The fracture toughness determined by this test method characterizes the resistance of a material to fracture by a slowly advancing steady-state crack (see 3.2.5) in a neutral environment under severe tensile constraint. The state of stress near the crack front approaches plane strain, and the crack-tip plastic region is small compared with the crack size and specimen dimensions in the constraint direction. A K_Iv or K_Ivj value may be used to estimate the relation between failure stress and defect size when the conditions described above would be expected, although the relationship may differ from that obtained from a K_Ic value (see Note 1). Background information concerning the basis for development of this test method in terms of linear elastic fracture mechanics may be found in Refs (1-15).

The K_Iv, K_Ivj, or K_IvM value of a given material can be a function of testing speed (strain rate) and temperature. Furthermore, cyclic forces can cause crack extension at K_I values less than K_Iv, and crack extension can be increased by the presence of an aggressive environment. Therefore, application of K_Iv in the design of service components should be made with an awareness of differences that may exist between the laboratory tests and field conditions.

Plane-strain fracture toughness testing is unusual in that there can be no advance assurance that a valid K_Iv, K_Ivj, or K_IvM will be determined in a particular test. Therefore, it is essential that all the criteria concerning the validity of results be carefully considered as described herein.

This test method can serve the following purposes:

To establish 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.

For specifications of acceptance and manufacturing quality control, but only when there is a sound basis for specification of minimum K_Iv, K_Ivj, or K_IvM values, and then only if the dimensions of the product are sufficient to provide specimens of the size required for valid K_Iv determination (5). The specification of K_Iv values in relation to a particular application should signify that a fracture control study has been conducted on the component in relation to the expected history of 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.

To provide high spatial resolution in measuring plane strain fracture toughness variations in parent pieces of material (14).

Note 2—The high spatial resolution is possible because of the small allowable specimen size criterion, B ≥ 1.25 (K_Iv /σ_YS)² (5), and because the toughness is measured at approximately the midline of the specimen, and only in the material covered by the crack's lateral extent, which is about one third of the specimen's lateral dimension, B.

1.1 This test method covers the determination of plane-strain (chevron-notch) fracture toughnesses, K_Iv or K_IvM, of metallic materials. Fracture toughness by this method is relative to a slowly advancing steady state crack initiated at a chevron-shaped notch, and propagating in a chevron-shaped ligament (Fig. 1). Some metallic materials, when tested by this method, exhibit a sporadic crack growth in which the crack front remains nearly stationary until a critical load is reached. The crack then becomes unstable and suddenly advances at high speed to the next arrest point. For these materials, this test method covers the determination of the plane-strain fracture toughness, K_Ivj or K_IvM, relative to the crack at the points of instability.

Note 1—One difference between this test method and Test Method E 399 (which measures K_Ic) is that Test Method E 399 centers attention on the start of crack extension from a fatigue precrack. This test method makes use of either a steady state slowly propagating crack, or a crack at the initiation of a crack jump. Although both methods are based on the principles of linear elastic fracture mechanics, this difference, plus other differences in test procedure, may cause the values from this test method to be larger than K_Ic values in some materials. Therefore, toughness values determined by this test method cannot be used interchangeably with K_Ic.

1.2 This test method uses either chevron-notched rod specimens of circular cross section, or chevron-notched bar specimens of square or rectangular cross section (Figs. 1-10). The terms “short rod” and “short bar” are used commonly for these types of chevron-notched specimens.

1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.

1.4 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 and health practices and determine the applicability of regulatory limitations prior to use.