Weakest-link failure prediction for ceramics

Weakest-link failure prediction for ceramic components is usually based on strength data obtained from 3- and 4-point bend tests. However, in the route from test specimen to actual component several pitfalls are encountered. Firstly, the bend tests themselves do have their problems. Secondly, there is the problem of the interpretation of the uniaxial strength data in terms of a failure criterium for multiaxial stress states, for which at this juncture no universally applicable model is available. In this contribution the difficulties encountered will be summarized. Besides alternatives will be mentioned which are based on microstructural insight and anisotropic deformation models

Thermomechanical modelling of laser surface glazing for H13 tool steel

A two-dimensional thermomechanical finite element (FE) model of laser surface glazing (LSG) has been developed for H13 tool steel. The direct coupling technique of ANSYS 17.2 (APDL) has been utilised to solve the transient thermomechanical process. A H13 tool steel cylindrical cross-section has been modelled for laser power 200 W and 300 W at constant 0.2 mm beam width and 0.15 ms residence time. The model can predict temperature distribution, stress–strain increments in elastic and plastic region with time and space. The crack formation tendency also can be assumed by analysing the von Mises stress in the heat-concentrated zone. Isotropic and kinematic hardening models have been applied separately to predict the after-yield phenomena. At 200 W laser power, the peak surface temperature achieved is 1520 K which is below the melting point (1727 K) of H13 tool steel. For laser power 300 W, the peak surface temperature is 2523 K. Tensile residual stresses on surface have been found after cooling, which are in agreement with literature. Isotropic model shows higher residual stress that increases with laser power. Conversely, kinematic model gives lower residual stress which decreases with laser power. Therefore, both plasticity models could work in LSG for H13 tool steel