The methods used to predict crack initiation life (durability) and crack growth life (damage tolerance) of the United States Air Force/Northrop Grumman B-2 Bomber are discussed. The test matrix used to verify the methodologies is presented, followed by a discussion of each method and the results of the verification tests.
Durability analysis on the B-2 is based on a notch strain analysis method. The method uses the cyclic stress-strain properties of a material combined with the Neuber equation for strain concentration at a notch to predict crack initiation lifetimes. The method results in a design-allowable curve for durability.
Damage tolerance analyses must account for the effects of crack retardation due to overload cycles in a flight spectrum. The methods used for crack growth analysis rely on empirical parameters that are dependent on material; however, they are also often dependent on the type of spectrum. If the spectrum changes drastically, it is often necessary to recalibrate the crack growth model. A model based on phenomenon of crack closure has been developed that overcomes the need to recalibrate the model for each new spectrum. The model, referred to as the “constant closure model,” was used for crack growth life prediction of metallic components in the B-2. In the constant closure model, the closure factor (CF) is the only empirical parameter. The other parameters needed to use the constant closure model are obtained from the load spectrum.