Closure Behavior of Fatigue Crack

Experimental Study Based on the Measured Results by the S Shaped Unloading Curve Method

Abstract

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, ΔK_eff^op=K_max-K_op, or ΔK_eff^cl=K_max-K_cl, independently of the stress ratio and the loading history. The inclination of the line of log(dl/dN) vs. log ΔK_eff^cl 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 u_op(=ΔK_eff^op/K_max) and u_cl(=ΔK_eff^cl/K_max) plotted against K_max 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 u_op and u_cl increase linearly with K_max in the range of K_max=7.5MPa√m to 12.5MPa√m. Especially, the u_cl based on K_cl is nearly in direct proportion to K_max. 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.