Comparison of a New Equation Describing Fatigue Crack Growth Cruves with the Narsgro Equation

Comparison of a New Equation Describing Fatigue Crack Growth Cruves with the Narsgro Equation

Peer-reviewed article not indexed in WoS or Scopus

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 delta 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 delta K(R) = delta K(0)(1 - R)-m which is valid also for threshold value delta 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.

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 delta 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 delta K(R) = delta K(0)(1 - R)-m which is valid also for threshold value delta 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.

Fatigue, Fatigue crack Nasgro equation

Fatigue, Fatigue crack Nasgro equation

KOHOUT, J.; VĚCHET, S.; HRUBÝ, V.

01.11.2006

EIDS - Žilinská univerzita

Žilina

1335-4205

Communications

8

4/2006

Slovak Republic

29

33

5