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(a) For a material which obeys power law creep, the creep Incubation time for initiation time) may be written as, (tolary = Identify each term in the equation and outline the key assumptions in the derivation of the equation. Explain why s has a different value under plane stress and plane 6 strain 14 (b) Nickel discs of diameter 250 mm rotating at an angular velocity of 415 rad/s are used in a gas turbine engine. Estimate the time required during service for a creep crack to initiate from a 5 mm Internal defect that exists from the start of life, assuming plane strain conditions. At the operating temperature the material exhibits power law creep & = (%)". with 69 = 45 MPa, » = 13 and 2 is creep strain rate in hand o is stress in MPa. The average grain size of the steel is 50 um and the creep failure strain at the operating temperature under uniaxial conditions is 0.15. The value of I, in the HRR solution for n = 13 is 4.54. Additional information for Question 4(b): The value of C* may be estimated using the reference stress with C Keref Ores o and the reference stress taken to be the maximum principal stress o given by: (3+)pR2 8 where Ris dise radius, vis Poisson's ratio, is the angular velocity in rad/s and p is the density of the material (for nickel, p= 8,900 kg/ml, v0.3). Additional stresses due to er stress concentrations or temperature gradients may be neglected.