INITIATION AND PROPAGATION OF FATIGUE CRACKS IN PARTIALLY-PENETRATED LONGITUDINAL WELDS

Conventionally, evaluations of safety against fatigue of structures have been made based on S-N diagrams obtained in fatigue tests of structural members and specimens of joint portions. Current fatigue designs of bridges also are made using allowable stresses set up taking into account suitable safety factors on fatigue limits (in most cases 2 x 106-cycles strength) obtained from such fatigue tests'. Such a method is direct, and reliability is assured to some extent. However, even though structural members and joints themselves are tested, the configurations and dimensions of specimens are subject to considerable restrictions because of limitations to capabilities of testing machines, and thorough care must be exercised in the application of the results. Fatigue failure is a phenomenon which can be subdivided into initiation of fatigue crack and propagation of the crack up to failure of the member. Initiation of fatigue crack is a localized phenomenon which occurs at a location in the member where stress conditions are unfavorable, and the life up to that time (crack initiation life: N0) is greatly influenced by localized stress concentrations and weld defects in the member. In contrast, the life up to failure of the member upon propagation of fatigue crack (crack propagation life: Np) varies depending on the dimensions of the member and overall stress distribution. In case of being subject to variable stresses fluctuations such as actual stresses by vehicle, it is possible to estimate Nc applying the linear damage law, but the application is difficult for Np2. Accordingly, it is thought that fatigue safety of a structure should be evaluated distinguishing between N0 and N. Various definitions may be given Nc depending on the dimensions of the fatigue crack used as the basis, but it is practical to define Nc as the number of cycles of stressing until occurrence of a macroscopic fatigue crack of dimensions of the degree that it could be taken up as a subject in fracture mechanics. This is thought to correspond to the time when the propagation mechanism of fatigue cracking has entered its second stage3 . The fatigue crack propagation life can be estimated by fracture mechanics techniques using the stress intensity factor and the fatigue crack growth rate, and applications to various joint portions have been attempted4-8. In such cases, it is necessary to assume the configurations and dimensions of the initial crack, the configurations of cracks during propagation, and the final configurations and dimensions of cracks, and therefore, having a grasp of the initiation of fatigue crack at actual joint portions and subsequent conditions of propagation will be most fundamental for application of fracture mechanics. In the present study, examinations are made of partially-penetrated single-bevel groove joints used for corner welds of box-section truss members. The authors have already clarified the great influence of residual welding stress on the fatigue strength of a joint, and that fatigue crack is initiated with the portion of irregularity at the root of the weld as the starting point9. However, the initiation of fatigue crack and the conditions of propagation had not been adequately grasped. In order to confirm the initiation and propagation of a fatigue crack, (1) the surface of the specimen may be directly inspected with a magnifying glass or microscope, (2) a sample may be collected from the surface of the specimen and inspected by microscope, (3) ultrasonic waves may be employed, (4) marking may be done with * Dr. Eng., Associate Professor, of Dept., Civil