Fatigue thresholds have been determined for Inconel 617 and Ti-6Al-4V that are unaffected by any closure phenomenon. These results show that a nonpropagation condition exists between K_op and K_max of unique amplitude ΔK_T which is unaffected by the position between K_op and K_max. This experimental fact opens up the possibility of measuring the dividing point K_op (fixing the ΔK_eff) based on a propagation/nonpropagation criterion. Three such test methods are proposed. Results of these measurements are compared with K_op determinations via load-displacement curves made in parallel on Inconel 617. Comparison of both results show that they contradict each other.

It is concluded that the two different experimental procedures determine and characterize two independent phenomena affecting the fatigue crack growth process differently. Characterizing the two different phenomena by K_op is incorrect.