Fatigue Damage And Life Prediction Based On Dynamic Response Characteristics For Spot-Welded Joints Under Variable Amplitude Loading

In this paper, based on fatigue test and dynamic response measurement under variable amplitude and random loading, relationship between the variable ratio of natural frequency and current life cycles was studied for single spot-welded joint, made of galvanized low carbon steel materials (Steel grade as BJXH50) for automotive body supplied by Beijing Benz– Daimler Chrysler Automotive Co. Ltd, and then a fatigue damage parameter and fatigue life prediction methods were proposed. Meanwhile, the change characteristics of natural frequency were simulated by using three-dimensional (3D) finite element analysis for spot-welded joints during the fatigue failure process. By comparing between the experimental and simulated results, it is showed that the fatigue crack propagation process and the change of natural frequency can be simulated with the 3D finite element analysis for spot-welded joints.Firstly, fatigue loading test was carried out for spot-welded specimen by the MTS810 test machine, and data of frequency were extracted during the fatigue process with dynamic response measurement. Then crack initiation and propagation for spot-welded specimen were studied in the aspect of microstructures, hardness, and fracture analysis.Secondly, a fatigue damage parameter was proposed by the change characteristics of frequency during the fatigue process under variable amplitude loading. Experimental results showed that the fatigue damage process for spot-welded joints can be described by the proposed fatigue damage parameter.Thirdly, finite element models were set up based on specimen shape and the rules of crack initiation and propagation, and simulated characteristics of change in frequency by using the finite element method for spot-welded joints. It showed that 3D finite element simulation results have good agreement with the experimental results and can reflect real changes of the dynamic response frequency for the specimen during the fatigue damage process.Fourthly, fatigue life prediction model was proposed based on the relationship between frequency range variable ratio (FRVR) and current life cycles for spot-welded specimen under variable amplitude loading.Finally, on the basis of finite element simulation, the frequency range falling velocity model was developed under random loading in according with the relationship between the fatigue crack and the natural frequency, and used to estimate fatigue life for spot-welded specimens. The results showed that the proposed frequency range falling velocity model can describe the characteristics of the frequency, and have a good agreement between the calculated and experimental results. In addition, with the fatigue damage parameter proposed above, the formula of equivalent load range was established for random load statistics. Based on the software Matlab, random load information was counted and equivalent load range was calculated with program.