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 values of K_Ic can be obtained for materials that fail by cleavage fracture; for example, ferritic steels in the ductile-to-brittle transition region or below, where the crack front length affects the measurement in a stochastic manner independent of crack front constraint. The present test method does not apply to such materials and the user is referred to Test Method E1921 and E1820. Likewise this test method does not apply to high toughness or high tearing-resistance materials whose failure is accompanied by appreciable amounts of plasticity. 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 (2).

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 adversely affect the indicated K_Q and K_Ic values. The effect can be especially significant for specimens removed from as-heat treated or otherwise non-stress relieved stock, from weldments, from complex wrought parts, or from parts with intentionally induced residual stresses. 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, methods for minimizing the effects of residual stress during testing, and guidelines for correction and interpretation of data.

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 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.

1.1 This test method covers the determination of fracture toughness (K_Ic and optionally K_Isi) 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 K_Ic 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 K_Ic 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 K_Isi test procedure is described in Appendix X1 and is an optional part of this test method. The K_Isi test procedure is based on a fixed amount of crack extension of 0.5 mm, and as a result, K_Isi is less sensitive to specimen size than K_Ic. This less size-sensitive fracture toughness, K_Isi, 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, K_Isi.

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 K_Ic 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 K_Isi test procedure, which provides an optional additional analysis procedure for the test data collected as part of the K_Ic test procedure.

1.3 General information and requirements common to all specimen configurations:

Section

Referenced Documents

2

Terminology

3

 Stress-Intensity Factor

3.1.1

 Plane-Strain Fracture Toughness

3.1.2

 Crack Plane Orientation

3.1.4

Summary of Test Method

4

Significance and Use

5

 Significance

5.1

 Precautions

5.1.1 – 5.1.5

 Practical Applications

5.2

Apparatus (see also 1.4)

6

 Tension Machine

6.1

 Fatigue Machine

6.2

 Loading Fixtures

6.3

 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

7.3.2.2

General Procedure

8

 Specimen Measurements

  Thickness

8.2.1

  Width

8.2.2

  Crack Size

8.2.3

  Crack Plane Angle

8.2.4

 Specimen Testing

  Loading Rate

8.3

  Test Record

8.4

Calculation and Interpretation of Results

9

 Test Record Analysis

9.1

 P_max/P_Q Validity Requirement

9.1.3

 Specimen Size Validity Requirements

9.1.4

Reporting

10

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 K_Ic and K_Isi Specimens

Annex A8

Hot-Pressed Beryllium Testing

Annex A9

Rapid-Force Testing

Annex A10

Determination of K_Isi

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.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.