Effect of Grain Size on the Fatigue Crack Growth in Steels at Temperatures 295 and 77 K

Fatigue crack growth (FCG) and threshold stress intensity in low carbon and HSLA steels with remarkably different ferrite grain sizes (df) at room temperature and temperature of liquid nitrogen were investigated. The FCG rates were found to decrease with decreasing temperature for all investigated microstructural states and the influence of temperature decrease was the most significant in coarse grain microstructures. The threshold values of the stress intensity factor, (ΔKth), below which cracks do not propagate, decreased by between 29.1% (df=2.7 μm) and 49.3% (df=88.4 μm) when specimens were tested at the cryogenic temperature.A general relationship between the FCG rate, the effects of grain boundary blocking on the plastic zone size and/or the crack-tip opening displacement and the effect of changing temperature, is discussed. Furthermore, the concept of a functional relation between tensile and fatigue data at cryogenic temperatures was also investigated. It was shown that the ratio of ΔKth at 77 K to ΔKth at 295 K is proportional to the second root of the ratio of the tensile strength values at these temperatures i.e. ΔKth 77 K/ΔKth 295K≈(σu 77 K/σu 295 K)1/2.