A very large plate in an engineering component, which is in continuous operation for 10 hours each day, is subjected to a mean tensile stress of 200 MPa during service. Superimposed on this operating stress are fluctuations arising from vibrations, which have an amplitude of 15 MPa (i.e. total range of 30 MPa) and a cyclic loading frequency of 50 Hz. Nondestructive crack detection techniques, which have a resolution of 0.2 mm in crack size, do not reveal any flaws in the component. If a crack were to exist at the critical location, the operating stress would be normal to the plane of the centrally located, through-thickness crack and small-scale yielding conditions would prevail in the plate. The plane strain fracture initiation toughness of the material from which the plate is made is 100 MPa. The fatigue threshold stress intensity factor range, ?K0, of the material is 3 MPa for load ratio R = 0 and 1.5 MPa for R = 0.85 and can be assumed to be independent of the cyclic frequency.

(a) Which of the following factors is/are most significant in the safe design of the component against catastrophic fracture:

(i) the plane strain fracture toughness, Kic,

(ii) the threshold stress intensity factor range, ?K0, or

(iii) the rate of crack growth in the Paris regime? Why?

(b) Suggest a procedure for the calculation of fatigue life in the plate