Fatigue life evaluation model of friction self-piercing riveted aluminum alloy sheets based on the structural stress method

Abstract

Friction self-piercing riveting (F-SPR) provides an enabling technology for achieving high-efficiency riveting without prefabricated holes in aerospace. Hence it is of great significance to evaluate the fatigue life of the novel F-SPR joints under different loads. For this purpose, three types of F-SPR joints are designed to account for different fatigue loaded states, including high-load stress state joints, zero-load stress state joints and low-load stress state joints. Fatigue tests with different load levels are conducted to acquire the fatigue life data of F-SPR joints. The typical failure location of the F-SPR joints is determined as the region surrounding the rivet tip in the lower sheet. The structural stress on the critical cross-section of F-SPR joints is analytically derived via structural stress theory. A simplified model with beam-shell element of the F-SPR joint is proposed to calculate the structural stress. The contribution of different structural stress components to the effective structural stress is quantified, and the normal structural stress is identified as the critical factor for all the joints. Finally, the master S-N curve for different joints is fitted, achieving the unified life evaluation for F-SPR joints. Meanwhile, the high correlation of the fitting results proves the validity of the proposed model. The proposed method is useful for the analysis of both F-SPR and other spot-welded structures.