Determining mode-I cohesive parameters for nugget fracture in ultrasonic spot welds

Ultrasonic spot welds have been used as a model system to investigate how to determine the mode-I cohesive parameters associated with interfacial fracture of a spot weld. Numerical analyses indicated that, while multiple combinations of the two cohesive parameters (characteristic strength, σ̂ , and toughness, I ) could result in virtually indistinguishable behaviors for individual geometries, only a single pair of parameters can provide a unique set of behaviors for different test geometries. This provides the basis for determining the cohesive parameters by comparing numerical predictions to experimental observations. In particular, a direct uniaxial tensile test was found to be particularly useful for measuring the characteristic strength of an ultrasonic weld. With the characteristic strength known, the toughness of the weld was determined by fitting numerical predictions to experimental observations of the load–displacement curves obtained from T-peel specimens bonded with the ultrasonic weld. These two parameters were then used without modification to predict the performance of welded U-peel specimens. The numerical predictions for this third configuration were in excellent agreement with the experimental results, verifying that it may be possible to use cohesive-zone parameters to predict the behavior of different geometries of spot welds formed under nominally similar conditions.