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Affiliations
The Northwest Composites Centre
National University of Science and Technology
The University of Manchester
American Institute of Aeronautics and Astronautics
Pakistani Student Forum
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Abstract
Bonded joints are key structural elements and for parts made from fibre reinforced polymeric matrix composites (FRC), these are preferred over mechanical fasteners such as rivets and bolts. Low velocity impact loading of structures is frequently encountered both in-service and during maintenance and when an object made from FRC is subjected to impact loading, the resulting damage is often quite complex in terms of both multiplicity and unpredictability of damage modes. If the structure involves bonded joints the complexity is increased further. Since the use of composite materials is continuously increasing in all type of structural components the impact threat can no longer be dealt with simplified design approaches and unrealistic in-service operating requirements. All the major commercial Aircraft manufacturers are looking at fibre polymeric composites as the material of choice for the future aircrafts, however, the poor understanding of impact induced damage mechanisms necessitate the use of generous safety factors for product design. This often results in offsetting the weight saving advantage promised by composite materials, which is a consequence of their higher in-plane specific modulus and strength. Hence, from both an academic and industrial point of view it is important to increase the understanding of impact damage in bonded joints of composites.
Single lap joints which are the focus of this study are less suitable for many types of loading due to their tendency of lateral deflection (or out of plane bending) that results in increased peel stresses as compared with their double lap counterparts. Practical considerations such as geometry requirements and manufacturing limitations, however, can often mean that they are the only design choice. A review of recent literature related to damage of bonded joint of composites has highlighted that firstly only a few authors have specifically looked at impact induced damage. Secondly even when impact response is studied; it is not with a view to characterize and establish the damage modes arising due to such loading. Thirdly most of the studies have not attempted to take a holistic view of the problem (that is considering both adherend and adhesive failure and taking in account both macro and micro failure mechanisms). In fact most of these studies consider a relatively weak adhesive with stronger adherends that limits the crack propagation to be in either entirely the adhesive layer or at best the adhesive adherend interface. The techniques that have generally been used for identifying and visualising damage are 2D and hence limited in scope. On the modelling side there are so many different possible failure theories and modelling methodologies that one simply gets overwhelmed. A clear comparison of the capabilities of these theories for real life situations is lacking.
The current study goes beyond the approaches found in literature as it tries to observe and model the impact induced damage modes in three dimensions and in its entirety i.e. both adherend and bond failure, taking stock of both macro and micro failure mechanisms. The view that has been taken in this study is this that the problem of damage mode characterization in bonded joints of composites should not be viewed in isolation of the problem of damage mode characterization in laminated composites; in fact it will become clear in this thesis that in many ways it is a subset of the same problem with added geometric and material constraints that dictate the overall damage modes. This study uses a combination of non-destructive inspection techniques such as C-scan and X-ray micro tomography to observe damage modes at macro and micro levels. At the same time using finite element analysis it compares various modelling approaches and failure theories to identify the most suitable approach to model this scenario. Based on the damage mode observations and finite element analysis the effects of design parameters such as overlap width and impact location have been studied. And a new conept of characteristic overlap width is introduced, which relates dominant damage mechanism with overlap width. This concept can be very useful as a design guideline for such joints. In short this study is a completely new chapter in our understanding of damage process as it happens in bonded joints of composites under impact loading.
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