(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
600oC 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 In =4.66 for n =7.63. Based on the
reference stress approach [50], the fracture mechanics parametersC* can be calculated using