THE ROLE OF THE INTERFACE IN THE INITIATION OF FATIGUE CRACKS IN SCS-6 TITANIUM MMCS

The role of the interface in the initiation of fatigue cracks is discu ssed for the case of two unidirectional monofilament fibre composites, Textron SCS-6/Ti-6-4 and SCS-6/Ti-15-3. Parallel-sided fatigue specim ens of the materials in the 'as-fabricated' condition were tested at a load ratio of 0.1 in laboratory air at room temperature. Surface crac k growth was monitored using an acetate replication technique and frac ture surfaces were examined in the electron microscope. Examination of metallographic sections through specimens tested at various stress le vels has allowed the identification of crack initiation mechanisms. Cr acks were found to grow from breaks in the interfacial reaction layer and from the unfractured reaction layer. These cracks were not associa ted with fibre damage. In the SCS-6/Ti-6-4 material, cracks growing fr om fibre breaks were observed to propagate at a faster rate than those from the reaction layer. In SCS-6/Ti-15-3, cracks growing from the in terface were seen to propagate at a comparable rate to those from fibr e breaks.