Effect of MSD on Large Damage Residual Strength

Abstract

The purpose of this paper is to point out that a need exists to re-visit component test data used to substantiate large damage residual strength capability for those commercial transport aircraft now considered to be aging. Many of these aircraft are operating well beyond their initial design life goals due to the current economic environment and are therefore prone to the formulation of widespread fatigue damage (WFD) which may considerably reduce the large damage tolerance capability used to support initial certification of the aircraft. The purpose of this paper is to re-visit such a component test, as an example, to illustrate the considerable effect that in-service uninspectable multiple-site-damage (MSD), a sub-element of WFD, can have on lead crack residual strength. The stiffened panel test was originally performed to substantiate the residual strength capability in the presence of a two-bay circumferential crack with a broken central stiffener in the most critical location on the crown of a pressurized fuselage. This particular test was chosen because it illustrates the need to consider the non-linear load displacement characteristics of the skin to stiffener rivets as the skin crack approaches the intact crack arresting stiffeners. The results of an elastic/plastic displacement compatibility analysis illustrates that the lead crack tip stress intensity factor becomes non-linear with respect to applied gross stress well before the failure stress due to rivet yielding. The resulting crack tip stress intensity factor, a function of both crack length and applied gross stress, is used with an intuitive lead crack/MSD crack link-up criterion to determine the loss in residual strength caused by in-service uninspectable MSD. The analysis illustrates that residual strength is very sensitive to exact rivet location with respect to the lead crack tip and also to the model used to describe the crack tip plastic zone size. There appears to be an urgent need to conduct further research to determine the effect on plastic zone size of stiffening materials having much higher yield strengths than the cracked skin.