Rilevamento del segnale termoelastico a campo intero mediante analisi lock-in in modalità off-line

Il presente lavoro propone una procedura di post-processamento del segnale termografico misurato su provini sollecitati in modo ciclico al fine di ricavare il segnale termoelastico. Dopo aver descritto la Correlazione Lock-In, usata tradizionalmente dai sistemi commerciali per ricavare il segnale termoelastico, il lavoro mostra come tale procedura sia numericamente equivalente alla determinazione delle armoniche della Trasformata di Fourier Discreta (TFD) del segnale campionato. Viene quindi proposta ed implementata una procedura di filtraggio del segnale termoelastico basata sulla applicazione diretta della TFD, che ha il vantaggio di poter in alcune circostanze fare a meno di un segnale di riferimento esterno. Il lavoro mostra quindi i risultati di una campagna sperimentali condotta su provini con campo tensionale noto, dimostrando come la procedura proposta sia semplice da implementare, versatile e robusta

IMPLEMENTAZIONE DI TECNICHE NDT DI TERMOGRAFIA AD INFRAROSSI SU PANNELLI IN COMPOSITO GRP PER APPLICAZIONI NAUTICHE

Il presente lavoro descrive l’implementazione di due tecniche di termografia attiva ad infrarossi per il controllo non distruttivo (IRNDT) di delaminazioni sub-superficiali su pannelli in composito (GRP) di grosso spessore, tipicamente impiegati per uso navale. Si è in particolare fatto uso di attrezzature a basso costo consistenti in comuni lampade alogene a bassa potenza come sorgenti di calore esterne, ed una termocamera ad infrarossi a singolo sensore a bassa risoluzione termica e basso frame rate. Le due tecniche implementate sono la Pulsed Thermography (PT) ed una nuova variante di Lock-In Thermography denominata Pulse-Modulated Lock-In Thermography (PMLT), basata sull’analisi in frequenza del segnale termografico del provino riscaldato da una serie di impulsi modulati. Il lavoro descrive in particolare il trattamento del segnale termografico messo a punto al fine di ottimizzare la tracciabilità dei difetti e ricavare informazioni utili sulla loro natur

IMPLEMENTAZIONE DI TECNICHE DI TERMOGRAFIA ATTIVA IRNDT SU COMPOSITI GRP MEDIANTE LA DEPOSIZIONE DI IMPULSI TERMICI DI LUNGA DURATA

Nel presente lavoro viene analizzata l\u2019implementazione di tecniche attive di termografia ad infrarossi per analisi NDT su pannelli in composito. In particolare si propone l\u2019uso di lampade alogene come sorgente di calore esterna, le quali richiedono tempi di accensione dell\u2019ordine di diversi secondi al fine di depositare efficaci quantit\ue0 di calore. Si discutono quindi le problematiche connesse all\u2019implementazione di tecniche di Transient Thermography e di Lock-In Thermography in presenza di tempi lunghi di deposizione di calore. Viene implementata una tecnica di modulazione della deposizione di calore mediante treni di onde quadre di lunga durata (PMLT - Pulse Modulated Lock-In Thermography). Viene in particolare analizzata l\u2019influenza del periodo dell\u2019onda quadra, e del rapporto di acceso-spento del treno di onde quadre, attraverso l\u2019analisi dell\u2019andamento del contrasto di fase al variare della frequenza di correlazione (lock-in frequency)

Using FEM simulation to predict structural performances of a sailing dinghy

The use of finite element method (FEM) tools is proposed to investigate the structural response of an eco-sustainable sailing yacht to different loading conditions, typical of those acting during regattas. The boat is, in particular, a 4.60 m dinghy with the hull and the deck made of an hybrid flax\ue2\u80\u93cork sandwich and internal reinforcements made of marine plywood. A preliminary activity has consisted in the refitting of an existing model in order to reduce the hull weight and to improve performances during manoeuvrings. These tasks have been interactively simulated in the virtual environment of the boat CAD model, where longitudinal and transversal reinforcements were enlightened and the maximum beam reduced. At the same time, results of FEM simulations on the modified model were analysed in order to verify the structural integrity. Shape modifications have been applied to the real model in laboratory and the resulting hull has been instrumented with strain gauges and tested under rigging conditions to validate the numerical procedure. Finally, the FEM model was used to predict the response of the boat to loading systems typical of sailing conditions

Thermo-Mechanical Behaviour of Flax-Fibre Reinforced Epoxy Laminates for Industrial Applications

The present work describes the experimental mechanical characterisation of a natural flax fibre reinforced epoxy polymer composite. A commercial plain woven quasi-unidirectional flax fabric with spun-twisted yarns is employed in particular, as well as unidirectional composite panels manufactured with three techniques: hand-lay-up, vacuum bagging and resin infusion. The stiffness and strength behaviours are investigated under both monotonic and low-cycle fatigue loadings. The analysed material has, in particular, shown a typical bilinear behaviour under pure traction, with a knee yield point occurring at a rather low stress value, after which the material tensile stiffness is significantly reduced. In the present work, such a mechanism is investigated by a phenomenological approach, performing periodical loading/unloading cycles, and repeating tensile tests on previously \u201cyielded\u201d samples to assess the evolution of stiffness behaviour. Infrared thermography is also employed to measure the temperature of specimens during monotonic and cyclic loading. In the first case, the thermal signal is monitored to correlate departures from the thermoelastic behaviour with the onset of energy loss mechanisms. In the case of cyclic loading, the thermoelastic signal and the second harmonic component are both determined in order to investigate the extent of elastic behaviour of the materia

investigation of the crack tip stress field in a stainless steel sent specimen by means of thermoelastic stress analysis

Abstract In this work a Thermoelastic Stress Analysis (TSA) setup is implemented to investigates the Thermoelastic and Second Harmonic signals on a fatigue loaded Single Edge Notched Tension (SENT) specimen made of stainless steel AISI 304L. Three load ratios are in particular applied, R=-1, 0, 0.1. The thermoelastic signal is used to evaluate the Stress Intensity Factor via two approaches, the Stanley-Chan linear interpolation method and the over-deterministic least-square fitting (LSF) method using the Williams' series expansion. Regarding least-square fitting, an iterative procedure is proposed to identify the optimal crack tip position in the thermoelastic maps. The SIF and T-Stress are then evaluated considering the influence of the number of terms (up to 20) in the Williams' series function, and the extent and position of the area used for data input. The study also investigates the Second Harmonic signal observed on the wake of the crack with varying load ratio R. An interpretation is proposed that considers the rise of the Second Harmonic as the result of the modulation of the compression loads between the crack flanks, rather than dissipation phenomena. This interpretation enables the possibility to use this parameter to reveal the presence and extent of crack-closure

Standard mechanical testing is inadequate for the mechanical characterisation of shape-memory alloys: source of errors and a new corrective approach

Thanks to its unique behaviour characterised by a superelastic response, Nitinol has now become the material of preference in a number of critical applications, especially in the area of medical implants. However, the reversible phase transformation producing its exceptional comportment is also responsible for a number of phenomena that make its mechanical characterisation particularly complex, by hindering the assumptions at the very basis of common uniaxial tensile testing. This necessarily reduces the level of safety and design optimization of current applications, which rely on incorrect mechanical parameters. In this study, the spurious effects introduced by the unconventional material behaviour during uniaxial tensile testing are analysed by means of digital image correlation (DIC), identifying the onset of undesirable material inhomogeneities and bending moments that are dependent on the test setup and strongly limit the reliability of standard characterisation. Hence, a more accurate and systematic testing approach, exploiting the ability of DIC to analyse the local mechanical response at specific regions of the test specimen, is presented and discussed

Standard mechanical testing is inadequate for the mechanical characterisation of shape-memory alloys: Source of errors and a new corrective approach

Thanks to its unique behaviour characterised by a superelastic response, Nitinol has now become the material of preference in a number of critical applications, especially in the area of medical implants. However, the reversible phase transformation producing its exceptional comportment is also responsible for a number of phenomena that make its mechanical characterisation particularly complex, by hindering the assumptions at the very basis of common uniaxial tensile testing. This necessarily reduces the level of safety and design optimization of current applications, which rely on incorrect mechanical parameters. In this study, the spurious effects introduced by the unconventional material behaviour during uniaxial tensile testing are analysed by means of digital image correlation (DIC), identifying the onset of undesirable material inhomogeneities and bending moments that are dependent on the test setup and strongly limit the reliability of standard characterisation. Hence, a more accurate and systematic testing approach, exploiting the ability of DIC to analyse the local mechanical response at specific regions of the test specimen, is presented and discussed

Photocrosslinkable polyaspartamide/polylactide copolymer and its porous scaffolds for chondrocytes

With the aim to produce, by a simple and reproducible technique, porous scaffolds potentially employable for tissue engineering purposes, in this work, we have synthesized a methacrylate (MA) copolymer of \u3b1,\u3b2-poly(N-2-hydroxyethyl)-DL-aspartamide (PHEA) and polylactic acid (PLA). PHEA-PLA-MA has been dissolved in organic solvent at different concentrations in the presence of NaCl particles with different granulometry, and through UV irradiation and further salt leaching technique, various porous scaffolds have been prepared. Obtained samples have been characterized by scanning electron microscopy and their porosity has been evaluated as well as their degradation profile in aqueous medium in the absence or in the presence of esterase from porcine liver. PHEA-PLA-MA scaffold that has shown homogeneous porosity and the best degradation profile has been further characterized to study its mechanical properties along with its capacity to incorporate and to control the release of dexamethasone. Finally, the ability to allow a three-dimensional culture of bovine articular chondrocytes have been also investigate