A study of automatic measurement method of RPG (Re-tensile plastic zone’s generated) load

Identification of the relationship between fatigue crack propagation rate (da/dN) and stress intensity factor range (∆K) is inevitable to apply the fracture mechanics approach to assess the growth of fatigue crack growth. The relationship between da/dN and ∆K is widely applied to evaluate fatigue crack propagation behavior. To evaluate the fatigue crack growth history under variable loading history, it is necessary to replace ∆K to the effective stress intensity factor range, which can quantitatively consider fatigue crack opening and closing behavior. ∆K_eff proposed by Elber is well known as the effective stress intensity factor range, but even if ∆K_eff is applied to evaluate the fatigue crack propagation behavior, a threshold value ((∆K_eff)_th)was occurred. On the other hand, it is known that fatigue cracks propagate even at ∆K_eff below (∆K_eff)_th under variable loading history. This implies that ∆K_eff is an insufficient parameter to describe fatigue crack propagation behavior. ∆K_RPG, which has a close relationship with the cyclic plastic behavior in the vicinity of the crack tip proposed by Toyosada and Niwa, can give the solution of the problem of ∆K_eff, and even under complicated variable loading history including multiple frequency components. To apply the fatigue crack propagation law with ∆K_RPG as a parameter, it is necessary to experimentally measure the RPG load and identify the propagation law constants (∆K_RPG=C(∆K_RPG)^m) C and m. A conventional method for identifying RPG loads requires the superposition of the hysteresis loop near the crack tip and its reversal loop as measured by the unloading elastic compliance method. However, advanced skills in this method, such as understanding the characteristics of measurement errors associated with loops, is required. In this study, we propose a method to automatically measure the RPG load equivalent to that of an expert, and compare the automatic measurement results by this method with the past conventional measurement results under multiple materials and loading conditions, and validate the automatic measuring method.