ASTM E3490-26

1.1 This test method is used to characterize conditions for the onset of fatigue crack growth expressed in terms of intrinsic threshold stress-intensity factor range, ?K_th,i. For the purpose of determination, ?K_th,i is associated with the onset of fatigue crack growth at a rate not exceeding da/dN = 10^-10 m/cycle under conditions that ensure that crack wake contact at threshold is either absent or of insignificant consequence.

1.2 The value, ?K_th,i, varies with near-tip residual stress, s*, computed at a particular distance, r*, from the crack tip. This test method characterizes the ?K_th,i versus s* relationship by determining ?K_th,i under a sufficiently wide range of controlled s* conditions. This relationship can be analytically transformed into one between ?K_th,i and applied force ratio under constant amplitude loading in the absence of crack wake contact.

1.3 This test method is intended to support such activities as materials research and development, engineering design, process and quality control, product performance, and failure analysis.

1.4 Mechanisms that induce the dependence of ?K_th,i on s* are likely to be environment sensitive. Therefore, it is important to ensure that the laboratory test environment be representative of the conditions to which obtained test results will be applied. The calculation of s* for conditions involving cycle or time-dependent crack tip stress relaxation is not considered in this test method.

1.5 Dedicated software is required on the test system to ensure reproducible automated implementation of the test procedure, which involves baseline cycling of varying duration and amplitude, periodic application of precision overloads and underloads, and crack size measurements.

1.6 This test method provides sufficient detail for testing compact tension, C(T) specimens, the drawings for which are provided in Test Method E647. The nature of the ?K_th,i versus s* relationship does not appear to preclude the use of other standard specimen geometries listed in Test Method E647, or other standardized sources, provided suitable instrumentation and software is available for automated crack size measurement and computation of stress-intensity factor.

1.7 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

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.

5.1 The value, ?K_th, determined using Test Method E647 does not partition contributions of intrinsic and extrinsic effects (?K_th,i and ?K_th,x, respectively as illustrated in Fig. 2) to crack growth rate. The ?K_th measurements can be unconservative because they include a varying and indeterminant extent of ?K_th,x caused by crack wake contact (1-9). The value ?K_th,x can be sensitive to the test method and test parameters used.

5.2 The ?K_th,i versus s* relationship describes cyclic loading conditions for the onset of fatigue crack extension in the absence of crack wake contact, as is the case for initial defects prior to the onset of crack growth. The value ?K_th,i can vary by a factor of 3 to 5 as a function of applied force ratio and load history (10-14).

5.3 Equations are described to calculate s* from the applied loading conditions at threshold.

5.4 Equations are provided to transform the relationship between ?K_th,i and s* into one between ?K_th,i and the applied force ratio, R, for conditions associated with the absence of crack wake contact. The transformed relationship also serves as ?K_th,i versus R_op for the case of long cracks likely to experience crack wake contact.

5.5 Results from this test method can serve as an input to: