Many types of ferrous metals can sustain an indefinite number of repeated loading cycles (N10 ~7 cycles) provided that the maximum imposed stresses do not exceed certain critical values usually referred to as fatigue or endurance limits. In current practice, these limits are primarily inferred from statistical analyses of numerous fatigue experiments that relate the number of cycles to failure, Nf, to the loading programs. Numerous attempts have been made to bypass these time consuming tests by the direct observation of changes in material microstructures utilizing a variety of physical effects ranging from neutron diffraction, x-ray radiography, acoustic emission and even positron radiation patterns; but none of these approaches has yielded any unambiguous indices of damage. Recently, it has been found that the evolution of piezomagnetic hysteresis, due to magnetization changes induced in ferromagnetic steels by tension and compression, is a reliable indicator of the development of fatigue damage and can lead to practical predictions of service life. Further detailed information concerning processes at the microstructural level can be obtained from measurements of flux jumps associated with the piezomagnetic fields. Sequences of flux variations of the order of 10 ~3Mx or 10 ~(11)Wb, comparable to those observed in conventional Barkhausen experiments, appear when ferromagnetic steels are subjected to tension or compression. The amplitude distribution of these piezo-Barkhausen pulses increases markedly in the vicinity of the endurance limit and appears to provide a rapid means for distinguishing between stable, i.e. safe, loading regimes and those terminating in fatigue failure.
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