1986 Volume 35 Issue 394 Pages 779-784
A usual unloading elastic compliance method for determining the crack opening point is based on an unloading curve whose uppermost part looks like a straight line. By this method there is the possibility of obtaining a wrong conclusion, since the crack closure behavior is always accompanied with plastic deformation. In this paper the tension-compression fatigue tests were carried out (stress ratio were taken as 0 and -1), and the opening and closure point is successfully measured by the S shaped unloading curve.
The crack closure point measured from the S shaped unloading curve does not coincide with the opening point. The crack growth rate can be expressed unifyingly by the effective stress intensity range, ΔKeffop=Kmax-Kop, or ΔKeffcl=Kmax-Kcl, independently of the stress ratio and the loading history. The inclination of the line of log(dl/dN) vs. log ΔKeffcl plot is approximately 2. This suggests that the proposed method for determining the crack closure point is rational.
It is shown that the values of effective stress range ratio uop(=ΔKeffop/Kmax) and ucl(=ΔKeffcl/Kmax) plotted against Kmax respectively get together on one narrow band (when R is constant), if the cracks were propagated into a steady state. The effective stress range ratios uop and ucl increase linearly with Kmax in the range of Kmax=7.5MPa√m to 12.5MPa√m. Especially, the ucl based on Kcl is nearly in direct proportion to Kmax. This is closely related to the background of the fact that the Paris crack growth low dl/dN=CΔK4 holds good in the present material.