Finite element modelling of fatigue crack growth of surface cracked plates - Part III: Stress intensity factor and fatigue crack growth life

Stress intensity factors (SIFs) and fatigue crack growth lives, obtained by a numerical simulation technique which is based on a multiple degree of freedom model, are presented. The results include those for both semi-elliptical static cracks and propagating cracks that are not necessarily semi-elliptical. Comparisons are made between the present results and the predictions from a two degree of freedom model based on the Newman and Raju closed-form SIF equation. The results show that the widely used Newman and Raju equation might be of relatively worse accuracy for cracks whose depth is larger than 90% of the plate thickness; that the present SIF predictions during crack growth are generally in good agreement with those obtained by the two degree of freedom method; and that most of the cracks approach, but cannot subsequently maintain, an iso-K profile because of the presence of both the front and back surfaces of the plate, or the bending component of the load. The free surface correction is also considered in both the multiple and two degree of freedom methods. The results show that the free boundary correction seems to have little influence on fatigue crack growth curves; and that agreement between the fatigue crack growth curves estimated from the present technique and the Newman and Raju 'two-point plus semi-ellipse' method, assuming the relation of C-C = 0.9(m)C(A), is excellent for pure tension, while it becomes slightly worse for pure bending. (C) 1999 Elsevier Science Ltd. All rights reserved.