PhD student: Kiho Cho
Title: Processing and Characterisation of S-Glass Fibres and Halloysite Nanotubes for Flowable Dental Composites
Year: 2020
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Prof. Martina Stenzel, Dr. Ginu Rajan, Paul Farrar, Dr Raju
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Abstract: Over the past few decades, various types of filler materials have been employed to develop the advanced resin-based dental composites, enhancing the lifetime of the restorations. However, further effort in the research on the multi-functional composite that is comparable to dental tissue in mechanical strength, as well as offering the improved antibacterial function and the better aesthetics, is continuously required. In this thesis, micro-sized short S-glass fibres and halloysite nanotubes (HNTs) are employed to serve as excellent load-carrying filler members and antibacterial agent in the dental composites. The mechanical reinforcement mechanism and the interfacial behaviours between filler and resin matrix have been precisely investigated through the multiscale analysis from atomistic to macro by utilising the combined experimental, theoretical, and computational methods. The surface modification process on the short S-glass fibres, named selective atomic-level metal etching, has been developed, which enables to strengthen the interfacial bond between resin matrix and glass fibre by increasing the surface roughness and reactive sites on the fibre. The influence of the surface treatment on the interfacial strength and mechanical properties of the resulted composites were examined through the single-fibre pull-out tests. Also, the modified Lewis-Nielsen model has been developed, where the effective fibre length factor is applied to accurately predict the modulus of the short fibre reinforced composites. For better understanding of the atomistic interfacial bonding and fracture behaviours between glass fibre and resin matrix, molecular dynamics simulations were conducted. The numerical results of the single fibre pull-out and the uniaxial composite tension simulations were validated with the experimental findings. The optimised computational design and analysis methods were established for developing new dental and bio-composites with the accurate prediction on the mechanical performances. The surface modification process on the HNTs was developed to promote the mechanical reinforcement effect and to add an antimicrobial functionality in the composites. The composite reinforced with 2.0 wt.% of chitosan grafted HNTs showed an increased efficacy in flexural strength and modulus up to 8.1% and 14.1%, respectively, and exhibited an improved antibacterial functionality against S. mutans with 39% reduction, making it a desirable dental material.
Title: Processing and Characterisation of S-Glass Fibres and Halloysite Nanotubes for Flowable Dental Composites
Year: 2020
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Prof. Martina Stenzel, Dr. Ginu Rajan, Paul Farrar, Dr Raju
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Abstract: Over the past few decades, various types of filler materials have been employed to develop the advanced resin-based dental composites, enhancing the lifetime of the restorations. However, further effort in the research on the multi-functional composite that is comparable to dental tissue in mechanical strength, as well as offering the improved antibacterial function and the better aesthetics, is continuously required. In this thesis, micro-sized short S-glass fibres and halloysite nanotubes (HNTs) are employed to serve as excellent load-carrying filler members and antibacterial agent in the dental composites. The mechanical reinforcement mechanism and the interfacial behaviours between filler and resin matrix have been precisely investigated through the multiscale analysis from atomistic to macro by utilising the combined experimental, theoretical, and computational methods. The surface modification process on the short S-glass fibres, named selective atomic-level metal etching, has been developed, which enables to strengthen the interfacial bond between resin matrix and glass fibre by increasing the surface roughness and reactive sites on the fibre. The influence of the surface treatment on the interfacial strength and mechanical properties of the resulted composites were examined through the single-fibre pull-out tests. Also, the modified Lewis-Nielsen model has been developed, where the effective fibre length factor is applied to accurately predict the modulus of the short fibre reinforced composites. For better understanding of the atomistic interfacial bonding and fracture behaviours between glass fibre and resin matrix, molecular dynamics simulations were conducted. The numerical results of the single fibre pull-out and the uniaxial composite tension simulations were validated with the experimental findings. The optimised computational design and analysis methods were established for developing new dental and bio-composites with the accurate prediction on the mechanical performances. The surface modification process on the HNTs was developed to promote the mechanical reinforcement effect and to add an antimicrobial functionality in the composites. The composite reinforced with 2.0 wt.% of chitosan grafted HNTs showed an increased efficacy in flexural strength and modulus up to 8.1% and 14.1%, respectively, and exhibited an improved antibacterial functionality against S. mutans with 39% reduction, making it a desirable dental material.
PhD student: Nimal Balasubramani
Title: Development of Novel Tools for Stochastic Multiscale Finite Element Analysis of Composite Structures
Year: 2021
University: UNSW SYDNEY
Supervisors: Associate Prof. Garth Pearce, Prof. Gangadhara Prusty
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Abstract: A modular and generally applicable stochastic multi-scale finite element analysis (FEA) framework for investigating the structural performance of unidirectional and multi-directional textile fibre reinforced polymer (FRP) composites is presented. The framework enables studying global (stiffness) and local responses (strength) under the influence of geometric and material uncertainties across length scales. As heterogeneous and hierarchically built-up materials, predicting the behaviour of advanced laminated composite structures reliably is challenging, limiting their full exploitation. Following decades of macro-mechanical approaches, the past decade has seen increased adoption of multi-scale methods thanks to the ability to carefully consider the uncertainties related to microstructural features and constituent thermo-mechanical properties. Yet, the optimised representation of uncertainties and the means to mitigate the computational intractability of such approaches are not fully addressed. For this purpose, the requirements of properly modelling the microstructural features of FRP composites using representative volume elements (RVE) were investigated. The lack of reliable data quantifying the variability of meso-scale geometric features in textile composites was mitigated by performing segmentation and statistical analysis of X-ray CT images. The sensitivity of the global stiffness and localised damage initiation due to the uncertainties were studied using Monte Carlo simulations to reduce the uncertainties. A surrogate modelling implementation via user subroutines was developed to accelerate the multi-scale analysis of composite structures. Full-field strain measurement, acoustic emission and X-ray CT imaging of rectangular and open-hole tension test coupons were used to validate the developed multiscale modelling framework. The novel contributions of this thesis include extensions to a modular multi-scale modelling technique incorporating uncertainties at two different length scales and the use of state-of-the-art machine learning models for composite microstructure characterisation and surrogate modelling. A software tool was also developed augmenting an existing FEA software to reduce multi-scale modelling workloads. These contributions pave the way for investigating the static and dynamic performance of composite structures, considering multi-scale uncertainties in a computationally efficient manner, to confidently exploit their many advantages.
Title: Development of Novel Tools for Stochastic Multiscale Finite Element Analysis of Composite Structures
Year: 2021
University: UNSW SYDNEY
Supervisors: Associate Prof. Garth Pearce, Prof. Gangadhara Prusty
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Abstract: A modular and generally applicable stochastic multi-scale finite element analysis (FEA) framework for investigating the structural performance of unidirectional and multi-directional textile fibre reinforced polymer (FRP) composites is presented. The framework enables studying global (stiffness) and local responses (strength) under the influence of geometric and material uncertainties across length scales. As heterogeneous and hierarchically built-up materials, predicting the behaviour of advanced laminated composite structures reliably is challenging, limiting their full exploitation. Following decades of macro-mechanical approaches, the past decade has seen increased adoption of multi-scale methods thanks to the ability to carefully consider the uncertainties related to microstructural features and constituent thermo-mechanical properties. Yet, the optimised representation of uncertainties and the means to mitigate the computational intractability of such approaches are not fully addressed. For this purpose, the requirements of properly modelling the microstructural features of FRP composites using representative volume elements (RVE) were investigated. The lack of reliable data quantifying the variability of meso-scale geometric features in textile composites was mitigated by performing segmentation and statistical analysis of X-ray CT images. The sensitivity of the global stiffness and localised damage initiation due to the uncertainties were studied using Monte Carlo simulations to reduce the uncertainties. A surrogate modelling implementation via user subroutines was developed to accelerate the multi-scale analysis of composite structures. Full-field strain measurement, acoustic emission and X-ray CT imaging of rectangular and open-hole tension test coupons were used to validate the developed multiscale modelling framework. The novel contributions of this thesis include extensions to a modular multi-scale modelling technique incorporating uncertainties at two different length scales and the use of state-of-the-art machine learning models for composite microstructure characterisation and surrogate modelling. A software tool was also developed augmenting an existing FEA software to reduce multi-scale modelling workloads. These contributions pave the way for investigating the static and dynamic performance of composite structures, considering multi-scale uncertainties in a computationally efficient manner, to confidently exploit their many advantages.
PhD student: Nikhil Garg
Title: Scaled Boundary Finite Element Method for Inter-ply Damage Prediction in Thick Laminated Composite
Year: 2022
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Prof. Chongmin Song, Dr Andrew Phillips
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Abstract: Adhesively bonded joints have been widely used to manufacture aircraft components. However, its application to single load-path airframe structure is costly to certify as extensive validation testing is required. Certification of bonded joints or patch repairs for primary aircraft structures requires demonstration of damage tolerance. In recent years, a damage slow growth management strategy has been considered acceptable by Federal Aviation Administration to reduce the maintenance cost. This thesis evaluates the applicability of a damage slow growth management strategy to bonded joints/patch repairs of primary aircraft structures through both experimental and computational study. The investigation was carried out first by 2-D strip specimen assessment and finally using 3-D analysis of wider panel specimen. This research was a collaborative project between ARC Training Centre for Automated Manufacture of Advanced Composites (AMAC) at the University of New South Wales (UNSW) and Defence Science and Technology (DST) Group. Fatigue tests of 2-D strip specimen were conducted to investigate the entire process of disbond growth from initiation up to joint ultimate failure. The residual static strength of the joint as a function of disbond length was established using finite element modelling based on the characteristic distance approach. A virtual crack close technique (VCCT) approach was utilised to assess the strain energy release rates (SERRs) as a function of disbond crack length. The measured disbond growth rates were correlated with the SERRs using a modified Paris law that enabled prediction of joint fatigue life. The fatigue test results indicated that for a joint having a sufficient static strength safety margin under a typical fatigue loading that would propagate disbond, the disbond growth would remain stable within a particular length range. Thus, the slow growth approach would be feasible for bonded joints/patch repairs if the patch is designed to be sufficiently large to allow extended damage propagation. Cohesive zone element (CZE) technique was utilised to assess the SERRs and estimate the disbond growth of 3-D wider panel specimen analysis. The impact of local or partial width disbond (load shedding effect) was investigated in detail. The results indicate that for a local or part width disbond, some load was redistributed to the adjacent regions that causes a slower disbond growth compared to the full width disbond.
Title: Scaled Boundary Finite Element Method for Inter-ply Damage Prediction in Thick Laminated Composite
Year: 2022
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Prof. Chongmin Song, Dr Andrew Phillips
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Abstract: Adhesively bonded joints have been widely used to manufacture aircraft components. However, its application to single load-path airframe structure is costly to certify as extensive validation testing is required. Certification of bonded joints or patch repairs for primary aircraft structures requires demonstration of damage tolerance. In recent years, a damage slow growth management strategy has been considered acceptable by Federal Aviation Administration to reduce the maintenance cost. This thesis evaluates the applicability of a damage slow growth management strategy to bonded joints/patch repairs of primary aircraft structures through both experimental and computational study. The investigation was carried out first by 2-D strip specimen assessment and finally using 3-D analysis of wider panel specimen. This research was a collaborative project between ARC Training Centre for Automated Manufacture of Advanced Composites (AMAC) at the University of New South Wales (UNSW) and Defence Science and Technology (DST) Group. Fatigue tests of 2-D strip specimen were conducted to investigate the entire process of disbond growth from initiation up to joint ultimate failure. The residual static strength of the joint as a function of disbond length was established using finite element modelling based on the characteristic distance approach. A virtual crack close technique (VCCT) approach was utilised to assess the strain energy release rates (SERRs) as a function of disbond crack length. The measured disbond growth rates were correlated with the SERRs using a modified Paris law that enabled prediction of joint fatigue life. The fatigue test results indicated that for a joint having a sufficient static strength safety margin under a typical fatigue loading that would propagate disbond, the disbond growth would remain stable within a particular length range. Thus, the slow growth approach would be feasible for bonded joints/patch repairs if the patch is designed to be sufficiently large to allow extended damage propagation. Cohesive zone element (CZE) technique was utilised to assess the SERRs and estimate the disbond growth of 3-D wider panel specimen analysis. The impact of local or partial width disbond (load shedding effect) was investigated in detail. The results indicate that for a local or part width disbond, some load was redistributed to the adjacent regions that causes a slower disbond growth compared to the full width disbond.
PhD student: Xie Li
Title: Mesoscale Numerical Modelling and Failure Prediction of Automated Fibre Placement Composites
Year: 2022
University: UNSW SYDNEY
Supervisors: Associate Prof. Garth Pearce; Dr. Sonya Brown
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Abstract: Fibre reinforced laminated composites offer many benefits over conventional materials and hence are now used in most engineering sectors. However, their intrinsic in-homogeneity and anisotropic properties make modelling damage initiation and propagation a challenging task. This thesis explores a relatively new semi-analytical approach, Scaled Boundary Finite Element Method (SBFEM), for accurate and efficient modelling of inter-ply damage in thick laminated composites, which is sometimes referred to as interfacial imperfection, debonding or delamination. SBFEM is a tool which combines the benefit of finite element method (FEM) and boundary element method (BEM). In SBFEM, boundaries are solved numerically, whereas an analytical solution is obtained inside the domain. This considerably reduces the computational effort required to solve the problem. In its framework, a 2D plane strain approach is first considered to model the laminated composite plates undergoing cylindrical bending. The work is then extended for 3D modelling to model the bi-axial bending of plates. Unlike 2D plate models, these approaches do not make any assumption on the displacement field and hence, provides superior results. Modelling has been performed in perfectly bonded conditions as well as with interfacial imperfections. Often thick laminates make up the load bearing components of engineering structures; however, they are challenging to model accurately, particularly in cases where out-of-plane loads are significant. Thus, the applicability of the presented modelling technique using SBFEM is assessed over traditional numerical approaches for modelling thick laminated composites. The model is then expanded to study progressive delamination growth using cohesive zone modelling, in pure as well as mixed mode fracture conditions. Finally, for experimental validation of the SBFEM predictions and to justify the application of theoretical approach in practical scenarios, in-house experimentations performed for the fracture studies are modelled. The SBFEM approach for analysis of inter-ply damage in laminated composites is found to be in good agreement with traditional methods while achieving significant reduction in the computational cost. Precise behaviour of laminates can be captured without the necessity of multiple sub-divisions in the through thickness direction of plies. Moreover, the requirement of having small interface elements can be fulfilled without refining the mesh in the adjoining regions. In this way, SBFEM reduces the computational cost of the model many folds without any compromise in the accuracy. The thesis provides a basis for future research on the application of SBFEM to model inter and intra-ply damage in complex laminated composite structures.
Title: Mesoscale Numerical Modelling and Failure Prediction of Automated Fibre Placement Composites
Year: 2022
University: UNSW SYDNEY
Supervisors: Associate Prof. Garth Pearce; Dr. Sonya Brown
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Abstract: Fibre reinforced laminated composites offer many benefits over conventional materials and hence are now used in most engineering sectors. However, their intrinsic in-homogeneity and anisotropic properties make modelling damage initiation and propagation a challenging task. This thesis explores a relatively new semi-analytical approach, Scaled Boundary Finite Element Method (SBFEM), for accurate and efficient modelling of inter-ply damage in thick laminated composites, which is sometimes referred to as interfacial imperfection, debonding or delamination. SBFEM is a tool which combines the benefit of finite element method (FEM) and boundary element method (BEM). In SBFEM, boundaries are solved numerically, whereas an analytical solution is obtained inside the domain. This considerably reduces the computational effort required to solve the problem. In its framework, a 2D plane strain approach is first considered to model the laminated composite plates undergoing cylindrical bending. The work is then extended for 3D modelling to model the bi-axial bending of plates. Unlike 2D plate models, these approaches do not make any assumption on the displacement field and hence, provides superior results. Modelling has been performed in perfectly bonded conditions as well as with interfacial imperfections. Often thick laminates make up the load bearing components of engineering structures; however, they are challenging to model accurately, particularly in cases where out-of-plane loads are significant. Thus, the applicability of the presented modelling technique using SBFEM is assessed over traditional numerical approaches for modelling thick laminated composites. The model is then expanded to study progressive delamination growth using cohesive zone modelling, in pure as well as mixed mode fracture conditions. Finally, for experimental validation of the SBFEM predictions and to justify the application of theoretical approach in practical scenarios, in-house experimentations performed for the fracture studies are modelled. The SBFEM approach for analysis of inter-ply damage in laminated composites is found to be in good agreement with traditional methods while achieving significant reduction in the computational cost. Precise behaviour of laminates can be captured without the necessity of multiple sub-divisions in the through thickness direction of plies. Moreover, the requirement of having small interface elements can be fulfilled without refining the mesh in the adjoining regions. In this way, SBFEM reduces the computational cost of the model many folds without any compromise in the accuracy. The thesis provides a basis for future research on the application of SBFEM to model inter and intra-ply damage in complex laminated composite structures.
PhD student: Veldyanto Tanulia
Title: Bonded Patch Repair Applications for Primary Aircraft Structures
Year: 2022
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Associate Professor Garth Pearce, Dr. John Wang, Dr. Alan Baker, Dr. Matthew David
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Abstract: Following a comprehensive literature review on the progress of abrasive waterjet (AWJ) machining, an experimental study of the AWJ machining of carbon fibre-reinforced polymers (CFRPs) of various thicknesses was conducted, showing that clean cuts can be achieved with good processing rates. The effect of process parameters on the machined kerf and hole characteristics is amply discussed in the thesis. It was demonstrated that AWJ machining is a good process for thick CFRPs that other processes may be unable to cut. However, material delamination in the form of edge pop-up in the jet entry and pushout at the jet exit caused by the initial pure waterjet impact of an AWJ piercing operation was observed. It was experimentally shown that using a steel mask on top of the workpiece can eliminate pop-up delamination, while push-out delamination at the jet exit can be reduced or eliminated by proper process parameters. However, the mechanisms involved require further investigation. Mathematical models for predicting the major machining performance indicators were developed using dimensional and regression analysis. Experimental verification confirms that the predictive models are reasonable and reliable for assisting in the planning of AWJ machining processes. A computational model is developed and verified experimentally to study the interaction between a pure waterjet and CFRPs. The behaviour of the waterjet is modelled using the smoothed particle hydrodynamics method while the CFRP is modelled by finite element using a continuum damage material model and cohesive zone method. A computational study using the developed model reveals that the material pop-up delamination is initiated due to the material’s elastic response to a rapid release of shock pressure to stagnation pressure and the traverse shear stresses induced by the downward bending of the laminated layers. The pure waterjet impact causes flow divergence and a hydro wedging effect between the material plies, which propagates the delamination. The delamination magnitude is found to increase initially with waterjet pressure up to a threshold after which a change in pressure does not affect the pop-up delamination significantly. The smallest pop-up delamination area occurs on the [0]12 laminate, followed by the [0/45/90/-45/0/45]s and [0/90]3s laminate. It is also found that the pushout loading towards the jet exit and the hydro wedging effect act jointly to result in pushout delamination.
Title: Bonded Patch Repair Applications for Primary Aircraft Structures
Year: 2022
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Associate Professor Garth Pearce, Dr. John Wang, Dr. Alan Baker, Dr. Matthew David
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Abstract: Following a comprehensive literature review on the progress of abrasive waterjet (AWJ) machining, an experimental study of the AWJ machining of carbon fibre-reinforced polymers (CFRPs) of various thicknesses was conducted, showing that clean cuts can be achieved with good processing rates. The effect of process parameters on the machined kerf and hole characteristics is amply discussed in the thesis. It was demonstrated that AWJ machining is a good process for thick CFRPs that other processes may be unable to cut. However, material delamination in the form of edge pop-up in the jet entry and pushout at the jet exit caused by the initial pure waterjet impact of an AWJ piercing operation was observed. It was experimentally shown that using a steel mask on top of the workpiece can eliminate pop-up delamination, while push-out delamination at the jet exit can be reduced or eliminated by proper process parameters. However, the mechanisms involved require further investigation. Mathematical models for predicting the major machining performance indicators were developed using dimensional and regression analysis. Experimental verification confirms that the predictive models are reasonable and reliable for assisting in the planning of AWJ machining processes. A computational model is developed and verified experimentally to study the interaction between a pure waterjet and CFRPs. The behaviour of the waterjet is modelled using the smoothed particle hydrodynamics method while the CFRP is modelled by finite element using a continuum damage material model and cohesive zone method. A computational study using the developed model reveals that the material pop-up delamination is initiated due to the material’s elastic response to a rapid release of shock pressure to stagnation pressure and the traverse shear stresses induced by the downward bending of the laminated layers. The pure waterjet impact causes flow divergence and a hydro wedging effect between the material plies, which propagates the delamination. The delamination magnitude is found to increase initially with waterjet pressure up to a threshold after which a change in pressure does not affect the pop-up delamination significantly. The smallest pop-up delamination area occurs on the [0]12 laminate, followed by the [0/45/90/-45/0/45]s and [0/90]3s laminate. It is also found that the pushout loading towards the jet exit and the hydro wedging effect act jointly to result in pushout delamination.
PhD student: Yiwen Gu
Title: A Study of the Abrasive Waterjet Machining Process for Carbon Fibre-Reinforced Polymers
Year: 2022
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Prof. Jun Wang
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Abstract: Automated fibre placement (AFP) is an advanced and fully automated composites manufacturing technique and offers a huge design space for lightweight composite structures through flexible fibre distribution and orientation. Advanced placed ply (AP-Ply) and variable stiffness laminate (VSL) are typical examples and are called Advanced AFP Laminates in this thesis. However, due to the machine tolerance and novel tow path manipulation, a great variety of intrinsic mesoscale geometric features (gaps, overlaps, tow drops, tow crimping, etc.) can be produced, which may have a great impact on the laminate strength depending on specific applications. Despite the increasing awareness of the significance of these features, understanding the corresponding effect on part performance is still challenging due to the huge parameter space, particularly for advanced AFP laminates. This research has developed a finite element (FE) method to predict the mechanical properties of AFP composites at coupon or part scale while retaining the intrinsic geometric features. The AP-Ply is an example used to validate this technique due to the sophisticated fibre architecture. Thus, several experimental programs including short beam shear, low-velocity impact, and compression-after-impact were conducted to facilitate understanding the effect of these geometric features on the structural performance of AP-Ply. The finite element method provided in this thesis was developed at mesoscale, specifically at a length scale of tows rather than plies or laminates in conventional methods. This method significantly improves the geometric fidelity of the model with the potential of depicting each tow and geometric feature individually. To improve the efficiency of model generation, an automated tow-wise modelling (TWM) algorithm was developed, aiming to build the part virtually following the robotic kinematics. The downstream use of TWM in the prediction of different failures is achieved with the implementation of a novel cohesive network approach, which greatly eases the pre-processing effort of explicitly allocating cohesive elements or developing complex fracture criteria. This method allows greater mesh size to be used in the crack front compared to conventional methods. The feasibility and accuracy of TWM in the prediction of mechanical properties of AFP composites were validated with AP-Ply experiments, specifically the short beam shear and low-velocity impact tests.
Title: A Study of the Abrasive Waterjet Machining Process for Carbon Fibre-Reinforced Polymers
Year: 2022
University: UNSW SYDNEY
Supervisors: Prof. Gangadhara Prusty, Prof. Jun Wang
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Abstract: Automated fibre placement (AFP) is an advanced and fully automated composites manufacturing technique and offers a huge design space for lightweight composite structures through flexible fibre distribution and orientation. Advanced placed ply (AP-Ply) and variable stiffness laminate (VSL) are typical examples and are called Advanced AFP Laminates in this thesis. However, due to the machine tolerance and novel tow path manipulation, a great variety of intrinsic mesoscale geometric features (gaps, overlaps, tow drops, tow crimping, etc.) can be produced, which may have a great impact on the laminate strength depending on specific applications. Despite the increasing awareness of the significance of these features, understanding the corresponding effect on part performance is still challenging due to the huge parameter space, particularly for advanced AFP laminates. This research has developed a finite element (FE) method to predict the mechanical properties of AFP composites at coupon or part scale while retaining the intrinsic geometric features. The AP-Ply is an example used to validate this technique due to the sophisticated fibre architecture. Thus, several experimental programs including short beam shear, low-velocity impact, and compression-after-impact were conducted to facilitate understanding the effect of these geometric features on the structural performance of AP-Ply. The finite element method provided in this thesis was developed at mesoscale, specifically at a length scale of tows rather than plies or laminates in conventional methods. This method significantly improves the geometric fidelity of the model with the potential of depicting each tow and geometric feature individually. To improve the efficiency of model generation, an automated tow-wise modelling (TWM) algorithm was developed, aiming to build the part virtually following the robotic kinematics. The downstream use of TWM in the prediction of different failures is achieved with the implementation of a novel cohesive network approach, which greatly eases the pre-processing effort of explicitly allocating cohesive elements or developing complex fracture criteria. This method allows greater mesh size to be used in the crack front compared to conventional methods. The feasibility and accuracy of TWM in the prediction of mechanical properties of AFP composites were validated with AP-Ply experiments, specifically the short beam shear and low-velocity impact tests.
PhD student: Victoria Zinnecker
Title: Manufacture of laser textured steel-Carbon/PA6 hybrids using laser assisted automated tape placement
Year: 2022
University: The Australian National University
Supervisors: Prof. Paul Compston, Dr. Chris Stokes-Griffin
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Abstract: Metal-composite hybrids provide great potential to manufacture lightweight automotive and aerospace components and combine the most desirable characteristics, such as fatigue, impact, and overall strength, especially in fibre direction, of each material. Selectively with thermoplastic composites reinforced metal parts can be manufactured using a robot-guided laser-assisted tape placement process (ATP). However, manufacturing metal-composite hybrids is challenging, largely due to the one order of magnitude difference in power required to heat the metal substrate and the composite tape to the same processing temperature. Surface pre-treatments such as grit blasting combined with a film coating have been reported to increase the processability due to an increased absorptance, diffuse reflection of the laser radiation and providing mechanical interlocking for the polymer. With limited suitability of grit blasting for automotive and aerospace production lines owing to introduced contaminants, new surface pre-treatments and their feasibility to manufacture strong metal-composite hybrids need to be investigated. Femtosecond laser ablation is applied to the steel substrate before bonding to engineer surfaces with enhanced absorptance, broader scattering and superior mechanical interlocking. Efficiency optimisation of the laser ablation process has been performed to manufacture advanced surface structures. Tape placement trials for the manufacture of selectively reinforced metal-composite hybrids were executed, in a first step, with an additional PA6 film interlayer between the composite and the metal and in a second step without an interlayer, directly on the steel. The impact of distinct surface structures on the manufacturing process was analysed with thermal imaging and thermocouple measurements. Ray tracing simulation findings were in good agreement with preliminary experimental results and predicted the laser radiation distribution for various surface textures. It was shown that each surface structure required the determination of individual processing parameters for the laser assisted tape placement process. For high heat flux distributions induced by the reflected laser radiation from the substrate on the feed tape, the feed tape needed shielding with a reflector from overexposure to radiation and ultimately overheating and degradation of the polymer. For medium to low heat flux distributions, no reflector was needed. For laser textured surfaces, up to 25% less laser power was required to manufacture metal-composite hybrids compared to a grit blasted surface preparation. The bonding strengths of the metal-composite hybrids were determined with a novel application for the compression shear strength test and compared to the lap shear strength test. They yielded equivalent shear strength with less scatter for the compression shear test. Bond line characterisation showed good nesting of the fibres into the asperities of the grooves and wet out of the polymer before testing. The highest shear strength for directly bonded CF/PA 6 to steel of 10.2±2.9 MPa was obtained with a 70 μm deep top hat structured groove with a 300 μm distance between adjacent grooves. The grit blasted reference sample exceeds the compression shear strength of the strongest laser textured sample by 57%. The specimens with laser textured surfaces showed mixed failure with adhesive and cohesive failure modes, whereas the grit blasted surface pre-treatment resulted in cohesive failure of the composite with fibre-tear failure over the whole bond line. So, the successful and efficient manufacture of strong metal-composite hybrids with standard ATP equipment is reported for the first time.
Title: Manufacture of laser textured steel-Carbon/PA6 hybrids using laser assisted automated tape placement
Year: 2022
University: The Australian National University
Supervisors: Prof. Paul Compston, Dr. Chris Stokes-Griffin
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Abstract: Metal-composite hybrids provide great potential to manufacture lightweight automotive and aerospace components and combine the most desirable characteristics, such as fatigue, impact, and overall strength, especially in fibre direction, of each material. Selectively with thermoplastic composites reinforced metal parts can be manufactured using a robot-guided laser-assisted tape placement process (ATP). However, manufacturing metal-composite hybrids is challenging, largely due to the one order of magnitude difference in power required to heat the metal substrate and the composite tape to the same processing temperature. Surface pre-treatments such as grit blasting combined with a film coating have been reported to increase the processability due to an increased absorptance, diffuse reflection of the laser radiation and providing mechanical interlocking for the polymer. With limited suitability of grit blasting for automotive and aerospace production lines owing to introduced contaminants, new surface pre-treatments and their feasibility to manufacture strong metal-composite hybrids need to be investigated. Femtosecond laser ablation is applied to the steel substrate before bonding to engineer surfaces with enhanced absorptance, broader scattering and superior mechanical interlocking. Efficiency optimisation of the laser ablation process has been performed to manufacture advanced surface structures. Tape placement trials for the manufacture of selectively reinforced metal-composite hybrids were executed, in a first step, with an additional PA6 film interlayer between the composite and the metal and in a second step without an interlayer, directly on the steel. The impact of distinct surface structures on the manufacturing process was analysed with thermal imaging and thermocouple measurements. Ray tracing simulation findings were in good agreement with preliminary experimental results and predicted the laser radiation distribution for various surface textures. It was shown that each surface structure required the determination of individual processing parameters for the laser assisted tape placement process. For high heat flux distributions induced by the reflected laser radiation from the substrate on the feed tape, the feed tape needed shielding with a reflector from overexposure to radiation and ultimately overheating and degradation of the polymer. For medium to low heat flux distributions, no reflector was needed. For laser textured surfaces, up to 25% less laser power was required to manufacture metal-composite hybrids compared to a grit blasted surface preparation. The bonding strengths of the metal-composite hybrids were determined with a novel application for the compression shear strength test and compared to the lap shear strength test. They yielded equivalent shear strength with less scatter for the compression shear test. Bond line characterisation showed good nesting of the fibres into the asperities of the grooves and wet out of the polymer before testing. The highest shear strength for directly bonded CF/PA 6 to steel of 10.2±2.9 MPa was obtained with a 70 μm deep top hat structured groove with a 300 μm distance between adjacent grooves. The grit blasted reference sample exceeds the compression shear strength of the strongest laser textured sample by 57%. The specimens with laser textured surfaces showed mixed failure with adhesive and cohesive failure modes, whereas the grit blasted surface pre-treatment resulted in cohesive failure of the composite with fibre-tear failure over the whole bond line. So, the successful and efficient manufacture of strong metal-composite hybrids with standard ATP equipment is reported for the first time.
PhD student: Puneet Garg
Title: Urethane-acrylate based Polymeric Superhydrophobic Coatings
Year: 2023
University: The Australian National University
Supervisors: Prof. Antonio Tricoli
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Abstract: The study of self-cleaning lotus leaf surface in 1997 by Barthlott and Neinhuis combined with the knowledge of surface wetting phenomenon investigated by Thomas Young, Wenzel, and Cassie-Baxter paved the way for the development of artificial superhydrophobic surfaces. The novel surfaces exhibited immense application potential and ability to solve numerous ubiquitous challenges by functioning as self-cleaning surfaces, demonstrating anti-corrosion, anti-icing, anti-fouling, and drag reduction properties, and ability for oil-water separation, and material degradation prevention. Several fabrication methods have been implemented in literature to develop superhydrophobic surfaces by utilizing materials such as metals, ceramics, polymers, and composites. However, the importance of polymers in the development and commercialization of durable superhydrophobic surfaces is often overlooked. The use of polymer binders such as cross-linked polyurethane-poly(methyl methacrylate) (PU-PMMA) system demonstrate significant durability enhancement of superhydrophobic surfaces compared to bare low surface energy coatings. However, the long-term storage stability of colloidal dispersions of such polymeric binders is an essential requirement to develop superhydrophobic coatings with consistent properties. In this work, the significance of polymers is highlighted by reviewing the numerous techniques involved in the fabrication of superhydrophobic surfaces and their potential to achieve novel properties such as reversible switching, durability enhancement, flame retardancy, UV and chemical stability due to the ability of polymers to be synthesized with tunable properties. Further, the colloidal stability and gelation prevention of PU-PMMA is investigated by varying the isocyanate to hydroxyl index (NCO:OH) and examining the role of water in the system while considering the side reactions of isocyanate-based urethane systems. In addition, the study of rheological and mechanical behaviour of the urethane-acrylate system for formulations with varying water content further explains the colloidal stability and gelation of PU-PMMA system via packing theory. Lastly, a green eco-friendly aqueous poly(urethane-acrylate) system is synthesized as a sustainable alternative for solvent-borne PU-PMMA colloid while completely eliminating the use of volatile organic compounds and exhibiting excellent coating durability and superhydrophobic performance Keywords: polymer coatings, superhydrophobic surfaces, eco-friendly synthesis, urethane-acrylate systems, material enhancement.
Title: Urethane-acrylate based Polymeric Superhydrophobic Coatings
Year: 2023
University: The Australian National University
Supervisors: Prof. Antonio Tricoli
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Abstract: The study of self-cleaning lotus leaf surface in 1997 by Barthlott and Neinhuis combined with the knowledge of surface wetting phenomenon investigated by Thomas Young, Wenzel, and Cassie-Baxter paved the way for the development of artificial superhydrophobic surfaces. The novel surfaces exhibited immense application potential and ability to solve numerous ubiquitous challenges by functioning as self-cleaning surfaces, demonstrating anti-corrosion, anti-icing, anti-fouling, and drag reduction properties, and ability for oil-water separation, and material degradation prevention. Several fabrication methods have been implemented in literature to develop superhydrophobic surfaces by utilizing materials such as metals, ceramics, polymers, and composites. However, the importance of polymers in the development and commercialization of durable superhydrophobic surfaces is often overlooked. The use of polymer binders such as cross-linked polyurethane-poly(methyl methacrylate) (PU-PMMA) system demonstrate significant durability enhancement of superhydrophobic surfaces compared to bare low surface energy coatings. However, the long-term storage stability of colloidal dispersions of such polymeric binders is an essential requirement to develop superhydrophobic coatings with consistent properties. In this work, the significance of polymers is highlighted by reviewing the numerous techniques involved in the fabrication of superhydrophobic surfaces and their potential to achieve novel properties such as reversible switching, durability enhancement, flame retardancy, UV and chemical stability due to the ability of polymers to be synthesized with tunable properties. Further, the colloidal stability and gelation prevention of PU-PMMA is investigated by varying the isocyanate to hydroxyl index (NCO:OH) and examining the role of water in the system while considering the side reactions of isocyanate-based urethane systems. In addition, the study of rheological and mechanical behaviour of the urethane-acrylate system for formulations with varying water content further explains the colloidal stability and gelation of PU-PMMA system via packing theory. Lastly, a green eco-friendly aqueous poly(urethane-acrylate) system is synthesized as a sustainable alternative for solvent-borne PU-PMMA colloid while completely eliminating the use of volatile organic compounds and exhibiting excellent coating durability and superhydrophobic performance Keywords: polymer coatings, superhydrophobic surfaces, eco-friendly synthesis, urethane-acrylate systems, material enhancement.