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Fibre reinforced polymer composites are frequently replacing metals and alloys in structural applications for the aerospace, automotive and marine industries due to their high specific strength and stiffness and excellent corrosion resistance. However, a key limitation of composite laminates is their low resistance to out-of-plane loading and susceptibility to delamination. Improving the out-of-plane resistance to delamination in composites is achievable through the insertion of a z-direction reinforcement. There are several available techniques to achieve this, of which, through-thickness stitching is considered an effective, low cost option. Despite its popularity, limitations to the current stitch geometries employed for composites often result in significant geometrical defects and degradation of the in-plane properties. The aim of this research was to optimise the stitching geometry in order to effectively improve the impact resistance of textile composites without causing excessive damage to the in-plane tensile properties. This research details the employment and optimisation of a novel stitch type for the composite industry, the ISO-401 double-thread chain-stitch. Although this stitch type is used regularly throughout the textile industry, no research to date considers its suitability to composite reinforcement. Adjustment of the junction positon of the upper and lower stitching ISO-401 threads demonstrated that defects such as resin pocket size and fibre waviness can be reduced through optimisation. Thus, at high density stitching, which is reported as beneficial to improving the out-of-plane properties, the tensile properties are not significantly affected and therefore maintained. Under low velocity impact, stitching was found to significantly reduce the damage area through an arrest and bridging technique. This resulted in improved compressive strength after impact in stitched composites due to the smaller initiated damage size and further bridging effects of the stitches under loading. Considering the importance of the ISO-401 junction location, a final case study also revealed that the preform fabric characteristics can significantly affect the junction position under the same stitching conditions. Therefore, careful consideration of the sewing conditions and parameters must be taken to achieve the desired geometry. The work completed for this thesis demonstrates that ISO-401 shows good compatibility for the through-thickness reinforcement of textile composites and that it can address some of the limitations of other popular stitch types.

Composite materials offer considerable advantages in the impetus towards lightweighting and therefore the exploitation of fibre-reinforced composites in engineering structures has been steadily diversifying from e.g. sports equipment and racing cars, to helicopters and commercial aeroplanes. Furthermore, recent advancements in composite manufacturing technology have facilitated the use of complex configurations inindustrial design applications. For reliable design of multilayered structures, accurate stress analysis tools are required. However, with an increasing structural complexity, predicting structure's response can be non-intuitive and often can not be modelled adequately using classical lamination theory. High-fidelity finite element methods (FEM) are often employed to obtain reliable three-dimensional (3D) stress analyses with the desired level of accuracy. However, these models are computationally expensive and are prohibitive for iterative design studies. Consequently, over the years, several one-dimensional (1D) and two-dimensional (2D) models based on higher-order theories have been proposed for the analysis of multilayered composite structures with the aim of predicting accurate 3D stress fields in a computationally efficient manner. The majority of these numerical models either lack kinematic fidelity or accuracy in capturing localised regions of the structure, or have limited capabilities to model complex structures. The work presented here uses the Unified Formulation (UF) that supersedes classical theories by exploiting a compact, hierarchical notation that allows most classic and recent theories to be retrieved from one, hence unified, model. Importantly, and unlike many classic theories, the UF applies to the partial differential equations governing three-dimensional elasticity. Full stress and strain fields are, therefore, recovered by its implementation. Although current implementations are found wanting in this respect, in a UF setting, complex geometries could easily be analysed. This is because the displacement field is expressed by means of classic 1D (beam-like case) and 2D (plate- and shell-like cases) finite element elements that need not be prismatic. Additional expansion functions are employed to approximate 3D kinematics over cross-sections (beam-like case) and through-thickness (plate- and shell-like cases). In the present work, the 1D UF is adopted and developed further by introducing a hierarchical, Serendipity Lagrange polynomials-based, cross-sectional expansion model. The 3D stress predicting capabilities of the proposed model is verified against high-fidelity finite element models and other numerical and experimental results available in the literature by means of static analyses of isotropic, constant- and variable stiffness laminated composite, beam-like and stiffened structures. Special attention is given to the accuracy of the model in capturing 3D stress response in the localised regions, such as near geometric or constitutive discontinuities, constraints and point of load application, and using these further for predicting the structure's failure response. Finally, to showcase a possible application, the model is applied to analyse non-prismatic and curved structures. The general formulation presented herein is well-suited for accurate and computationally efficient stress analysis for industrial design applications.

Guadua angustifolia Kunth (Guadua) is a bamboo species native to South and Central America that has been widely used for structural applications in small and large-scale buildings, bridges and temporary structures. Currently, its structural use is regulated within seismic resistant building codes in countries such as Peru and Colombia. Nevertheless, Guadua remains a material for vernacular construction associated with high levels of manual labour and structural unpredictability. Guadua buildings are limited to two storeys due to the overall flexibility of the slender and hollow culms and its connection systems. Its axial specific stiffness is comparable to that of steel and hardwoods, but unlike wood, Guadua’s hollow structure and lack of ray cells render it prone to buckling along the grain and to transverse crushing. As a result, Guadua’s mainstream use in construction and transformation into standard sizes or engineered Guadua products is scarce. Therefore, this work focussed on the development of standardised flat industrial structural products from Guadua devising replicable manufacturing technologies and engineering methods to measure and predict their mechanical behaviour. Cross-laminated Guadua panels were developed using thermohydro-mechanically modified and laminated flat Guadua strips glued with a high performance resin. Guadua was subjected to thermo-hydro-mechanical (THM) treatments that modified its microstructure and mechanical properties. THM treatment was applied to Guadua with the aim of tackling the difficulties in the fabrication of standardised construction materials and to gain a uniform fibre content profile that facilitated prediction of mechanical properties for structural design. Densified homogenous flat Guadua strips (FGS) were obtained. Elastic properties of FGS were determined in tension, compression and shear using small-clear specimens. These properties were used to predict the structural behaviour of G-XLam panels comprised of three and five layers (G-XLam3 and G-XLam5) by numerical methods. The panels were assumed as multi-layered systems composed of contiguous lamellas with orthotropic axes orientated at 0º and 90º. A finite element (FE) model was developed, and successfully simulated the response of G-XLam3 & 5 panels virtually loaded with the same boundary conditions as the following experimental tests on full-scale panels. G-XLam3 and G-XLam5 were manufactured and their mechanical properties evaluated by testing large specimens in compression, shear and bending. Results from numerical, FE predictions and mechanical testing demonstrated comparable results. Finally, design and manufacturing aspects of the G-XLam panels were discussed and examples of their architectural and structural use in construction applications such as mid-rise buildings, grid shells and vaults are presented. Overall, this research studies THM treatments applied to Guadua in order to produce standardised engineered Guadua products (EGP), and provides guidelines for manufacturing, testing, and for the structural analysis and design with G-XLam panels. These factors are of key importance for the use of Guadua as a mainstream material in construction.

Woven glass composites have been used for many years in commercial applications due to their light weight, competitive price and good engineering properties. Absorption of energy by laminated composite material results in damage in various forms, the most common of which is delamination. Inter-laminar fracture causes the layers of composite to separate, resulting in a reduction in stiffness and strength of the composite structure, matrix cracking and in some cases fibre breakage takes place. The aim of this project was to improve the inter-laminar bond strength between woven glass fabric and resin. Air jet texturing was selected to provide a small amount of bulk to the glass yarn. The purpose was to provide more surface contact between the fibres and resin and also to increase the adhesion between the neighbouring layers. These were expected to enhance the resistance to delamination in the woven glass composites.Glass yarns were textured by a Stähle air jet texturing machine. Core-and-effect yarn was produced instead of a simple air textured yarn. Hand loom and vacuum bagging techniques were used for making the fabric and composite panels from both textured and non-textured yarns. Density and fibre volume content were established for physical characterisation. Breaking strength (tenacity) of the yarns and tensile, flexure, inter-laminar shear strength (ILSS) and fracture toughness (mode 1) properties of the composites were determined. Projection microscopy and SEM imaging techniques were used to assess the fractured surfaces of the composite specimens. The yarn tenacity and the tensile properties of the composites were significantly reduced after the texturing process, whereas flexure properties were unchanged. However, significant improvement was observed in the ILSS and fracture toughness of the composites after the texturing process. It was also observed that the composites made from the fabrics with textured yarns in only the weft direction are the most advantageous as they maintained the tensile and flexure properties but have significantly higher inter-laminar shear strength.

Fibre reinforced laminated composites offer many benefits over conventional materials and hence are now used in most engineering sectors. However, their intrinsic in-homogeneity and anisotropic properties make modelling damage initiation and propagation a challenging task. This thesis explores a relatively new semi-analytical approach, Scaled Boundary Finite Element Method (SBFEM), for accurate and efficient modelling of inter-ply damage in thick laminated composites, which is sometimes referred to as interfacial imperfection, debonding or delamination. SBFEM is a tool which combines the benefit of finite element method (FEM) and boundary element method (BEM). In SBFEM, boundaries are solved numerically, whereas an analytical solution is obtained inside the domain. This considerably reduces the computational effort required to solve the problem. In its framework, a 2D plane strain approach is first considered to model the laminated composite plates undergoing cylindrical bending. The work is then extended for 3D modelling to model the bi-axial bending of plates. Unlike 2D plate models, these approaches do not make any assumption on the displacement field and hence, provides superior results. Modelling has been performed in perfectly bonded conditions as well as with interfacial imperfections. Often thick laminates make up the load bearing components of engineering structures; however, they are challenging to model accurately, particularly in cases where out-of-plane loads are significant. Thus, the applicability of the presented modelling technique using SBFEM is assessed over traditional numerical approaches for modelling thick laminated composites. The model is then expanded to study progressive delamination growth using cohesive zone modelling, in pure as well as mixed mode fracture conditions. Finally, for experimental validation of the SBFEM predictions and to justify the application of theoretical approach in practical scenarios, in-house experimentations performed for the fracture studies are modelled. The SBFEM approach for analysis of inter-ply damage in laminated composites is found to be in good agreement with traditional methods while achieving significant reduction in the computational cost. Precise behaviour of laminates can be captured without the necessity of multiple sub-divisions in the throughthickness direction of plies. Moreover, the requirement of having small interface elements can be fulfilled without refining the mesh in the adjoining regions. In this way, SBFEM reduces the computational cost of the model many folds without any compromise in the accuracy. The thesis provides a basis for future research on the application of SBFEM to model inter and intra-ply damage in complex laminated composite structures.

Following a comprehensive literature review on the progress of abrasive waterjet (AWJ) machining, an experimental study of the AWJ machining of carbon fibre-reinforced polymers (CFRPs) of various thicknesses was conducted, showing that clean cuts can be achieved with good processing rates. The effect of process parameters on the machined kerf and hole characteristics is amply discussed in the thesis. It was demonstrated that AWJ machining is a good process for thick CFRPs that other processes may be unable to cut. However, material delamination in the form of edge pop-up in the jet entry and push-out at the jet exit caused by the initial pure waterjet impact of an AWJ piercing operation was observed. It was experimentally shown that using a steel mask on top of the workpiece can eliminate pop-up delamination, while push-out delamination at the jet exit can be reduced or eliminated by proper process parameters. However, the mechanisms involved require further investigation. Mathematical models for predicting the major machining performance indicators were developed using dimensional and regression analysis. Experimental verification confirms that the predictive models are reasonable and reliable for assisting in the planning of AWJ machining processes. A computational model is developed and verified experimentally to study the interaction between a pure waterjet and CFRPs. The behaviour of the waterjet is modelled using the smoothed particle hydrodynamics method while the CFRP is modelled by finite element using a continuum damage material model and cohesive zone method. A computational study using the developed model reveals that the material pop-up delamination is initiated due to the material’s elastic response to a rapid release of shock pressure to stagnation pressure and the traverse shear stresses induced by the downward bending of the laminated layers. The pure waterjet impact causes flow divergence and a hydro wedging effect between the material plies, which propagates the delamination. The delamination magnitude is found to increase initially with waterjet pressure up to a threshold after which a change in pressure does not affect the pop-up delamination significantly. The smallest pop-up delamination area occurs on the [0]12 laminate, followed by the [0/45/90/-45/0/45]s and [0/90]3s laminate. It is also found that the push-out loading towards the jet exit and the hydro wedging effect act jointly to result in push-out delamination.

Delamination is a common failure mode in composite (fiber reinforced and layered) structures subject to low-velocity impacts by foreign objects. To maximize the design capacity, it is important to have reliable tools to predict delamination evolution in laminated composites. The focus of this research is to analyze flexural responses and delamination evolution in laminated composites subject to flexural loading. Analytical solutions were derived from linear elasticity theory and structural mechanics of beam and plate configurations. Formulations and evaluations of the proposed analytical approaches were validated by comparing with results of finite element (FE) simulations in similar settings and published experiment data. Two-dimensional (2D) elasticity theory for laminated panels was extended to analyze elastodynamic responses of pristine panels and quasi-static responses of pre-delaminated panels. A highlight of the approach is exact solutions of displacement and stress fields it provides. Further investigations showed that the 2D elasticity theory is not amenable to a closed-form solution for laminates containing off-axis angle plies due to three-dimensional (3D) states of stress. Closed-form solutions of cohesive zone modeling (CZM) were developed for popular delamination toughness tests of laminated beams. A laminate was modeled as an assembly of two sub-laminates connected by a virtual deformable layer with infinitesimal thickness. Comprehensive parametric studies were performed, offering a deeper understanding of CZM. The studies were further simplified so that closed-form expressions can be obtained, serving as a quick estimation of the flexural responses and the process zone lengths. Analytical CZM solutions were extended analyze quasi-static impact tests of laminated composite plates with arbitrary stacking sequences, aiming to predict critical load, critical interfaces and extent of delamination at that interface. The Rayleigh-Ritz method was used to determine approximate solutions. The predicted results were found in good agreement with FE simulations.

Torsion of cylindrical shafts has long been a basic subject in the classical theory of elasticity. In 1998 Swanson proposed a theoretical solution for the torsion problem of laminated composites. He adopted the traditional formulation of the torsion problem based on Saint Venant's torsion theory. The eigenfunction expansion method was employed to solve the formulated problem. The analytical method is proposed in this study enabling one to solve the torsion problem of laminated composite beams. Instead of following the classical Saint Venant theory formulation, the notion of effective elastic constant is utilized. This approach uses the concept of elastic constants, and in this context the three-dimensional non-homogeneous orthotropic laminate is replaced by an equivalent homogeneous orthotropic material. By adopting the assumptions of constant stress and constant strain, the effective shear moduli of the composite laminates are then derived. Upon obtaining the shear moduli of the equivalent homogeneous material, the effective torsional rigidity of the laminated composite rods can be determined by employing the theory developed by Lekhnitskii in 1963. Finally, the predicted results based on the present analytical approach are compared with those by the finite element, the finite difference method and Swanson's results.

Dimensional stability of tufted carpets has been a continuing problem in the industry for years. When a tufted carpet is installed by the stretch method, it experiences stress relaxation over time which can cause the carpet to buckle, wrinkle and become loose with the only option being a costly re-stretching of the carpet. Since woven carpets seldom require re-stretching, the carpet buckling problem seems to be linked primarily to the tufted construction. This research seeks to analyze the various components of the tufted carpet composite structure and identify the role each component plays in the phenomenon of stress relaxation. Since a carpet is always stretched in both dimensions simultaneously during installation, understanding its biaxial stress decay is important. To this end, a biaxial loading system has been used to test various samples of the primary backing alone (before tufting), primary backing after tufting (with tufts), the secondary backing alone, and the finished carpet after attaching the backings with various binder weights per area. The four variables under consideration include: primary and secondary backing constructions, tufting density, and latex weight, with the secondary backing and latex weight expected to have the greatest effects on stress decay. In order to collect the most information, the biaxial test system was connected to a computer-based data acquisition system to continuously monitor stress levels and generate stress relaxation curves over a 20-hour testing period. A viscoelastic model that included representations of each component in the carpet structure was used to analyze and understand the influence of the components on the stress relaxation of carpets.

Understanding the mechanisms of failure of composite structures and developing mechanism based failure criteria are important considerations in designing structures made of composite materials. The compressive response of composite materials and structures has received considerable attention due to their significance in the aerospace industry and the complexity associated with compressive failure. Several competing failure mechanisms such as fiber instability, fiber/matrix interfacial failure, fiber microbuckling/kinking, delamination initiation and delamination buckling may become active in compressive loading. Environmental effect such as an elevated temperature can alter and affect these failure mechanisms. In this thesis, a micromechanics based finite element predictive model for notched strength of multidirectional laminates is presented. The in-situ shear response of the matrix, the fiber mechanical properties, the lay-up (stacking sequence) and fiber volume fraction serve as input to the model. The prediction of the model is found to match favorably with experimental data. The effect of ply angle and its influence on the failure mechanism are quantified and compared with a set of available experimental data. The present work is the first development of a non-empirical mechanics based failure prediction methodology for notched compressive strength of composite laminates. Both an experimental and an analytical study are presented herein.