FATIGUE-CRACK GROWTH OF SM-1240 TIMETAL-21S METAL-MATRIX COMPOSITES AT ELEVATED-TEMPERATURES/

A series of high-temperature fatigue crack growth experiments was cond ucted on a continuous-fiber-reinforced SM1240/TIMETAL-21S composite us ing three different temperatures, room temperature (24 degrees C), 500 degrees C, and 650 degrees C, and three loading frequencies, 10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated alon g a single mode I crack for which the principal damage mechanism was c rack bridging and fiber/matrix debonding, The matrix transgranular fra cture mode was not significantly influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based on the knowledge of the fiber pullout lengths measure d along the fracture surfaces of the test specimens. The average pullo ut length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found to lead to a de crease in the crack growth rate. The mechanism responsible for this be havior is discussed in relation to the interaction of a number of temp erature-dependent factors acting along the bridged fiber/matrix debond ed zone. These factors include the frictional stress, the radial stres s, and the debonding length of the fiber/matrix interface. In addition , the crack growth speed was found to depend proportionally on the loa ding frequency. This relationship, particularly at low frequencies, is interpreted in terms of the development of a crack tip closure induce d by the relaxation of the compressive residual stresses developed in the matrix phase in regions ahead of the crack lip during the time-dep endent loading process.