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ASTM E3490-26
Standard Test Method for Relationship between Intrinsic Threshold Stress-Intensity Factor and Near-Tip Residual Stress
Standard Test Method for Relationship between Intrinsic Threshold Stress-Intensity Factor and Near-Tip Residual Stress
E3490-26
ASTM|E3490-26|en-US
Standard Test Method for Relationship between Intrinsic Threshold Stress-Intensity Factor and Near-Tip Residual Stress
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E3490 Standard Test Method for Relationship between Intrinsic Threshold Stress-Intensity Factor and Near-Tip Residual Stress>
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BOS Vol. 03.01 Committee E08
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Significance and Use
5.1 The value, ΔKth, determined using Test Method E647 does not partition contributions of intrinsic and extrinsic effects (ΔKth,i and ΔKth,x, respectively as illustrated in Fig. 2) to crack growth rate. The ΔKth measurements can be unconservative because they include a varying and indeterminant extent of ΔKth,x caused by crack wake contact (1-9). The value ΔKth,x can be sensitive to the test method and test parameters used.
5.2 The ΔKth,i versus σ* 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 ΔKth,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 σ* from the applied loading conditions at threshold.
5.4 Equations are provided to transform the relationship between ΔKth,i and σ* into one between ΔKth,i and the applied force ratio, R, for conditions associated with the absence of crack wake contact. The transformed relationship also serves as ΔKth,i versus Rop 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:
5.5.1 Construct Kitagawa-Takahashi diagrams (15) for different applied force ratio and load history on the consideration that initial defects are likely to respond to the entire applied force cycle even in compression;
5.5.2 Establish permissible defect size in research and development in material refinement, casting, forging, extrusion, and additive manufacturing to ensure defects, if any, do not exceed specified limits in input quality control;
5.5.3 Establish the origin of the da/dN versus ΔKeff curve on the assumption that a long crack at threshold is likely to respond to the same fraction of applied ΔK as at higher values of da/dN;
5.5.4 Determine the closure component, ΔKth,x (ΔKop or ΔKcl) in legacy data presented as ΔKth versus applied force ratio, R or Kmax(16); and
5.5.5 Interpret and model load history effects in near-threshold variable amplitude fatigue crack growth.
Scope
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, ΔKth,i. For the purpose of determination, ΔKth,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, ΔKth,i, varies with near-tip residual stress, σ*, computed at a particular distance, r*, from the crack tip. This test method characterizes the ΔKth,i versus σ* relationship by determining ΔKth,i under a sufficiently wide range of controlled σ* conditions. This relationship can be analytically transformed into one between ΔKth,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 ΔKth,i on σ* 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 σ* 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 ΔKth,i versus σ* 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.