It is generally believed that Δ
Keff is the most appropriate parameter which controls the fatigue crack propagation rate. On the other hand, the difference of the crack closing load from the crack opening load or the difference of the re-tensile plastic zone's generated load (RPG Load) from the crack opening load was also pointed out. However, the accuracy of usual measurements seems to be not sufficient to discuss quantitatively the above mentioned difference, in other words, the superiority of one of the fatigue crack propagation parameters Δ
Kop, Δ
Kcl, and Δ
KRP.
Under this background, successful measurement of the change in strain near fatigue crack tip with high accuracy was made in our previous report, and in this paper, matured consideration on the fatigue crack propagation parameter on the basis of the experimental results was carried out. From dividing the loading and unloading processes into several characteristic areas on the basis of the tendency which the compliance and the change rate of compliance show, the following observations have been obtained as conclusions.
(1) In the unloading process, it seems to be impossible to isolate the area corresponding to the creation of the re-compressive plastic zone and there is also doubt about using the stress intensity factor because of the influence of crack closing. So it seems to be rational to adopt the area in the loading process, corresponding to the creation of the re-tensile plastic zone, for the fatigue crack propagation parameter.
(2) According to the experimental results, it is clear that the re-tensile plastic zone's generated point disagrees with the crack opening point.
(3) From the facts described above (1) & (2), it may be concluded that Δ
KRP, not Δ
Kop nor Δ
Kcl, is considered the most suitable parameter which controls the fatigue crack propagation.
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