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Fiber-reinforced polymer composites, such as carbon-epoxy composites, are found to exhibit non-linear behavior when mechanically loaded in the transverse and shear directions. Experimental studies suggest that the non-linear behavior is due to two mechanisms: (i) damage in the matrix in the form of cracks and (ii) yielding of the matrix followed by plastic deformation. In this study, a bimodulus-plastic model that includes these two different damage mechanisms to simulate the non-linearity prior to failure is proposed. The pre-failure, onset of failure and post-failure analysis with the proposed model is discussed in detail with emphasis put on the transverse and shear components. The process of determining the material properties and parameters required for defining the new model is discussed and demonstrated. The newly developed model is then validated against the experimental results from three-point flexure tests on the composites. The correlation was good showing that the proposed model was able to simulate accurately the non-linear behavior of the composites and thus predict the failure. Finally, the model is applied to a case study on the failure of a composite riser under internal pressure loads. Accepted version

Textile non-crimp carbon fabrics (NCCFs) are made of several dry uni-directional (UD) tows which are aligned straight and stitched together. The use of these reduces resin rich areas and stress concentrations that are likely found in woven fabrics composites during manufacturing. The use of spread tows to manufacture the thin plies leads to the fibre areal weight (FAW) of lower than 150 g/m2. This current research aims at investigating the tensile, flexure and interlaminar shear response of thin ply carbon composites with a view to deduce the failure mechanisms. The baseline comparison is carried out with conventional thick ply composites and differences in the failure mechanisms are studied. For tensile tests, reduced number of acoustic emission counts in the case of thin ply composites indicated that the stresses were unaffected until the laminate ultimate strength. In flexural tests, it was noticed that the resin rich sites near the stitches first initiated compression failure followed by translaminar cracking or longitudinal fibre fracture. This is due to the strong fibre-matrix adhesion in the case of thin ply composites. No interlaminar shear fracture was seen to occur throughout the laminate thickness and across the width, leading to a significant improvement in the flexural properties of the thin ply composites. These thin laminates have better interlaminar shear properties. It was observed through microscopy that thin ply laminates first undergo interlaminar shear followed by inelastic deformation at the ultimate strength of the material.

Fiber reinforced thermoplastic composites are gaining popularity in many industries due to their short consolidation cycles, among other advantages over thermoset-based composites. Computer aided manufacturing processes, such as filament winding and automated fiber placement, have been used conventionally for thermoset-based composites. The automated processes can be adapted to include in situ consolidation for the fabrication of thermoplastic-based composites. In this paper, a detailed literature review on the factors affecting the in situ consolidation process is presented. The models used to study the various aspects of the in situ consolidation process are discussed. The processing parameters that gave good consolidation results in past studies are compiled and highlighted. The parameters can be used as reference points for future studies to further improve the automated manufacturing processes.

One of the main downsides of fiber reinforced polymer composite laminates is their susceptibility to impact damage. Many different methods have been devised to either modify or engineer the composites to improve their impact resistance and tolerance. In this chapter, three types of engineered composites and their impact performances are presented and discussed. The engineered composites are (i) composites with core–shell polymer particles, (ii) composites with carbon nanotubes/nanofibers and (iii) composites with thermoplastic film interleaves. Core–shell polymer particles absorb large amounts of energy during impact, thus limit the damage done to the composite laminate. Carbon nanotubes and nanofibers have excellent stiffness and strength, making them suitable for reinforcing the interlaminar regions of composite laminates, leading to improved impact resistance. For composites with thermoplastic film interleaves, thermoplastic films with high intrinsic toughness modify the interlaminar regions of the composite laminate and enhance the overall toughness. The three types of engineered composites exhibit varying levels of improved impact performances compared to the unmodified composites. For each type of engineered composite, its impact response, the strengthening mechanisms and factors affecting its impact performance are deliberated.

Advanced manufacturing techniques, such as automated fiber placement and additive manufacturing enables the fabrication of fiber-reinforced polymer composite components with customized material and structural configurations. In order to take advantage of this customizability, the design process for fiber-reinforced polymer composite components needs to be improved. Machine learning methods have been identified as potential techniques capable of handling the complexity of the design problem. In this review, the applications of machine learning methods in various aspects of structural component design are discussed. They include studies on microstructure-based material design, applications of machine learning models in stress analysis, and topology optimization of fiber-reinforced polymer composites. A design automation framework for performance-optimized fiber-reinforced polymer composite components is also proposed. The proposed framework aims to provide a comprehensive and efficient approach for the design and optimization of fiber-reinforced polymer composite components. The challenges in building the models required for the proposed framework are also discussed briefly.

Interlaminar fracture or delamination is one of the major failure modes for fiber reinforced polymer (FRP) and fiber metal laminate (FML) composites. Many studies have been carried out to investigate the mechanisms affecting this interlaminar failure and ways to improve the interlaminar fracture toughness (ILFT) of FRP composites and FMLs. However, information on the topic is scattered, with studies having different emphases. Therefore, in this review, the studies are surveyed and put into perspective in terms of the improvements to ILFT achieved. A wide range of methods to improve the ILFT of FRP composites and FMLs is presented. For FRP composites, the methods include modifications to the constituents or interlaminar regions of the composite, applications of nano-fillers in multiscale composites approaches, and through thickness reinforcement techniques. For FMLs, studies on various surface treatment methods for the metal in FMLs are included. The strengthening mechanisms involved in each of the methods are also discussed. This review provides a comprehensive picture of the different methods to improve interlaminar interfaces in laminated composites, including their performances and drawbacks, as well as the future trends in the related topics.

The bonding between metal and carbon fiber-reinforced polymer (CFRP) composites is important in structures such as composite risers. Surface treatment of the metal is required to improve the metal-composite bonding. In this study, the effects of two mechanical surface treatment methods on the bonding between carbon/epoxy composites and aluminum are studied. The treatment methods investigated are grit blasting and grooving. Mechanical tests are performed and the test results are used with finite element simulations to study the failure and damage mechanisms at the composite-metal interface. The mechanical response and damage propagation at the metal-composite interface are modeled successfully using surface-based cohesive behavior. The finite element simulations gave insight into the way that the grooving treatment method improved the composite-metal bonding.

The use of composite materials such as carbon/epoxy for the construction of deep water risers in place of metals can lead to significant weight reduction and cost savings. In order to realize this potential, a thorough understanding of the mechanical behavior and failure modes of composite risers is required. In this study, the progressive damage analysis technique and finite element simulations are used extensively to investigate the failure of composite risers. The nonlinear behavior of composites in the pre-failure stage is taken into consideration in the bimodulus-plastic model developed in this study. The inclusion of the nonlinear behavior can result in better predictions in the finite element simulations. A predictive model is also developed to determine the tensile properties of composite materials affected by moisture. The model is based on the use of additive group contributions for polymers. The tensile properties calculated using the model agreed well with experimental data. The effect of moisture on the compressive properties of composite materials is also investigated in experiments and simulations. Failure at the liner-composite interface is also considered in this study. The failure and damage mechanisms at the interface for the grooving and grit blasting mechanical surface treatment methods are investigated and discussed. The behavior and failure of the liner-composite interface are modeled successfully in finite element simulations. The local analysis of a composite riser segment using finite element simulations is demonstrated. The progressive damage analysis method is effective in predicting failure, especially for cases where damage due to accidental load has occurred. Doctor of Philosophy (MAE)

The study of crystal growth using molecular dynamics is very difficult due to the slow rate of growth and the complex intermolecular interactions involved. In order to perform molecular dynamics si...

A model to estimate the tensile properties of carbon fiber-reinforced polymers (CFRP) based on their moisture contents and molecular structures is formulated and studied. In the proposed model, water molecules absorbed by polymers are considered congregated near the sub-micro-cavities within the polymers. The absorbed water molecules form hydrogen bonds with the polymer molecules and cause the regions around the sub-micro-cavities to have a reduced elastic modulus. The additive group contributions for polymers are used to determine the contributions of hydrophobic and hydrophilic functional groups separately. This enables the effects of water molecules on the mechanical properties of polymers to be quantified and estimated. For CFRP laminae, an additional effect of moisture on the fiber-matrix interface is considered in the calculation of the transverse tensile strength. The tensile properties are calculated using the proposed model agreed well with the experimental values. It is expected that the time and materials required to obtain the mechanical properties of CFRP exposed to humid environments can be reduced with the help of this model.