High temperature mechanical behavior of AI/SiC nanoscale multilayers

Nanoscale Al/SiC composite laminates are metal-ceramic multilayers with unique mechanical properties at ambient temperature, such as high strength, high toughness, and damage tolerance, due to the nm scale thickness of their layers. Nevertheless, nothing is known about their high temperature mechanical properties and this is a key issue both from the fundamental viewpoint as well as from the in-service behavior. This lack of information is mainly due to the difficulties associated with the characterization of the mechanical behavior of thin-films at high temperature, a rather unexplored area. High temperature instrumented nanoindentation and micropillar compression were carried out in this thesis to study the mechanical properties of Al/SiC nanolaminates as a function of layer thickness from room temperature up to 300ºC. Mechanical tests were complemented with detailed transmission electron microscopy (TEM) analysis of the deformed structures to elucidate the effect of temperature on the deformation mechanisms at the nm scale. In addition, finite element simulations of the multilayer deformation were used to clarify the influence of the Al flow stress and of the interface properties (strength, friction coefficient) on the overall stress-strain response of Al/SiC multilayers. The combination of nanoindentation, micropillar compression tests, TEM observations and numerical simulations provided a better understanding of the key parameters influencing the high temperature mechanical behavior of Al/SiC nanoscale multilayers. It was found that the mechanical behavior at ambient temperature was controlled by the high strength of the Al nanograins and the constraint induced by the stiff SiC nanolayers on the Al plastic flow. Changes in the Al-SiC interface behavior, in the form of interface sliding, limited the constraint on plastic flow at 100ºC. This phenomenon, together with the softening of the Al nanolayers, resulted in a marked reduction in the flow stress and in the strain hardening capacity of the nanoscale multilayers. The role of the Al-SiC interfaces in plastic flow was also apparent in the creep activation energies, which showed a marked decrease with the reduction in Al layer thicknesses, reaching values close to the activation energy for grain boundary diffusion in Al for layer thicknesses of 10 nm. Finally, above 200ºC, chemical reactions between Al and SiC promoted a large degradation in the mechanical properties of the nanoscale multilayers. ----------------------Los nanolaminados de Al/SiC son materiales multicapa metal-cerámicos con propiedades mecánicas singulares a temperatura ambiente, combinando alta resistencia, alta tenacidad y tolerancia al daño, debido al espesor nanométrico de las capas. Sin embargo, su comportamiento mecánico a alta temperatura no es conocido y éste es un aspecto clave tanto desde el punto de vista fundamental como para su comportamiento en servicio. Ello es principalmente debido a la dificultad inherente a la caracterización mecánica a altas temperaturas de películas delgadas, un área que está en desarrollo en la actualidad. En este trabajo, se han empleado técnicas de nanoindentación instrumentada y compresión de micropilares a alta temperatura para estudiar las propiedades mecánicas de nanolaminados de Al/SiC, en función del espesor de las capas, entre temperatura ambiente y 300ºC. Los ensayos mecánicos se complementaron con una caracterización detallada de las microestructuras deformadas mediante microscopía electrónica de transmisión (TEM), con el objetivo de determinar el efecto de la temperatura en los mecanismos de deformación. Por último, se han empleado simulaciones por elementos finitos para entender mejor la influencia de la tensión de fluencia del Al y de las propiedades de las intercaras (resistencia, coeficiente de fricción) en el comportamiento global tensión-deformación de los nanolaminados de Al/SiC. La resultados obtenidos en los estudios de nanoindentación, compresión de micropilares, caracterización por TEM y simulaciones numéricas permitieron adquirir una mejor compresión de los parámetros que determinan el comportamiento mecánico a alta temperatura de nanolaminados de Al/SiC. Se concluyó que el comportamiento mecánico a temperatura ambiente está controlado por la alta resistencia plástica de las nanocapas de Al y la restricción a la deformación impuesta por las capas rígidas de SiC. A 100ºC, se observó un cambio en el comportamiento de la intercaras Al/SiC, dando lugar a la aparición de deslizamiento en las intercaras, limitando así la restricción a la deformación plástica de las capas de Al. Este cambio, junto al ablandamiento de las nanocapas de Al, dio lugar a una abrupta reducción de la tensión de fluencia y de la tasa de endurecimiento por deformación de los nanolaminados. El efecto de las intercaras Al-SiC en la deformación plástica también fue notable en las energías de activación a fluencia lenta, que decrecieron con la reducción del espesor de las capas de Al, hasta alcanzar valores cercanos a la energía de activación para difusión en las fronteras de grano del Al, para espesores de capa de 10 nm. Por último, por encima de 200ºC, reacciones químicas entre las capas de Al y SiC dieron lugar a una reducción importante de las propiedades mecánicas de los nanolaminados.Programa Oficial de Posgrado en Ciencia e Ingeniería de MaterialesPresidente: José Manuel Torralba Castelló; Vocales: Pedro Alberto Poza Gómez; Secretario: Ignacio Jiménez Guerrer

A critical review of wind power forecasting methods - past, present and future

The largest obstacle that suppresses the increase of wind power penetration within the power grid is uncertainties and fluctuations in wind speeds. Therefore, accurate wind power forecasting is a challenging task, which can significantly impact the effective operation of power systems. Wind power forecasting is also vital for planning unit commitment, maintenance scheduling and profit maximisation of power traders. The current development of cost-effective operation and maintenance methods for modern wind turbines benefits from the advancement of effective and accurate wind power forecasting approaches. This paper systematically reviewed the state-of-the-art approaches of wind power forecasting with regard to physical, statistical (time series and artificial neural networks) and hybrid methods, including factors that affect accuracy and computational time in the predictive modelling efforts. Besides, this study provided a guideline for wind power forecasting process screening, allowing the wind turbine/farm operators to identify the most appropriate predictive methods based on time horizons, input features, computational time, error measurements, etc. More specifically, further recommendations for the research community of wind power forecasting were proposed based on reviewed literature

Multi-criteria decision analysis of an innovative additive manufacturing technique for onboard maintenance

Access to spare parts in the maritime industry is limited throughout most of a ship’s life cycle. The limitation is caused by both the geographical distance of vessels from suppliers and the often limited turnaround time during which parts can be delivered. Manufacturing some parts onboard is possible, but it is a time-consuming and labour-intensive process. Advanced manufacturing techniques could be used to improve access to spare parts at sea by combining the desirable materials properties and flexibility of Direct Energy Deposition (DED) and the higher dimensional tolerances of Computer Numerical Control (CNC) manufacturing. The present study assesses the comparative viability of onboard implementation of advanced manufacturing techniques for offshore assets as a capital investment in different modes against an option of no onboard advanced manufacturing using a multi-criteria decision analysis method. To this end, a Technique to Order Preference by Similarity to Ideal Solution (TOPSIS) is employed considering the techno-economic and environmental aspects of the decision-making process as well as the inherent challenges that come with a new area of research. Finally, the challenges, opportunities, and pathways to onboard maintenance using additive manufacturing are discussed within the scope of the sustainable future for ship and offshore energy assets

Impacts of water depth increase on offshore floating wind turbine dynamics

This paper aims at investigating the effect of water depth increase on the global performance of a floating offshore wind turbine, with a special focus on the environmental loading effects and turbine operating status. An integrated aero-hydro-servo-elastic (AHSE) analysis was simulated in the time domain. The model was first validated against published results in terms of mooring system restoring force and platform natural frequencies. The considered water depth is between 200 and 300 m, which is the deep-water range used in the current floating offshore wind turbine (FOWT) industry. In this study, both normal operating and failure conditions were considered. Key conclusions from case studies indicated that, based on the current water depth range, platform heave motion with slack mooring configurations and mooring line top tension are more sensitive to water depth. Water depth increase influences the tower base bending force when the turbine has a high-speed shaft brake due to grid loss, but the effects are restricted to the high-frequency response range (>2 Hz) and less obvious than the influences on mooring lines

A review of challenges and opportunities associated with bolted flange connections in the offshore wind industry

The use of bolted flange connections in the offshore wind industry has steeply risen in the last few years. This trend is because of failings observed in other modes of joints such as grouted joints, coupled with enormous economic losses associated with such failures. As many aspects of bolted flange connections for the offshore wind industry are yet to be understood in full, the current study undertakes a comprehensive review of the lessons learned about bolted connections from a range of industries such as nuclear, aerospace, and onshore wind for application in offshore wind industry. Subsequently, the collected information could be used to effectively address and investigate ways to improve bolted flange connections in the offshore wind industry. As monopiles constitute an overwhelming majority of foundation types used in the current offshore wind market, this work focusses on large diameter flanges in the primary load path of a wind turbine foundation, such as those typically found at the base of turbine towers, or at monopile to transition piece connections. Finally, a summary of issues associated with flanges as well as bolted connections is provided, and insights are recommended on the direction to be followed to address these concerns

Analysis of radiation pressure and aerodynamic forces acting on powder grains in powder-based additive manufacturing

Selection of process parameters is an important step in Powder-Based Additive Manufacturing (PBAM) of metals. In order to achieve an optimal parameter set, current literature is mainly focused on the understanding of powder dynamics by analysing the aerodynamic forces. In this letter, however, we show the importance of the laser induced force (radiation pressure) on the powder dynamics. Generalised Lorenz-Mie theory has been employed to accurately estimate the radiation pressure and it is shown that its magnitude is significant in comparison to various aerodynamic forces and the grains weight, hence, can significantly contribute to denudation and spatter observed in the manufacturing process. Furthermore, the importance of compressibility and rarefaction effects on the magnitude of drag and lift forces that a particle experiences is demonstrated by estimating the Ma and Kn numbers under process conditions, which directly impact the powder dynamics

Effect of multi-pass friction stir processing on textural evolution and grain boundary structure of Al-Fe3O4 system

A mixture of pre-milled Fe 3O 4 and Al powder was added to the surface of an aluminum alloy 1050 substrate to obtain hybrid surface nanocomposites using friction stir processing. In situ nano-sized products were formed by the exothermic reaction of Al and Fe 3O 4. The reaction is triggered by hot working characteristics of the process. The microstructure and crystallographic microtexture transition and grain boundaries evolution of the fabricated nanocomposite were investigated using optical microscopy, X-ray diffraction, field emission scanning electron microscopy, and electron backscattered diffraction analyses. It is illustrated that matrix means grain size decreased in the specimens, which is processed without and with the introduction of the powder mixture to ∼8 and 2 μm, respectively. In addition, high angle grain boundaries showed marked increasing that demonstrates the happening of dynamic restoration phenomenon in the aluminum matrix. Moreover, the fraction of low ςCSL boundaries showed increasing (remarkably in the presence of hard particles); these boundaries play the main role in dynamic recrystallization. The incorporation of nano-sized products such as Al 13Fe 4 and Al 2O 3 in the dynamically recrystallized aluminum matrix produced a pre-dominantly Cube Twin texture component induced by the stirring function of the rotating tool. As a result, the effect of nano-sized products is constrained

Offshore wind power forecasting-a new hyperparameter optimisation algorithm for deep learning models

The main obstacle against the penetration of wind power into the power grid is its high variability in terms of wind speed fluctuations. Accurate power forecasting, while making maintenance more efficient, leads to the profit maximisation of power traders, whether for a wind turbine or a wind farm. Machine learning (ML) models are recognised as an accurate and fast method of wind power prediction, but their accuracy depends on the selection of the correct hyperparameters. The incorrect choice of hyperparameters will make it impossible to extract the maximum performance of the ML models, which is attributed to the weakness of the forecasting models. This paper uses a novel optimisation algorithm to tune the long short-term memory (LSTM) model for short-term wind power forecasting. The proposed method improves the power prediction accuracy and accelerates the optimisation process. Historical power data of an offshore wind turbine in Scotland is utilised to validate the proposed method and compare its outcome with regular ML models tuned by grid search. The results revealed the significant effect of the optimisation algorithm on the forecasting models' performance, with improvements of the RMSE of 7.89, 5.9, and 2.65 percent, compared to the persistence and conventional grid search-tuned Auto-Regressive Integrated Moving Average (ARIMA) and LSTM models

Development of damage tolerant composite laminates using ultra-thin interlaminar electrospun thermoplastic nanofibres

Carbon fibre-reinforced polymer (CFRP) composites are extensively used in high performance transport and renewable energy structures. However, composite laminates face the recurrent problem of being prone to damage in dynamic and impact events due to extensive interlaminar delamination. Therefore, interlaminar tougheners such as thermoplastic veils are introduced between pre-impregnated composite plies or through-thickness reinforcement techniques such as tufting are employed. However, these reinforcements are additional steps in the process which will add a degree of complexity and time in preparing composite lay-ups. A novel material and laying-up process is proposed in this paper that uses highly stretched electrospun thermoplastic nanofibers (TNF) that can enhance structural integrity with almost zero weight penalty (having 0.2gsm compared to the 300gsm CFRP plies), ensuring a smooth stress transfer through different layers, and serves directional property tailoring, with no interference with geometric features e.g. thickness. Aerospace grade pre-impregnated CFRP composite laminates have been modified with the TNFs (each layer having an average thickness of <1 micron) electrospun on each ply, and autoclave manufactured, and the effect of the nanofibers on the fracture toughness has been studied. Interlaminar fracture toughness specimens were manufactured for Mode I (double cantilever beam) and Mode II (end notched flextural) fracture tests. Such thin low-density TNF layers added an improvement of 20% in failure loads and fracture toughness in modes I and II

Towards the use of electrospun piezoelectric nanofibre layers for enabling in-situ measurement in high performance composite laminates

The aim of this research is to highlight the effects from composite manufacturing on the piezoelectric properties of fibre-reinforced composite laminates internally modified by layers of low-density piezoelectric thermoplastic nanofibres in association with a conductive electrode layer. for in-situ deformation measurement of aerospace and renewable energy composite structures through enabling electrical signal change. Several methods have been used to analyse the effects such as phase characterisation of the piezoelectric thermoplastic nanofibres and non-destructive inspection of the laminates, during processing an Inter Digital Electrode (IDE) made by conductive epoxy-graphene resin, and pre-preg autoclave manufacturing aerospace grade laminates. The purpose of fabrication of such IDE layer was to embed the same resin type (HexFlow® RTM6) for the conductive layer as that used for the laminates, in order to sustain the structural integrity via mitigation of downgrading effects on the bonding quality and interlaminar properties between plies, rising from materials mismatch and discontinuous interplay stress transfer. XRD, FTIR, EDS and SEM analyses have been carried out in the material characterisation phase, whereas pulsed thermography and ultrasonic C-scanning were used for the localisation of conductive resin embedded within the composite laminates. This study has shown promising results for enabling internally embedded piezoelectricity (and thus health monitoring capabilities) in high performance composite laminates such as those in aerospace, automotive and energy sectors