Probabilities of occurrence and detection of damage in airframe materials

The reliability of airworthiness assessment and the effective management of ageing fleets of aircraft depend critically on the quality of tools for predicting damage nucleation and accumulation and its detection, i.e. on the interrelationship between the probabilities of occurrence and detection. To illustrate these interrelationships, a mechanistically based probability approach involving localized pitting corrosion and subsequent fatigue cracking is presented. A probability of detection based on a typical state-of-the-art technique for non-destructive evaluation is used for comparison and probabilistic assessment. The results suggest that the probability of detection is inadequate, and information on damage size should be included as part of an effective airworthiness assurance methodology. An appropriate target for detecting and sizing damage of ≈ 0.10 mm with a probability of detection and a confidence level of at least 90% is suggested, versus the current capability of 1.27 mm at only 50%     

A model to assess the fatigue behaviour of ageing aircraft fuselage

A computer efficient model for assessing the fatigue behaviour of ageing aircraft components in the presence of multiple site damage is introduced. The model involves the computation of crack initiation scenarios on the basis of a probabilistic approach and the estimation of the fatigue behaviour of complex, highly loaded aircraft structures using a deterministic concept. To reduce the computing effort because of the model size and mesh difficulties in crack propagation calculations, a sub-structuring procedure under the FE method is utilized. An incremental approach for calculating crack initiation and crack propagation has been involved; it leads to a further essential reduction of the computing effort. Computer simulation is compared with experimental results for characteristic multiple site damage problems of aluminium 2024 lap and butt joints.     

Fatigue crack growth analysis of 2024 T3 aluminium specimens under aircraft service spectra

The fatigue crack growth behaviour of 2024 T3 aluminium was investigated experimentally. The fatigue experiments were performed under constant stress amplitude, constant amplitude with single and multiple overloads and aircraft service spectra. The fatigue spectra used correspond to the air-to-air, air-to-ground and instrumentation and navigation flight phases. They were applied for different stress levels. In total 11 different random flight service spectra were examined. The retardation effects caused by the overloads on the fatigue crack growth behaviour and the fatigue crack growth under aircraft service spectra were predicted using an in-house-developed code. The code makes use of the strip plastic zone approximation to account for material hardening effects along the path of prospective crack growth. Crack growth is treated incrementally and corresponds to failure of material elements ahead of an existing crack after a certain critical number of fatigue cycles. For the simulation of irregular service spectra by equivalent sequences of distinguished stress cycles a modified rainflow counting method is utilized. Spectrum simulation accounts also for non-linearity in fatigue damage accumulation and load sequence effects. The computed fatigue curves fit well with the experimental results.     

Analysing the onset of multiple site damage at mechanical joints

A detailed analysis has been performed on the onset of multiple site damage at mechanical joints. The crack initiation is analysed based on non-linear stress analyses and the crack closure model. It has been found that the stress-strain response at the joint can have a stabilised linear relation under fatigue loading even if considerable plastic yield has occurred. This makes it possible to analyse the post yield multiple crack initiation based on fracture mechanics solutions for the crack growth in a residual stress field. Weight function method has been used to solve stress intensity factors and Green's functions for the crack growth analyses according to a strip yield crack closure solution. In this analysis, the small crack behaviour as well as the post yield effect have been accounted for. Intrinsic material parameters have been extensively used so that the method is not limited to the specified problems. Together with the crack growth analyses, a probabilistic model has been developed to account for uncertainties in initial flaw, stochastic crack propagation, geometrical inconsistency, as well as variation in the fatigue load. According to the probabilistic solution, important information about the onset of multiple site damage, e.g. the probability of crack occurrence, and the probability of crack break-through etc., are predicted as a function of the stress and the construction of joint. One example is provided to illustrate the procedure and to highlight problems in dealing with the onset of multiple site damages at mechanical joints.     

The effect of compression stresses,stress level and stress order on fatigue crack growth of multiple site damage

Structural components are generally subjected to a wide stress spectrum over their lifetime. Service loads are accentuated at the areas of stress concentration, mainly at the connection of components. When there is a critical level of multiple site damage at connections, cracks link up to form a large crack which abruptly reduces the residual strength of the damaged structural member. Therefore, it is important to estimate the fatigue life before the cracks link up due to critical multiple site damage. In this study, the extended finite element method was applied to predict lifetime under constant amplitude cyclic loadings of fatigue tests on several multiple site damage specimens made of Al 2024‐T3. Then the multiple crack growths under service stress spectra are calculated to investigate the effects of compressive stress, stress orders and the effect of sequence cyclic loadings on stress levels by using Forman and NASGROW equations.     

拉压加载下纤维增强铝合金层板疲劳裂纹扩展的压载荷效应与预测模型

基于增量塑性损伤理论与纤维增强金属层板疲劳裂纹扩展唯象方法, 推导出在拉-压循环加载下, 纤维增强金属层板疲劳裂纹扩展速率预测模型。并通过玻璃纤维增强铝合金层板在应力比R=-1,-2的疲劳裂纹扩展实验对预测模型进行验证。结果表明, 纤维增强铝合金层板疲劳裂纹扩展的压载荷效应分为两种情况: 在有效循环应力比R_C>0时, 表现为压载荷对铝合金层所承受残余拉应力的抵消作用; 当R_C<0时, 表现为压载荷抵消残余拉应力后, 对纤维增强铝合金层板金属层的塑性损伤, 对疲劳裂纹扩展存在促进作用。纤维铝合金层板疲劳裂纹扩展的压载荷效应不可忽略, 本文中得出的在拉-压循环加载下疲劳裂纹扩展速率预测模型与实验结果符合较好。