ASTM E1457-13

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 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.

1.1 This test method covers the determination of time for a creep crack to grow on initial load (CCI) and its subsequent creep crack growth (CCG) rates in metals at elevated temperatures using pre-cracked specimens subjected to elevated temperatures under static or quasi-static loading conditions. The tests are validated for either base material (homogenous properties) or mixed base/weld material with inhomogeneous microstructures and creep properties. For CCI the time (CCI), t_0.2 to an initial crack extension δa_i = 0.2 mm from the onset of first applied force and CCG rate, a˙ or 1.1.1 The choice of the crack growth correlating parameter C*, C*(t), C_t, or 1.1.2 In any one test, two regions of crack growth behavior may be present (9, 10). The initial transient region where elastic strains dominate and creep damage develops and in the steady state region where crack grows proportionally to time. Steady-state creep crack growth rate behavior is covered by this standard. In addition specific recommendations are made in 1.1.3 In creep ductile materials, extensive creep occurs when the entire un-cracked ligament undergoes creep deformation. Such conditions are distinct from the conditions of small-scale creep and transition creep (1-7). In the case of extensive creep, the region dominated by creep deformation is significant in size in comparison to both the crack length and the uncracked ligament sizes. In small-scale-creep only a small region of the un-cracked ligament local to the crack tip experiences creep deformation.

1.1.4 The creep crack growth rate in the extensive creep region is correlated by the C*(t)-integral. The 1.1.5 An engineering definition of an initial crack extension size δa_i is used in order to quantify the initial period of crack development. This distance is given as 0.2 mm. It has been shown (38-40) that this initial period which exists at the start of the test could be a substantial period of the test time. During this early period the crack tip undergoes damage development as well as redistribution of stresses prior reaching steady state. Recommendation is made to correlate this initial crack growth period defined as t_0.2 at δa_i = 0.2 mm with the steady state C* when the crack tip is under extensive creep and with K for creep brittle conditions. The values for C* and K should be calculated at the final specified crack size defined as 1.1.6 The recommended specimens for CCI and CCG testing is the standard compact tension specimen C(T) (see 1.1.7 The state-of-stress at the crack tip may have an influence on the creep crack growth behavior and can cause crack-front tunneling in plane-sided specimens. Specimen size, geometry, crack length, test duration and creep properties will affect the state-of-stress at the crack tip and are important factors in determining crack growth rate. A recommended size range of test specimens and their side-grooving are given in 1.1.8 Material inhomogeneities, residual stresses and material degradation at temperature, specimen geometry and low-force long duration tests (mainly greater that one year) can influence the rate of crack initiation and growth properties 1.1.9 Stress relaxation of the residual stresses due to creep and crack extension should also be taken into consideration. No specific allowance is included in this standard for dealing with these variations. However the method of calculating C* presented in this document which used the specimen’s creep displacement rate to estimate C* inherently takes into account the effects described above as reflected by the instantaneous creep strains that have been measured. However extra caution should still be observed with the analysis of these types of tests as the correlating parameters K and C* shown in 1.1.10 Specimen configurations and sizes other than those listed in Table A1.1 which are tested under constant force will involve further validity requirements. This is done by comparing data from recommended test configurations. Nevertheless, use of other geometries are applicable by this method provided data are compared to data obtained from standard specimens (as identified in 1.2 The values stated in SI units are to be regarded as the standard. The inch-pound units given in parentheses are for information only.

1.3 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.