Fatigue crack growth in a Ti-15-3 matrix SCS-6 fibre metal matrix composite under tension-tension loading

The fatigue crack growth characteristics of a Ti-15-3 matrix/SCS-6 fibre composite containing an unbridged defect under tension-tension loading were investigated as a function of load range, load ratio, temperature, and specimen size. The results obtained and trends observed were compared with earlier data for the same composite tested using three point bending. There were no major mechanistic differences between crack growth under the two loading configurations, and the effects of load range, load ratio, and temperature were similar in each case. Variations between the results of tests using different loading configurations but identical starting conditions can be explained by differences in the change of nominal compliance with crack extension. The results demonstrate that fatigue cracks can behave in a range of different ways in this composite, from complete arrest at low stresses to rapid propagation through the specimen at high stresses. Both load range (which drives matrix crack propagation) and maximum load (which determines whether fibre failure occurs) are important in determining fatigue behaviour. Testing at an elevated temperature (350 degrees C) significantly reduced fatigue life and promoted premature fibre failure. The absolute distance of the the crack tip from the unbridged notch was more important in determining matrix crack growth rates than was the overall crack length relative to the specimen width (and hence the nominal value of applied stress intensity factor range). (C) 1996 The Institute of Materials.