Fatigue crack growth at negative stress ratios: On the uncertainty of using ΔK and R to define the cyclic crack tip load

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At negative stress ratios, the cyclic crack tip loading cannot be defined by ΔK and R. Little attention has been paid on this issue and most of the existing fatigue crack growth data at negative stress ratios is treated in the same way as positive stress ratios. The aim of this note is to call attention on this topic and to sensitize the reader for possible uncertainties in life time prediction. Simple examples are discussed which show that neither reversal crack tip plasticity nor the crack growth rate is controlled by Kmax and R at negative stress ratios. Instead the parameter σtip seems to be a controlling parameter for minimum crack tip load at negative stress ratios. A discussion is provided with the focus on transferring the findings to other specimen or structures. Perspectives for the treatment of negative stress ratios in the context of fatigue crack growth are briefly discussed.