Standard Test Method for Measurement of Creep Crack Growth Times in Metals

Importancia y uso:

6.1 Creep crack growth rate expressed as a function of the steady state C* or K characterizes the resistance of a material to crack growth under conditions of extensive creep deformation or under brittle creep conditions. Background information on the rationale for employing the fracture mechanics approach in the analyses of creep crack growth data is given in (11, 13, 30-35).

6.2 Aggressive environments at high temperatures can significantly affect the creep crack growth behavior. Attention must be given to the proper selection and control of temperature and environment in research studies and in generation of design data.

6.2.1 Expressing CCI time, t_0.2 and CCG rate, da/dt as a function of an appropriate fracture mechanics related parameter generally provides results that are independent of specimen size and planar geometry for the same stress state at the crack tip for the range of geometries and sizes presented in this document (see Annex A1). Thus, the appropriate correlation will enable exchange and comparison of data obtained from a variety of specimen configurations and loading conditions. Moreover, this feature enables creep crack growth data to be utilized in the design and evaluation of engineering structures operated at elevated temperatures where creep deformation is a concern. The concept of similitude is assumed, implying that cracks of differing sizes subjected to the same nominal C*(t), C_t, or K will advance by equal increments of crack extension per unit time, provided the conditions for the validity for the specific crack growth rate relating parameter are met. See 11.7 for details.

6.2.2 The effects of crack tip constraint arising from variations in specimen size, geometry and material ductility can influence t_0.2 and da/dt. For example, crack growth rates at the same value of C*(t), C_t in creep-ductile materials generally increases with increasing thickness. It is therefore necessary to keep the component dimensions in mind when selecting specimen thickness, geometry and size for laboratory testing.

6.2.3 Different geometries as mentioned in 1.1.6 may have different size requirements for obtaining geometry and size independent creep crack growth rate data. It is therefore necessary to account for these factors when comparing da/dt data for different geometries or when predicting component life using laboratory data. For these reasons, the scope of this standard is restricted to the use of specimens shown in Annex A1 and the validation criteria for these specimens are specified in 11.7. However if specimens other than the C(T) geometry are used for generating creep crack growth data, then the da/dt data obtained should, if possible, be compared against test data derived from the standard C(T) tests in order to validate the data.

6.2.4 Creep cracks have been observed to grow at different rates at the beginning of tests compared with the rates at equivalent C*(t), C_t or K values for cracks that have sustained previous creep crack extension (12, 13). This region is identified as ‘tail’. The duration of this transient condition, ‘tail’, varies with material and initially applied force level. These transients are due to rapid changes in the crack tip stress fields after initial elastic loading and/or due to an initial period during which a creep damage zone evolves at the crack tip and propagates in a self-similar fashion with further crack extension (12, 13). This region is separated from the steady-state crack extension which follows this period and is characterized by a unique da/dt versus C*(t), C_t or K relationship. This transient region, especially in creep-brittle materials, can be present for a substantial fraction of the overall life (35). Criteria are provided in this standard to quantify this region as an initial crack growth period (see 1.1.5) and to use it in parallel with the steady state crack growth rate data. See 11.8.8 for further details.

6.3 Results from this test method can be used as follows:

6.3.1 Establish predictive models for crack incubation periods and growth using analytical and numerical techniques (18-21).

6.3.2 Establish the influence of creep crack development and growth on remaining component life under conditions of sustained loading at elevated temperatures wherein creeps deformation might occur (23-28).

Note 1: For such cases, the experimental data must be generated under representative loading and stress-state conditions and combined with appropriate fracture or plastic collapse criterion, defect characterization data, and stress analysis information.

6.3.3 Establish material selection criteria and inspection requirements for damage tolerant applications.

6.3.4 Establish, in quantitative terms, the individual and combined effects of metallurgical, fabrication, operating temperature, and loading variables on creep crack growth life.

6.4 The results obtained from this test method are designed for crack dominant regimes of creep failure and should not be applied to cracks in structures with wide-spread creep damage which effectively reduces the crack extension to a collective damage region. Localized damage in a small zone around the crack tip is permissible, but not in a zone that is comparable in size to the crack size or the remaining ligament size. Creep damage for the purposes here is defined by the presence of grain boundary cavitation. Creep crack growth is defined primarily by the growth of intergranular time-dependent cracks. Crack tip branching and deviation of the crack growth directions can occur if the wrong choice of specimen size, side-grooving and geometry is made (see 8.3). The criteria for geometry selection are discussed in 5.8.

Subcomité:

E08.06

Referida por:

E1823-24C, E2760-19E03

Volúmen:

03.01

Número ICS:

77.040.10 (Mechanical testing of metals)