(11)

where (r , θ ) are the polar coordinate at the crack-tip;\({\tilde{\sigma}}_{\text{ij}}\) is a dimensionless stress function depending on m or n and θ ; for Grade 91 at 600^oC considered in this paper,σ₀ =311 MPa; αε₀ =0.002;\(\dot{\varepsilon}\)₀ = 6.47x10^-3 1/h; n =7.63; and the integration constant I_n =4.66 for n =7.63. Based on the reference stress approach [50], the fracture mechanics parametersC^* can be calculated using