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The fatigue crack growth (FCG) threshold behavior of elastic-plastic materials is analyzed theoretically by simplistic mechanical considerations. Emphasis was laid upon two of the key aspects of threshold, which are both still controversially discussed in the literature: intrinsic threshold and extrinsic crack tip shielding. The analytical relations derived from the model confirm experimental indications, that at R-ratios beyond a certain limit FCG and its threshold is governed by intrinsic mechanisms, whereas at lower R it is crucially influenced by extrinsic shielding mechanisms. The latter are postulated to consist of a crack-closure and a non-closure part, which is confirmed by preliminary experimental data as well as data from the literature. To measure crack closure independently, a new experimental technique, the so-called cut compliance method, is proposed, which is shown by some preliminary tests to work well even in the threshold regime. The model and the derived mathematical relations also enable one to distinguish between those parts of the threshold that are inherently associated with FCG, and the ones that are geometry- or load-history-dependent. Therewith conservatively transferable threshold data can be obtained.
crack tip shielding, crack closure, intrinsic threshold, experimental, residual stress, influence functions, cut compliance method, threshold chart, R-effect
Senior research engineer, Swiss Federal Labs, for Materials Testing and Research (EMPA), Duebendorf,