Issue 37

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 37 (2016) 193-199; DOI: 10.3221/IGF-ESIS.37.25 197 S TRUCTURAL INTEGRITY ASSESSMENT pplication of the strain energy density (SED) function for analyzing fatigue fracture has been made in work [11]. To extend the response to unloading, reloading and cyclic loading, making use of the cyclic stress and strain curves, a critical state of elastic-plastic hysteresis loop can be written in terms of the strain energy density as                4 2 m f f f c dW N dV (3) Substituting into the left part of formula (3) the total SED we have got                            2 1 4 2 m yn f f f S N E where                          2 1 2 1 11 1 1 2 1  n e n n K a S a K n w I (4) The crack growth rate per cycle, da/dN , is given by                   1 2 2 * * 2 4 m n th th f f S S da a dN E (5) The above rate of crack propagation law contains the mechanical properties of the material, E, cyclic properties **,   , n  , the governing parameters of elastic-plastic stress-strain field In and a length parameter c  associated with the fracture process zone size. More details to determine the SED functions  S , th S  , the In-factor and equivalent stress e  ~ for cracked body different configurations are given by Refs. [7-11]. We have paid our attention on the combined method including the numerical stress-strain distribution in the parts and components of power steam turbine by using finite element analyses and limited experimental data related to elastic- plastic material properties under uniaxial tension. Within the current investigation a fatigue life estimation approach (Eq.5) based on the FEA results and experimental data is applied to predict residual durability of power steam turbine disk with take into account of operating time. The fatigue crack growth analysis was performed under harmonic loading using the elastic and elastic-plastic SIF's distributions along different crack front profiles. An initial circumferential edge crack at the highest elastic-plastic stress location was chosen to be 1.6 mm in the depth and length direction, which is much smaller than the observed crack size at operation. It is found that the crack growth in the depth direction is much faster than that in the length direction as it shown in Figs. 7 and 8. Also it should be noticed that the two ends of the crack tend to place in the radial direction and this is believed to be due to the influence of the hoop stress. Figure 7: Crack growth rate comparison for elastic and elastic-plastic solutions. A