Fatigue fracture of tungsten monofilament-reinforced monocrystalline copper matrix composites

Studies on fatigue fracture have been conducted on tungsten monofilament-reinforced monocrystalline copper composites. Fatigue cracks were found exclusively to initiate at persistent slip bands (PSBs). After crack initiation, the propagation of the cracks persists in stage I, i.e. along the PSBs, for most of the fatigue life of the composite, which is an unique phenomenon of the composite caused by fiber-load bearing during cyclic deformation. Eventually, the fatal crack approaches and breaks the fiber. Soon after the fiber breaks, the stage I crack transforms into a stage II crack which, as usual, occupies a small percentage of the fatigue life at long life. The fatigue life of the composite is about the same as that of pure metal under plastic strain control. This result really indicates, on the basis of the Coffin–Mansion Law, superior life behavior on the part of the composite because the composite has less ductility than the pure metal. On a stress basis, the composite is clearly superior to the base metal. The superior fatigue behavior of the composite can be attributed to the following two factors: (1) Delay in the formation of a fatal crack because of the more uniform distribution of the PSBs; (2) maintenance of the crack growth in stage I for most of the fatigue life, through support by the fiber.