Finite element analysis of bonded crack retarders for integral aircraft structures

摘要

Trends in aircraft design and manufacture are towards the reduction of manufacturing cost and structural weight while maintaining high level of safety. Thesereductions can be achieved by using integral structures. However, integral structures lack redundant structural members, hence fail safety is not guaranteed.Bonded selective reinforcements (straps) can obviate this problem and improvethe damage tolerance capability of integral structures, although increase the design di±culties.The objective of this research is to develop an effective analysis method to predictthe fatigue crack growth (FCG) life of integral structures reinforced by bondedcrack retarders, determine the effectiveness of the reinforcements, and assess theimportant strap design parameters.The main mechanisms that influence the crack propagation have been identified,modelled, and discussed. When a crack propagates in the panel skin, bondedstraps delay the fracture growth by exerting bridging forces at the crack tip.Nevertheless damage also affects the strap due to the stiffness mismatch and highstress concentration, and the strap/substrate interface is affected by a progressivedelamination that advances together with the substrate crack and limits the strapbridging action. Tensile thermal residual stresses (TRS) in the cracked substrate,caused by the adhesive cure process, act to open the crack and hence increase thegrowth rate. Last but not least, secondary bending caused by the non-symmetricconfigurations induces a stress gradient along the crack front. This reduces theeffectiveness of the bridging action and causes a curved crack front.An enhanced 2D FE modelling technique that takes into account of these mechanisms and their interactions has been developed and implemented in a computerprogram that interfaces the commercial code NASTRAN. This program is usedto calculate the stress intensity factors and the FCG life of bonded strap reinforced integral structures.This modelling technique has been validated for a wide range of test samplesin terms of TRS and their redistribution with crack propagation, disbond areas, and FCG lives. The FCG life of a large scale integral skin-stringer panelreinforced by various bonded straps has also been predicted and compared withthe experiments. Numerical predictions have shown good agreement with theexperimental measurements.Parametric studies have been conducted to understand the effectiveness of different strap configurations on crack growth retardation; these include differentstrap materials, strap dimensions and locations on the substrate.A design tool has been developed aimed at achieving optimal crack retarderdesign in terms of prescribed fatigue life target and minimum structural weight.In conclusion, a novel modelling tool has been developed, the effectiveness ofbonded straps in retarding fatigue crack growth has been demonstrated and,following the parametric analysis, the most important parameters in the designof bonded straps have been identified.