Study of the duration of the fatigue crack growth in steels depending on the nature of variable loading
Duration of growth of the fatigue crack of various steels such as AISI 4030, 40X, 40 at external excitation of various character is investigated. It should be noted that kinetics of the crack growth depends on the nature of variable loading in many respects. Experimental data of kinetics of fatigue cracks’ growth of the wide spectrum of block and random loadings of investigated steels are received and an approach for cracks’ growth estimate depending on material properties and the nature of variable external input is offered. It is shown that the duration of growth of the fatigue crack on the middle-amplitude segment of its growth depends on the completeness (irregularity) of block or accidental loading. It is offered the dependence of the longevity evaluation of fatigue crack durability from a given target size to its critical value corresponding to the moment of material destruction. For this calculation it was used the value of the fatigue crack growth duration was used at a zero regular loading with the maximum load corresponding to the maximum load of the later used block or accidental loading, also the fatigue properties of the material determined from the slope of the endurance curve under soft symmetrical loading were taken into account. The second approach is related to the application of averaged parameters of the fatigue crack growth kinetics - the slope of curve of the kinetic fatigue fracture diagram (KFFD) at regular cyclic loading with the same force parameters. The linear principle of summation of crack growth without interaction of amplitudes was used in both cases. For the study the accepted assumptions it was made a calculation of the duration growth of cracks in the program of summation the crack growth for each loading cycle (cycle-by-cycle) without taking into account the interaction of load amplitudes.