Environmental and geometrical effects on the mechanics of bonded composite joints in aerospace structures

Abstract

Application of highly efficient bonded composite joints to flight critical aerospace structures is currently limited in use, and is generally applied to secondary structures where component failure is not detrimental to overall safety. Less efficient and more traditional fastened joints may be utilised as the behaviour is well understood and characterised thoroughly in literature. The limited use of bonded composite joints to primary components is partly due to the lack of damage tolerant assessment, particularly as a result from either adverse operational environments or defects in the bondline. It is currently problematic to evaluate the damage tolerant performance of bonded composite joints to an acceptable level due to the complexity of damage mechanisms and challenges in detection or monitoring of failure. The progression of damage is inherently difficult to appreciate because of different failure modes interacting and greatly influencing each other. These interactions can also be drastically affected by several features including changes in geometrical, material, boundary, and environmental conditions. Understanding the various failure modes under typical aerospace operational circumstances, with regards to types of bondline faults, is essential to analysing damage tolerance of bonded composite scarf joints in primary structural application. From thorough review of the literature, a comprehensive test program was conducted to investigate the quasi-static structural performance of bonded composite scarf joints under a vast array of geometrical and environmental conditions, representative of aerospace structures, which include the effects of pre-flawed bondlines. The identification of several critical failure mechanisms was highlighted, in both the adhesive layer and bonded adherends, where failure phenomena were captured through a variety of advanced imaging techniques including Scanning Electron Microscopy (SEM), optical microscopy, and micro Computed Tomography (μCT). The results of this program have highlighted the significance of not only the type of failure that occur for various joint conditions, but also the severity of each damage mechanism. Because of this work, the overall performance was directly compared to corresponding failure mechanisms, providing new insight towards damage tolerant assessment of bonded composite scarf joints. Extending from the knowledge gained in experimental test program, an extensive benchmark study was conducted to assess the “current-state-of-the-art” analysis of progressive damage modelling, towards bonded composite scarf joints, using the commercially available Abaqus CAE package. The results showed that current methods can provide great insight into the general performance and damage progression of bonded composite joints provided, however in certain aspects provided mixed results in terms of accurate strength predations, and the ability to represent failure mechanisms observed from experimental results. A new analysis methodology has been developed to overcome previous modelling limitations, which includes the combined effects of intralaminar adherend damage, interlaminar delamination, composite based debonding, inelastic bondline deformation and bulk failure of the adhesive layer. Using inbuilt Abaqus functions, Continuum Damage Mechanics (CDM) and Cohesive Zone Modelling (CZM) were used to describe the nature of damage progression in the current study. The use of CDM can be broken down further into the Abaqus fibre reinforce and ductile material failure models. CZM was used to simulate both interlaminar delamination and debonding at the adhesive adherend interface. To capture the simultaneous combination of all these non-linear failure methods, an explicit Finite Element (FE) integration scheme was used. Validation of the new methodology highlighted the requirement for simulating all failure mechanisms observed from experimental findings, to have a high-fidelity failure model which can accurately predict the behaviour of bonded composite scarf joints, over various geometrical and environmental conditions. For the first time, it can be seen how various geometries and environments have a significant effect on the development of damage, which causes overall failure in bonded composite scarf joints. For the conditions investigated in the experimental test program, significant insight has been provided into not only when and where critical damage initiation regions are located, but also the interactions of failure progression which lead to a loss in performance. This critical assessment of failure mechanisms has lead to the development of simple design guidelines which will aid in the critical analysis of bonded composite joints towards flight critical structures. These guidelines focus on critical areas where damage will initiate and progress, to provide joint design allowables which will not cause significant irreversible deformation, or minimise the effects of critical damage progression. The research presented within this thesis is significant to the development of future bonded composite scarf joints, towards application in flight critical structures, in terms of both aircraft safety and structural efficiency