Residual Strength of Stiffened LY12CZ Aluminum Alloy Panels with Widespread Fatigue Damage

Experimental and analytical investigations on the residual strength of the stiffened LY12CZ aluminum alloy panels with widespread fatigue damage （WFD） are conducted. Nine stiffened LY12CZ aluminum alloy panels with three different types of damage are tested for residual strength. Each specimen is pre-cracked at rivet holes by saw cuts and subjected to a monotonically increasing tensile load until failure is occurred and the failure load is recorded. The stress intensity factors at the tips of the lead crack and the adjacent WFD cracks of the stiffened aluminum alloy panels are calculated by compounding approach and finite element method （FEM） respectively. The residual strength of the stiffened panels with WFD is evaluated by the engineering method with plastic zone linkup criterion and the FEM with apparent fracture toughness criterion respectively. The predicted residual strength agrees well with the experiment results. It indicates that in engineering practice these methods can be used for residual strength evaluation with the acceptable accuracy. It can be seen from this research that WFD can significantly reduce the residual strength and the critical crack length of the stiffened panels with WFD. The effect of WFD crack length on residual strength is also studied.

The Strategy of Widespread Fatigue Damage in FMP System

As is known, widespread fatigue damage （WFD） is a common phenomenon of aging aircraft being a negative impact on safety and reliability. The current FAR 25. 571 （b） requires that ＂An LOV （ Limit of Validi- ty） must be established, during which it is demonstrated that widespread fatigue damage will not occur in the air- craft structures＂. The FAA issued AC 91-82A in August 2011 providing guidance on developing and implemen- ting a Fatigue Management Program （FMP） system to address in-service issues for metallic fatigue critical struc- tures. This action brings new challenges to aircraft manufacturers in China. This paper introduces the problem of widespread fatigue damage including the MSD（ multiple site damage） and MED（ multiple element damage） and the differences between them and the steps to establish the LOV/ELOV（ Extended limit of validity）. Authors in this paper is also proposing some management indicators to control WFD during the design phase and mainte- nance. Developing and implementing the management indicators to address WFD which are involved in the FMP system may provide guidance on the estimation and management to address WFD for present and future aircraft. Finally, some suggestions are put forward to provide the theoretical basis for large civil aircraft design and airwor- thiness.