Active fibre-reinforced composites with embedded shape memory alloys

This dissertation concerns active fibre-reinforced composites with embedded shape memory alloy wires. The structural application of active materials allows to develop adaptive structures which actively respond to changes in the environment, such as morphing structures, self-healing structures and power harvesting devices. In particular, shape memory alloy actuators integrated within a composite actively control the structural shape or stiffness, thus influencing the composite static and dynamic properties. Envisaged applications include, among others, the prevention of thermal buckling of the outer skin of air vehicles, shape changes in panels for improved aerodynamic characteristics and the deployment of large space structures. The study and design of active composites is a complex and multidisciplinary topic, requiring in-depth understanding of both the coupled behaviour of active materials and the interaction between the different composite constituents. Both fibre-reinforced composites and shape memory alloys are extremely active research topics, whose modelling and experimental characterisation still present a number of open problems. Thus, while this dissertation focuses on active composites, some of the research results presented here can be usefully applied to traditional fibre-reinforced composites or other shape memory alloy applications. The dissertation is composed of four chapters. In the first chapter, active fibre-reinforced composites are introduced by giving an overview of the most common choices available for the reinforcement, matrix and production process, together with a brief introduction and classification of active materials. The second chapter presents a number of original contributions regarding the modelling of fibre-reinforced composites. Different two-dimensional laminate theories are derived from a parent three-dimensional theory, introducing a procedure for the a posteriori reconstruction of transverse stresses along the laminate thickness. Accurate through the thickness stresses are crucial for the composite modelling as they are responsible for some common failure mechanisms. A new finite element based on the First-order Shear Deformation Theory and a hybrid stress approach is proposed for the numerical solution of the two-dimensional laminate problem. The element is simple and computationally efficient. The transverse stresses through the laminate thickness are reconstructed starting from a general finite element solution. A two stages procedure is devised, based on Recovery by Compatibility in Patches and three-dimensional equilibrium. Finally, the determination of the elastic parameters of laminated structures via numerical-experimental Bayesian techniques is investigated. Two different estimators are analysed and compared, leading to the definition of an alternative procedure to improve convergence of the estimation process. The third chapter focuses on shape memory alloys, describing their properties and applications. A number of constitutive models proposed in the literature, both one-dimensional and three-dimensional, are critically discussed and compared, underlining their potential and limitations, which are mainly related to the definition of the phase diagram and the choice of internal variables. Some new experimental results on shape memory alloy material characterisation are also presented. These experimental observations display some features of the shape memory alloy behaviour which are generally not included in the current models, thus some ideas are proposed for the development of a new constitutive model. The fourth chapter, finally, focuses on active composite plates with embedded shape memory alloy wires. A number of di®erent approaches can be used to predict the behaviour of such structures, each model presenting different advantages and drawbacks related to complexity and versatility. A simple model able to describe both shape and stiffness control configurations within the same context is proposed and implemented. The model is then validated considering the shape control configuration, which is the most sensitive to model parameters. The experimental work is divided in two parts. In the first part, an active composite is built by gluing prestrained shape memory alloy wires on a carbon fibre laminate strip. This structure is relatively simple to build, however it is useful in order to experimentally demonstrate the feasibility of the concept proposed in the first part of the chapter. In the second part, the making of a fibre-reinforced composite with embedded shape memory alloy wires is investigated, considering different possible choices of materials and manufacturing processes. Although a number of technological issues still need to be faced, the experimental results allow to demonstrate the mechanism of shape control via embedded shape memory alloy wires, while showing a good agreement with the proposed model predictions.

Experimental analysis and FE Modeling of Aluminium alloys Microstructural Evolution in Extrusion

Extrusion is a process used to form long products of constant cross section, from simple billets, with a high variety of shapes. Aluminum alloys are the materials most processed in the extrusion industry due to their deformability and the wide field of applications that range from buildings to aerospace and from design to automotive industries. The diverse applications imply different requirements that can be fulfilled by the wide range of alloys and treatments, that is from critical structural application to high quality surface and aesthetical aspect. Whether one or the other is the critical aspect, they both depend directly from microstructure. The extrusion process is moreover marked by high deformations and complex strain gradients making difficult the control of microstructure evolution that is at present not yet fully achieved. Nevertheless the evolution of Finite Element modeling has reached a maturity and can therefore start to be used as a tool for investigation and prediction of microstructure evolution. This thesis will analyze and model the evolution of microstructure throughout the entire extrusion process for 6XXX series aluminum alloys. Core phase of the work was the development of specific tests to investigate the microstructure evolution and validate the model implemented in a commercial FE code. Along with it two essential activities were carried out for a correct calibration of the model beyond the simple research of contour parameters, thus leading to the understanding and control of both code and process. In this direction activities were also conducted on building critical knowhow on the interpretation of microstructure and extrusion phenomena. It is believed, in fact, that the sole analysis of the microstructure evolution regardless of its relevance in the technological aspects of the process would be of little use for the industry as well as ineffective for the interpretation of the results.

Manufacturing and characterization of amorphous silicon alloys passivation layers for silicon hetero-junction solar cells

Nel presente lavoro di tesi magistrale sono stati depositati e caratterizzati film sottili (circa 10 nm) di silicio amorfo idrogenato (a-Si:H), studiando in particolare leghe a basso contenuto di ossigeno e carbonio. Tali layer andranno ad essere implementati come strati di passivazione per wafer di Si monocristallino in celle solari ad eterogiunzione HIT (heterojunctions with intrinsic thin layer), con le quali recentemente è stato raggiunto il record di efficienza pari a 24.7% . La deposizione è avvenuta mediante PECVD (plasma enhanced chemical vapour deposition). Tecniche di spettroscopia ottica, come FT-IR (Fourier transform infrared spectroscopy) e SE (spettroscopic ellipsometry) sono state utilizzate per analizzare le configurazioni di legami eteronucleari (Si-H, Si-O, Si-C) e le proprietà strutturali dei film sottili: un nuovo metodo è stato implementato per calcolare i contenuti atomici di H, O e C da misure ottiche. In tal modo è stato possibile osservare come una bassa incorporazione (< 10%) di ossigeno e carbonio sia sufficiente ad aumentare la porosità ed il grado di disordine a lungo raggio del materiale: relativamente a quest’ultimo aspetto, è stata sviluppata una nuova tecnica per determinare dagli spettri ellisometrici l’energia di Urbach, che esprime la coda esponenziale interna al gap in semiconduttori amorfi e fornisce una stima degli stati elettronici in presenza di disordine reticolare. Nella seconda parte della tesi sono stati sviluppati esperimenti di annealing isocrono, in modo da studiare i processi di cristallizzazione e di effusione dell’idrogeno, correlandoli con la degradazione delle proprietà optoelettroniche. L’analisi dei differenti risultati ottenuti studiando queste particolari leghe (a-SiOx e a-SiCy) ha permesso di concludere che solo con una bassa percentuale di ossigeno o carbonio, i.e. < 3.5 %, è possibile migliorare la risposta termica dello specifico layer, ritardando i fenomeni di degradazione di circa 50°C.

Spectroscopic investigations of functional bulk and thin film Heusler alloys

Functional and smart materials have gained large scientific and practical interest in current research and development. The Heusler alloys form an important class of functional materials used in spintronics, thermoelectrics, and for shape memory alloy applications. An important aspect of functional materials is the adaptability of their physical properties. In this work functional polycrystalline bulk and epitaxial thin film Heusler alloys are characterized by means of spectroscopic investigation methods, X-ray magnetic circular dichroism (XMCD) and energy dispersive X-ray analysis (EDX). With EDX the homogeneity of the samples is studied extensively. For some cases of quaternary compounds, for example Co2(MnxTi1−x)Sn and Co2(Mn0.5Dy0.5)Sn, an interesting phase separation in two nearly pure ternary Heusler phases occurs. For these samples the phase separation leads to an improvement of thermoelectric properties. XMCD as the main investigation method was used to study Co2TiZ (Z = Si, Sn, and Sb), Co2(MnxTi1−x)Si, Co2(MnxTi1−x)Ge, Co2Mn(Ga1−xGex), Co2FeAl, Mn2VAl, and Ni2MnGa Heusler compounds. The element-specific magnetic moments are calculated. Also, the spin-resolved unoccupied density of states is determined, for example giving hints for half-metallic ferromagnetism for some Co-based compounds. The systematic change of the magnetic moments and the shift of the Fermi energy is a proof that Heusler alloys are suitable for a controlled tailoring of physical properties. The comparison of the experimental results with theoretical predictions improves the understanding of complex materials needed to optimize functional Heusler alloys.

Measurement of LSP-induced residual stresses and fatigue life on thin-walled aluminium alloys specimens

This thesis work encloses activities carried out in the Laser Center of the Polytechnic University of Madrid and the laboratories of the University of Bologna in Forlì. This thesis focuses on the superficial mechanical treatment for metallic materials called Laser Shock Peening (LSP). This process is a surface enhancement treatment which induces a significant layer of beneficial compressive residual stresses underneath the surface of metal components in order to improve the detrimental effects of the crack growth behavior rate in it. The innovation aspect of this work is the LSP application to specimens with extremely low thickness. In particular, after a bibliographic study and comparison with the main treatments used for the same purposes, this work analyzes the physics of the operation of a laser, its interaction with the surface of the material and the generation of the surface residual stresses which are fundamentals to obtain the LSP benefits. In particular this thesis work regards the application of this treatment to some Al2024-T351 specimens with low thickness. Among the improvements that can be obtained performing this operation, the most important in the aeronautic field is the fatigue life improvement of the treated components. As demonstrated in this work, a well-done LSP treatment can slow down the progress of the defects in the material that could lead to sudden failure of the structure. A part of this thesis is the simulation of this phenomenon using the program AFGROW, with which have been analyzed different geometric configurations of the treatment, verifying which was better for large panels of typical aeronautical interest. The core of the LSP process are the residual stresses that are induced on the material by the interaction with the laser light, these can be simulated with the finite elements but it is essential to verify and measure them experimentally. In the thesis are introduced the main methods for the detection of those stresses, they can be mechanical or by diffraction. In particular, will be described the principles and the detailed realization method of the Hole Drilling measure and an introduction of the X-ray Diffraction; then will be presented the results I obtained with both techniques. In addition to these two measurement techniques will also be introduced Neutron Diffraction method. The last part refers to the experimental tests of the fatigue life of the specimens, with a detailed description of the apparatus and the procedure used from the initial specimen preparation to the fatigue test with the press. Then the obtained results are exposed and discussed.

Chemical composition modification of casting aluminium alloys for engine applications

The research activities were focused on evaluating the effect of Mo addition to mechanical properties and microstructure of A354 aluminium casting alloy. Samples, with increasing amount of Mo, were produced and heat treated. After heat treatment and exposition to high temperatures samples underwent microstructural and chemical analyses, hardness and tensile tests. The collected data led to the optimization of both casting parameters, for obtaining a homogeneous Mo distribution in the alloy, and heat treatment parameters, allowing the formation of Mo based strengthening precipitates stable at high temperature. Microstructural and chemical analyses highlighted how Mo addition in percentage superior to 0.1% wt. can modify the silicon eutectic morphology and hinder the formation of iron based β intermetallics. High temperature exposure curves, instead, showed that after long exposition hardness is slightly influenced by heat treatment while the effect of Mo addition superior to 0,3% is negligible. Tensile tests confirmed that the addition of 0.3%wt Mo induces an increase of about 10% of ultimate tensile strength after high temperature exposition (250°C for 100h) while heat treatments have slight influence on mechanical behaviour. These results could be exploited for developing innovative heat treatment sequence able to reduce residual stresses in castings produced with A354 modified with Mo.

Varying spin state composition by the choice of capping ligand in a family of molecular chains: detailed analysis of magnetic properties of chromium(III) horseshoes

We report a detailed physical analysis on a family of isolated, antiferro-magnetically (AF) coupled, chromium(III) finite chains, of general formula (Cr(RCO(2))(2)F)(n) where the chain length n = 6 or 7. Additionally, the chains are capped with a selection of possible terminating ligands, including hfac (= 1,1,1,5,5,5-hexafluoropentane-2,4-dionate(1-)), acac (= pentane-2,4-dionate(1-)) or (F)(3). Measurements by inelastic neutron scattering (INS), magnetometery and electron paramagnetic resonance (EPR) spectroscopy have been used to study how the electronic properties are affected by n and capping ligand type. These comparisons allowed the subtle electronic effects the choice of capping ligand makes for odd member spin 3/2 ground state and even membered spin 0 ground state chains to be investigated. For this investigation full characterisation of physical properties have been performed with spin Hamiltonian parameterisation, including the determination of Heisenberg exchange coupling constants and single ion axial and rhombic anisotropy. We reveal how the quantum spin energy levels of odd or even membered chains can be modified by the type of capping ligand terminating the chain. Choice of capping ligands enables Cr-Cr exchange coupling to be adjusted by 0, 4 or 24%, relative to Cr-Cr exchange coupling within the body of the chain, by the substitution of hfac, acac or (F)(3) capping ligands to the ends of the chain, respectively. The manipulation of quantum spin levels via ligands which play no role in super-exchange, is of general interest to the practise of spin Hamilton modelling, where such second order effects are generally not considered of relevance to magnetic properties.

Nano-engineering of composite material via reactive mechanical alloying/milling (RMA/M)

Attempts to strengthen a chromium-modified titanium trialuminide by a combination of grain size refinement and dispersoid strengthening led to a new means to synthesize such materials. This Reactive Mechanical Alloying/Milling process uses in situ reactions between the metallic powders and elements from a process control agent and/or a gaseous environment to assemble a dispersed small hard particle phase within the matrix by a bottom-up approach. In the current research milled powders of the trialuminide alloy along with titanium carbide were produced. The amount of the carbide can be varied widely with simple processing changes and in this case the milling process created trialuminide grain sizes and carbide particles that are the smallest known from such a process. Characterization of these materials required the development of x-ray diffraction means to determine particle sizes by deconvoluting and synthesizing components of the complex multiphase diffraction patterns and to carry out whole pattern analysis to analyze the diffuse scattering that developed from larger than usual highly defective grain boundary regions. These identified regions provide an important mass transport capability in the processing and not only facilitate the alloy development, but add to the understanding of the mechanical alloying process. Consolidation of the milled powder that consisted of small crystallites of the alloy and dispersed carbide particles two nanometers in size formed a unique, somewhat coarsened, microstructure producing an ultra-high strength solid material composed of the chromium-modified titanium trialuminide alloy matrix with small platelets of the complex carbides Ti2AlC and Ti3AlC2. This synthesis process provides the unique ability to nano-engineer a wide variety of composite materials, or special alloys, and has shown the ability to be extended to a wide variety of metallic materials.

The Electrical Resistance of Metals and Alloys in Their Hard and Soft States

It is known that the electrical resistance of annealed metals is usually smaller than that of metals in their cold worked state. The curve showing the relation between electrical resistance and annealing temperature reaches a minimum; continued annealing at higher temperature produces an increase in the electrical resistance. In the case of alloys it has been noted that a second decrease occurs at higher annealing temperature. The following work corroborates the observance of previous investigations. The electrical resistance of cold worked copper, gold, nickel, and iron decreased with annealing and then increased, the minimum being around 300° C. or 400° C. Monel metal showed a minimum resistance followed by an increase which in turn was followed by a second decrease.

The Electrolytic Deposition of Chromium Upon Aluminum

The purpose or this investigation is primarily to determine the best conditions for plating chromium on aluminum. The work was carried out with the hope of obtaining coherent deposits, and of determin­ing the conditions under which such deposits may be duplicated.