FRACTOGRAPHIC INTERPRETATION OF FAILURE MECHANISMS IN TITANIUM MATRIXCOMPOSITES

Titanium matrix composites (TMC) offer a combination of good mechanica l properties and high temperature durability that make them attractive candidate materials for advanced engine components and high temperatu re structural applications. In such applications the material will be subjected to changing mechanical loads and temperature fluctuations, r esulting in complex stress states within the constituents of the compo site, This study examines how the various loading conditions on the TM Cs are reflected in the fracture behaviour to gain insight into the da mage mechanisms active in these materials. A fractographic study was c onducted on several different TMC specimens, fabricated from Ti-15-3 a nd Timetal 21S alloys and SCS-6 fibres, that have been subjected to va rious thermomechanical loading conditions. The analysis showed that th e Ti-15-3 composites were more susceptible to damage during sustained load at elevated temperature than the Timetal 21S composites. For both materials, striations only appear during elevated temperature fatigue when the residual processing stresses are relieved. During the Generi c Hypersonic Flight Profile (GHFP) tests, the higher temperatures of t he Mission I profile induce more damage in the Timetal 21S composites. The striations appear in the composites containing centre holes, unli ke the unnotched specimens, indicating that the combined effect of str ess concentration due to the hole and an underdeveloped fibre bridging zone may have resulted in crack closure.