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.
Spalling experiments with alumina bars performed at thperatures up to 1500°C have been conducted in a new apparatus. A cylindrical uninstrumented specimen is l o c a f d in an open-end furnace and dynamically loaded in tension-by the reflection of a compressive pulse from its free end. A bridging piece of the same impedance covers the transition zone between the measuring rod and the specirneh, which is positioned in the homogeneous part of the thermal field within the furnace. The data acquisition c o n s m of the measuring of the incoming and reflected pulses in the transmitter, the temperature distribution in the furnace, and the positions of the spalling sites in the specimen. The evaluation procedure takes into account the high-tmperature corrections, due to temperarure-induced drop in the wave velocity and impedance of the bridging piece,'This results in the change of the dispersion and the shape of the pulse. The results of the experiments show that the suength drop with the temperature growing up to 1500°C.Resum6 : Des experiences de rupture dy~iamique de barres d'alumilie effectutcs h des tempCratures jusqu'h 1500°C ont kt6 ronduites 3. I'aide d'un nouvel appueil. L'iprouvette cylindrique est placie dans un fourneau et est charger dynamiquement sous rclisloli par la retlex~on d'u~lc impulsio~i de compression. L'acqu~s~tioii des caractt!ristiques colisiste d'une part en le calcul des impulsions transmises et rCtl2chies dans le tr;lnsmelteur et la distribution de 1; 1 temperature dans le fourneau, d'auue p u t en l a mesure des positions des sites de rupture dynmique dans I'Cprouvette. La procedure d'evaluauon comprend les corrections pour de ul\s haurcs temperatures. h cause du changement Lhermiquc qui afiecte la vitesse des ondes et Itimpedance des connexions. P;lr consequent. 1;1 d~spcrsion et 1;1 forme de I'impulsion son[ nlodifi9es. Les resultats expkrimentaux monuent que la resist:mce diminue simultanemrnt avec I';lugrnent;~tion de 1;) ternptrature jusqu'i 1500°C.
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