FATIGUE CRACK-GROWTH IN A FIBER-REINFORCED TITANIUM MMC AT AMBIENT AND ELEVATED-TEMPERATURES

Fatigue crack growth rates in an SCS6 fibre-reinforced Ti-15-3 based metal-matrix composite have been measured at a number of different temperatures, frequencies and load ratios. In specimens fatigued under a constant load range, crack growth rates decrease with increasing crack length, despite an increase in the nominal applied stress intensity range (DELTAK). At room temperature the crack arrests after growing for < 1 mm when the initial nominal DELTAK is 16 MPa m1/2, whereas at elevated temperatures the deceleration of crack growth is interrupted by discrete events, which lead to instantaneous rises in propagation rate of an order of magnitude or greater. These events are associated with the failure of fibres, and crack arrest does not occur at elevated temperatures (200-500-degrees-C). In general, fatigue cracks grow faster at higher temperatures, higher load ratios and lower frequencies.