A ceramic has a fracture toughness of Kic=1.5 MPam. Its strength is controlled by random semi-circular flaws of radius a distributed over the surface. The mode-I stress-intensity factors for these flaws is given by K, = 0.50, where o is the stress in the ceramic. The fracture strength of the ceramic is controlled by these surface flaws; it follows a Weibull distribution with m = 10, So = 200 MPa (with A. = 1000 mm). The yield strength of the ceramic is 4.0 GPa. In the presence of moisture, the ceramic exhibits stress-corrosion cracking, with the crack velocity being a function of K/: da dt =1.0 10 *K m/s (where Kris in MPam) The threshold-stress intensity factor, below which crack growth does not occur is given by Kth 0.5 MPam. In the questions below, ceramic disks are tested in biaxial tension. There is a circular surface area of 15,700 mm, in which the stress is uniform and biaxial. - a) If the disks are subjected to a proof test of 125 MPa in a dry environment, calculate the percentage of disks that fail the proof test. b) Calculate the minimum time before any of the remaining specimens fail in a subsequent test at 50 MPa in a moist environment. c) If the disks were not subjected to any proof test, calculate the percentage of disks that will exhibit no crack growth during the course of testing at 50 MPa in a moist environment.