Structural Integrity Assessment of ORC Turbine Tie-Rods: An Analysis Based on Elastic Shakedown and Fracture Mechanics

Fatigue life assessment of turbine rotating components is usually performed taking into account the presence of potential defects and their propagation, together with an accurate description of the stress/strain fields acting at components critical regions. In this work, such an approach was applied on a radial outflow turbine: the component investigated in this paper is the tie-rod. An axisymmetric FEM model of the whole turbine was implemented to evaluate the stress field acting at the root of tie-rod thread, the component most critical region. Three different preload procedures were investigated, in order to evaluate the effects of plastic strains on fatigue cycles. It was found that an initial overtightening, performed before machine assembly, induces a larger plastic zone at the root of the thread, which lowers the peak stress and lessens the stress ratio of the fatigue cycle. Stress gradients for the three different preloading conditions were the starting point of a crack propagation model, which calculated crack advancement as a function of service time. The model, which considered crack closure and stress ratio effects, allowed structural integrity assessment and showed that the initial overtightening had also the effect of crack growth rates reduction.