Fracture of welded joints under impulsive loads by a local damage criterion

The broad aim of this work is to introduce a method for the evaluation of the strength and fracture resistance of welded joints under impulsive load, using a local criterion approach integrating damage softening and thermal softening, and accommodating strain rate effects. Within the field of continuum mechanics a local criterion is used in a simulation of the damage occurring at a crack tip. Basically, this type of criterion states that a given piece of metal will break when the stress and strain satisfy a certain function. The local criterion analysis is based on local mechanical values. Local criterion concepts are discussed in Chapter 3 and they are fundamentally different from other common methods such as the K-method or the J-method, which are normally based on energy release rate considerations. Furthermore, previous works published on impact-induced fracture typically did not take into account important physical phenomena such as viscoplasticity, strain-rate effects on damage, and kinematic hardening. In the past, dynamic fracture of welds has been simulated with limited models, which were developed under quasistatic assumptions and without thermal effects. The materials of interest are assumed to be initially isotropic.; This dissertation has two major contributions. The first is the development of two constitutive models integrating damage via a local criterion with thermal softening associated with rate effects, while also accommodating kinematic hardening. The second is the application of this theory to dynamic fracture of weldments.; The developed models have been implemented into the finite element code LS-DYNA, using the code's user interface. The code calculates local damage in each element as a function of stress state and plastic strain increments, allowing discrete separation of the elements. That means that, when an element fails, it is deleted from the subsequent calculations. The predicted separation process in the weldment is visually the same as the observed crack propagation in experimental results. This element separation criterion gives rise to mesh dependence when performing numerical analyses. This issue is also discussed in this dissertation.