Effect of Incomplete Penetration on Bending Fatigue Strength

Abstract

This study is carried out to clarify the relation between stress concentration and fatigue strength, and between initiation and propagation of crack and fatigue strength.
For this purpose, double-Vee butt joints with incomplete penetration are used. As non-destructive inspection, ultrasonic inspection and X-ray inspection are used and compared with the characteristics.
The results obtained are summarized as follows.
(1) By the bend fatigue test of specimen with incomplete penetration, it is found that the fatigue strength decreases rapidly as the size of incomplete penetration is increased.
(2) The failure do not occur lower than a certain value of initial range in stress intensity factor ΔK_i dependent upon size of incomplete penetration in this experiment.
(3) By the ultrasonic inspection, the range in fatigue strength is estimated by the maximum echo height.
(4) By the ultrasonic inspection, fatigue crack initiation is identified up to about 30% in total fatigue life. On the other hand, by the X-ray inspection, the total fatigue life is divided into three regions. In 1st region, incomplete penetration is undetectable, in 2nd region, incomplete penetration is detectable and in 3rd region, initiation and propagation of crack is identified.
(5) The relation between the crack propagation rate da/dN and the range in stress intensity factor ΔK is given by the following expression da/dN=5.48×10^-14(ΔK)^5.7.
(6) By the bend fatigue test of specimen with incomplete penetration, stress concentration factor a and fatigue notch factor β is rapidly increased as the size of incomplete penetration is increased.