B.X. Xu

Investigation of residual stress by the indentation method with the flat cylindrical indenter

Experiments and numerical simulations have been carried out to study residual stress of copper specimens by the identation method with a flat cylindrical indenter. Copper specimens were annealed at different temperatures for 35 min to obtain different residual stress levels. The experiments carried out on these specimens demonstrated the influence of residual stress on indentation behavior. The influence of annealing temperature on the elastic-plastic transition region is quite obvious. A method has been presented to determine material properties, such as elastic modulus and Poisson ratio. This method can also be applied to determine residual stress with the assumption of knowing the yield stress in advance. The advantage of this method is that it can avoid calculating the contact area. In the finite element modeling (FEM), residual stresses on copper specimens are simulated by preapplying stresses. The influence of residual stress on the indentation load-depth curves has been studied by FEM. There is good agreement between experimental and FEM numerical results. A numerical method has also been presented to determine residual stress. In addition, Mises stress and plastic distribution ahead of the indenter have also been studied to help us further understand the influence of residual stress.

Analysis of damage during bending creep tests

The bending creep test (with overhang beam geometry) is investigated, incorporating the important contribution of damage. A simplified analytical model shows that the evolution of damage is analogous to the classic K–R creep damage law. Finite-element simulations incorporating the creep damage law are employed to investigate the behaviour of the bending creep specimen. Good agreement of the lifetime is found between theoretical analysis and FEM simulation. We also propose several useful methods of determining the creep damage parameters directly from the bending creep tests. The results may shed some light on damage evolution during bending creep, as well as providing an efficient way of determining material damage characteristics.