applied fracture mechanics

Good Practice for Fatigue Crack Growth Curves Description 223approximating courses as in Fig. 16b. The curve illustrating the Kc * = f(R) dependence is inthis case a broken line shown in Fig. 15b, which – easy to see – considerably strays awayfrom the linear dependence. This proves that, among other things, one cannot ad hoc imposeany form upon it. It can be assumed that for a larger number of experimentally gainedcurves, including the wider scope of values of R, the suggested method of determining therelationship Kc = f(R) will offer better results that better correspond to the actual dependenceand will remain useful for approximating the da/dN-K curves. The broken-line curve, asthat resulting from the optimisation process, may be described with, e.g. a straight line ora quadratic equation (as in Fig. 17) and used as a component of the theoretical (analytical)description of the test data with the NASGRO equation. In the case of a straight line, thecorrelation coefficient increases up to approx. 0.78 for the polynomial. Obviously, with threepoints Kc * the correlation is complete, but if the scope of values of the asymmetry coefficientwas greater, i.e. there would be more experimentally gained curves of different values of R(then the number of these points would increase), one should also expect high correlationfor the relationship Kc = f(R).Figure 16. Approximation of curves da/dN-K with the NASGRO equation, modified by formula (32)LSM, with extrapolated mapping points according to the MRCM: a) with regression applied as in Fig.15, b) with optimisation for values of coefficients Kc*