Experimental and numerical investigations of fatigue crack growth in various specimen geometries
This paper presents some experimental results demonstrating the geometry dependence of fatigue crack growth (FCG) curves for the steel EA4T (25CrMo4). The experimental results exhibit considerable differences in FCG rates measured on M(T) and C(T) standard specimens, as well as specimens with surface cracks. To explore the possibility of an analytical description of these effects, a numerical analysis is applied to simulate crack growth behaviour for the M(T) and C(T) geometries with special emphasis put on modelling plasticity induced crack closure. The analysis results provide an adequate qualitative description of the crack propagation in different specimens and suggest an explanation for the difference in respective FCG data. On the other hand, a re-evaluation of the experimental results is undertaken to explore a correlation between crack growth rates and the amount of crack tip yielding, with the latter being quantified in terms of the plasticity parameter Lr of t he failure assessment diagram. Such an analysis demonstrates that, even though small scale yielding conditions prevail at the crack tip, the FCG curves for individual specimens depend upon and can be arranged according to the plasticity level. Thus, the results suggest that engineering calculations of fatigue crack propagation can be facilitated by incorporating the Lr factor as an additional influencing parameter to take into account the geometry and load effects on FCG rates.