Fracture mechanical characterization of the initiation and growth of interior fatigue cracks

Abstract

The formation and growth of interior fatigue cracks have been studied thoroughly during the last decades, due to the high practical importance of accurate predictions about the fatigue lives of structures and machine components. This is of special interest in the VHCF regime, for which the ultrasonic fatigue method is especially useful. Fracture mechanical values and lifetimes can be measured at constant and variable amplitudes. Endurance limits and ΔK thresholds can also be determined very quickly (e.g. 1010 cycles within a few days). Three different materials were studied. For chromium steel, internal crack formation and a fracture mechanical analysis of its growth were carried out. For a specially manufactured Mg alloy, microstructural features leading to interior cracks were analysed. Finally, for polycrystalline high‐purity copper, the conditions for interior‐crack formation and growth were determined. The utilization of FE‐SEM microscopy assisted in interpreting the prevailing mechanisms responsible for endurance limits, lifetimes and fatigue‐crack growth thresholds.

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

“…Knowing the effect of environment on the VHCF strength is also very important to better understand internal crack initiation . It is well known that crack growth rate is greater in humid air than in vacuum . This is in agreement with the observation of a very long incubation period for crack initiation which is characteristic of the gigacycle regime.…”

“…Very recently, Stanzl‐Tschegg showed that on “Type I” material (high‐purity copper), internal fatigue crack initiation can be observed in the VHCF regime, but 10 10 cycles were needed to observe that. The reason of such initiation is still unknown even if short cracks (20 μm long) seem to play the role of newly existing defects.…”

Fatigue strength of spring steel with small scratches

“…In practice, the interior fatigue cracks predominantly can be formed at a very high number of cycles. The formation and the growth of interior fatigue cracks depend on numerous factors, such as a composition of materials, microstructure, and surface condition as well as various loading parameters and the environment . Correspondingly, Sandaiji et al conducted the torsional fatigue tests in the VHCF regime, and they showed the different internal fatigue crack behaviors associated with the different types of inclusion for the shot‐peened specimens.…”

The influence of defect and temperature on the fatigue behaviours of Al‐Si‐Cu‐Mg‐Ni alloy

“…It occurs under stress lower than the 10 7 lifetime defined fatigue strength. It was revealed that smaller defects with length up to 40 μm could cause VHCF failure …”