Abstract:
The deterministic approach in engineering design and analysis compensates for the
uncertainty in specific system parameters through an empirical factor of safety. The
probabilistic approach incorporates this randomness or uncertainty and ensures optimal use
of resources without compromising safety. This paper presents a probabilistic model to
determine the fatigue life of a low-pressure steam turbine blade under the influence of
transient loading conditions characteristic of a start-up. The methodology consists of
simulating the randomness in the values of structural damping, rotational speed, and the
blade material's elastic modulus through Finite Element Analysis (FEA). Monte Carlo
simulation is utilized to generate random stress and fatigue cycle values obtained from the
results of Finite Element Analysis. The Palmgren-Miner rule is then used to determine the
probabilistic fatigue life. Results show that the deterministic approach underestimates
fatigue life. Also, it fails to demonstrate the relative impact of these random variables on
fatigue life. The probabilistic model developed for this study clearly establishes that
randomness in the values of turbine rotational velocity has the most significant effect on
the probabilistic fatigue life of a low-pressure steam turbine blade.
