The influence of heat treatment on the tensile strength of single fibre composites (SiC fibre/titanium alloy)

摘要

Different models are used to predict the tensile strength distribution of single fibre composites (SCS-6-fibre coated with Ti-6Al-4V matrix, V{sub}f=0.49). At first the strength distribution of uncoated fibres is analysed with the aid of Weibullstatistics. After this the fibre and matrix stresses in single fibre composites on cooling down from fabrication temperature to test temperature and upon subsequent axial loading are calculated by nonlinear static analysis.An analytical concentric cylinder model is used to determine the stress field in the composite. Using Weibull statistics it is possible to predict the tensile strength distribution of single fibre composites assuming, that composite failure is caused byfibre rupture. A good agreement between a measured Weibull dustribution and a predicted Weibull distribution for the single fibre composite strength showed the validity of the assumption. A heat treatment at 850°C leads to the formation of a TiC reactionlayer between fibre and matrix. With increasing treatment time the thickness of the brittle reaction layer increases (to 3.3μm after 100 h at 850°C), resulting in a decrease of the tensile strength (maximum≈8%). A finite element method is used tocalculate the stress distribution in the composite as a result of a cracked reaction layer. Also in this case the strength distribution can be predicted by calculating the fibre stresses and application of Weibull statistics, again assuming, that fibrefailure leads to composite failure.Good agreement between the calculated Weibull strength distribution for the shortest heat treatment (25 h, reaction layer thickness 1.7μm) and the measured distribution is found. The analysis for thicker reaction layers is at the moment not yet completed.