As composite materials are now used in load conditions with increasing complexity and thickness, all the three-dimensional stress components become important and should be taken into account when predicting failures. In particular, the through-thickness stresses can play a crucial role in determining the in-plane behaviours and strength of a composite, laminate. The work presented in this PhD thesis aims to investigate failures due to complex stress fields at the root of a composite component in a dovetail assembly, where highly concentrated through-thickness stresses as well as in-plane tensile and interlaminar shear stresses are present. The problem was decoupled into two simpler multiaxial load cases which were studied separately: (1) through-thickness compression with interlaminar shear, and (2) through-thickness compression with longitudinal tension. They were investigated experimentally using new loading configuration in a biaxial test machine. This bridges the gap in reliable multiaxial experimental data which is lacking in the open literature. This was then combined with a finite element (FE) modelling approach to, develop simple failure criteria which are validated for engineering design purposes. A simple constitutive law which takes into account the effect of transverse compression and analytical tools which can be I easily utilised to predict stresses and failures in composites were also developed. The findings of this thesis were finally applied to a severely tapered dovetail composite specimen, together with some mitigation strategies, to predict its ultimate fibre failure load and the failure locations.

This thesis describes the development of stochastic analysis frameworks for use in engineering design and optimisation. The research focuses on fibre-reinforced composites, with the stochastic analyses of an existing analytical failure model for unidirectional composites and of a unit cell numerical model of a 2D 5-Harness satin weave. Stochastic failure envelopes are generated through parallelised Monte Carlo Simulation of deterministic, analytical, physically based failure criteria for unidirectional carbon fibre/epoxy matrix composite plies. Monte Carlo integration of global variance-based Sobol sensitivity indices is performed and utilised to decompose observed variance within stochastic failure envelopes into contributions from physical input parameters. It is observed how the interaction effect can be used to identify domains of bi-modal failure, within which the predicted failure probability is governed by multiple failure modes. A reduced unit cell (rUC) model of a 5-Harness satin weave is constructed and analysed deterministically in uniaxial and biaxial loading conditions. An algorithm is developed and implemented to fully automate the rUC construction such that stochastic variations of the crimp angle can be evaluated. Monte Carlo Simulation is employed to propagate the effect of the crimp angle through the deterministic model and the probabilistic response compared with data obtained experimentally. It is observed how simulated variability compares well in uni-axial compression, but under-predicts observed experimental variability in uni-axial tension. The influence of vertical stacking sequence of plies is also demonstrated through the study of in-phase and out-of-phase periodic boundary conditions. The research highlights various, potential advantages that stochastic methodologies offer over the traditional deterministic approach, making a case for their application in engineering design and providing a springboard for further research come the day when greater computational power is available.

Failure predictions for a fibre-reinforced composite with unidirectional (UD) plies can only be relied upon provided the stress state is accurately known. This requires a prediction of the constitutive response to be made when the material is loaded. When failure does occur, matrix cracking is frequently the first mode of failure. Cracking results in a reduction of the material properties of the structure and can lead to other forms of damage. In this context, an elasto-plastic constitutive model that can accurately represent the full non-linear mechanical response of UD composites is developed, as well as the implementation of an improved model for matrix cracking. Unlike many existing constitutive models in the literature, the developed model captures some key features that are often neglected in constitutive modelling. These include the effect of hydrostatic pressure on both the elastic and non-elastic response. A novel yield function is formulated specifically for polymer-matrix fibre-reinforced composites, taking into account the presence of fibres in the material. The developed model is able to predict the non-linear response under complex loading combinations, given only the experimental response from two uniaxial tests. A non-associative flow rule is used to capture the pressure sensitivity of the material. The translation of subsequent yield surfaces under complex loading regimes is modelled by the inclusion of a non-linear kinematic hardening rule, which also allows for simulation of material unloading. The implementation of the model as a user defined material subroutine in a commercial finite element package is described. Regarding the modelling of matrix cracking, several methods are available in the literature. These models are reviewed and an existing model is combined with suitable failure criteria for the simulation of stiffness loss and crack accumulation in laminates. This model is then used to make predictions of crack accumulation and loss in stiffness of composite materials.

A simple mathematical model (cylinder model) is proposed and used to evaluate various constitutive laws used in finite element codes to simulate fatigue-driven delamination in composite materials. The cylinder model has one degree of freedom and allows the evaluation of several parameters of the constitutive laws in an ffcient and accurate way without worrying about the complex interactions that may arise while using a nite element code. This cylinder model enabled a large number of analysis to be performed in a relatively short time and made possible parametric studies with tens of thousands of dfferent sets of parameter values of the constitutive laws. Different fatigue degradations strategies have been implemented into the cylinder model and investigated. A systematic comparison of the different strategies has been performed and it was found that the behaviour of the different strategies is similar but with some variations in the sensitivity to the spacing of the springs and to the size of the increment in the number of loading cycles. The results confirm that the more accurate results are found for smaller values of these two variables. A new fatigue degradation strategy is also proposed. It is based on the construction of a 3-D surface that represents the behaviour of the interface under fatigue loading. This surface has been incorporated into the cylinder model and its performance has been evaluated.

The work presented within this thesis concerns measurement of the fracture toughness associated with the translaminar, fibre-breaking, failure modes of composites. Loading cases of mode I tension, mode I compression and mixed mode I/II tension and shear are considered. Fracture toughness measurement for translaminar tensile failure is investigated using the compact tension specimen. A detailed analysis of data reduction schemes concludes that a modified compliance calibration technique is the most appropriate, in terms of reproducibility of results and simplicity. Investigation of specimen in-plane size, thickness and lay-up effects indicates a thickness dependence. Specimen fracture surfaces reveal that the increase in measured toughness for specimens with thicker 0° plies is due to an increased amount of fibre pull-out. Furthermore, it is found that true measurements of fracture toughness are obtained from specimens with initial notch radii less than 250 μm, for the IM7/8552 material system. Fractography reveals that the critical notch root radius is dictated by the 0° plies alone. A four-point bend specimen is used to measure the fracture toughness associated with translaminar compressive failure. This method enables the failure mode to be triggered in isolation, making it superior to other methods in the literature. Microscopy of failed specimens reveals a shear driven fibre failure at 45° to the initial notch; the cause of this failure mode is investigated using a micromechanical finite element model. Mixed-mode translaminar tensile/shear fracture is investigated using a modified compact tension configuration and a specially developed fixture. SEM of the fracture surfaces of failed specimens reveals damage mechanisms specifically caused by the introduction of a mode II component of loading which accompany a significant increase in the damage zone size. R-curves are generated which suggest that the specimen configuration used here is appropriate for characterising fracture toughness at low proportions of mode II.

Glass matrix composites based on NextelTM alumina fibre reinforced borosilicate glass have been fabricated to improve their mechanical property and fracture toughness. In this work, a novel processing technique, which is called “sandwich” hot-pressing, has been used. It consists of arranging the reinforcing fibres in two directions with a periodic interspacing between glass slides, and submitting the material to a heat-treatment for consolidation into highly dense and transparent composites, which were proved by XRD analysis and SEM observations. These composites’ mechanical, optical and microstructural properties were studied and compared to those of the unidirectional fibre reinforced borosilicate glass composite and unreinforced glass matrix produced under the same conditions. Furthermore, a hybrid sol-gel technique has been employed for coating the fibres with a smooth and crack free ZrO2 interfacial layer to provide a weak bonding at the fibre/matrix interface to promote fibre pull-out during fracture. ZrO2 coated and uncoated fibre-reinforced borosilicate glass matrix composites were fabricated, with different sizes of optical windows including 4x4, 5x5 and 6x6 cm2. Moreover, a geometry based equation was derived to evaluate the expected light transmittance of the composites. These multi-directional fibre reinforced glass matrix composites retained at least 50% of the light transmittance and higher flexural strength compared with the unreinforced glass matrix. The highest measured flexural strength value of these composites was 56 ± 7 MPa. The composites reinforced by ZrO2 coated fibres had higher flexural strength (approx. 36%) and lower standard deviation (approx. 47%) compared with those reinforced by uncoated fibres. The introduction of a ZrO2 interfacial layer was to improve the mechanical properties and to retain the composites’ integrity, which was proved by the observations of fibre pull-out and crack deflection upon failure during mechanical tests. To investigate the microstructure of the interfacial layer in the composite, SEM, FIB-SIMS and TEM were employed. The present composites show potential for applications in architecture and special machinery requiring strong transparent windows.

The thesis starts off with an introductory chapter on composite materials. This includes a definition of composites, a brief history of composite materials, their use in aerostructures (primarily as stiffened structures), and also optimization of composite structures. A literature review is then presented on postbuckling stiffened structures. This includes both experimental investigations on stiffened composite panels and investigations into secondary instabilities and mode jumping as well as their numerical modelling. Next, the Finite Element (FE) modelling of posthuckling stiffened structures is discussed, relating how ABAQUS models are set up in order to trace stiffened composite panels' buckling and postbuckling responses. An experimental programme conducted on an I-stiffened panel is described, where the panel was tested in compression until collapse. The buckling and postbuckling characteristics of the panel are presented, and then an FE model is described together with its predicted numerical behaviour of the panel's buckling and postbuckling characteristics. Focus then shifts to the modelling of failure in composites, in particular delamination failure. A literature review is conducted, looking at the use of both the Virtual Crack Closure Technique (VCCT) and interface elements in delamination modelling. Two stiffener runout models, representing two specimens previously tested experimentally, are then developed to illustrate how interface elements may be used to model mixed mode delamination. The previously discussed panel is revisited, and a global-local modelling approach used to model the skin-stiffener interface. FE models of a stiffened cylindrical shell are also considered, and again the postbuckling characteristics of the shell are compared with experimental results. . The thesis then moves on to optimization of composite structures. This starts off with a literature review of existing optimization methodologies. A Genetic Algorithm (GA) is devised to increase the damage resistance of the I-stiffened panel. The global-local ABAQUS model discussed earlier is used in conjunction with the GA in order to find a revised stacking sequence of both the panel flanges and skin so as to minimize skin-stiffener debonding subject to a variety of design constraints. A second optimization is then presented, this time linked to the FE model of the stiffened cylindrical shell. The objective is to increase the collapse load of the shell, again subject to specific design constraints. The thesis concludes by summarising the importance of the work conducted. FE models were created and validated against experimental work in order to model a variety of composite stiffened structures in their buckling and postbuckling regimes. These models were able to capture the failure characteristics of these structures relating to delamination at the skin-stiffener interface, a phenomenon widely observed experimentally. Various optimizations, able to account for failure mechanisms which may occur prior to overall structural collapse, were then conducted on the analysed structures in order to obtain more damage resistant designs.

The demand for oil in the world is expected to rise by 1.7% in the fourth quarter of 2012 compared to fourth quarter of 2011. In order to cater for this increasing demand, the oil and gas industry continues to explore and develop deep-sea oilfields where oil and gas risers and pipelines encounter extreme conditions. The combination of high pressure and temperature with aggressive media which contains of hydrocarbon, alkanes, acids, sour gas (H2S), and CO2, etc., requires superior material performance and durability. Conventional engineering materials, such as steel are heavy and require corrosion protection, which are currently used as risers, flowlines and choke and kill lines have reached their limits. This is because of the poor chemical resistance and damage tolerance and the high costs involved in supporting their own weight. This has motivated the industry to explore non-corroding and lighter alternative materials if deeper sea reservoirs are to be explored. One such material that has the potential to overcome such limitations thus enabling new design strategies for cost effective, weight and energy saving materials is fibre reinforced composites. The remarkable properties and the tailorability of fibre reinforcement along load paths to achieve excellent performance of the composites is an attribute not found in any other material. The aim of this research was to manufacture novel carbon fibre reinforced polyvinylidene fluoride (PVDF) composites by incorporating atmospheric plasma fluorination of the carbon fibres. Powder impregnation method was adapted for the manufacturing of continuous unidirectional (UD) carbon fibre reinforced PVDF composite prepregs. The resulting composite laminates were characterised through various macro-mechanical tests. The impact of atmospheric plasma fluorination of the carbon fibre on the tensile, flexural, short beam shear and tearing properties of the UD composites were investigated to determine whether the improvements observed in the single fibre model composite can be translated to macro-level composite laminates. Apart from this, the impact of combining both fibre and matrix modifications on the composite were studied and the preliminary results on micro-mechanical scale are presented. Finally, composite pipe structures, made by filament winding technique using unidirectional carbon fibre reinforced PVDF composite prepregs onto a pure PVDF liner were fabricated, and characterised with respect to its mechanical properties.

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