Creep-induced residual stress strengthening in a laminated 0 degrees/90 degrees Nicalon-fiber-reinforced BMAS glass-ceramic matrix composite
High temperature creep-conditioning of a continuous fiber-reinforced ceramic matrix composite, in which the fibers are more creep-resistant than the matrix, should cause the matrix to transfer its load to the fibers. Upon removal of the load after cooling to room temperature, a compressive residual stress should be induced in the matrix by the elastic recovery of the fibers. This compressive residual stress in the matrix should increase the matrix cracking stress of the composite, and hence, improve the reliability of the composite. To test the validity of this creep-load transfer technique in inducing a compressive residual stress in the matrix, laminated 0$\sp\circ$/90$\sp\circ$ Nicalon-fiber-reinforced BMAS glass-ceramic matrix composite specimens were crept in tension at 1100$\sp\circ$C under constant stress, cooled under load to room temperature, and then unloaded. The applied creep stress ranged from 35 MPa to 90 MPa, and the creep times varied from 12 hours to 100 hours. The resulting room temperature tensile stress-strain behavior was determined and acoustic emission activity was monitored during the tensile test to detect cracking activity. The residual stress in the matrix was independently measured with X-ray diffraction. The creep-load transfer results showed that the increase in the proportional limit, i.e. the onset of significant matrix cracking, of the creep-conditioned specimens was attributed to the effectiveness of the creep-load transfer treatment in inducing a compressive residual stress in the matrix. The X-ray diffraction data confirmed that the increase in the observed proportional limit of the creep-conditioned specimens was due to an increase in the compressive residual stress in the matrix. The compressive residual stress in the matrix was found to be dependent upon the applied creep stress; however, it was not significantly affected by the creep time. Fractography revealed that the creep-conditioned specimens exhibited similar fiber pull-out behavior to that of the as-received specimens, indicating that the creep-load transfer treatment did not embrittle the fiber/matrix interface, and hence, the "graceful" failure behavior of the composite was preserved.
0548: Mechanical engineering