Fatigue Damage And Life Prediction Based On Dynamic Response Analysis For Spot-Welded Joints

Resistance spot-welding is the widely used technology in modern machinery manufacturing industry, such as structures in automotive, locomotive, subway works, especially the automotive manufacturing. The fatigue crack is hardly detected when the spot welds are fatigued, and mechanics analysis of spot welds is hard to carry out while spot welds usually experience complicated stress state. How to accurately predict the fatigue life of spot welds is still a difficult issue to research.As a non-destructive evaluation method for structures, modal analysis technology has become one of the important effective methods for failure analysis, and very effective. This thesis studied the damage and life prediction of spot welds with the modal analysis method from experimental and theoretical aspects. The relation between fatigue damage and natural frequency, as well as life prediction model was established by experimental results with damage mechanics, modal analysis and finite elements analysis. Fatigue lives of spot welds were predicted with the local stress-strain approach by using finite element analysis results. The contents of the study including: Firstly, two types of tensile-shear spot-welded specimen were tested under constant amplitude cyclic loading, obtained fatigue lives under certain loading, measured the changes of natural frequency with fatigue life in the fatigue process, and observed the shape and location of fatigue crack. The results showed that the fatigue crack initiates at the nugget periphery, the shapes of cracks are typical semi-elliptical shape. Natural frequency shows a decrease tendency as fatigue life increase, initially, at the beginning of the fatigue life, natural frequency almost relatively unchanged, and later, more noticeable changes in the natural frequency begin to occur, and change dramatically at the end of fatigue life, shows an accelerated rate in total fatigue process. However, damping ratio does not show explicit changes in the total fatigue process.Secondly, fatigue crack propagation behaviour was studied based on the dynamic response simulation by the finite-element analysis. The changes of natural frequency in the crack propagation process are simulated. Comparison of the first several mode natural frequencies both from finite element analysis and experimental results showed that the first mode natural frequency is the most sensitive to the existing of cracks, so changes of the first mode natural frequency can be used to describe the fatigue damage extent. Based on the damage represented by crack area, a damage parameter was established using natural frequency change ratio. The damage evolution equation was established with the damage described by changes of natural frequency, as well as life prediction equation and life prediction model using natural frequency measured at certain loading cycles. Fatigue lives under constant loading were predicted using natural frequency data at a certain life stage, with damage evolution equation, giving agreeable fatigue life prediction results.Thirdly, the changes of natural frequency were measured with fatigue life in fatigue process under two-level loading, multi-level variable amplitude loading and random history loading. Using the defined damage, damage were analyzed under two-level loading, multi-level variable amplitude loading and random history loading, and fatigue lives were predicted as well. Damage accumulation by linear rule under two -level loading showed the effect of notch for spot welds nugget root. And the effect of loading sequence on accumulative damage was shown under two level loading, but not distinct, and it also showed a similar behavior for the damage accumulation under two-level and multi-level loading. The results of life prediction showed that the proposed life prediction model can well used for life prediction under random loading. Finally, the nugget root is taken as a sharp notch with two slope faying faces as practical specimen, using the relation between hardness and material cyclic properties, finite element analyses were carried out for spot welds. Using the relation between hardness and material fatigue properties, with the stress strain results obtained from finite element analysis, fatigue lives of spot welds under different loading levels were predicted by local stress strain approach.