UN MODELLO MECCANICO PER LA DELAMINAZIONE PROMOSSA DA INSTABILITÀ

Il fenomeno della delaminazione è uno dei problemi principali cui sono soggetti i laminati compositi. Tale fenomeno può manifestarsi a diversi livelli, coinvolgendo sia le lamine di FRP, sia l'adesivo nel caso di giunti incollati. Nel presente lavoro analizziamo gli effetti della delaminazione, promossa da instabilità locale, in un laminato, contenente una delaminazione centrale, soggetto a flessione su quattro punti. Il modello considera due lamine di FRP, aventi spessori diversi, parzialmente connesse mediante un'interfaccia elastica, costituita da un letto di molle normali e tangenziali. L'interfaccia elastica non è presente sul tratto delaminato in corrispondenza della mezzeria. Il modello è descritto da un sistema di equazioni differenziali, dotato di opportune condizioni al contorno, che viene risolto analiticamente, ricavando espressioni esplicite per le principali grandezze di interesse (spostamenti, caratteristiche della sollecitazione, etc.). In particolare, il modello è stato suddiviso in tre tratti a differente comportamento: una prima zona, tra l'appoggio e il punto di applicazione del carico, in cui le due lamine sono schematizzate come un'unica trave estensibile e flessibile, in regime elastico di piccole deformazioni; una seconda zona, tra il punto di applicazione del carico e la zona danneggiata, in cui l'interfaccia viene schematizzata con un letto di molle e le lamine come travi estensibili e flessibili, sempre in regime di piccole deformazioni; un'ultima zona decoesa dove il laminato compresso è schematizzato secondo la teoria di Eulero per travi presso-inflesse, ed il laminato teso come una trave ordinaria. La diversa modellazione meccanica è giustificata dalla minore rigidezza della zona decoesa, infatti, essendo il laminato sottoposto a flessione su quattro punti, la zona più compressa e più snella è proprio quella decoesa. Il modello prevede una risposta meccanica complessiva non lineare. Nella fase post-critica, si valuta inoltre il rateo di rilascio dell'energia e il rapporto di modo misto, da confrontare con la tenacità a frattura per predire la crescita della delaminazione

Delamination of thin layers promoted by local buckling

This dissertation investigates the combined phenomena of buckling and fracture, which occur in thin superficial layers subjected to compressive forces. As a representative case, the four-point bending test on laminated specimens with midspan, through-the-width delaminations, is taken into consideration: a mechanical model of the test was developed and experimental tests on carbon fiber laminates were conducted. The thesis is subdivided into six Chapters. Chapter 1 presents a general introduction to the investigated problem. Chapter 2 illustrates the state of the art on delamination in composite materials with focus on buckling-driven delamination and four-point bending tests. Chapter 3 illustrates the experimental Mode II fracture toughness characterization of the carbon fiber laminates investigated in the thesis. In Chapter 4, the experimental campaign with four-point bending tests is illustrated. Chapter 5 presents the analytical model to simulate and interpret the four-point bending tests. Chapter 6 shows a comparison between the analytical prediction and the experimental evidence. In Chapter 7, results are summarized and future developments are outlined

An elastic-interface model for buckling-driven delamination growth in four-point bending tests

The paper presents a mechanical model of a four-point bending test on a delaminated specimen, considered as an assemblage of laminated beams partly connected by an elastic interface. A differential problem with suitable boundary conditions is formulated to describe the model. Then, an analytical solution is determined for both the pre- and post-critical stages. A mixed-mode fracture criterion is applied to predict the onset of delamination growth. The model is il-lustrated through comparison with some experimental results taken from the literature

An elastic-interface model for buckling-driven delamination growth in composite panels under bending

Delamination is a major failure mode for composite laminates. Such phenomenon can have multiple causes, such as manufacturing defects and low-energy impacts. Delamination cracks propagate under both static and cyclic loads [1]. In this paper, we analyse the delamination growth promoted by local buckling in a laminate subjected to four-point bending [2]. The model considers the specimen as an assemblage of sublaminates of different thicknesses, partly connected through an elastic interface, consisting of a continuous distribution of normal and tangential springs. In particular, the specimen is subdivided into three zones with different behaviour: a first zone, between the support and the load application point, where the laminate is schematised as a single extensible and flexible beam undergoing small elastic deformations; a second zone, between the load application point and the delamination front, in which the laminate consists of two sublaminates connected by the elastic interface, modelled as extensible and flexible beams, again under small deformations; one last zone, where the two sublaminates are considered as extensible and flexible beams undergoing large displacements. The different modelling assumptions in the three zones are justified by the lower stiffness of the delaminated region, which turns out to be the region with higher compression and slenderness. The model is described by a system of differential equations, accompanied by suitable boundary conditions. The differential problem is solved analytically and the value of the critical load of instability is determined through the numerical solution of a suitable transcendental equation. The model provides an overall non-linear mechanical response. In the post-critical regime, also the energy release rate and mode mixity are evaluated. Such quantities are then compared with the fracture toughness to predict the growth of the delamination crack

DELAMINATION BUCKLING IN FOUR-POINT BENDING TESTS – AN EXPERIMENTAL INVESTIGATION

A set of carbon-fibre laminated specimens have been subjected to (static) four-point bending tests to assess the conditions for delamination induced by local buckling events of the compressed lamina. Testing conditions prescribed by the ASTM D7264/D7264M-1 standard, valid for monolithic samples, have been followed. It is worth noticing that the delaminated area, which is loaded in compression, at a certain point reaches a critical value and snaps upwards. During the experimental campaign, the applied load and the corresponding displacements have been recorded, as well as the length of the crack and the deformation of the delaminated area by using a Digital Image Correlation (DIC) technique

Experimental characterisation of buckling-driven delamination growth in four-point bending tests

Delamination cracks in composite laminates may originate from manufacturing defects, lowenergy impacts, and many other causes. Once present, a delamination crack may propagate due to local buckling producing high interlaminar stresses at the crack front. To investigate this phenomenon, many authors have suggested carrying out four-point bending tests on composite laminated specimens with mid-span, through-the-width delamination cracks. In previous work, we developed an analytical solution for an elastic-interface model of such a test. Herein we present the results of an experimental campaign aimed at validating the abovementioned mechanical model. A 300 x 400 mm2 laminated plate was manufactured using quasi-unidirectional carbon-fibre fabric and epoxy resin by Microtex Composites for a total of 16 plies and a nominal thickness of 4.8 mm. Ten 220 x 13 mm2 specimens were cut from the plate to conduct four-point bending tests according to the ASTM standard. A 40 mm long artificial delamination was created at the mid-span of each specimen by introducing a thin layer of polytetrafluoroethylene (PTFE) between the second and third plies. The experimental tests were conducted using the facilities of the Multi-scale Analysis of Materials Laboratory (MUSAM-Lab) at IMT. A Zwick-Roell universal testing machine with 10 kN load cell was used for displacement control with a rate of 1 mm/min. The Correlated Solution kit for 2D Digital Image Correlation (DIC) was employed to measure the full-field displacement and the delamination length during the test. Snapping instability was observed, followed by crack propagation. Plots of the load vs. crack opening displacement and delamination length have been obtained and compared to the theoretical predictions of our previous model

Nanogravimetric and Optical Characterizations of Thrombin Interaction with a Self-Assembled Thiolated Aptamer

Efficient biorecognition of thrombin (TB), a serine protease with crucial role in physiological and pathological blood coagulation, is a hot topic in medical diagnostics. In this work, we investigate the ability of synthetic thrombin aptamer (TBA), immobilized on a gold substrate, to bind thrombin by two different label-free techniques: the quartz crystal microbalance (QCM) and the spectroscopic ellipsometry (SE). By QCM characterization in the range from 20 to 110 nM, we demonstrate high specificity of TBA-TB interaction and determine affinity constant (Kd) of 17.7 ± 0.3 nM, system sensitivity of 0.42 ± 0.03 Hz nM-1, and limit of detection (LOD) of 240 ± 20 pM. The interaction between TBA and TB is also investigated by SE, an all-optical method, by quantifying the thickness increase of the TBA film assembled on gold substrate. AFM characterization of TBA and TB molecules deposited on flat silicon surface is also supplied

PNA-based graphene oxide/porous silicon hybrid biosensor: towards a label-free optical assay for Brugada Syndrome

Peptide nucleic acid (PNA) is a synthetic DNA mimic that outperforms the properties of traditional oligonucleotides (ONs). On account of its outstanding features, such as remarkable binding affinity towards complementary DNA or RNA as well as high thermal and chemical stability, PNA has been proposed as a valuable alternative to the ON probe in gene-sensor design. In this study, a hybrid transducer made-up of graphene oxide (GO) nano-sheets covalently grafted onto a porous silicon (PSi) matrix has been investigated for the early detection of a genetic cardiac disorder, the Brugada syndrome (BS). A functionalization strategy towards the realization of a potential PNA-based device is described. A peptide nucleic acid (PNA), able to detect the SCN5A associated with the BS has been properly synthesized and used as a bioprobe for the realization of a proof-of-concept label-free optical PNA-biosensor. PSi reflectance and GO photoluminescence (PL) signals were simultaneously exploited for the monitoring of the device functionalization and response

Liquid biopsy beyond cancer: a miRNA detection in serum with electrochemical chip for non-invasive coeliac disease diagnosis

Coeliac disease is a very common autoimmune disease estimated to affect 1 in 100 people worldwide. It occurs in genetically predisposed people where the ingestion of gluten leads to damage in the small intestine, and it is accurately diagnosticated through duodenal biopsy, an invasive diagnostic method. The liquid biopsy, generally used for monitoring cancer, is an appealing alternative even for autoimmune pathology such as coeliac disease, allowing for detecting disease progression or resistance to treatment. For this reason, an electrochemical peptide nucleic acid (PNA) device combined with a smartphone-assisted potentiostat for non-invasive coeliac disease diagnosis is proposed, by measuring the selected overexpressed miRNA-486-5p in serum, enlarging the application of liquid biopsy in nontumor pathologies. For highly sensitive detection, the polyester-based printed sensor is nanomodified with gold nanoparticles and a synthetic customized PNA probe. The designed sensor can detect the target analyte in the range of 10–100 nM with a limit of detection of 0.7 nM by measuring the variation of the response of the electrochemical mediator hexaammineruthenium in the presence of PNA–miRNA duplex on the nanostructured working electrode surface. The analyses testing serum samples are found in agreement with ones obtained by realxtime quantitative polymerase chain reaction (RT-qPCR), demonstrating the reliability of this innovative electrochemical chip developed