A new approach to the correlation between the coefficient and the exponent in the power law equation of fatigue crack growth

The presented experimental results for 2xxx, 6xxx and 7xxx type aluminium alloys show a variability of the fatigue crack growth rates in the Paris regime to an extent important for the materials selection according to the damage tolerant design concept in the aircraft industry. This paper briefly reviews the controversial views on the correlation between coefficient and exponent in the power law equation of fatigue crack growth. A new approach to the correlation based on a scaling factor of the stress intensity factor range is outlined. It is shown that the correlation is an algebraic one that can be suppressed by choosing a particular scaling factor characteristic of the set of materials under investigation. The approach allows the variability of the power law coefficient to be quantified more clearly and the responsible influence factors to be identified.