Abstract
This paper discusses the appropriateness of crack length as a reference dimension for fatigue damage. Current discussion on short crack versus long crack data is still divided between various approaches to model small crack growth. A proper physical explanation of the probable cause of the apparent differences between short crack and long crack data is not yet provided. Long crack data often comprises crack growth in constant thickness specimens, with a through crack of near constant crack front geometry. This is not true for corner cracks or elliptical surface crack geometries in the small crack regime where the crack front geometry is not symmetric or through-thickness. This affects similitude parameters that are based on the crack length. The hypothesis in this paper is that a comparison between long crack data and short crack data should be made using similar increments in crack surface area. The work applied to the specimen is dissipated in generation of fracture surface, whereas fracture length is a result. The crack surface area approach includes the two-dimensional effect of crack growth geometry in the small crack regime. A corner crack and a through crack are shown to follow the same power law relationship when using the crack area as base parameter. The crack front length is not constant, and its power law behaviour for a corner crack is shown.
Highlights
Most fatigue crack growth data is published as ddaa/dddd versus ∆KK.a The data often appears as one or more straight lines in a graph with log-log scales, and is known as the ‘Paris law’
The choice of crack length aa as base parameter for fatigue crack growth measurements is questionable for crack types other than through cracks
It is shown that corner cracks behave differently in the small crack growth region
Summary
Most fatigue crack growth data is published as ddaa/dddd versus ∆KK.a The data often appears as one or more straight lines in a graph with log-log scales, and is known as the ‘Paris law’. In industry, this Paris relation is widely used for its simplicity. Alderliesten [1,2] questions the general idea that the Paris relation in all its forms should be taken as a ‘law’, as there is no physical basis for the power law He suggests that a better understanding of fatigue crack growth could be obtained by looking at the energy balance throughout fatigue cycles. Another choice of base parameter might improve the understanding of fatigue crack growth, and could help in understanding the energy balance cycle during crack growth
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