A Modified Stress Severity Factor Approach For Lifetime Prediction Of Joints

Because there are numerous joints in the aerospace structures, the joint pontes become the main area where fatigue damage happens. The damage of the joints will lead to the loss of the load-carry function of the structure, which means that the life of the joints determines the life of the aircraft, therefore a more accurate prediction of the fatigue life of the joints is of great significance. Based on the research achievements of stress concentration at the edge of the hole in the previous literature, an improvement of the stress severity factor (SSF) approach which is used for the fatigue life analysis of joints were accomplished in this paper.Taking two-dimension finite element method, the stress distribution at the edge of the hole in the joints was obtained. According to the phenomena of fatigue damage, the extrusion stress concentration factor at the position which is vertical to the overall load direction is acquired by a modification to the factor at the maximum stress position given in the stress severity factor approach. Consequently, a revised formula used for the extrusion stress concentration factor fitting was obtained.Furthermore, by using three-dimension elastic finite element contact algorithm, the stress distribution at the edge of the hole was also analyzed. Analogously, the extrusion distribution factor at the position vertical to the overall load direction is acquired by considering the conversion coefficient between two-dimensional and three-dimensional stress concentration. Consequently, a revised formula used for the extrusion distribution factor fitting was obtained.Making use of the modified extrusion stress concentration factor, extrusion distribution factor and conversion factor between two-dimensional and three-dimensional stress concentration, a revised stress severity factor approach was obtained. Numerical examples demonstrate that the result using the revised approach is much closer to the experimental result. Moreover, an improvement to the equivalent SSF approach which is used for fatigue life prediction of complex joints was also taken. A fatigue life estimation of the joint pontes of an aircraft was carried out by virtue of the revised facor and equivalent SSF approach. The calculation result is quite satisfactory.