Damage Evoluation and Dynamic Mechanical Properties in a SiC Fibre Reinforced Ti Alloy Composite

Abstract

Damage evolution in a SiC fibre reinforced Ti alloy composite (SCS-6/Ti-6Al-4V) has been assessed by modulus loss, change in internal friction (mechanical damping) and acoustic emission (AE). Damage was introduced into unidirectionally reinforced unnotched specimens by four point bending using constant cyclic amplitude loading. Dynamic Young's modulus and internal friction were measured by a flexural resonant method after a predetermined number of cycles, and AE signals were also monitored during the tests. Damage induced in the composites changed progressively primarily as a function of the applied maximum stress levels (σ max ). Fibre fracture dominated at σ max values greater than 0.5 σB (where σ B is the flexural strength of the composite). High amplitude AE signals associated with fibre fracture were observed, and internal friction measurements increased rapidly just before failure of the specimen. In contrast, matrix cracking dominated at σ max values lower than 0.35 σ B . As matrix cracks bridged by intact fibres) grow in these composites, internal friction measurements increase. Small amplitude AE signals were observed to be associated with matrix crack growth. Also, fibre fracture occurred in the bridged zone at σ max values between 0.35 and 0.5σ B . In all tests, the Young's modulus decreased by only 1% of its initial value until just before failure of the specimen. However, internal friction measurements increased rapidly prior to catastrophic failure of the specimen at all stress levels investigated in this study. This result shows that an increase in internal friction is related to damage accumulation in the composites, and suggests that measurement of changes in internal friction may be applicable to the assessment of damage in such composites.