Analisi della rugosità della superficie di rottura di materiali policristallini. An analysis on crack surface roughness in polycrystals

La tesi tratta l'analisi della rugosità della superficie di frattura di un materiale policristallino portato a rottura secondo il modo I. Il continuo viene discretizzato con la tassellazione di Voronoi e la duale triangolazione di Delaunay, da cui si ottiene un traliccio equivalente ovvero il modello del problema. Viene poi effettuata un'analisi elastica incrementale che porta, ad ogni passo, al raggiungimento della soglia di rottura per un elemento del traliccio, delineando così il profilo di rottura. La rugosità del profilo di rottura viene stimata attraverso il calcolo dell'esponente di Hurst, ottenuto dallo studio della funzione di correlazione delle altezze.

Crack measurements in structural concrete with D.I.C. system and validation of a tensile constitutive law

Compared with other mature engineering disciplines, fracture mechanics of concrete is still a developing field and very important for structures like bridges subject to dynamic loading. An historical point of view of what done in the field is provided and then the project is presented. The project presents an application of the Digital Image Correlation (DIC) technique for the detection of cracks at the surface of concrete prisms (500mmx100mmx100mm) subject to flexural loading conditions (Four Point Bending test). The technique provide displacement measurements of the region of interest and from this displacement field information about crack mouth opening (CMOD) are obtained and related to the applied load. The evolution of the fracture process is shown through graphs and graphical maps of the displacement at some step of the loading process. The study shows that it is possible with the DIC system to detect the appearance and evolution of cracks, even before the cracks become visually detectable.

Influence of microstructure variability on short crack growth behavior

Fatigue life in metals is predicted utilizing regression analysis of large sets of experimental data, thus representing the material’s macroscopic response. Furthermore, a high variability in the short crack growth (SCG) rate has been observed in polycrystalline materials, in which the evolution and distributionof local plasticity is strongly influenced by the microstructure features. The present work serves to (a) identify the relationship between the crack driving force based on the local microstructure in the proximity of the crack-tip and (b) defines the correlation between scatter observed in the SCG rates to variability in the microstructure. A crystal plasticity model based on the fast Fourier transform formulation of the elasto-viscoplastic problem (CP-EVP-FFT) is used, since the ability to account for the both elastic and plastic regime is critical in fatigue. Fatigue is governed by slip irreversibility, resulting in crack growth, which starts to occur during local elasto-plastic transition. To investigate the effects of microstructure variability on the SCG rate, sets of different microstructure realizations are constructed, in which cracks of different length are introduced to mimic quasi-static SCG in engineering alloys. From these results, the behavior of the characteristic variables of different length scale are analyzed: (i) Von Mises stress fields (ii) resolved shear stress/strain in the pertinent slip systems, and (iii) slip accumulation/irreversibilities. Through fatigue indicator parameters (FIP), scatter within the SCG rates is related to variability in the microstructural features; the results demonstrate that this relationship between microstructure variability and uncertainty in fatigue behavior is critical for accurate fatigue life prediction.

Laser shock peening treatment to control and moderate fatigue crack growth in aircraft structure based on residual stress engineering approach

Laser Shock Peening (LSP) is a surface enhancement treatment which induces a significant layer of beneficial compressive residual stresses of up to several mm underneath the surface of metal components in order to improve the detrimental effects of the crack growth behavior rate in it. The aim of this thesis is to predict the crack growth behavior in metallic specimens with one or more stripes which define the compressive residual stress area induced by the Laser Shock Peening treatment. The process was applied as crack retardation stripes perpendicular to the crack propagation direction with the object of slowing down the crack when approaching the peened stripes. The finite element method has been applied to simulate the redistribution of stresses in a cracked model when it is subjected to a tension load and to a compressive residual stress field, and to evaluate the Stress Intensity Factor (SIF) in this condition. Finally, the Afgrow software is used to predict the crack growth behavior of the component following the Laser Shock Peening treatment and to detect the improvement in the fatigue life comparing it to the baseline specimen. An educational internship at the “Research & Technologies Germany – Hamburg” department of AIRBUS helped to achieve knowledge and experience to write this thesis. The main tasks of the thesis are the following: •To up to date Literature Survey related to “Laser Shock Peening in Metallic Structures” •To validate the FE model developed against experimental measurements at coupon level •To develop design of crack growth slowdown in Centered Cracked Tension specimens based on residual stress engineering approach using laser peened strip transversal to the crack path •To evaluate the Stress Intensity Factor values for Centered Cracked Tension specimens after the Laser Shock Peening treatment via Finite Element Analysis •To predict the crack growth behavior in Centered Cracked Tension specimens using as input the SIF values evaluated with the FE simulations •To validate the results by means of experimental tests

Debonding and crack propagation in cracked steel beams strengthened with CFRP and subjected to cyclic loads.

The reinforcement methods used to restore or increase the bearing capacity of metal structures are based on the application of steel plates to be bolted or welded to the original structure, which can cause problems to the integrity of the original structure. These difficulties can be overcome with the introduction of fiber-reinforced composite materials. FRPs are characterized by high strength to weight ratio, and they are very resistant to corrosion. In this dissertation a cracked steel I-beam reinforced with Carbon Fiber-Reinforced Polymer will be studied by performing a numerical evaluation of the structure with the commercial Finite Element Method software ABAQUS. The crack propagation will be computed using XFEM, while the debonding of the reinforcement layer will be found by considering a cohesive contact interface between the beam and the CFRP plate. The results will show the efficiency of the strengthening method in increasing the load carrying capacity of the cracked beam, and in reducing the crack opening of the initial notch.

Caratterizzazione meccanica di calcestruzzi fibrorinforzati con fibre d'acciaio

L’utilizzo di materiali compositi come i calcestruzzi fibrorinforzati sta diventando sempre più frequente e diffuso. Tuttavia la scelta di nuovi materiali richiede una approfondita analisi delle loro caratteristiche e dei loro comportamenti. I vantaggi forniti dall’aggiunta di fibre d’acciaio ad un materiale fragile, quale il calcestruzzo, sono legati al miglioramento della duttilità e all'aumento di assorbimento di energia. L’aggiunta di fibre permette quindi di migliorare il comportamento strutturale del composito, dando vita ad un nuovo materiale capace di lavorare non solo a compressione ma anche in piccola parte a trazione, ma soprattutto caratterizzato da una discreta duttilità ed una buona capacità plastica. Questa tesi ha avuto come fine l’analisi delle caratteristiche di questi compositi cementizi fibrorinforzati. Partendo da prove sperimentali classiche quali prove di trazione e compressione, si è arrivati alla caratterizzazione di questi materiali avvalendosi di una campagna sperimentale basata sull’applicazione della norma UNI 11039/2003. L’obiettivo principale di questo lavoro consiste nell’analizzare e nel confrontare calcestruzzi rinforzati con fibre di due diverse lunghezze e in diversi dosaggi. Studiando questi calcestruzzi si è cercato di comprendere meglio questi materiali e trovare un riscontro pratico ai comportamenti descritti in teorie ormai diffuse e consolidate. La comparazione dei risultati dei test condotti ha permesso di mettere in luce differenze tra i materiali rinforzati con l’aggiunta di fibre corte rispetto a quelli con fibre lunghe, ma ha anche permesso di mostrare e sottolineare le analogie che caratterizzano questi materiali fibrorinforzati. Sono stati affrontati inoltre gli aspetti legati alle fasi della costituzione di questi materiali sia da un punto di vista teorico sia da un punto di vista pratico. Infine è stato sviluppato un modello analitico basato sulla definizione di specifici diagrammi tensione-deformazione; i risultati di questo modello sono quindi stati confrontati con i dati sperimentali ottenuti in laboratorio.

L'effetto della pressione sulle perdite in condotte con fessure longitudinali - The effect of pressure on leakage in longitudinally crecked pressurized pipes.

La crescente attenzione verso un utilizzo attento, sostenibile ed economicamente efficiente della risorsa idrica rende di primaria importanza il tema delle perdite idriche e della gestione efficiente dei sistemi idrici. La richiesta di controlli dell’uso dell’acqua è stata avanzata a livello mondiale. Il problema delle perdite idriche nei Paesi industrializzati è stato così affrontato con specifiche normative e procedure di best practice gestionale per avanzare una valutazione delle perdite idriche e una limitazione degli sprechi e degli usi impropri. In quest’ambito, la pressione gioca un ruolo fondamentale nella regolazione delle perdite reali. La regolazione delle pressioni nelle diverse ore del giorno consente, infatti, di poter agire su queste ultime perdite, che aumentano all’aumentare della pressione secondo una cosiddetta legge di potenza. La motivazione della presente tesi è originata dalla necessità di quantificare il livello di perdita idrica in un sistema acquedottistico in relazione alla pressione all’interno del sistema stesso. Per avere una stima realistica che vada al di là della legge della foronomia, si vuole valutare l’influenza della deformabilità della condotta in pressione fessurata sull’entità delle perdite idriche, con particolare attenzione alle fessurazioni di tipo longitudinale. Tale studio è condotto tramite l’introduzione di un semplice modello di trave alla Winkler grazie al quale, attraverso un’analisi elastica, si descrive il comportamento di una generica condotta fessurata longitudinalmente e si valuta la quantità d’acqua perduta. I risultati ottenuti in condizioni specifiche della condotta (tipo di materiale, caratteristiche geometriche dei tubi e delle fessure, etc.) e mediante l’inserimento di opportuni parametri nel modello, calibrati sui risultati forniti da una raffinata modellazione tridimensionale agli elementi finiti delle medesime condotte, verranno poi confrontati con i risultati di alcune campagne sperimentali. Gli obiettivi del presente lavoro sono, quindi, la descrizione e la valutazione del modello di trave introdotto, per stabilire se esso, nonostante la sua semplicità, sia effettivamente in grado di riprodurre, in maniera realistica, la situazione che si potrebbe verificare nel caso di tubo fessurato longitudinalmente e di fornire risultati attendibili per lo studio delle perdite idriche. Nella prima parte verrà approfondito il problema della perdite idriche. Nella seconda parte si illustrerà il semplice modello di trave su suolo elastico adottato per l’analisi delle condotte in pressione fessurate, dopo alcuni cenni teorici ai quali si è fatto riferimento per la realizzazione del modello stesso. Successivamente, nella terza parte, si procederà alla calibrazione del modello, tramite il confronto con i risultati forniti da un’analisi tridimensionale agli elementi finiti. Infine nella quarta parte verrà ricavata la relazione flusso-pressione con particolare attenzione all’esponente di perdita, il cui valore risulterà superiore a quello predetto dalla teoria della foronomia, e verrà verificata l’effettiva validità del modello tramite un confronto con i risultati sperimentali di cui è stata fatta menzione in precedenza.

Shear behaviour of reinforced cconcrete slab under concentrated load: an investigation through non-linear and sequentially linear analysis

English: The assessment of safety in existing bridges and viaducts led the Ministry of Public Works of the Netherlands to finance a specific campaing aimed at the study of the response of the elements of these infrastructures. Therefore, this activity is focused on the investigation of the behaviour of reinforced concrete slabs under concentrated loads, adopting finite element modeling and comparison with experimental results. These elements are characterized by shear behaviour and crisi, whose modeling is, from a computational point of view, a hard challeng, due to the brittle behavior combined with three-dimensional effects. The numerical modeling of the failure is studied through Sequentially Linear Analysis (SLA), an alternative Finite Element method, with respect to traditional incremental and iterative approaches. The comparison between the two different numerical techniques represents one of the first works and comparisons in a three-dimensional environment. It's carried out adopting one of the experimental test executed on reinforced concrete slabs as well. The advantage of the SLA is to avoid the well known problems of convergence of typical non-linear analysis, by directly specifying a damage increment, in terms of reduction of stiffness and resistance in particular finite element, instead of load or displacement increasing on the whole structure . For the first time, particular attention has been paid to specific aspects of the slabs, like an accurate constraints modeling and sensitivity of the solution with respect to the mesh density. This detailed analysis with respect to the main parameters proofed a strong influence of the tensile fracture energy, mesh density and chosen model on the solution in terms of force-displacement diagram, distribution of the crack patterns and shear failure mode. The SLA showed a great potential, but it requires a further developments for what regards two aspects of modeling: load conditions (constant and proportional loads) and softening behaviour of brittle materials (like concrete) in the three-dimensional field, in order to widen its horizons in these new contexts of study.

Evolution of the structural and fracture design of the ISS payloads due to the new launchers, specifically applied to Biolab IEC-2 projects

The relatively young discipline of astronautics represents one of the scientifically most fascinating and technologically advanced achievements of our time. The human exploration in space does not offer only extraordinary research possibilities but also demands high requirements from man and technology. The space environment provides a lot of attractive experimental tools towards the understanding of fundamental mechanism in natural sciences. It has been shown that especially reduced gravity and elevated radiation, two distinctive factors in space, influence the behavior of biological systems significantly. For this reason one of the key objectives on board of an earth orbiting laboratory is the research in the field of life sciences, covering the broad range from botany, human physiology and crew health up to biotechnology. The Columbus Module is the only European low gravity platform that allows researchers to perform ambitious experiments in a continuous time frame up to several months. Biolab is part of the initial outfitting of the Columbus Laboratory; it is a multi-user facility supporting research in the field of biology, e.g. effect of microgravity and space radiation on cell cultures, micro-organisms, small plants and small invertebrates. The Biolab IEC are projects designed to work in the automatic part of Biolab. In this moment in the TO-53 department of Airbus Defence & Space (formerly Astrium) there are two experiments that are in phase C/D of the development and they are the subject of this thesis: CELLRAD and CYTOSKELETON. They will be launched in soft configuration, that means packed inside a block of foam that has the task to reduce the launch loads on the payload. Until 10 years ago the payloads which were launched in soft configuration were supposed to be structural safe by themselves and a specific structural analysis could be waived on them; with the opening of the launchers market to private companies (that are not under the direct control of the international space agencies), the requirements on the verifications of payloads are changed and they have become much more conservative. In 2012 a new random environment has been introduced due to the new Space-X launch specification that results to be particularly challenging for the soft launched payloads. The last ESA specification requires to perform structural analysis on the payload for combined loads (random vibration, quasi-steady acceleration and pressure). The aim of this thesis is to create FEM models able to reproduce the launch configuration and to verify that all the margins of safety are positive and to show how they change because of the new Space-X random environment. In case the results are negative, improved design solution are implemented. Based on the FEM result a study of the joins has been carried out and, when needed, a crack growth analysis has been performed.

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.

Influence of adhesive thickness on adhesively bonded joints under fatigue loading

Research on adhesive joints is arousing increasing interest in aerospace industry. Incomplete knowledge of fatigue in adhesively bonded joints is a major obstacle to their application. The prediction of the disbonding growth is yet an open question. This thesis researches the influence of the adhesive thickness on fatigue disbond growth. Experimental testing on specimens with different thickness has been performed. Both a conventional approach based on the strain energy release rate and an approach based on cyclic strain energy are provided. The inadequacy of the former approach is discussed. Outcomes from tests support the idea of correlating the crack growth rate to the cyclic strain energy. In order to push further the study, a 2D finite element model for the prediction of disbond growth under quasi-static loading has been developed and implemented in Abaqus. Numerical simulations have been conducted with different values of the adhesive thickness. The results from tests and simulations are in accordance with each other. According to them, no dependence of disbonding on the adhesive thickness has been evidenced.

Numerical simulations of the effects of residual stresses in adhesively bonded composite specimen

In this work the problem of performing a numerical simulation of quasi-static crack propagation within an adhesive layer of a bonded joint under Mode I loading affected by stress field changes due to thermal-chemical shrinkage induced by cure process is addressed. Secondly, a parametric study on fracture critical energy, cohesive strength and Young's modulus is performed. Finally, a particular case of adhesive layer stiffening is simulated in order to verify qualitatively the major effect.

In-situ detection of defect formation in organic flexible electronics by Kelvin Probe Force Microscopy

Organic semiconductor technology has attracted considerable research interest in view of its great promise for large area, lightweight, and flexible electronics applications. Owing to their advantages in processing and unique physical properties, organic semiconductors can bring exciting new opportunities for broad-impact applications requiring large area coverage, mechanical flexibility, low-temperature processing, and low cost. In order to achieve highly flexible device architecture it is crucial to understand on a microscopic scale how mechanical deformation affects the electrical performance of organic thin film devices. Towards this aim, I established in this thesis the experimental technique of Kelvin Probe Force Microscopy (KPFM) as a tool to investigate the morphology and the surface potential of organic semiconducting thin films under mechanical strain. KPFM has been employed to investigate the strain response of two different Organic Thin Film Transistor with active layer made by 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pentacene), and Poly(3-hexylthiophene-2,5-diyl) (P3HT). The results show that this technique allows to investigate on a microscopic scale failure of flexible TFT with this kind of materials during bending. I find that the abrupt reduction of TIPS-pentacene device performance at critical bending radii is related to the formation of nano-cracks in the microcrystal morphology, easily identified due to the abrupt variation in surface potential caused by local increase in resistance. Numerical simulation of the bending mechanics of the transistor structure further identifies the mechanical strain exerted on the TIPS-pentacene micro-crystals as the fundamental origin of fracture. Instead for P3HT based transistors no significant reduction in electrical performance is observed during bending. This finding is attributed to the amorphous nature of the polymer giving rise to an elastic response without the occurrence of crack formation.