the KIc test procedure. The second part consists of Annexes that give specific information on displacement gage and loading fixture design, special requirements for individual specimen configurations, and detailed procedures for fatigue precracking. Additional annexes are provided that give specific procedures for beryllium and rapid-force testing, and the KIsi test procedure, which provides an optional additional analysis procedure for the test data collected as part of the KIc test procedure.
1.3 General information and requirements common to all specimen configurations:
|Plane-Strain Fracture Toughness||3.1.2|
|Crack Plane Orientation||3.1.4|
|Summary of Test Method||4|
|Significance and Use||5|
|Precautions||5.1.1 – 5.1.5|
|Apparatus (see also 1.4)||6|
|Displacement Gage, Measurement||6.4|
|Specimen Size, Configurations, and Preparation (see also 1.5)||7|
|Specimen Size Estimates||7.1|
|Standard and Alternative Specimen Configurations||7.2|
|Fatigue Crack Starter Notches||7.3.1|
|Fatigue Precracking (see also 1.6)||7.3.2|
|Crack Extension Beyond Starter Notch||126.96.36.199|
|Crack Plane Angle||8.2.4|
|Calculation and Interpretation of Results||9|
|Test Record Analysis||9.1|
|Pmax/PQ Validity Requirement||9.1.3|
|Specimen Size Validity Requirements||9.1.4|
|Precision and Bias||11|
1.4 Specific requirements related to test apparatus:
|Double-Cantilever Displacement Gage||Annex A1|
|Testing Fixtures||Annex A2|
|Bend Specimen Loading Fixture||Annex A2.1|
|Compact Specimen Loading Clevis||Annex A2.2|
1.5 Specific requirements related to individual specimen configurations:
|Bend Specimen SE(B)||Annex A3|
|Compact Specimen C(T)||Annex A4|
|Disk-Shaped Compact Specimen DC(T)||Annex A5|
|Arc-Shaped Tension Specimen A(T)||Annex A6|
|Arc-Shaped Bend Specimen A(B)||Annex A7|
1.6 Specific requirements related to special test procedures:
|Fatigue Precracking KIc and KIsi Specimens||Annex A8|
|Hot-Pressed Beryllium Testing||Annex A9|
|Rapid-Force Testing||Annex A10|
|Determination of KIsi||Appendix X1|
1.7 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.