Fatigue strength of titanium matrix composites depending on fibre volume content and fibre orientation

Abstract

SiC-fibre reinforced titanium matrix composites are marked by exceptional high specific strength and stiffness at temperatures up to 550°C. That is why these materials are considered as promising materials for components of future aeronautic gas turbines. Tensile and fatigue properties have been investigated precisely in the past. The influence of fibre volume content and fibre orientation on the fatigue strength has been investigated less systematically. The tensile strength of titanium matrix composites under loading in fibre direction can be predicted appropriate by the rule of mixture. For fatigue loading the mechanism is more complex; especially in the high cycle fatigue regime. Residual stresses and matrix cracks bridged by fibres are influencing the behaviour significantly. The reinforcing effect of high fibre volume contents diminishes with increasing number of cycles to failure. Four different fibre volume contents between 25% and 58% were investigated experimentally. In the low cycle fatigue region the strengths are marked by the fibre volume content while the strengths in the high cycle fatigue region beyond 100,000 cycles become more and more similar. This behaviour can be understood by an analytical model which will be presented, too. Fatigue loading transverse to the fibre direction leads to a relatively low level of strength which is about one third of the unreinforced matrix alloy. However, many intended applications require high strength in one loading direction. Thus, the transverse fatigue strength is still sufficient. The results of transverse fatigue tests were compared with the longitudinal fatigue tests.