Spring(s) in harsh environments: micromechanical modelling of UHTCMCs

The strength retention of ultra-high temperature ceramic matrix composites (UHTCMCs) based on ZrB2-matrix reinforced with 45 vol% of unidirectional continuous carbon fibers was investigated after inducing thermal and indentation damaging. The results proved that UHTCMCs kept their load bearing capability in both cases, and showed damage insensitivity although tested in fully matrix-dominated loading configuration (off-axis configuration). The thermal damage accumulation and residual stresses were evaluated through dilatometric analysis, studying the thermal mechanical hysteresis. The analysis carried out allowed to extrapolate the Young's modulus of the matrix, which steadily decreased from 195 GPa to 24 GPa. This decrease was ascribed to the matrix cracking induced by thermal cycling between RT and 1300 °C. On the other hand, the analysis suggested that carbon fibers kept their Young's modulus constant at 780 GPa. As last point raised by dilatometric analysis, residual stresses due to t thermal expansion mismatch between matrix and carbon fibers let us to justify the Young's modulus of 230 GPa, which cannot be explained with the so-called "rule of mixtures", generally valid and widely used in the composite science. Finally, the investigation of 10 kg Vickers imprints allowed a better understanding of the correlation between residual stresses and mechanical behavior.