The threshold behavior of metallic materials cannot be described by a single numerical value, but a complete threshold curve is necessary for this purpose. This curve has to be determined from a larger number of measurements. Methods for determining the threshold are described and discussed as well as the ins and outs of the different curves that can be derived from the test results. Typical material behavior can be represented by simple analytic equations. Care has to be taken when threshold data are used to interpret crack initiation and crack propagation phenomena. This concerns especially short crack behavior and calculation of fatigue life of technical parts.
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