PSI - Issue 7

E. Vacchieri et al. / Procedia Structural Integrity 7 (2017) 182–189

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E. Vacchieri et al. / Structural Integrity Procedia 00 (2017) 000–000

Fig. 4. Weibull cumulative function and determining of the confidence bands, as a normalized NSU function from field data about crack of a length l , (a) suction side and (b) pressure side.

These curves give an indication about the number of blade within the set that shows the minimum level of damage. The confidence bands allows the evaluation of the data out of this trend. The Weibull fits for the di ff erent cracks length are shown in Fig. 5.

Fig. 5. Weibull plots obtained for di ff erent cracks lengths, (a) suction side and (b) pressure side.

For both blade sides, the curves that defines the cumulative distribution for the di ff erent levels of crack shows a similar slope and they are shifted to higher number of NSU for bigger crack length. This means that a relationship between crack length and NSU exists and can be used to evaluate the crack propagation on the blade surface. The probability function of crack initiation for a single blade has been determined by Zaratesky et al. (2012). The reliability level, S set , of a set with n blades is given by the product of n and the reliability levels of each blade in the set. If all components have the same S value, that is the same probability to have a crack, the relation between L x − set for the blade set and L x − blade can be calculated as shown in Eq. 5. L x − blade = L x − set n ( 1 β ) (5) where n is the number of blade in a set ( n = 80) and x is the selected probability level. From the analysis performed on available data and shown in Fig. 5, there is no di ff erence in the Weibull slopes describing the cumulative curves for the cracks at the suction side and a small deviation in the slopes for the cracks at the pressure side. For both locations the mean β values range between 5 and 5.5 as observed by Zaratesky et al. (2012) and they have been used to calculate L x − blade , following Eq. 5. L x − set is always less than L x − blade with the same probability of survival. Weibull statistics applied to the field data allows calculation of the cumulative distribution describing the percentage of blades with a defined crack size. This approach, using the maximum acceptable crack size, defined in the repair