Reasons for nonlinear stress-strain curves in shear of unidirectional glass fibre composite are analysed. Laminate with stacking sequence [45/-45]s is used in tensile quasi-static as well as in tensile creep and strain recovery tests to study the development of viscoelastic and viscoplastic shear strains in local coordinates of the ply. It is shown that Zapa’s integral representation of viscoplasticity is applicable for this material and methodology for parameter determination is given. Schapery’s nonlinear viscoelastic material model was used for shear response characterization and the nonlinearity parameters’ dependence on the shear stress was determined and described by fitting functions. Microdamage development is quantified by measuring axial modulus and Poisson’s ratio of the laminate. The obtained nonlinear viscoelastic, viscoplastic model with included effect of microdamage was successfully used to predict the nonlinear shear stress-strain curve in strain controlled tensile loading.

The potential of accurate modelling of the shear modulus reduction of laminates with cracked 90-layers using models based on the minimization of the complementary energy with improved stress description in the constraint layers is evaluated. This group of models refine Hashin’s model by introducing shape functions with unknown parameters to represent the out-of-plane shear stress distribution across the constraint layer thickness. The Hashin’s model becomes a particular case of the presented when the shape parameter approaches to zero. The most accurate shape parameters are found in the result of minimization. Three models are compared: the present variational model, Hashin’s model and the shear lag model introduced by Soutis which assumes linear out-of-plane shear stress distribution over an unknown part of the layer. It is shown in this paper that the size of this part may be determined by minimization of the complementary energy. The present model is the most accurate amongst the three, whereas Soutis’ model is more accurate than the Hashin’s model for laminates with constraint layer, thicker than the cracked layer. The comparison with finite element method results shows reasonable agreement. Agreement can be improved developing models with better description of the stress state in the cracked layer.

The effect that different curing time/temperature conditions bring to the final properties of a polymeric resin, along with measurements of their chemical shrinkage have been investigated in the present study.

In the present thesis, the inelastic behavior of polymer composites is investigated. This investigation concerns three fields of study; time dependent behavior during the lifetime of the composite material, influence of micro-damage on its overall mechanical performance and development of chemical shrinkage strains during the curing process. The significance of this work is related to the nature of all composite materials. All polymer composites tend to indicate an inelastic behaviour. This behaviour can be either linear or non-linear. No matter what it is, is very important to be taken into account in the analysis, since it is related to strain rate effects, micro-damage induced to the structure of the composite and/or irreversible plastic strains.The first part of this thesis consists of the time dependent behavior of polymer composites. There are two main assumptions; irreversible strains in a damaged state are higher and that the strains can be decoupled into viscoelastic and viscoplastic response. Each assumption is investigated and a material model that includes all the above is compiled. In order to examine its validity, different material categories have been examined. Pure polymer (paper I), polymer reinforced with short fibres (Paper II), polymer reinforced with continuous fibres (Paper III). As a step further on, the time dependent behavior within a ply level was examined. A [45/-45]s laminate was used and the non-linear shear stress strain response was studied (Paper IV). In the first part of the thesis, damage was only quantified in terms of elastic modulus development after high stress application without going into detail in what is causing it. In Paper V, the effect of damage, in terms of crack density on shear elastic modulus was studied. More accurate expressions for stress calculations in the damaged lamina were suggested, by incorporating shape functions and by checking validity with the principle of minimum complementary energy. Finally, the results from the suggested model are compared with existing models and with results from finite element analysis. A small improvement is observed at all cases. Finally, in Paper VI, the effect of curing parameters and development of chemical strains during the curing process was investigated. A relation between curing time, degree of cure and mechanical performance was drawn. What is more, different procedures for measuring chemical strains were used and a testing methodology is suggested.

Nanocomposites based in polypropylene (both syndiotactic and isotactic) and multiwall carbon nanotubes (MWNTs) directly blended in the melt via micro-extrusion, were studied. The aim was to determine the role of MWNTs, on the creation of the plastic zone and crack propagation in double-cracked tensile specimens. Essential work of fracture toughness testing was performed on these nanocomposites. In all cases, the effect of MWNT weight fraction on the above properties was extensively studied. Data obtained have shown a non-linear relationship of almost all properties with MWNTs content.Scanning electron microscopy revealed that a micro crack/craze bridging mechanism can be detected in polymer matrix/ MWNT nanocomposites. This mechanism was observed in the fibrillated areas of the specimens' fractured surfaces. The results obtained showed a promising role of the nanotubes as tougheners for future nanocomposites' applications. Ductile-to-brittle transition was confirmed for MWNT contents above 1 wt%.

The nonlinear and time dependent stress-strain response of non-crimp fabric (NCF) [±45] s laminates in tension is studied. Testing methodology is suggested to separate and quantify the effect on the inelastic response of damage development, nonlinear viscoelastic effects and viscoplasticity. Material model based on Schapery’s work on viscoelasticity and Zapas viscoplastic functional with added damage terms is used. Simulation is performed and validated with constant stress rate tensile tests, identifying the nonlinear viscoelasticity and viscoplasticity as the major sources of the nonlinear response.

The most common damage mode and the one examined in this work is the formation of intralaminar cracks in layers of laminates. These cracks can occur when the composite structure is subjected to mechanical and/or thermal loading and eventually lead to degradation of thermo-elastic properties. In the present work, the shear modulus reduction due to cracking is studied. Mathematical models exist in literature for the simple case of cross-ply laminates. The in-plane shear modulus of a damaged laminate is only considered in a few studies. In the current work, the shear modulus reduction in cross-plies will be analysed based on the principle of minimization of complementary energy. Hashin investigated the in-plane shear modulus reduction of cross-ply laminates with cracks in inside 90-layer using this variational approach and assuming that the in-plane shear stress in layers does not depend on the thickness coordinate. In the present study, a more detailed and accurate approach for stress estimation is followed using shape functions for this dependence with parameters obtained by minimization. The results for complementary energy are then compared with the respective from literature and finally an expression for shear modulus degradation is derived.

Non Crimp Fabrics (NCF) are promising new generation composite materials. They are now being used in some sections of composite industry, for example in wind turbine blades and boat hulls. The aerospace industry also shows an increasing interest in this material, thanks to the low cost of its manufacturing process. NCFs are special types of textile composites, made of layers of parallel fiber bundles oriented in different directions and separated by resin. Due to the manufacturing process the fiber bundles are not perfectly straight. They show a certain degree of waviness which decreases the stiffness and the strength of the material. The heterogeneous mesostructure affects the mechanical properties of the material and the failure mechanisms. This was studied using both numerical and experimental methods. In our experimental approach, a carbon fiber/epoxy resin laminate with uniform fiber distribution was manufactured by voluntarily introducing waviness to simulate the NCF composites. The displacement map was studied against the thickness of a sample loaded in tension, using ESPI (Electronic Speckle Pattern Interferometry). This can give us a primary idea of the micro damage initiation and the cracks' shapes.

The effect of maleic anhydride grafted polypropylene (MAPP) coupling agents on properties of a new composite made of recycled carbon fibers and recycled polypropylene (rCF/[rPP + MAPP]) was studied experimentally. This new material presented significantly improved properties, compared to the previous generation, without the addition of MAPP (Giannadakis K, Szpieg M and Varna J. Mechanical performance of recycled carbon fibre/PP. Exp Mech 2010; published online.). This was mostly attributed to improvement of the fiber/matrix interface. The inelastic and time-dependent behavior of the MAPP modified composite material in tension was analyzed. A series of quasi-static tensile and creep tests were performed to identify the material model, which accounts for: (a) damage-related stiffness reduction, (b)development of stress and time-dependent irreversible strains described as viscoplasticity, (c) nonlinear viscoelastic behavior. The damage-related stiffness reduction was found to be less than 10%. Although damage-dependent stiffness was not the main source of nonlinearity, it was included in the inelastic material model. In creep tests, it was found that the time and stress dependence of viscoplastic strains follows a power law, which makes the determination of the parameters in the viscoplasticity model relatively simple. The viscoelastic response of the composite was found to be linear in the investigated stress domain. The material model was validated in constant stress rate tensile tests.

A composite made of recycled carbon fibres in recycled polypropylene matrix is studied experimentally to describe the features of the elastic and time dependent nonlinear mechanical behaviour. The properties of the developed material have a large variability to be addressed and understood. It was found that the stress-strain curves in tension are rather nonlinear at low strain rate and the strength is sensitive to strain rate. The elastic properties' reduction for this composite after loading to high strains is rather limited. More important is that even in the "elastic region" due to viscoelastic effects the slope of loading-unloading curve is not the same and that at higher stress large viscoplastic strains develop and creep rupture is typical. The time and stress dependence of viscoplastic strains was analysed and described theoretically. The viscoelastic response of the composite was analysed using creep compliance, which was found to be slightly nonlinear