Enhanced fracture energy and effects of temperature at spalling in ceramics: continuum damage modelling

Abstract

Enhanced fracture energy losses at spalling and the temperature dependence of the spalling strength of alumina ceramic bars are analysed on the basis of the experimental tests conducted both in room temperature and within the temperature range up to 1500°C at strain rates of some 500 s )1 . The experimental method and the measurements are ®rst shortly outlined. The mechanical response of ceramic bars is modelled then as a heterogeneous distribution of brittle-elastic mesoelements undergoing continuum damage at the known strain history, corresponding to that registered in the experiments. The mesoelements are characterised by the values of initial damage randomly¯uctuating within a given band-width superposed on a deterministic distribution, which corresponds to the fabrication conditions of the ceramic bars. The model has been tested in the evaluation of room-temperature experiments, its parameters: the average value of the initial damage, Young's modulus of the undamaged material and the energy absorption capacity in continuum damage are taken from the calibration ®tting the experimental data. The registered energy losses at spalling, which exceed the static values of fracture energy by almost an order of magnitude, can be explained thus by the enhancement of the dissipation due to bulk damage, which is computed on the basis of the above parameters. The temperature change of the Young's modulus of the matrix material is taken as corresponding to the measured change of the uniaxial wave velocity in the bar, and corrected by the temperature change of the mass density. The analysis of the model shows that the drop in the spalling strength of the specimens with the increase of the temperature is phenomenologically related to the falling energy absorption capacity within the continuum damage mechanism. An explanation of this phenomenon is attempted, based on the grain-sizerelated mechanisms of the microfracture from pre-existing intergranular¯aws distributed over the bulk of ceramics.

Effect of material’s randomness on scaling of crack propagation in ceramics

“…The latter is a temperature-dependent material property linked to the process energy of damage evolution. The magnitude of this parameter was obtained from the treatment of the experimental data on dynamic failure of alumina at various temperatures in the interval from 20 to 1500 • C, for details see Najar et al (2000).…”