How Likely is Multiple Site Damage?

One way to avoid multiple site damage in airframes of the future is to screen design details by means of coupon tests. A simple conceptual model for estimating the risk of multiple site damage is presented. The risk is expressed as the fraction of similar details which can be expected to form fatigue cracks at times close enough to each other to allow a fracture cascade after the damage has propagated. Examples based on an approximate representation of typical transport category fuselage skin properties and stress environments show that the risk becomes significant for details which exhibit much less fatigue life scatter than is observed in plain fatigue tests and for which the characteristic life is of the order of the service life. Results for confidence limits are also presented to show that at least 40 replicate details must be tested to obtain adequate estimates of scatter and characteristic life.