Weight Function Method For Aircraft Structure Ultiple Site Damage Analyses And Residual Strength Rediction

Multiple Site Damage (MSD) has been a serious threat to moderntransport airplane structure safety, and therefore has become a great concernin aircraft damage tolerance design and airworthiness certification. Because ofits high complexity, MSD is hardly tractable by classic analytical methods.Consequently most current approaches rely on numerical methods, especiallyFinite Element Method (FEM).In this thesis, an analytical method, the Weight Function Method (WFM),has been proposed and further developed, for the first time, to analyze theMSD problems and to determine the related various fracture mechanicsparameters. The results are extensively validated. Combined with Crack TipOpening Displacement criterion (CTOD), the present WFM is employed topredict the stable crack growth and residual strength of aircraft aluminumsheets containing four types of MSD-configurations. The predicted resultscompared very well with the corresponding experimental data. Thereby anovel analytical approach with high efficiency and reliability has beendevised for MSD analyses in aircraft structures.The main accomplishments and advances made by this thesis include thefollowing.1. Weight function method is employed to analyze a number of specialMSD configurations, including a periodic array of collinear cracks，twoequal-length collinear cracks in an infinite sheet and strip yield model forthree collinear cracks with coalesced plastic zones in a finite sheet. Bytreating these MSD configurations as a single crack and using thecorresponding weight functions, stress intensity factors, crack opening displacements and the Dugdale strip yield plastic zone sizes are obtained.Extensive comparisons show excellent agreement with existing exactsolutions and FEM results in the literature.2. For other MSD configurations, which cannot be treated as single crackproblem, weight function method for general collinear cracks is devised.Significant differences between the weight functions for general collinearcracks and a single crack configuration are found. Based on the WFM forgeneral collinear cracks, weight functions for three collinear cracks in aninfinite sheet are derived. Stress intensity factors and crack openingdisplacements for arbitrary load conditions as well as strip yield plastic zonesfor the three cracks are determined through simple quadrature.3. Based on the weight function of single crack, a unified method isproposed to solve the strip yield models for collinear cracks in infinite,semi-infinite and finite sheet. The method is used to solve the strip yieldmodels for two and three symmetrical collinear cracks in infinite,semi-infinite and finite sheets to obtain plastic zones, crack openingdisplacements and stress distributions along the elastic ligaments betweencracks. These results are widely compared with exact solutions and FEMresults, perfect agreements are observed. This method is simple, efficient,reliable, and versatile.4. Two types of specimens, C(T) and M(T), are designed to determinethe basic mechanical properties of2024-T3aluminum as well as the CrackTip Opening Angle (CTOA) during stable crack extension. Finite width sheetswith two, three, five and seven collinear cracks are tested to obtain the crackgrowth processes and residual strengths. By proper design of MSDgeometries, various fracture scenarios are achieved. The experimental resultsprovide important information for assessing the WFM capability to determineligament failure loads and residual strength.5. Combined with CTOD criterion, the WFM is used to predict the stablecrack growth behaviors and residual strengths of four types of MSDconfigurations in finite-width sheets subjected to monotonic loading. The predicted stable crack growth processes, ligament failure loads and maximumresidual strengths are in good agreement with the corresponding experimentalresults (within11%and16%for residual strength and ligament failure load,respectively). The solution accuracy and efficiency are significantly betterthan the FEM.In summary, the present WFM for MSD provides a powerful analyticalapproach for fracture mechanics analyses and residual strength assessment forMSD-contained aircraft structures.