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Title: Dynamic behaviour of blast loaded hybrid structural systems
Author: Nwankwo, Ebuka
ISNI:       0000 0005 0733 0456
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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A hybrid system consists of two or more different constituent materials combined to form a single system to achieve increased mechanical properties and structural performances. The combinations of constituent materials are on a macroscopic level. The improved performances achieved in hybrid systems are in fatigue, impact, corrosion resistance, weight savings, and improved strength to weight performances. The increasing demand for high-performance and lightweight structures forms the motivation for this thesis. In the light of these, three different hybrid systems under different blast scenarios have been studied and the reason for their high-performance over monolithic systems discussed. The possibility of debonding of the strengthening composite patch from the stainless steel panel in a hybrid system of strengthened blast wall leads to the study of fibre metal laminates (FMLs) and lap joints. Since composites, form a significant part of these hybrid systems, simplified damage models for composites are developed and applied to the various hybrid systems studied in order to investigate their overall response. First, this thesis presents a hybrid system of a stainless steel blast wall with retrofitting composite patches. An analytical model, which allows for multiple deformation modes, is developed to study the hybrid system of strengthened blast wall. Maximum displacements predicted by the analytical models correlated well with maximum displacements predicted by the numerical models of the proposed hybrid system in Abaqus. It is observed that fibre fracture, which is a more detrimental failure mode, did not occur in the composite patch in the numerical model. The hybrid system of composite strengthened blast walls allows for increased energy absorption by the development four plastic hinges compared to the development of three plastic hinges of the monolithic system. This behaviour renders it superior to a monolithic system in a gas explosion scenario. In order to simplify the system presented in Chapter 3, an analytical solution for evaluating the maximum displacement of a continuous system with semi-rigid supports subjected to pulse loads is presented. The maximum elastic displacement presented by the numerical models in Chapter 3 is compared with the maximum displacement presented by the simplified model. The limitation of the simplified model is subsequently discussed. Using the simplified model, an elastic pressure-impulse diagram for the blast wall studied in Chapter 3 is presented under typical hydrocarbon explosions. In addition, unique pulse-shape independent pressure-impulse diagrams for elastic and elastic-plastic responses are developed using dimensionless parameters for typical high explosive events. However, the major limitations of this model are its inability to account for membrane effect, travelling plastic hinge, support shear hinge and connection pull-in. Secondly, the response of an FML is studied in order to obtain an insight into debonding between composite and metal, which was assumed to be prevented in Chapter 3. An FML was chosen because of the availability of experimental data on the blast response for this kind of hybrid system in the open literature. In addition, other researchers have proven that FMLs performed better the monolithic aluminium with similar areal density. A modified Hashin model is used to model damage in the composite layers of fibre metal laminates (FMLs) under blast loads. The FML studied comprises 2024-O aluminium alloys (O represents the temper of the aluminium heat treatment) and woven glass-fibre/polypropylene composites. Thus, this work presents an improved and simplified model to analyse the damage initiation, damage progression, and failure of the aluminium layers and the three-dimensional woven composite layers. In order to gain an insight on how bonded substrates influence the stress in adhesive layers and because interfacial stresses cannot be obtained directly from cohesive elements in Abaqus (i.e. adhesive layers in the studied FML), an analytical model to predict the maximum peel and shear stresses in an elastic adhesive in a single lap joint (metal-metal adherends) subjected to transverse pulse loads is presented. The analytical model for a metal-adhesive-metal system, which was validated with numerical models in Abaqus, gave an insight into the relationship of interfacial stresses in adhesive layers with bonded layers. Inference drawn from this model supports the assumption that bonded materials with similar in-plane stiffness would result in minimal interfacial stresses under blast scenarios as originally assumed in Chapter 3. Finally, a lap joint with similar adherends under in-plane blast load is compared with a hybrid system of metal and composite lap joint. The interfacial stresses produced by the hybrid system showed some reduction and fibre failure was not observed in the composite. This reinforces the improved performance of hybrid systems.
Supervisor: Louca, Luke Sponsor: Commonwealth Scholarship Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral