Impact of residual stress on thermal damage accumulation, and Young's modulus of fiber-reinforced ultra-high temperature ceramics

Highlights: • Thermal cycles induce matrix cracking which decreases the Young's modulus of the ZrB₂-based matrix from 195 GPa to 24 GPa. • The longitudinal thermal dilatation of the composites decreases by thermal cycling and approaches that of bare carbon fibers. • Contrary to linear elasticity the composite Young's modulus is affected by residual stresses even in absence of micro-damage. • A new equation is proposed to account for residual stresses effect and damage frequency on Young's modulus. Ultra-high temperature ceramic matrix composites (UHTCMCs) based on ZrB₂-matrix reinforced with 45 vol% of unidirectional continuous carbon fibers are studied through the thermal mechanical hysteresis in order to investigate the thermal damage accumulation. The analysis carried out allowed to extrapolate the Young's modulus of the matrix from thermal expansion measures. It was found that the initial matrix Young's modulus of 195 GPa steadily decreases by thermal cycling the samples between RT and 1300 °C as a consequence of matrix cracking. On the other hand, the analysis suggested that carbon fibers keep their Young's modulus constant at 780 GPa. Finally, the residual stresses due to the different coefficient of thermal expansion between matrix and carbon fibers are discussed and let 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.