A general equation for Kop and KPR description

A general equation for description of the stress intensity factor for crack opening, Kop, is presented. The equations has three coefficients which can be determined using three reference values at load ratios equal −1, 0, and a limiting value 1. The same type of equation is applicable also to express the stress intensity factor for crack propagation, KPR. Similarly, three reference values at load ratios equal −1, 0, and a limiting value 1 are sufficient to determine equation’s coefficients. This general equation follows closely the trend of experimental data reported in literature for both Kop and KPR. Hence, unlike some equations obtained by least square nonlinear regression, the proposed equation is not limited to a specific range of load ratios.