The mechanical behavior of the novel fiber CeraFib75 after various thermal exposures is examined. This fully crystalline mullite fiber was developed to exceed the thermal stability of commercially available oxide fibers. Therefore, heat treatments at temperatures ranging from 1000°C to 1400°C for 25 h were performed and results compared to the well-established Nextel TM 720 fibers. Mechanical characterization was realized with bundle tensile tests using acoustic emission sensors to determinate the fiber failure distributions. Investigations showed that the initial fiber microstructure of mullite grains with traces of alumina transforms starting at 1200°C. Changes include dissociation of the alumina-rich mullite phase and grain growth. Thus, strength reduction is measured as a result of these microstructure transformations. Remarkably, at 1400°C, fibers become more fragile and Weibull statistics can no longer describe the failure distribution. A relation between the distribution shape and the load redistribution capability of fibers is suggested. This is more pronounced for Nextel TM 720 fibers, which present much bigger grains and retain only 10% of their original strength. However, CeraFib75 fibers are more stable and exhibit a strength retention of 50% at the same conditions, which is attributed to the higher amount of mullite phase.

The influence of hygrothermal conditioning on mechanical properties of Carall laminates have been investigated by tensile and compression tests. The environmental factors can limit the applications of composites by deteriorating the mechanical properties during service. The importance of temperature at the time of conditioning plays an important role in environmental degradation of such composite materials. In this work, the results show that for carbon fiber/epoxy composites tensile and compression values decrease after hygrothermal conditioning. However, the changes on mechanical properties of Carall are negligible, regardless the hygrothermal conditioning.

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