42 research outputs found
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
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
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
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