Fatigue crack growth in Si−Fe

Fatigue crack growth rates were measured in vacuum in the temperature range from 100° to 250°C. At 200°C and above, crack growth occurs by a ductile mechanism. The ductile crack growth rate is proportional to the stress intensity factor amplitude raised to the 5/2 power, is equal to about one-tenth of the crack opening displacement per cycle, and is not measurably dependent on the peak stress intensity factor in the range measured. Local crack growth occasionally becomes arrested by large scale blunting of the crack tip. The fracture surface has many “river markings” and often adheres to specific growth planes on a scale of the grain size. The overall crack growth is approximately in the plane of maximum tensile stress. Inclusions and grain boundaries are not of importance in the growth mechanism in the range of crack growth rates measured. Growth rates in air at 200°C are at most twice those in vacuum. Although a mechanism, using continuum mechanical concepts, can explain the external features of the ductile crack growth rate, the local features showing a combination of all three modes of straining suggests that the actual process of crack growth must be far more complicated than hitherto realized. At 150°C and below, the ductile growth mechanism is augmented by the wholesale cleavage of single grains or grain clusters. The growth rates at these temperatures are higher and more sensitive to stress intensity factor amplitude.