The effect of loading history on the crack closure behavior was investigated during K-increasing and K-decreasing processes in fatigue crack propagation tests. It was found that the crack opening point, Kop, which had been once raised by the prior loading did not easily decrease in the following K-decreasing process, so Kop in the K-decreasing process was higher than that in the K-increasing or K-constant process, which resulted in to give the low crack propagation rate and the high threshold condition for crack propagation in the K-decreasing test. This is the reason for the fact that the Kth determined by the conventional K-decreasing test is usually higher than that calculated from the fatigue strength of a cracked specimen.
From the above mentioned testing results and discussion, it may be concluded that it is dangerous to estimate the criterion whether a flaw will grow or not based on the Kth determined by the K-decreasing test and it should be determined by the K-increasing method using an ideally pre-cracked specimen.
