Many turbines in operation today were designed at a time when little was known about the unsteady loading on blades or about damping. As a result, the blades were designed using a tuning criterion and a steady stress criuterion. More recent approaches to this problem have used the concept of :"stimulus." The stimulus approach assumes that the unsteady loading is proportional to the steady loading on the blade. While this approach is more satisfactory than the steady stress criterion, it is a one mode at a time approach and leaves open the question of vibratory stresses in more than one mode combining to produce a stress greater than the reduced endurance strength. This paper summarizes the deterministic approach described above and extends the analysis, using probabilistic design methods, to include the possibility of stresses in multiple modes combining. The method presented also includes the effect of statistical variations in other design parameters. It is shown that, at the reliability levels of interest, it is necessary to consider multiple mode contributions to the total vibratory stress. The output of this new approach also gives a probability of failure that can be used to make rational decisions when considering different design alternatives.
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