Abstract: On the basis of the dynamic response measurement during the fatigue process, fatigue damage and life prediction are studied for single spot-welded joint made of galvanized low carbon steel materials (Steel grade as BJXH50) for automotive body under variable amplitude loading. Fatigue test is carried out under variable amplitude loading，and the data of dynamic response frequency is extracted by measuring dynamic response during the fatigue damage process. The characteristics of natural frequency change are analyzed during the fatigue process under constant and variable amplitude loadings. The natural frequency ratio (NFR) is defined，and the damage is denoted by the NFR. It is proposed that the effect of load level is reflected by the NFR and current cycles. By the NFR and current cycles, fatigue life prediction model is proposed, which can avoid errors generated from load spectra analysis and damage calculation. The results show that good agreements are achieved with the calculated fatigue life.

Key words: Dynamic response, Life prediction, Natural frequency ratio, Spot-welded joints, Variable amplitude loading