Investigation of the high-temperature mechanical behaviour of oxide-oxide ceramic-matrix composites

Abstract

The adoption of oxide-oxide ceramic-matrix composites (CMCs) for aerospace applications requires a detailed understanding of the relationship between their inherently complex microstructures, mechanical behaviour and fracture mechanisms. To this end, novel approaches using high temperature mechanical testing with in situ X-ray computed microtomography and digital volume correlation were explored. Subsequently, residual stresses were quantified using photoluminescence piezospectroscopy.
Materials with different sintering temperatures based on Nextel^TM 720 fibres displayed average flexural strengths between 55 and 104 MPa when tested at room temperature and 1050 °C. Materials based on Nextel^TM 610 fibres, with different reinforcement architectures, displayed higher flexural strengths between 161 and 226 MPa when tested at room temperature, 800 °C and 1100 °C. Furthermore, the strength retention of a pre-fatigued material based on Nextel^TM 610 fibres was found to be greater than 100%. A categorisation of cracking mechanisms including interfacial cracking, intra-tow cracking and matrix cracking was proposed based on tomographic observations of microstructural fracture phenomena. The observed patterns showed a strong dependency on the materials’ manufacturing parameters, construction and, to a lesser extent, testing temperature. Subsequently, correlations between fracture phenomena and local strains were identified and quantified using digital volume correlation.
To assess the suitability of the digital volume correlation technique for use on oxide-oxide CMCs, multiple sensitivity studies were carried out including repeat scans, virtual translation and virtual deformation, revealing high accuracy and precision.
Using room temperature photoluminescence piezospectroscopy, compressive stresses were found in the fibres of the Nextel^TM 720 CMCs ranging from -312 MPa to -126 MPa. The matrix tensile stress was found to increase considerably in the vicinity of the fibres. A stress range of 45-55 MPa was present in the fibres of the Nextel^TM 610 CMCs and a highly linear decrease in the position of the luminescence response was observed up to 250 °C.

Date of Award	27 Sept 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Martin H H Kuball (Supervisor), Giuliano Allegri (Supervisor) & Dong Liu (Supervisor)