Analysis of Fatigue Damage in Unidirectional Carbon Fibre Reinforced Polymer Material

Carbon fibre reinforced polymer (CFRP) is widely used across different industries, mainly aviation, marine, automotive, sports equipment, and many more. This is attributed to its outstanding high strength-to-weight ratio compared to other materials such as metals. In addition, this material also features design customizability to fit various applications, which makes it one of the most versatile materials today. In real-life applications, CFRP structures are subjected to fatigue loading. The material response towards this loading is very complex. This is due to the evolution of damage within the material that occur through various failure mechanisms. The intricacy of fatigue damage in CFRP material is also contributed by the individual failure mechanisms and the synergy effects by these failure modes that occur concurrently at the microscopic level. Due to this complexity, macroscopic and microscopic material level response knowledge is required to comprehensively understand fatigue damage in CFRP. However, this crucial knowledge is not easily obtainable from standard testing alone. This thesis discusses the analysis of damage methods for unidirectional (UD) CFRP under fatigue loading through experimental and modelling work. A new experimental method to study fatigue on micro-level material response was introduced. The fatigue test requires a unique sample preparation process that involves micro-specimens of different fibre orientations (0°, 30°, 45°, 60° and 90°). The test was done under room temperature and $R=0.1$ stress ratio. The results obtained from the test are presented as an S-N curve, and in-situ observations of the micro-specimens are analysed. The fracture surface of selected broken micro-specimens is also presented and discussed here. Based on the comparison with standard macro-specimens testing, it can be deduced that the micro-fatigue test is of comparable quality to the macro-specimens standard testing. It also offers additional advantages in supplying a direct possibility to observe several critical phenomena under loading. A new continuum damage mechanics model (CDM) for fatigue and degradation of CFRP has also been proposed. The model is formulated based on anisotropic linear elastic Hooke’s law brittle model. The model includes three damage variables, each representing damage effect oriented with respect to the three coordinate axes. The formulation concept based on the dissipation of microplastic work within the material gives a physical sense of how the damage evolves in CFRP under fatigue. It is found to be numerically efficient due to the possible estimation of microplastic work from the elastic components of stress and strain. The model has been implemented in Abaqus user-subroutine and validated against two different types of fatigue specimens (macro-specimens and micro-specimens). In the case of macro-specimens, the model has been validated against filament wound carbon fibre epoxy matrix for the case of UD and multidirectional (MD) composite under tension, compression and alternating cyclic loading. On micro-specimens, a UD composite was analysed under tension fatigue load. The model proves a good prediction which indicates the model capability to accurately predict the fatigue lifetime of CFRP material at both macro and micro-level response. The thesis also discusses a new formulation to improve the current fatigue damage model further. The combination of experimental and modelling methods presented here provides high potential tools in the analysis of fatigue damage on CFRP material

Optimization of polypropylene cellular films for piezoelectric applications

Cette thèse comporte deux objectifs principaux: la production en continu de films de polypropylène (PP) moussés ayant une structure cellulaire de forme oculaire, suivie par la préparation de films PP ferroélectrets par décharge corona pour des applications piézoélectriques. Dans la première partie de ce travail, une production en continu par extrusion-calandrage a été développée pour produire des films de PP moussés pour des applications piézoélectriques. Le système est basé sur un moussage physique en utilisant de l'azote supercritique (SC-N2) et le carbonate de calcium (CaCO3) comme agent de nucléation. Les paramètres de mise en œuvre (conception de vis, profil de température, agent gonflant et de nucléation ainsi que leur contenu, et la vitesse d'étirement) ont été optimisés pour obtenir une forme spécifique (oculaire) comme structure cellulaire avec une distribution uniforme de la taille des cellules. Les résultats ont montré qu'une structure cellulaire avec un plus grand rapport d'aspect (AR) des cellules possède un plus faible module de Young, ce qui est approprié pour les films cellulaires piézoélectriques. Dans la deuxième partie, des films PP ferroélectrets ont été produits. Suite à l'optimisation du procédé de décharge corona (tension de charge, distance de l'aiguille, temps de charge), les propriétés piézoélectriques des films obtenus ont été caractérisées et le coefficient piézoélectrique quasi-statique d33 a produit une valeur de 550 pC/N. Afin de mieux caractériser le comportement du film, l’analyse mécanique dynamique (DMA) a été proposée comme une méthode simple pour relier les propriétés piézoélectriques des films PP cellulaires à leur morphologie (taille, géométrie et densité des cellules). Finalement, grâce à un post-traitement basé sur la saturation du film PP moussé avec le SC-N2, une procédure en température et pression a été développée afin d’améliorer la structure cellulaire (cellules plus allongées). Ce traitement a permis d’augmenter de 45% le coefficient d33 (800 pC/N).This thesis is composed of two main objectives: the continuous production of thin foamed polypropylene (PP) films having an eye-like cellular structure, followed by the preparation of ferroelectret PP films through corona discharge for piezoelectric applications. In the first part of this work, a continuous extrusion-calendaring setup was developed to produce PP foamed films for piezoelectric applications. The setup is based on physical foaming using supercritical nitrogen (SC-N2) and calcium carbonate (CaCO3) as nucleating agent. The processing parameters (screw design, temperature profile, blowing agent and nucleating agent content, and stretching speed) were optimized to achieve a specific stretched eye-like cellular structure with a uniform cell size distribution. The results showed that a cellular structure with higher cell aspect ratio (AR) has lower Young’s modulus, which is appropriate for piezoelectric cellular films. In the second part, ferroelectret PP films were produced. After optimization of the corona discharge process (charging voltage, needle distance, charging time), the piezoelectric properties of the resulting films were characterized and the optimum quasi-static piezoelectric d33 coefficient value was 550 pC/N. To better characterize the film behavior, dynamic mechanical analysis (DMA) was proposed as a simple method to relate the piezoelectric properties of the cellular PP films to their morphology (cell size, geometry and density). Finally, through a post-processing treatment based on the saturation of the foamed PP film with SC-N2, a temperature-pressure procedure was developed to improve the cellular structure (more stretched eye-like cells). This treatment was shown to increase by 45% the d33 coefficient (800 pC/N)

Milling/routing of carbon fibre reinforced plastic (CFRP) composites

The research relates to a study on the routing/slotting of CFRP composites of the type used in aerospace applications. Following a literature review, 3 phases of experimental work were undertaken to evaluate the effects of key process variables on the machinability of CFRP. The influence of varying operating parameters, tool material and cutting environment were initially investigated in Phase 1 work. The results showed that use of PCD was critical and highlighted the importance of chilled air in maintaining adequate tool life and acceptable workpiece integrity. Delivery of chilled air through a single-nozzle arrangement generally led to an increase in forces and delamination with the twin-nozzle configuration showing superior workpiece surface roughness. Phase 2 work detailed the effect of workpiece lay-up configuration on cutting forces, temperature and surface integrity following slotting and routing. Plies in the 45 direction generally exhibited the highest level of surface damage following machining. Experiments in Phase 3 showed that relatively small helix angles (± 3) had a negligible effect on tool life, forces and temperature. In addition, cutters with a single relief angle were found to have lower stability in operation compared to tools with a secondary clearance angle, with detrimental effects on surface roughness

Aeronautical engineering: A continuing bibliography with indexes (supplement 257)

This bibliography lists 560 reports, articles, and other documents introduced into the NASA scientific and technical information system in September 1990. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Manufacture, repair and recycling of thermoplastic composite boats

The design and construction of boats using thermoplastic composites (TPCs) is an emerging industry derived from the advantages these materials offer. Short manufacturing cycle times, virtually infinite shelf life, increased toughness, no volatiles emission, and the ability to be re‐processed and recycled, lead to improved processes and open new and more sustainable manufacturing possibilities for boats and other structures. However, the manufacture, repair and actual recycling of TPCs still present a number of technical challenges. This thesis addresses the five most important of these challenges, from both the academic and industrial points of view. The manufacturing of TPC structures involves the impregnation of reinforcing fibres with melted resin. This process, known as consolidation, is still to be fully understood. In order to contribute to this understanding, a consolidation model based on existing and newly developed sub‐models was developed and applied to experimental data. The results obtained proved that the non‐isothermal consolidation of laminates of a thickness typical of boatbuilding, can be approached by applying this model locally on a discretised laminate, fitting well experimental data. The choice of a cost‐effective moulding material is one of the factors currently preventing the widespread use of TPCs in boatbuilding. The vacuum forming of TPCs requires moulds which have considerable strength, and allow high service temperatures and the shape freedom which is typical of boat moulds. A review of commercial and experimental materials and laboratory experimentation on a novel glass‐reinforced ceramic composite was carried out, showing that a range of metals and composites are useful for TPC‐capable moulds, and that a cost‐effective free‐shape mould capable of processing any TPC is achievable. After hull shell manufacturing stiffeners and other internal structure are often required. The manufacturing of such a reinforced and subdivided hull involves the use of a joining technology. Adhesive joining, widely used in thermosetting resin composite boats, cannot be easily used on TPCs due to their low energy surfaces. However, the re‐melting ability of thermoplastic resins enables the use of welding, fusion bonding and other joining methods involving molecular diffusion at the bond line. Experiments carried out on lap and T‐joints showed that vacuum‐assisted local heating can be used for structural assemblies such as reinforced boat hulls, obtaining strengths that are comparable to existing thermosetting designs. A TPC boat manufactured and assembled in such way would still require a suitable repair technique that provides a long product life. An emergency repair method capable to return the boat to the water in less than 24 hours without using any mould was devised and tested on a prototype TPC rigid inflatable boat. This was achieved by fusion bonding the edges of a pre‐manufactured flat panel to the hull. The flat panel adapted to the hull double curvature by means of vacuum pressure, delivering the required bond quality and strength. Finally, the disposal of a TPC boat must be addressed after the end of its service life. Current policies and innovative business thinking are leading companies into reusing and recycling instead of landfilling materials. While the mechanical recycling of TPCs, achieved by means of resin re‐melting, has been largely studied, the recycling of a real boat containing paint and core material raise questions on how these materials would affect the recyclate. An experimental study on the recycling of a TPC real boat was carried out to answer these questions, revealing that despite the deleterious effect of core and paint, the final properties of injection moulded samples were in the region of those of virgin materials.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Study of the selective laser sintering process : materials properties and effect of process parameters

Additive manufacturing is attractive because it allows to reduce significantly the development and industrialization phases of part design. Among the promising technologies for thermoplastic parts, the SLS (Selective Laser Sintering) process stands out because of its ability to produce geometries with low dimensional tolerances. This process is based on the displacement of a laser beam that interacts with the powder bed. The attractiveness of additive manufacturing counterbalances, however, with the choice of currently available materials: these are mainly polyamides. Polyaryletherketones (PAEK) suitable to SLS process are still rare on the market and expensive. In this work, various powders have been characterized to deeper understand the properties necessary for their use in SLS and to define their processability temperature window. The absence of suitable PEEK powder led us to develop a new material by blending PEEK with an amorphous thermoplastic, polyethersulfone (PESU). The initially immiscible blends have been compatibilized in order to improve their mechanical properties and to delay their crystallization on cooling. During manufacturing, many process parameters control the melting of the powder, and thus the properties of the parts and their dimensional accuracy. Thus, a statistical analysis of the response of the parameters was led by a design of experiments to extract the most influential parameters. The parametric study, done with the polyamide powder, was carried out by varying five parameters and by looking at their influence on five groups of responses relating to the physical, mechanical and thermal properties as well as to the printing duration of the parts. The design of experiments made it possible to establish the mathematical models of the response surfaces linking the responses to factors and their interactions. These statistical models were used to define an optimal set of parameters. Finally, a combined experimental and numerical simulation approach was conducted to estimate the influence of each laser pass on the degree of crystallinity and the mechanical properties of each layer. The results show that the heating due to the successive laser passes cover a thickness equivalent to 14 deposited layers. However, only the 4 upper layers are significantly thermally affected by the laser pass on a powder layer and thus show an evolution of their degree of crystallinit

Devulcanisation of truck tyre tread vulcanisates in supercritical carbon dioxide using diphenyl disulphide and 2,2- dithiobis(benzothiazole)

A lot of work has been done in the recycling industry in an effort to increase the amount of reclaimed rubber used in new tyre formulations. The major drawback has been inferior physical and mechanical properties of reclaimed/virgin rubber blends in comparison to the virgin rubber material. Deterioration in these properties has been identified to be a result of chain degradation during reclamation processes as well as presence of crosslinks in the final reclaim product. Devulcanisation techniques have gained precedence due to the relatively improved properties of devulcanised/virgin rubber blends. The concept of devulcanisation is to reverse vulcanisation, resulting in total or partial cleavage of crosslinks. In this way, chain degradation is minimised while crosslink scission is maximised, thereby resulting in good quality devulcanised rubber. However, due to the persistence of chain degradation and crosslinks during devulcanisation processes, a very limited number of reports have claimed success in achieving this goal. Therefore there is still the need to develop a devulcanisation method that ensures improved quality and productivity of devulcanised rubber. Typical truck tyre tread vulcanisates were used for optimisation of time, temperature, heating rate, pressure and amount of devulcanising agent while monitoring percentage devulcanisation in supercritical carbon dioxide medium. Optimisation of the devulcanisation conditions was done by employing a twolevel central composite design in the isothermal and non-isothermal heating stages. This was followed by a single factor analysis of devulcanisation conditions in the non-isothermal stage. The effect of the presence of carbon black was investigated by comparing the percentage devulcanisation of carbon black filled and unfilled samples. The results show that supercritical carbon dioxide is an effective medium of devulcanisation using diphenyl disulphide (DD) and 2,2-dithiobis(benzothiazole) (MBTS). The relatively higher degree of devulcanisation observed during the non-isothermal stage compared to the isothermal stage, led to a shift of focus to non-isothermal devulcanisation. Temperature and time were found to have a significant antagonistic effect on the percentage devulcanisation, while changes in pressure above critical point and mass of devulcanising agent showed no effect on percentage devulcanisation. The heating rate was determined by the set-point, of which 180 ℃ set-point temperature resulted in desirable degree of devulcanisation for both DD and MBTS. 76.18 ± 5.50 % devulcanisation in 5 minutes at 102 ℃ was observed for DD whilst 70.92 ± 4.10 % devulcanisation in 4 minutes at 97 ℃ was observed for MBTS. Changes in pressure above critical point and mass of devulcanising agent used in devulcanisation showed no significant effect in the percentage devulcanisation and so they were kept constant at 80 bars and 1.00 % v (of weight of rubber sample) devulcanisation agent, respectively. The presence of carbon black was found to have an effect on the degree of devulcanisation; 87.95 % and 81.33 % devulcanisation was observed for unfilled samples devulcanised using DD and MBTS respectively. Thermogravimetric analysis of the natural rubber/styrene butadiene rubber (NR/SBR respectively) relative composition of devulcanisates indicated uneven devulcanisation when using DD, whereas MBTS did not show any form of preference. DD showed preference for NR devulcanisation over SBR. Further analysis of the sol and gel fractions were performed using; Differential Scanning Calorimetry, Fourier Transform Infrared Spectroscopy, Gel Permeation Chromatography and Gas Chromatography coupled with Mass Spectroscopy. Application of the optimised conditions to devulcanise ground tyre rubber (GTR) resulted in relatively lower degrees of devulcanisation for both DD and MBTS; 41.22 ± 4.22 and 22.41 ± 1.97 respectively. The differences in the degree of devulcanisation of the laboratory prepared vulcanisates and the GTR was determined to be due to sample differences; i.e. sample constituents, particle dimensions and crosslink network (crosslink distribution in particular)

Advances in Microfluidics Technology for Diagnostics and Detection

Microfluidics and lab-on-a-chip have, in recent years, come to the forefront in diagnostics and detection. At point-of-care, in the emergency room, and at the hospital bed or GP clinic, lab-on-a-chip offers the potential to rapidly detect time-critical and life-threatening diseases such as sepsis and bacterial meningitis. Furthermore, portable and user-friendly diagnostic platforms can enable disease diagnostics and detection in resource-poor settings where centralised laboratory facilities may not be available. At point-of-use, microfluidics and lab-on-chip can be applied in the field to rapidly identify plant pathogens, thus reducing the need for damaging broad spectrum pesticides while also reducing food losses. Microfluidics can also be applied to the continuous monitoring of water quality and can support policy-makers and protection agencies in protecting the environment. Perhaps most excitingly, microfluidics also offers the potential to enable entirely new diagnostic tests that cannot be implemented using conventional laboratory tools. Examples of microfluidics at the frontier of new medical diagnostic tests include early detection of cancers through circulating tumour cells (CTCs) and highly sensitive genetic tests using droplet-based digital PCR.This Special Issue on “Advances in Microfluidics Technology for Diagnostics and Detection” aims to gather outstanding research and to carry out comprehensive coverage of all aspects related to microfluidics in diagnostics and detection

Generation of heterogeneous cellular structures by sonication

Many materials require functionally graded cellular microstructures whose porosity (i.e. ratio of the void volume to the total volume of a material) is engineered to meet specific requirements and for an optimal performance in diverse applications. Numerous applications have demonstrated the potential of porous materials in areas ranging from biomaterial science through to structural engineering. Polymeric foams are an example of a cellular material whose microstructure can be considered as a blend of material and nonmaterial zones. While a huge variety of foams can be manufactured with homogenous porosity, for heterogeneous foams there are no generic processes for controlling the distribution of porosity throughout the resulting matrix. Motivated by the desire to create a flexible process for engineering heterogeneous foams, this work has investigated how ultrasound, applied during some of the foaming stages of a polyurethane melt, affects both the cellular structure and distribution of the pore size. After reviewing the literature concerning foam chemistry, ultrasound and sonochemistry, series of experiments were performed that used an ultrasonic field created by a sonotrode irradiating in a water bath containing a strategically placed vessel filled with foaming reactants. Prior to this, the acoustic field in the bath had been accurately mapped so that the acoustic pressure conditions within the foam container were known. During the foam polymerisation reaction, the acoustic pressure in the water bath varied causing the bubbles to pulsate in a state of ‘stable cavitation’ (i.e. rectified diffusion). This pulsation of the bubbles pumped gas from the liquid to the gas phase inducing them to increase in volume. The eventual solidification resulted in a porous material with a cellular structure that reflected the acoustic field imposed upon it. The experimental results revealed how the parameters of ultrasound exposure (i.e. frequency and acoustic pressure) influenced the volume and distribution of pores within the final polyurethane matrix: it was found that porosity varies in direct proportion to both the acoustic pressure and the frequency of the ultrasound signal. The effects of ultrasound on porosity demonstrated by this work offer the prospect of a manufacturing process that can control and adjust the cellular geometry of foam and hence ensure that the resulting characteristics of the heterogeneous material match the functional requirements.Engineering and Physical Sciences Research Council (EPSRC)Neilson Endowment Fund, in the Department of Mechanical Engineerin

Aeronautical engineering: A continuing bibliography with indexes (supplement 246)

This bibliography lists 690 reports, articles, and other documents introduced into the NASA scientific and technical information system in November, 1989. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics