Creep-induced residual stress strengthening in a nicalon-fiber-reinforced BMAS-glass-ceramic-matrix composite

The feasibility of inducing a compressive residual stress in the matrix of a Nicalon-fiber-reinforced BMAS-glass-ceramic-matrix composite through a cr eep-load transfer treatment was studied. Specimens mere crept at 1100 degre es C under constant tensile load to cause load transfer from the matrix to the fibers, then cooled under load. Upon removal of the load at room temper ature, the matrix was put into compression by the elastic recovery of the f ibers. This compressive residual stress in the matrix increased the room-te mperature proportional limit stress of the composite. The increase in the p roportional limit stress was found to be dependent upon the applied creep s tress, with an increase in creep stress resulting in an increase in the pro portional limit stress. Acoustic emission results showed that the onset of significant matrix cracking correlated closely to the proportional limit st ress. Changes in the state of residual stress in the matrix were supported by X-ray diffraction results. Fracture surfaces of all specimens exhibited fiber pullout behavior, indicating that the creep-load transfer process did not embrittle the fiber/matrix interface.