Nonlinear stress intensity factors in fracture mechanics and their applications

A new parameters for characterization of the crack growth resistance for a power law hardening materials and structures under normal and elevated temperature are introduced in the form of plastic and creep stress intensity factors. A numerical method is proposed for estimating the governing parameter of the elastic-plastic and creep crack tip fields in the form of an In-integral along the through-the-thickness straight and curved crack fronts. Equations are also proposed for calculating the In-factor at both the deepest point along the crack front and the crack tip on the surface. Static and fatigue crack growth and in-plane and out-of-plane constraint effects are studied through experiments and computations for the material different properties. It is further demonstrated that for moderately large-scale yielding conditions or plastic deformations, the fracture process can be controlled by the single parameter KP based on the elastic-plastic numerical solutions and, therefore, is reflected in the influence of the cracked body geometry and loading conditions. Keeping in mind the purpose of practical application of the introduced nonlinear stress intensity factors, an engineering approach to the prediction of residual lifetime of cracked steam turbine rotors components which operate at maintenance under cyclic loading conditions is proposed. Using the nonlinear stress intensity factors approximate estimations of carrying capacity are presented for the different stress-strain state of steam turbine disks at the operation. As result it is stated that a one-parameter approach based on the plastic and creep stress intensity factors are more convenient for practical use in assessing the fracture resistance under monotonic and cyclic loading of materials and structural elements with respect to the two parameters fracture theories.