A.R. Boccaccini et al. Materials Characterization 54(2005)75-83 5 Second measurement Temperature(C) Fig 4. Curves representing the thermal expansion of a composite material sample submitted to thermal shock 20 times(47-630C):(a)first neasurement,(b) second meast nt. Heating rate=s K min the intemal friction of the composite [10]. In the The sample subjected to thermal aging (100 h, 700 present study, two samples were thermally cycled with C)presents the most severe damage. This sample did I and 17 cycles, expecting to find microstructural amage in its early stage of development. However, no significant variations of the thermal expansion coefficients of the samples that could be attributed to microstructural damage were found(see Fig. 3) 3.3. Evaluation of the mechanical behaviour by mpact strength tes Fig. 5(a, b) shows images of the fracture surface of as-received sample broken by impact test(impact energy=4 J). Evidence of extensive fibre pull-out can be observed, which is indicative of the favourable 1mm quasi-ductile" fracture behaviour characteristic of this composite material. The samples subjected to thermal shock presented fracture surfaces very similar to those of the as received samples, with evidence of significant fibre pull-out, as shown in Fig. 6. In this investigation, no significant variations in the impact strength of the samples were found even after 21 thermal shocks with △7=630-650°C. Assuming that fibre pull-out depends fundamentally on the properties of the fibre/matrix interface and because thermal shock induces damage mainly in the form of matrix micro- cracking [12], the"quasi-ductile"mechanical behav iour induced by the fibre pull-out phenomenon is no altered under the thermal shock conditions inves tigated. Therefore, the composite material retains its Fig. 5. SEM micrographs showing the fracture surface of an as- received sample submitted to impact test (energy 4 J)at (a) low and high resistance to impact loads, even after having b) high magnification. The typical fibre pull-out effect, character been subjected to thermal shocks of AT=630 istic of these composite materials, is observed.the internal friction of the composite [10]. In the present study, two samples were thermally cycled with 1 and 17 cycles, expecting to find microstructural damage in its early stage of development. However, no significant variations of the thermal expansion coefficients of the samples that could be attributed to microstructural damage were found (see Fig. 3). 3.3. Evaluation of the mechanical behaviour by impact strength tests Fig. 5(a,b) shows images of the fracture surface of an as-received sample broken by impact test (impact energy=4 J). Evidence of extensive fibre pull-out can be observed, which is indicative of the favourable bquasi-ductileQ fracture behaviour characteristic of this composite material. The samples subjected to thermal shock presented fracture surfaces very similar to those of the as￾received samples, with evidence of significant fibre pull-out, as shown in Fig. 6. In this investigation, no significant variations in the impact strength of the samples were found even after 21 thermal shocks with DT=630–650 8C. Assuming that fibre pull-out depends fundamentally on the properties of the fibre/matrix interface and because thermal shock induces damage mainly in the form of matrix micro￾cracking [12], the bquasi-ductileQ mechanical behav￾iour induced by the fibre pull-out phenomenon is not altered under the thermal shock conditions inves￾tigated. Therefore, the composite material retains its high resistance to impact loads, even after having been subjected to thermal shocks of DT=630 8C. The sample subjected to thermal aging (100 h, 700 8C) presents the most severe damage. This sample did Fig. 4. Curves representing the thermal expansion of a composite material sample submitted to thermal shock 20 times (DT=630 8C): (a) first measurement, (b) second measurement. Heating rate=5 K min1 . Fig. 5. SEM micrographs showing the fracture surface of an as￾received sample submitted to impact test (energy 4 J) at (a) low and (b) high magnification. The typical fibre pull-out effect, character￾istic of these composite materials, is observed. 80 A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83