A.R. Boccaccini et al. Materials Characterization 54(2005)75-83 a), b), c)Samples in the as-received condition. d), e) Samples thermally aged for 100 hs at 600 and 700 C, respectively 20-300°C ample mechanically pre-stressed above the fracture load j, k) Samples mechanically pre-stressed above the fracture load subjected to thermal shock. 4 3,5 a) b) c) d) e) f g) h) i) 1) k) Fig. 2. values obtained for the thermal expansion coefficient in the range 20-300C for all composites investigated (as-received and after thermomechanical loading, as indicated in the legend): (O) first measurement, (O)second measurement, (O) third measurement. The error average in the measurements was±1×10-7K-1 significant oxidation of the interface at those inter-wave"in the thermal expansion curve recorded mediate temperatures. Matrix microcracking occurs during the first measurement of ac(Fig. 4) because in a quenching test, the sample surface is Regarding the samples thermally cycled in air,a brought to the temperature of the cooling medium previous study [10] has shown that microstructural rapidly, whereas the interior of the samples remains at damage can be mainly attributed to partial oxidation high temperature. This temperature gradient creates of the interfaces and to softening of the glass matrix internal stresses, which are tensile at the surface and during the exposure to a high-temperature oxidising compressive in the interior. With an increasing environment. In the previous study [10, thermal number of quenching cycles, the tensile stresses at cycling in air from 700C to room temperature the surfaces can reach a critical value sufficient to resulted in the generation of microstructural damage microcracks in the matrix. In this investigation, which was detected after 77 cycles by the simulta- nulated residual stresses are detected by the neous decrease in Youngs modulus and increase 2 200300 Temperature(C) Fig. 3. Curve representing the thermal expansion behaviour of the composite material investigated in the initial state(as-received). Heating rate=s K minsignificant oxidation of the interface at those inter￾mediate temperatures. Matrix microcracking occurs because in a quenching test, the sample surface is brought to the temperature of the cooling medium rapidly, whereas the interior of the samples remains at high temperature. This temperature gradient creates internal stresses, which are tensile at the surface and compressive in the interior. With an increasing number of quenching cycles, the tensile stresses at the surfaces can reach a critical value sufficient to create microcracks in the matrix. In this investigation, the accumulated residual stresses are detected by the bwaveQ in the thermal expansion curve recorded during the first measurement of ac (Fig. 4). Regarding the samples thermally cycled in air, a previous study [10] has shown that microstructural damage can be mainly attributed to partial oxidation of the interfaces and to softening of the glass matrix during the exposure to a high-temperature oxidising environment. In the previous study [10], thermal cycling in air from 700 8C to room temperature resulted in the generation of microstructural damage, which was detected after 77 cycles by the simulta￾neous decrease in Young’s modulus and increase in Fig. 2. Values obtained for the thermal expansion coefficient in the range 20–300 8C for all composites investigated (as-received and after thermomechanical loading, as indicated in the legend): (o) first measurement, (n) second measurement, ( R ) third measurement. The error average in the measurements was F1
107 K1 . Fig. 3. Curve representing the thermal expansion behaviour of the composite material investigated in the initial state (as-received). Heating rate=5 K min1 . A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83 79