Anoush Poursartip

This work extends the Integrated Flow-Stress (IFS) model framework to new frontiers, including capturing the complex two-way interactions between the deformation of solid fiber-bed and the flow of fluid resin, as well as predicting the effect of an additional compressible phase (gas or porosity) on resin distribution and deformation of the part. The first generation of IFS models presented a sequential coupling between the flow model, based on Darcy’s law, and the stress model, based on Terzaghi’s principle, to find the distortion of composite part and its fiber volume fraction during prepreg processing. As the sequential coupling failed to account for the effect of solid deformation on resin flow, the 2nd generation 3-Phase IFS (3PIFS) model introduced a state variable called the solidification factor, which implicitly accounts for pressure sharing between the fluid and solid phases and controls the effective shear and bulk moduli of the composite system.The current Multiscale Viscoelastic 3PIFS model benefits from newly developed computational modules and material characterization procedures. Firstly, the set of governing equations in the original 3PIFS are reformulated in mixed form involving displacements and pressure as elemental degrees of freedom in order to facilitate their implementation in commercial finite-element codes. Secondly, this work uses a differential form viscoelastic material model as the constitutive equation of the resin to account for resin hardening and stress relaxation in complex multi hold cure cycles.Moreover, to find permissible values of the solidification factor during consolidation, the effect of this parameter on solid-fluid pressure-sharing is studied. This parameter is also characterized as a function of both the temperature and degree of cure. Finally, a multiscale porosity model is developed to capture the effects of gas entrapment and capillary pressure on the porosity distribution at micro- and macroscale in the prepreg. This model is verified and validated by comparing the predicted transverse strain, pressure, and porosity distribution to the results of several numerical and experimental tests in the literature involving prepreg’s cure and consolidation under one- and two-hold cure cycles.

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Carbon fibre reinforced thermoset composites make for lighter and stronger structures and are widely used for secondary and primary structures of commercial airplanes. The cure cycle is important to achieve a high quality of composite components. Cure path dependency for thermoset composites has received increasing interest from automotive and aerospace researchers as fast cure cycles are required to achieve high-volume and low-cost composites manufacturing. Furthermore, toughened composites are using increasingly complex resin formulations and laminate microstructures to increase fracture and damage resistance. These advances raise the question whether modifying the cure cycle affects microstructures and chemical, physical and mechanical properties of toughened composites.This dissertation studies the processing-microstructure-property relationship for the interlayer toughened thermoset prepreg composite T800SC/3900-2B. The work has three main parts. The first part investigates curing effects on interlaminar microstructure and properties of composite constituents. Laminates were processed to the same degree of cure using different cure cycles. Interlaminar microstructure and particle morphology were examined using optical microscopy and scanning electron microscopy. In-situ elastic modulus of the base resin and toughening particles was characterized using nanoindentation. The second part studies curing effects on elastic properties during cure. Lamina shear modulus development was measured using dynamic mechanical analysis. The third part investigates curing effects on mechanical properties after complete cure. Mode I and Mode II interlaminar fracture toughness were measured using double cantilever beam and end-notched flexure tests. Damage resistance was examined using quasi-static indentation and low-velocity impact test.The work revealed that cure path dependency for T800SC/3900-2B prepreg composites is mainly driven by the glass transition of interlaminar toughening particles. Microstructural characteristics such as the interlayer thickness, particle shape and particle volume fraction, and manufacturing anomalies such as fibre migration into the interlayer, are affected by particle deformation which is dependent on the material state of particles in the pre-gelation stage. Cure-dependent interlaminar microstructures affect both elastic properties during cure and mechanical properties after complete cure. This dissertation discovered a new mechanism for cure path dependency of interlayer toughened composites, glass transition of the toughening particles, which is governed by the cure path in a conversion-temperature-transformation process map.

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Advanced composites are materials growing in importance. In recent years, all major aerospace original equipment manufacturers (OEMs) have invested significantly in this technology, and its use in automotive, alternative energy and industrial applications is rapidly growing. Increases in product size and production scaling, given radically larger and more complex structures and the sheer volume of composites manufacturing, are leading to challenging problems concerning manufacturing risk, such as increasing development time frames and program costs. The use of manufacturing science to address these problems has always been a rational and promising strategy with most research efforts focusing on automation to improve production efficiencies, the development of multiphysics based models exercised in manufacturing simulation software, and the promise of production ‘big data’ analytics given improvements in sensor technologies and machine based learning algorithms. However, it is no longer sufficient to keep adding to this science base without explicitly addressing how manufacturing practice should be changed. In this thesis, qualitative research analysis of two industrial small and medium sized enterprises (SMEs) based in Western Canada is first performed to investigate the use of the composites manufacturing science base to manage technological and market uncertainty, and how the needs and receptor capabilities of OEMs and SMEs differ. Next, a manufacturing outcomes taxonomy explicitly linking the science–technology–practice levels of activity and a hierarchical knowledge model (Equipment–Tool–Part–Material factory ontology) that defines a common nomenclature for organizing composites manufacturing domain knowledge are introduced. A series of high-level manufacturing scenarios are presented to demonstrate this developed framework. Finally, case studies based on the thermal analysis of thick thermoset composites data sets using manufacturing simulation are presented. These case studies represent a starting point for how science based approaches can be used to directly support manufacturing decisions at all stages of the development design cycle. This work represents efforts to introduce a new translational research strategy aimed at both the composites manufacturing research community and the composites industry. Its focus is to encourage the systematic use of composites manufacturing science to transform manufacturing practice, and to support the effective management of increasing manufacturing risk.

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The properties of reinforced composite materials are dominated by the orientation of their reinforcing fibres. This orientation is typically assumed, often implicitly, to be perfect leading to over-predictions of the final part’s strength. Traditional approaches for measuring misalignment are laborious which has limited their adoption.Further, as the desire for manufacturers to collect more and more data increases, both the data collection and reduction techniques must become automated in order to create value.In this thesis, several large data sets are created and ultimately analyzed using purpose-built automated scripts. One of these techniques analyzed over 2500 high resolution micrographs and returned information on over 200,000 fibres. Using the information from this analysis allowed the creation of an analytical model for the fibre bed. This phenomenological model uses the calculated excess length distribution to individually assign a unique excess length to each fibre in the system. It is shown that only with a distribution of excess lengths can the experimental unimodal misalignment distributions be properly modelled.Homogeneously dispersed variability was associated with each of the measured values which included in-plane and out-of-plane fibre alignment, cured and uncured ply thickness, and fibre volume fraction. Save the alignment distribution which lacks a standardized quality descriptor, the other metrics bounded the manufacturer’s data sheet values; however, these measurements showed that a non-trivial amount of variability should be expected in even high quality, aerospace grade, prepregs.A separate series of tests were developed which were able to impart small compressive strains into the compliant uncured prepreg. The localization of the uniformly distributed wrinkles was partially attributed to the homogeneous variability of the prepreg’s underlying architecture. These slow forming wrinkles were shown to have a consistent set of mechanics as fast tool-part debonding.A hypoelastic shear-lag relationship was developed which was able to predict the excess length introduced into the prepreg from the tool. This shear-lag approach predicts a zone of influence for the wrinkles which was experimentally determined using wrinkle initiators. Reducing tool-part interaction or rapid quenching were proposed as mitigation strategies for wrinkle management.

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An accurate and efficient predictive tool for process modelling of fibre-reinforced polymeric composite materials is highly desirable because of high production cost and substantial risk involved in their manufacturing. Process modelling of composite materials is complex due to multitude of physical and chemical processes such as flow of resin and gaseous volatiles through the fibre-bed, thermochemical changes, and stress development. These phenomena are usually simulated sequentially in a so-called “integrated sub-model” framework.In the sequential method, (i) mapping of the results from one state to another is performed in a tedious and inefficient process, (ii) the concurrent occurrence of resin flow and stress development is ignored, (iii) the history of pressure during the flow regime of processing is relinquished, and (iv) the local spatial and temporal variations in resin gelation is not captured. Incorporating the main steps of process modeling into a unified module that captures the various phenomena in a continuous manner would help overcome the foregoing drawbacks of the sequential approach.In this thesis, a new methodology is presented to integrate the simulation of resin and gas flow and stress development into a unified computational modelling framework. The governing equations are developed for the general case of a composite system that initially is regarded as a three-phase liquid-gas-solid system and, as a consequence of curing, the resin undergoes a transition from a fluid-like state into an elastic solid forming a solid cured composite material. The employed constitutive equations provide a continuous representation of the evolving material behaviour while maintaining consistency with the formulations that are typically used to represent the material at each of the two processing extremes. The model is implemented in a 2D plane strain displacement-velocity-pressure (u-v-P) finite element code developed in MATLAB. Various numerical examples are presented to demonstrate the capability of the Integrated Flow-Stress (IFS) model to predict the flow-compaction and stress development throughout the curing process of thermoset composite materials. The interactive effects of resin flow, gas flow, and stress development are investigated and comparisons are made with the predicted results obtained from the application of the stress model alone.

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Process models of composite materials are useful tools for understanding the effect of process parameters and variables and reducing manufacturing risks and costs. The sub-model approach for process modelling has been applied to thermoset composites since the early 1980s. In this approach, analysis is performed with different sub-models such as thermochemical, flow, void and stress, and the analysis results are sequentially transferred from one sub-model to the next, until the analysis is complete. In recent years there has been growing use of high performance thermoplastics such as PEEK and PEKK in aircraft structures, and hence process models for thermoplastics are increasingly of interest.During processing of thermoplastic materials, the material undergoes both melting and crystallization. Therefore a major component of the thermochemical/thermophysical sub-model for process modelling of thermoplastics is the crystallization/melt kinetics model. Most of the crystallization kinetics models in the literature are only valid for either constant temperatures or cooling at constant cooling rates. Furthermore, the number of melt kinetics models is very limited and their application restricted to small heating rates. As a material point in the part may undergo complex temperature cycles, a rate-type crystallization/melt kinetics model which is independent of the temperature cycle is desired.Another problem in processing is development of residual stresses and distortions, which are analyzed in the stress sub-model using mechanical response constitutive models such as thermo-elastic, CHILE and viscoelastic. Most thermoplastic materials such as PEEK are indeed viscoelastic, however their unrelaxed values of moduli are temperature dependent, ie their behaviour is ‘thermo-rheologically complex’.In this thesis the crystallization and melt behaviour of PEEK carbon fibre composites is investigated using DSC experiments. A rate type crystallization kinetics model is developed for prediction of degree of crystallinity during crystallization process. A concept of ‘master melt curve’ is introduced and is used along with the crystallization kinetics model for prediction of crystallinity change during an arbitrary process. Thermo-viscoelastic behaviour of the material is studied using DMA experiments. A thermo-viscoelastic (TVE) constitutive model is developed and is generalized to three dimensional cases. Some case studies are analyzed and validity of models are investigated.

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Modern composites such as advanced fibre-reinforced and strand-based wood composites are increasingly being used in the new generation of aerospace and civil engineering structures. The structural analysis of such composites often requires knowledge of their effective (homogenized) properties. Several micromechanical models have been developed and are available in the literature for predicting the effective elastic properties of fibre-reinforced solid composites. However, the underlying assumptions in these models somewhat limit their application in solving some practical problems related to the viscoelastic behaviour of composite materials. Two seemingly different classes of composites, i.e. thermoset fibre-reinforced composites and strand-based wood composites with distinct viscoelastic properties are considered in this work due to their wide application in aerospace and construction industry. For viscoelastic analysis of such materials, aspects which require further investigations at the micro-scale are identified first. Specifically, available analytical micromechanics models are extended to predict the shear properties of thermoset fibre-reinforced composites during cure where the resin evolves from a viscous fluid to a viscoelastic solid. For strand-based composites consisting of high volume fraction of orthotropic wood strands, analytical micromechanics models are developed. These models are employed for predicting the effective elastic and viscoelastic properties of strand-based composites. The validity ranges of these models are then examined using experimental data or numerical reference solutions that employ the computational homogenization technique.To enable viscoelastic analysis of large scale composite structures with generally orthotropic properties, an efficient and easy-to-implement approach in the context of 3-D multi-scale modelling, is presented. A multi-scale modelling framework involving analyses at different scales for composites with two difference microstructures is developed and implemented in a general purpose finite element code, ABAQUS®. The accuracy of the developed multi-scale approach is demonstrated for some practical applications involving MOE (apparent modulus of elasticity in bending) prediction of strand-based wood composites. Using this approach, the effect of microstructural parameters (e.g. fibre geometry, orientation, waviness, volume fraction, etc.) on the time-dependent macroscopic response of orthotropic composite structures can be investigated, quantitatively.

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The usual approach to the process modeling of thermoset matrix composites is to divide the analysis into two distinct and sequential steps, first of flow-deformation behaviour and then of stress-deformation. In the current processing models, each of these two aspects is dealt with in a separate sub-model, typically called the flow module and stress module respectively. The flow module is relevant to the pre-gelation behaviour of resin, while the stress module is valid for the post-gelation composite material. In this thesis, the framework to integrate the flow and the stress modules into a unified module in finite element processing models is presented. The work is based on a two-phase model for analysis of resin flow and its resulting deformations in the composite material. Special measures are introduced to provide for additional capability of this model to account for the development of stresses in the curing composite material. These modifications are needed to ensure the accuracy of the model in both of resin flow and stress development regimes, and include the introduction of consistent compressibility in the mass conservation equation of the two-phase system, and a special decomposition of stresses of the system.The formulation is implemented for a pseudo-viscoelastic stress model in a 2D plane strain FE code in MATLAB. The approach may readily be extended to fully viscoelastic models. Various examples from single-element problems dealing with the development of residual stresses throughout a single-hold cure cycle to more geometrically complex composite laminates undergoing standard cure cycles are modeled by the integrated model and comparisons are made in one extreme to the flow-compaction behaviour by the standard flow models, and in the other extreme to the results obtained by the pseudo-viscoelastic approach.The model developed here is a promising tool for simulating processing of large-scale composite structures continuously from the very early stages of the process when the resin behaves in a fluid-like manner all the way to the final stage when it behaves as a 3D solid.

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