ASTM E647-22b

5.1?Fatigue crack growth rate expressed as a function of crack-tip stress-intensity factor range, d a/dN versus ?K, characterizes a material's resistance to stable crack extension under cyclic loading. Background information on the ration-ale for employing linear elastic fracture mechanics to analyze fatigue crack growth rate data is given in Refs 5.1.1?In innocuous (inert) environments fatigue crack growth rates are primarily a function of ?K and force ratio, R, or K_max and R (Note 1). Temperature and aggressive environments can significantly affect da/ dN versus ?K, and in many cases accentuate Note 1:??5.1.2?Expressing da/dN as a function of ?K provides results that are independent of planar geometry, thus enabling exchange and comparison of data obtained from a variety of specimen configurations and loading conditions. Moreover, this feature enables d a/dN versus ?K data to be utilized in the design and evaluation of engineering structures. The concept of similitude is assumed, which implies that cracks of differing lengths subjected to the same nominal ?K will advance by equal increments of crack extension per cycle.

5.1.3?Fatigue crack growth rate data are not always geometry-independent in the strict sense since thickness effects sometimes occur. However, data on the influence of thickness on fatigue crack growth rate are mixed. Fatigue crack growth rates over a wide range of ?K have been reported to either increase, decrease, or remain unaffected as specimen thickness is increased. Thickness effects can also interact with other variables such as environment and heat treatment. For example, materials may exhibit thickness effects over the terminal range of da/ dN versus ?K, which are associated with either nominal yielding (Note 2) or as Note 2:?This condition should be avoided in tests that conform to the specimen size requirements listed in the appropriate specimen annex.

5.1.4?Residual stresses can influence fatigue crack growth rates, the measurement of such growth rates and the predictability of fatigue crack growth performance. The effect can be significant when test specimens are removed from materials that embody residual stress fields; for example weldments or complex shape forged, extruded, cast or machined thick sections, where full stress relief is not possible, or worked parts having complex shape forged, extruded, cast or machined thick sections where full stress relief is not possible or worked parts having intentionally-induced residual stresses. Specimens taken from such products that contain residual stresses will likewise themselves contain residual stress. While extraction of the specimen and introduction of the crack starting slot in itself partially relieves and redistributes the pattern of residual stress, the remaining magnitude can still cause significant error in the ensuing test result. Residual stress is superimposed on the applied cyclic stress and results in actual crack-tip maximum and minimum stress-intensities that are different from those based solely on externally applied cyclic forces or displacements. For example, crack-clamping resulting from far-field 3D residual stresses may lead to partly compressive stress cycles, and exacerbate the crack closure effect, even when the specimen nominal applied stress range is wholly tensile. Machining distortion during specimen preparation, specimen location and configuration dependence, irregular crack growth during fatigue precracking (for example, unexpected slow or fast crack growth rate, excessive crack-front curvature or crack path deviation), and dramatic relaxation in crack closing forces (associated with specimen stress relief as the crack extends) will often indicate influential residual stress impact on the measured da/dN versus ?5.1.5?The growth rate of small fatigue cracks can differ noticeably from that of long cracks at given ?5.1.6?Crack closure can have a dominant influence on fatigue crack growth rate behavior, particularly in the near-threshold regime at low stress ratios. This implies that the conditions in the wake of the crack and prior loading history can have a bearing on the current propagation rates. The understanding of the role of the closure process is essential to such phenomena as the behavior of small cracks and the transient crack growth rate behavior during variable amplitude loading. Closure provides a mechanism whereby the cyclic stress intensity near the crack tip, ?K_eff, differs from the nominally applied values, ?K. This concept is of importance to the fracture mechanics interpretation of fatigue crack growth rate data since it implies a non-unique growth rate dependence in terms of ?Note 3:?The characterization of small crack behavior may be more closely approximated in the near-threshold regime by testing at a high stress ratio where the anomalies due to crack closure are minimized.

5.2?This test method can serve the following purposes:

1.1?This test method1.9?Special requirements for the various specimen configurations appear in the following order:

1.10?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.

1.11?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.