Microdamage in the layers of CF Thornel^® T650 8-harness satin-weave composites with a thermosetting NEXIMID^® MHT-R polyimide resin, designed for high service temperatures, is analyzed. After cooling down to room temperature (RT), a multiple intrabundle cracking due to tensile transverse thermal stresses was observed in the [(+45/–45)/(90/0)]_2s laminates studied. Then, the composite was subjected to two ramps of thermal cycling quantifying the increase in crack density in its layers. A comparison of two ramps with the same lowest temperature showed that the highest temperature in the cycle where thermal stresses were low had a significant detrimental effect on the thermal fatigue resistance of the composite. The effect of holding it at 288°C for 40 days was also studied: many new cracks formed in it after cooling down to RT. During the time at the high temperature, the mechanical properties degraded with time, and the crack density versus aging time was measured at RT. Then, both aged and nonaged specimens were tested in uniaxial quasi-static tension quantifying the damage development in layers of different orientation. Cracking in the layers was analyzed using fracture mechanics arguments and probabilistic approaches: a) a simple one, not considering crack interaction; b) Monte Carlo simulations. It is shown that cracking in the off-axis layers which are not in contact with the damaged 90°-layer can be predicted based on the Weibull analysis of the 90°-layer, whereas in the off-axis layer contacting the 90°-layer, the crack density is much higher due to the local stress concentrations caused by cracks in the 90°-layer. The thermal treatment degraded the cracking resistance in the surface and adjacent layer, whereas the composite close to the midplane was not changed.

Carbon fibre T650 8-harness satin weave fabric composites with thermosetting polyimide resin designed for high service temperatures are solidified at 340 °C. High thermal stresses develop after cooling down to room temperature, which lead to multiple cracking in bundles of the studied quasi-isotropic composite. The composites are subjected to two thermal cycling ramps and the increase of crack density in each bundle is quantified. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant effect on thermal fatigue resistance. During thermal aging tests at 288 °C the mechanical properties are degrading with time and the crack density after certain aging time is measured. Aging and fatigue effects are separately analysed showing that part of the cracking in thermal cycling tests is related to material aging during the high temperature part of the cycle. Numerical edge stress analysis and fracture mechanics are used to explain observations. The 3-D finite element edge stress analysis reveals that there is large edge effect that induces a large difference in the damage state between the different layers on the edge. The linear elastic fracture mechanics explains the higher initiated and propagated crack density in the surface layers comparing to the inner layers.

Many types of composite materials are today used in various types of load carrying structures, due to their excellent strength and stiffness to weight ratio. Simplicity, reliability and low cost of the material processing are important factors affecting the final selection.With the textile reinforced composites, the cost-efficiency is reached by using dry preforms which are impregnated by resin infusion, resin transfer molding etc.; this have made a break-through and have been widely used. Textile composites with bundle meso-structure have been studied in this thesis for elastic properties and damage investigations. The first part of this thesis deals with elastic properties modeling for Non-crimp fabric (NCF) based composites for investigating the effect of meso-structure defects on mechanical properties degradation. The objective of the work is to formulate a model for the NCF composite mesostructure in an attempt to investigate the effect of the waviness on stiffness reduction. Moreover, the stiffness calculation methods for the complex geometry are explained and justified and finally, the different geometrical parameters changes are taken into consideration and included in the calculation.The damage initiation and development is presented is the second part, where woven fabric composites designated for high temperature application were investigated under severe thermal conditions to study their thermal stability and their resistance to thermal damage. The mechanical performance of the same composites was studied. The effect of aging was also investigated. 3D models were realized with Finite elements in order to explain the edge effect on the evolution of the cracks observed during the tensile tests. In addition, the differences and similarities in cracking in different layers were analysed using probabilistic approaches (a simple one as well as Monte Carlo simulations with Hashin’s and also shear lag model) and fracture mechanics arguments.

Microdamage in layers of CF Thornel® T650 8-harness satin woven composite with thermosetting polyimide NEXIMID® MHT-R resin was analysed. After cooling to room temperature multiple intra-bundle cracking due to tensile transverse thermal stresses was observed in the studied [(+45/-45)/(90/0)]2s composite. The composite was subjected to thermal cycling quantifying the increase of crack density in layers. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant detrimental effect. Exposure for 40 days to 288°C caused many new cracks after cooling down to room temperature. Both aged and not aged specimens were tested in uniaxial quasi-static tension. Cracking was analysed using fracture mechanics and probabilistic approaches. Cracking in off-axis layers was predicted based on Weibull analysis of the 90- layer. The thermal treatment degraded the cracking resistance of the surface layer and of the next layer.

CF Thornel® T650 8-harness satin weave fabric composite with thermosetting polyimide NEXIMID® MHT-R resin designed for high service temperatures is produced at around 390°C and therefore high thermal stresses develop after cooling down to room temperature. Thermal transverse stresses in bundles/layers are tensile and lead to multiple intra-bundle /intra-laminar cracking. When the composite plate is subjected to large and repeated temperature variations, new cracks can appear due to thermally induced fatigue stress. Experimental results show that the highest temperature inthe cycle, where thermal stresses are low, has a significant detrimental effect on thermal fatigue resistance. Another observed phenomenon is thermal aging: at high temperature the mechanical properties are degrading with time. Aging and fatigue effects were separately analyzed for quasi-isotropic laminates with lay-up [(+45/-45)/(90/0)]2s.

The effect of the 0°-tow waviness on axial stiffness of cross-ply non-crimp fabric composites is analysed using multiscale approach. The curved 0°- and 90°-layers are represented by flat layers with effective stiffness properties and classical laminate theory is used to calculate the macroscopic stiffness. The effective 0°-layer stiffness is calculated analysing isolated curved 0°-layers subjected not only to end loading, but also to surface loads. The surface loads are identified in a detailed finite element analysis and approximated by a sinus shaped function with amplitude depending on the waves parameters. The sinus shaped surface loads are then applied to an isolated curved 0°-layer finite element model together with end loading to calculate the effective stiffness of the layer. Finally, the effective 0°-layer stiffness was successfully used to calculate the macroscopic stiffness of the composite proving validity of the approach being used and showing that, without losing accuracy, elastic properties in the 90°-layers with bundle structure can be replaced by the transverse stiffness of the homogenised 90°-layer material.

The effect of 0∘-tow out-of-plane waviness on the biaxial Non-Crimp-Fabric (NCF) composite axial stiffness is investigated. Homogenizing, the bundle mesostructure of the NCF composite is replaced by layers. Then the composite is represented by a laminate with flat layers with effective stiffness properties representing the curved 0∘-layer and the 90∘-layer with varying thickness. It is shown that the NCF composite knock-down factor characterizing the stiffness degradation has almost the same dependence on wave parameters as the knock-down factor for the curved 0∘-layer. Numerical analysis showed that 90∘-layer knock-down factor versus amplitude curves for different wavelength can be reduced to one master curve which can be described by a one-parameter expression with the parameter dependent on the used material. This observation is used to obtain high accuracy for analytical predictions for knock-down factors for cases with different wavelength and amplitudes based on two FE calculations only.

Lightweight materials with high stiffness and damage tolerance are requested for aerospace, marine and automotive industries. Many types of composite materials are today used in various types of load carrying structures, due to their excellent strength and stiffness to weight ratio. Simplicity, reliability and low cost of the material processing are important factors affecting the final selection. In the last years new types of composites; Non-crimp-fabric (NCF) reinforced composites, where the cost-efficiency is reached by using dry preforms which are impregnated by resin infusion, resin transfer molding etc.; have made a break-through and have been widely used.As its names indicates, NCF composites consist of layers with ideally straight fiber bundles oriented in different directions, knitted by secondary yarn and separated by resin. This technique of dry preforms impregnated by resin infusion or RTM combine a perfect placement of reinforcement with easy, cheap and automated manufacturing. It produces a composite that can be formed easily in complex shapes, with improvement in damage tolerance as well as the out-of-plane fracture toughness. However, the stitching distorts and crimps the fiber bundles, which leads to large out-of-plane waviness. This deviation affects the mechanical properties of NCF composites. The bundle crimps reduces the stiffness and causes incorrect predictions of the laminate elastic properties employing assumption of the classical laminate theory (CLT).In the present study, the fiber tow waviness is assumed as sinusoidal and the undulation effect on the stiffness reduction is analyzed using Finite Element Method (FEM). The waviness parameters i.e. wavelength and amplitude as well as geometrical parameters like bundle thickness are used in modeling the elastic properties of the representative volume element of the waved structure using meso-scale FEM analysis.The possibility of applying CLT for cross-ply NCF composite stiffness determination is approved, by replacing the curved structure by idealized straight one using effective stiffness for the 0⁰- and the 90⁰- layers. The cross-ply NCF stiffness reduction is dominated by the stiffness reduction of the 0⁰-layer. The 0⁰-layer effective stiffness can be determined either by modeling a single curved tow subjected to distributed load, to reproduce its interaction with the neighboring layers, together with symmetry boundary conditions, or using a master curve approach, where a knock down factor is introduced to characterize the stiffness reduction and analytical expression is suggested. This expressions allows for determination of knock down factor for any given wavelength and amplitude of the waviness.

Composite laminates during service undergo complex combinations of thermal and mechanical loading leading to microdamage accumulation in the plies. The most common damage mode and the one examined in this work is intralaminar cracking in layers. The crack opening displacement (COD) and the crack sliding displacement (CSD) during loading reduce the average stress in the damaged layer, thus reducing the laminate stiffness. These parameters depend on material properties of the damaged layer and surrounding layers, on layer orientation and thickness. Previously these parameters have been calculated using finite element method (FEM) assuming linear elastic material with idealized geometry of cracks. To validate these assumptions experimentally the displacement field on the surface of a [90/0/90] carbon fiber/epoxy laminate specimens with multiple intralaminar cracks in the surface layer is studied and the COD dependence on the applied mechanical load is measured. The specimen full-field displacement measurement is carried out using ESPI (Electronic Speckle Pattern Interferometry). The displacement jumps corresponding to cracks are clearly visible and can be used to determine the opening displacement along the cracks. The effect of crack interaction on the COD at high crack density is also investigated.

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.