Standard Test Method for KR Curve Determination

Importancia y uso:

5.1 The K_R curve characterizes the resistance to fracture of materials during slow, stable crack extension and results from the growth of the plastic zone ahead of the crack as it extends from a fatigue precrack or sharp notch. It provides a record of the toughness development as a crack is driven stably under increasing applied stress intensity factor K. For a given material, K_R curves are dependent upon specimen thickness, temperature, and strain rate. The amount of valid K_R data generated in the test depends on the specimen type, size, method of loading, and, to a lesser extent, testing machine characteristics.

5.2 For an untested geometry, the K_R curve can be matched with the applied-K curves (crack driving curves) to estimate the degree of stable crack extension and the conditions necessary to cause unstable crack propagation (2). In making this estimate, K_R curves are regarded as being independent of initial crack size a_o and the specimen configuration in which they are developed. For a given material, material thickness, and test temperature, K_R curves appear to be a function of only the effective crack extension Δa_e (3).

5.2.1 To predict crack behavior and instability in a component, a family of applied-K curves is generated by calculating K as a function of crack size for the component using a series of force, displacement, or combined loading conditions. The K_R curve may be superimposed on the family of applied-K curves as shown in Fig. 1, with the origin of the K_R curve coinciding with the assumed initial crack size a_o. The intersection of the applied-K curves with the K_R curve shows the expected effective stable crack extension for each loading condition. The applied-K curve that develops tangency with the K_R curve defines the critical loading condition that will cause the onset of unstable fracture under the loading conditions used to develop the applied-K curves.

FIG. 1 Schematic Representation of K_R curve and Applied K Curves to Predict Instability; K_c, P₃, a_c, Corresponding to an Initial Crack Size, a_o

5.2.2 Conversely, the K_R curve can be shifted left or right in Fig. 1 to bring it into tangency with applied-K curve to determine the initial crack size that would cause crack instability under that loading condition.

5.3 If the K-gradient (slope of the applied-K curve) of the specimen chosen to develop the K_R curve has negative characteristics (see Note 1), as in a displacement-controlled test condition, it may be possible to drive the crack until a maximum or plateau toughness level is reached (4, 5, 6). When a specimen with positive K-gradient characteristics (see Note 2) is used, the extent of the K_R curve which can be developed is terminated when the crack becomes unstable.

Note 1: Fixed displacement in crack-line-loaded specimens results in a decrease of K with crack extension.

Note 2: With force control, K usually increases with crack extension, and instability will occur at maximum force.

Subcomité:

E08.07

Referida por:

B0646-19, E1823-24C, F3439-22, E0740_E0740M-23, B0645-21, B0909-21A, E0647-24, D7779-20, E2472-12R18E04, C1940-24E01

Volúmen:

03.01

Número ICS:

77.040.10 (Mechanical testing of metals)

Palabras clave:

effective crack extension; fracture mechanics; fracture resistance; fracture toughness; KR ; KR curve; linear elastic; plane stress; plastic zone; standard test method; stress intensity factor;