Abstract
The so-called NASGRO equation allows a very good description of fatigue crack growth curves, i.e. the dependence of crack length increase per one fatigue cycle da/dN on the stress intensity factor range K. In 1999 the first author of this paper published quite similar equation, which written for given loading cycle asymmetry with positive stress ratio R contains the same parameters having similar meaning as in the NASGRO equation. In most cases studied by the authors this equation leads to a better fit than the NASGRO equation, above all when the experimental curve contains relatively long Paris straight line and/or relatively sharp bend from this line to the threshold stress intensity factor range. The generalization of the NASGRO equation for various values of stress ratio R was made in a quite complicated way. The new equation was generalized using the Walker model based on the relation K(R) K(0)(1 R)m which is valid also for threshold value Kth but not for critical stress intensity Kc being a constant independent of R. Then the shift of the growth curves with the change of R ratio is described only by one parameter m (0 m 1) both for positive and for negative values of R. It means that the crack closure models are very important for explanation and deep study of fatigue crack growth mainly in negative R region but play no crucial role in a phenomenological description of the growth in the case of short-term tests in non-aggressive media.
Highlights
The NASGRO is a commercial complex of very sophisticated and deeply developed computer programs covering all necessary procedures dealing with the growth of long fatigue cracks, including extensive database of experimentally determined fatigue crack growth curves for various structural materials
Paris straight line, ⌬Kth is threshold stress intensity factor range, Kc is critical stress intensity, and p, q are exponents describing the bends of the curve from the Paris straight line to ⌬Kth and Kc, respectively
The equations (1) and (2) are written in very useful forms in which the description of single regions is separated: the term before the fraction describes the Paris straight line typical for the stable propagation region, the numerator of the fraction describes the bend of a curve in the initiation region to the threshold stress intensity factor range, and the denominator of the fraction describes the region of unstable fracture where the growth rate increases over all limits
Summary
The NASGRO is a commercial complex of very sophisticated and deeply developed computer programs covering all necessary procedures dealing with the growth of long fatigue cracks, including extensive database of experimentally determined fatigue crack growth curves for various structural materials. The equations (1) and (2) are written in very useful forms in which the description of single regions is separated: the term before the fraction describes the Paris straight line (if the log-log fit is considered) typical for the stable propagation region, the numerator of the fraction describes the bend of a curve in the initiation region to the threshold stress intensity factor range, and the denominator of the fraction describes the region of unstable fracture where the growth rate increases over all limits These special forms of the equations can be used as a building kit, because they allow simple adjustment for the cases when only two of all three regions are covered with experimental results and only the terms corresponding to the mentioned regions are included into the equations.
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