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No description available.

The aim of this study is to investigate a novel toughening approach for liquid mouldable carbon fibre/epoxy composites. The toughening mechanism is based on the use of thermoplastics for the toughening of epoxy resins in which polymer blends are formed, leading to phase separated morphologies which allows for various toughening mechanisms to take place. Instead of standard melt or solution blending, the thermoplastic in this study is introduced as solid phenoxy fibres, which are combined with dry carbon fabric preforms. These phenoxy fibres remain solid during resin infusion and dissolve when the laminates are heated and phase separation takes place before curing completed. The main benefits of this approach are that the viscosity of matrix resin remains low, which makes liquid moulding of these laminates possible. Localised and selective toughening of particular regions within a structure can also be achieved. Process time and cost can also be reduced by eliminating the polymer blending process. It was found that modification with phenoxy improved composite Mode-I interlaminar toughness significantly, with an increase of up to 10-folds for bifunctional epoxy composite and 100% for tetrafunctional epoxy composite, while tensile properties were not adversely affected. It was found that it is possible to combine the dissolvable phenoxy fibres with an undissolved aramid interleaf to improve toughness and damage properties. However, the phenoxy-epoxy systems had lowered environmental stability and degraded after hot-wet and solvent conditioning.

This thesis outlines the development of composite reinforcement technology for a ship's aluminium alloy superstructure. The work objective aimed to alleviate stress concentration in parts of the superstructure prone to fatigue-induced cracking. This is a novel approach to ship repair, which promises reduction in the cost of maintenance primarily due to greater efficiency and lower cost of repair. The work was conducted over approximately 12 years. It commenced in the late 80s with laboratory research and development and concluded in 2000 after completion of a seven-year trial on board a navy ship. Two carbon fibre composites, (5 m x 1 m consisting of a 25-ply laminate), were adhesively bonded to the 02-deck on the port and starboard sides. It was found that upgrading the structure using composites was effective, making it able to withstand service fatigue stresses. Finite element modelling and strain measurements on board the ship showed that critical stress concentration could be alleviated through stress redistribution. For the duration of the trial, no cracking of aluminium alloy deck in the vicinity of the reinforcements was reported. Both composite reinforcements exhibited good performance and remained in service after the end of the trial. However, the marine environment did cause some non-structural, edge debonding of the glass fibre reinforced overlay at the composite-metal interface. This overlay was designed to provide surface protection to the underlaying carbon reinforcement. Bond degradation was patchy. It occurred after about three years in service, most probably due to a combination of thermal cycling (solar heating/cooling) and water ingress at the interface. A new edge sealing method restored its durability and it required no further attention. This experiment was successfully demonstrated on board an active navy ship. The work proved that an effective and durable repair of a ship structure using non-metallic repair technology is feasible. Composite reinforcements prevented deck cracking and removed any need for welded repairs, thereby reducing the cost of ship maintenance. For further cost reduction it is recommended to adopt the principle of reverse engineering to simplify the technology for dockyard use.

Herein is contained details of a comprehensive finite element survey and experimental investigation into the buckling and postbuckling characteristics of thin laminated square Carbon-Epoxy panels with various cutout geometries, subjected to uniaxial compression. The plate edges are considered to be fully fixed with constant edge displacement loading. The panels were quasi isotropic in nature with a stacking sequence of (0/90/±45)2,. Square, circular and elliptical centrally located cutouts were considered with cutout dimension/panel widths ranging from 0.1 to 0.7 in increments of 0.1. Eccentrically located circular and square cutouts were considered for cutout dimension/panel width ratios ranging from 0.1 to 0.4 with vertical and horizontal eccentricity varying from 0 to 20% of the panels width. Multiple circular cutouts with cutout dimension/panel width ranging from 0 to 0.3 with separation distance/panel width ratios ranging from 0.2 to 0.65. A finite element eigenvalue analysis was adopted to determine the critical buckling loads and buckle mode shapes for the panels. The postbuckling response of the panels were investigated by adopting a non-linear finite element analysis approach using an Incremental Newton-Raphson Iterative solution scheme. A limited experimental test programme was undertaken to act as verification to the finite element solutions. A purpose built buckling rig was designed and manufactured for the purposes of the tests. It has been confirmed that the critical buckling loads for centrally located circular and square cutouts initially reduces as the cutout size increases. After reaching a minimum value it thereafter increases with large cutout sizes, the exact changeover point being dependant upon the shape of the cutout. The orientation of ellipse major axis significantly affects the critical buckling load of a panel. A horizontally aligned ellipse exhibits similar behaviour as that to a circular or square cutout. However when the major axis is rotated relative to the horizontal axis its buckling capacity reduces monotonically till it has a buckling load less than that for an unperforated panel when vertical aligned. It has been shown when a circular cutout is eccentrically placed in a panel, for small cutout sizes the buckling load reduces with horizontal eccentricity while a small increase is experienced for vertical eccentricity. Multiple circular cutouts significantly reduce the buckling capacity of the panel for all cutout sizes and separation distances. Initial geometric imperfection in the panel does not affect the critical buckling load significantly. The postbuckled response of such panels are also insensitive to the magnitude of imperfection. Panels with circular, square and elliptical cutouts exhibit substantial postbuckled strength. The post buckling response of such panels are insensitive to cutout geometry shape.

No description available.

No description available.

This thesis focuses on closed-cell aluminium foams (ALPORAS and ALULIGHT) and on sandwich panels comprising these foams laminated with 2/2 twill carbon fibre (MTM56/0300) skins. The thesis experimentally and analytically investigates the response of foam-only panels (ALPORAS) to indentation with various indenter sizes and shapes; and also studies the behaviour of sandwich panels to contact damage caused quasi-statically or by impact. Quasi??static uniaxial compression testing is used to determine the mechanical properties of the foams (ALPORAS and ALULIGHT). It is revealed that the plastic collapse strength (σ* pl) obtained from the stress??strain curves is lower than the values predicted by the Gibson-Ashby theoretical model. This phenomenon is explained by the fact that the aluminium foams tested are imperfect, non-homogeneous and non-isotropic, and show a distinct cell elongation. Whereas, the Gibson-Ashby theoretical model was based on the finite element method applied to the response of a unit tetrakaidecahedral closed cell having flat faces. The experimental work shows that the deformation of the foam-only panels to indentation is caused by progressive crushing of the cell bands and by shearing and tearing of the cell walls. This thesis presents new analytical models for the response of the foam-only panels and estimates the applied deformation load in all types of indentation. By fitting the experimental load-displacement curves, the shear strength (τ* pl) and the tear energy (γ) are deduced. Compared to the literature, more consistent results are obtained for the shear strength (τ * pl) and the tear energy (γ) from all types of indentation. It is also suggested to determine (τ * pl) and (γ) through indentations with long punches (FEP and LCP), instead of hemi-spherical or cylindrical indenters, because indentation on enclosed areas shows some indenter size dependence. It is concluded that thinner panels are not suitable for the determination of the tear energy (γ) since the densification of the foam is achieved before the tear resistance is fully engaged. Another objective of this thesis is to study the response of sandwich panels comprising a closed??cell aluminium foam core and laminated with carbon fibre skin to quasi-static and impact local damage. Special attention is paid to the residual (remnant) strength in bending of the already indented sandwich panels (quasi-statically or by impact) up to the failure point. The remnant strength in bending is determined by carrying out four point bending strength tests. The local damage is located on either the compressive or on the tensile side of the sandwich panels. Thus, the capacity of the panels to resist transverse loads after they have been locally damaged at contact is investigated. The contact damage on the sandwich panels is experimentally simulated using spherical indenters. The quasi-static indentation is carried out at a low constant velocity (0.5mm/min) ?? the induced contact damage is found to be independent on the sample thickness but dependent on the indenter diameter. On the contrary, the impact test indicates velocity-dependence of the failure mode of the sandwich panel (i.e. skin breakage or punch through) which is found from the load-displacement curves. The results reveal that there is a correlation between the area of the contact damage and the remnant strength, and that the use of metal foam cores leads to high contact damage resilience of composite structures.

This thesis focuses on closed-cell aluminium foams (ALPORAS and ALULIGHT) and on sandwich panels comprising these foams laminated with 2/2 twill carbon fibre (MTM56/0300) skins. The thesis experimentally and analytically investigates the response of foam-only panels (ALPORAS) to indentation with various indenter sizes and shapes; and also studies the behaviour of sandwich panels to contact damage caused quasi-statically or by impact. Quasi??static uniaxial compression testing is used to determine the mechanical properties of the foams (ALPORAS and ALULIGHT). It is revealed that the plastic collapse strength (σ* pl) obtained from the stress??strain curves is lower than the values predicted by the Gibson-Ashby theoretical model. This phenomenon is explained by the fact that the aluminium foams tested are imperfect, non-homogeneous and non-isotropic, and show a distinct cell elongation. Whereas, the Gibson-Ashby theoretical model was based on the finite element method applied to the response of a unit tetrakaidecahedral closed cell having flat faces. The experimental work shows that the deformation of the foam-only panels to indentation is caused by progressive crushing of the cell bands and by shearing and tearing of the cell walls. This thesis presents new analytical models for the response of the foam-only panels and estimates the applied deformation load in all types of indentation. By fitting the experimental load-displacement curves, the shear strength (τ* pl) and the tear energy (γ) are deduced. Compared to the literature, more consistent results are obtained for the shear strength (τ * pl) and the tear energy (γ) from all types of indentation. It is also suggested to determine (τ * pl) and (γ) through indentations with long punches (FEP and LCP), instead of hemi-spherical or cylindrical indenters, because indentation on enclosed areas shows some indenter size dependence. It is concluded that thinner panels are not suitable for the determination of the tear energy (γ) since the densification of the foam is achieved before the tear resistance is fully engaged. Another objective of this thesis is to study the response of sandwich panels comprising a closed??cell aluminium foam core and laminated with carbon fibre skin to quasi-static and impact local damage. Special attention is paid to the residual (remnant) strength in bending of the already indented sandwich panels (quasi-statically or by impact) up to the failure point. The remnant strength in bending is determined by carrying out four point bending strength tests. The local damage is located on either the compressive or on the tensile side of the sandwich panels. Thus, the capacity of the panels to resist transverse loads after they have been locally damaged at contact is investigated. The contact damage on the sandwich panels is experimentally simulated using spherical indenters. The quasi-static indentation is carried out at a low constant velocity (0.5mm/min) ?? the induced contact damage is found to be independent on the sample thickness but dependent on the indenter diameter. On the contrary, the impact test indicates velocity-dependence of the failure mode of the sandwich panel (i.e. skin breakage or punch through) which is found from the load-displacement curves. The results reveal that there is a correlation between the area of the contact damage and the remnant strength, and that the use of metal foam cores leads to high contact damage resilience of composite structures.