Durability prediction from service crack experience

Modern aircraft structures are designed to be damage tolerant and to have an adequate durability. The Damage Tolerance provides structural safety in case of inadvertent cracking, the Durability determines the time that the structure can be kept in service in an economically acceptable way. Many aircraft are planned today to be kept in service considerably longer than the original design life, and knowledge of the actual durability of the aircraft becomes important. Unfortunately, only a lower limit of that durability, determined in a full scale durability test is-known, however. The present report investigates how estimates of the actual durability can be made on the basis of the absence of cracks at a certain lifetime. Durability is loosely defined: In the present study, two Durability Criteria are investigated. In the one, it is assumed that the Durability Life ends when a specific number of cracks has occurred. The other criterion assumes that Durability life ends when a certain crack rate, that is the number of new cracks developing per flight hour, is reached. Using a simple statistical model, the concept is quantified and the influence of the various parameters involved is investigated. Some examples quantify the predictions that can be made and indicate that the methodology defined can be applied in practical cases.