Anoush Poursartip


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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - April 2022)
The effect of cure cycle on microstructure and mechanical properties of interlayer toughened composites (2021)

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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A framework for formalizing science based composites manufacturing practice (2019)

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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Misalignment defects in unidirectional composite materials (2019)

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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A three-phase integrated flow-stress framework for process modelling of composite materials (2017)

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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Crystallization and thermo-viscoelastic modelling of polymer composites (2017)

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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Efficient Multi-Scale Modelling of Viscoelastic Composites with Different Microstructures (2014)

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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Integrating resin flow and stress development in process modeling of themostat composites (2013)

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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Master's Student Supervision (2010 - 2021)
Process induced deformation of composite materials: an experimental methodology, systematic review and meta-analysis (2021)

The ability to consistently produce composite structures with controlled tolerance remains a challenge for commercial aerospace applications. Dimensional discrepancies between the true geometry and designed geometry leads to custom shimming processes or forced assembly, which can be costly and/or reduce structural performance. Thus, further understanding and better management of process-induced deformation (PID) is needed.In the present work, process induced deformation, manifesting itself as spring-in and warpage of L-shapes, is studied experimentally. A rigorous methodology is developed to address the gaps in the literature and identify the variabilities in the composite manufacturing processes. The methodology includes an accurate and automated analysis method whereby point cloud data of the L-shapes obtained by laser CMM is processed to distinguish the nuances of spring-in and warpage. Processing parameters such as laminate dimensions and cure cycles, which have been under-studied or the cause of disagreement, are shown to have meaningful impact on PID. The second half of the work presents a systematic review conducted for 94 experimental studies and over 2000 process induced deformation specimens from the open literature. This dataset is believed to be representative and as thorough as possible. A meta-analysis was performed on a subset of specimens made with three materials systems: HEXCEL AS4/8552, TORAY T800/3900-2 and CYCOM IM7/5320-1. This systematic review reveals disagreements within the PID literature and highlights the high variability in the composite manufacturing process which hinders direct comparison across studies and full understanding of PID. The meta-analysis investigates the data consistency, and probes the influences of laminate thickness, layup type, gelation temperature and other processing parameters, providing insight into the spring-in phenomena as seen by the combined literature.

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The effect of impactor mass and velocity on the structural response and damage modes of non-crimp and 2D braided CFRP panels (2021)

The market demand for reduced operating costs and carbon emissions, together with competition and regulatory changes are primary drivers for improving vehicle efficiency. One method of improving efficiency is lightweighting, where composite materials can play a role due to their high specific stiffness and strength.Aircraft manufacturers have made use of these properties for many years, but they have the benefit of longer design programs (4-8 years) and production run lifetimes (20-30 years) to make extensive prototype testing feasible and economical. Automotive manufacturers do not have such long timescales or price-inelastic customers and must largely rely on computer simulation to design fit-for-purpose components in compressed timelines. Unfortunately, accurate structural simulation of composite materials is a challenge, owing to various path dependent failure modes and the influence of manufacturing processes on the material properties throughout the part, among other complexities. Consequently, new composite product design still relies heavily on physical testing.The aim of this thesis is to assist with improving modelling capabilities by providing insight into how damage develops in composite materials of interest to the automotive industry, through instrumented testing and damage characterization. These tests range from quasi-static deflection to high-velocity transverse impact. The materials investigated are non-crimp fabric (NCF) and 2D braid carbon fibre reinforced epoxy because they show promise for economical, high-rate manufacturing.It is found that the 2D braid does not show a delamination test rate sensitivity, owing to the stiffness mismatch being constrained within a single layer that can arrest cracks with its intra-ply fibres. The result is more localized fragmentation in the 2D braid material. Conversely, NCF materials delaminate substantially more under high-velocity impact conditions due to large shear stresses early on, combined with large stiffness mismatches between plies and the lack of crack arresting features. Damage resistance and permanent dent depth decrease substantially under high-velocity impact in both materials; making the damage easier to create and less detectable with surface-only inspection methods.A numerical model of an impacted NCF panel is also developed and compared to the experimental results, showing good agreement with the structural response (force-displacement), but under-prediction of delamination area.

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Experimental study of the pre-gelation behaviour of composite prepreg (2020)

As the composite industry grows, the science base that supports the industry must also grow. In the past, most of the work done on thermoset composite prepreg systems have studied the post –gelation response, leaving the initial condition of the material relatively unexplored. The initial, or pre-gelation, state of the material is becoming increasingly relevant as the industry transitions to new manufacturing processes, mainly automatic fibre placement (AFP) and forming, that handles and manipulates the raw prepreg. Recent works investigating manufacturing defects in composite parts have shown a strong connection between the initial condition of the material and the final part quality. As research and industry shift to working with raw prepreg, it is time to re-examine the pre-gelation behaviour in order to improve processes in the future.This work studies the pre-gelation behaviour of unidirectional Hexcel AS4/8552 prepreg through several novel test methods. The methods presented offer a different perspective into the behaviour of the material. Initially a microscopic perspective was taken, and SEM microscopy was used to characterize the initial morphology of a ply of prepreg. This study presents the significance of variations in morphology and the impact of capillary flow driven ply consolidation. Next, to evaluate the macroscopic behaviour of prepreg, a non-contact approach using Digital Image Correlation (DIC) was developed. This method was able to capture global and local strain responses in the prepreg throughout processing. Findings show that the pre-gelation response is not as simple as previously thought. The material experiences a strong consolidating force when heated due to the effects of surface tension. Further tests performed in this study attempt to make connections between local morphology, resin variations and local variations in strain. These tests used DIC, IR thermography, optical microscopy, and SEM.New insights into prepreg conditions are gained by evaluating the common trends seen with this suite of testing. This work concludes that pre-gelation behaviour in prepreg is a highly complex state dependent on the variability of the material and the liquid-solid interfaces in the ply, as opposed to the traditional approach which considers the material response through a solid mechanics lens.

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Porosity in Configured Structures Effect of Ply Drops and Caul Sheets in the Processing of Composite Parts (2016)

Fiber reinforced polymer composites offer a variety of processing advantages in the manufacturing of high performance structures. However, due to a variety of potential quality defects, there is a great deal of risk associated with composite processing. Amongst these numerous defects, porosity has always been one of critical concern. Although there exists a great deal of literature on the subject, the bulk of existing research to date is restricted to the processing of flat uniform parts. As such, there exists a discrepancy between the current academic understanding and the practical knowledge needed in current practice. The objective of this thesis is to advance our knowledge of porosity to processing scenarios commonly seen in current practice. This has been done by conducting a comprehensive examination of the relationship between the mechanisms driving void evolution and the use of two commonly used structural configurations. These are ply drops and caul sheets. In this study, a series of configured composite parts were manufactured to parametrically assess the effect of ply drops and caul sheets both separately and in combination. The porosity content and final thickness profile of the parts were evaluated through optical microscopy and thickness measurements. The results from this parametric study show that resin pressure shielding due to lack of compliance between the laminate and the caul sheet can be a primary cause of porosity. It has also been found that lack of compliance caused resin migration which carries with it adverse effects on final part quality. The resin pressure distribution of ply-drop laminates processed with and without caul sheets was tracked in-situ through the use of instrumented tooling. The results from these experiments support the findings of the parametric study and provide a comprehensive understanding of the dominant mechanisms. These mechanisms where simulated with state of the art finite element software. These simulations demonstrate that commercially available software packages can be used to enhance our understanding of void evolution in complex processing scenarios. As such, the findings presented in this thesis are of great engineering value to current practices since they can be applied to a wide variety of applications.

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Experimental characterization of the viscoelastic behavior of a curing epoxy matrix composite from pre-gelation to full cure (2013)

Process models have been used to predict the flow and stress development during manufacturing of composite structures for over 20 years. To date, these models have been treated separately, and the required material property models (viscosity and modulus) have also been treated separately. The latest breakthrough in process modeling of composite structures is an integrated stress-flow model. However, a consistent viscosity and viscoelastic material model required by the integrated stress-flow model has not been developed. Presented in this thesis is a consistent material model that predicts the viscoelastic liquid and viscoelastic solid behavior of a commercially available thermoset polymer, namely MTM45-1 epoxy. The goal here is to show that a single material model can predict the viscosity and viscoelastic modulus for all temperature, degree of cure and time scales encountered in composite manufacturing. The model was generated by fitting a generalized Maxwell model to test results from both dynamic mechanical analysis and rheological tests. Both resin and prepreg samples were examined. Thermo-rheological complex behavior was captured by applying linear temperature dependence to the un-relaxed modulus. The effect of cure was accounted for by applying a degree of cure dependent shift function. The relaxed modulus was predicted using the cross-link concentration and the theory of rubber. Excellent agreement was found when comparing predictions from the model to experimental data ranging from temperatures of -50°C to 245°C, degree of cure of 0.01 to 1.0 and frequencies of 0.01Hz to 10Hz.

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Thermal modeling validation techniques for thermoset polymer matrix composites (2011)

Process modeling is becoming a widely-accepted tool to reduce the time, cost, and risk in producing increasingly large and complicated composite structures. Process modeling reduces the need for physical parts, as it is not practical or economical to design and fabricate large composite structures using a trial-and-error approach. The foundation of the composite manufacturing process, and thus of process models, is the thermal history of the composite part during cure. Improperly curing the composite part will compromise its mechanical properties. Consequently, proper validation of the thermal model input parameters is critical, since the simulation output depends on the accuracy of the input. However, there are no standard methods to validate thermal process model input parameters. In this work, repeatable and robust methods were developed to isolate and validate the conductive heat transfer, thermochemical, and convective heat transfer sub-models. By validating the sub-models, the uncertainty of the complete thermal simulation was significantly reduced. Conductive and thermochemical material models were validated by comparing the thermal response of a material surrounded by rubber bricks to a 1-D simulation of the same materials. Four composite prepreg systems and their respective material models were tested, with agreement ranging from excellent (errors less than 1.0 °C) to poor (errors greater than 5.0 °C).Calorimetery, visual monitoring, and CFD were used to characterize the convective heat transfer environment inside the UBC autoclave. The validation methods were also used to better understand the capabilities and limitations of the autoclave. Local variations in airflow patterns and heat transfer coefficients showed that heat transfer can be highly variable in an individual piece of equipment. Simple procedures for characterization of an autoclave or oven were demonstrated. The developed methods can be used individually, or in combination, to validate thermal models and reduce uncertainties associated with the cure of composites. With further refinement, the demonstrated methods can be developed into validation standards for thermal modeling of composite materials.

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