Three-Dimensional Boundary Element Analysis of DElaminated Composite Structures with Attached Piezoelectric Patch

Sommario Gli elevati valori di resistenza e rigidezza specifica unitamente alla possibilità di gestire i percorsi di carico rendono tali materiali particolarmente indicati nella progettazione delle strutture aeronautiche. Il crescente interesse nell'uso dei materiali compositi trova anche la sua ragion d'essere nell'opportunità di realizzare pannelli di grandi dimensioni con un minor numero di giunzioni rivettate, il che implica una semplificazione del componente da realizzare e da luogo ad una riduzione sia dei tempi che dei costi di manifattura ed ispezione. Indipendentemente dai vantaggi visti, grande attenzione va posta nell'uso di tali materiali in strutture molto caricate, specialmente in presenza di discontinuità geometriche, quali fori o cutout, poichè in prossimità di tali discontinuità il materiale composito tende a delaminare. Il comportamento dei materiali compositi risulta quindi molto complesso specialmente in presenza di danneggiamenti, per tale motivo risulta necessario stabilire delle strategie di modellazione numerica per caratterizzare la risposta strutturale in prossimità delle suddette discontinuità. Va infatti evidenziato che il comportamento a frattura per le delaminazioni, diversamente da quanto avviene per cricche in mezzi omogenei, è caratterizzato usualmente da una compresenza dei tre modi elementari di frattura e che, inoltre, i campi di tensione e deformazione al fronte di delaminazione sono caratterizzati da un andamento oscillatorio, oltre che singolare, che si esplica, matematicamente, in una rappresentazione complessa sia dei campi di tensione e deformazione che dei fattori d'intensificazione delle tensioni (SIFs). La rappresentazione in termini di funzioni complesse dei campi al fronte di cricca è in realtà un artefatto matematico introdotto dall'ipotesi di meccanica della frattura elastico lineare. Ciò causa anche delle difficoltà nel calcolare in modo univoco gli angoli di fase tra i modi di frattura, poichè tutte le possibili definizioni degli angoli di fase, basati cioè sui rapporti tra componenti di tensione, di campo di spostamenti o dei SIFs, producono risultati non-convergenti e che dipendono dall'entità della discontinuità elastica all'interfaccia. In questa tesi si è sviluppato un approccio agli elementi al contorno per l'analisi 3D di cricche all'interfaccia tra mezzi anisotropi ricorrendo alla implementazione multi-dominio delle equazioni BEM per modellare sia solidi non omogenei come i laminati sia le superfici di frattura all'interfaccia tra le lamine. Per caratterizzare il comportamento a frattura delle delaminazioni si è implementato un metodo basato sull'integrale di chiusura della cricca, così da sfruttare direttamente la rappresentazione al contorno dei campi tensione e spostamento in prossimità del fronte di cricca. La caratterizzazione a frattura di una cricca all'interfaccia tra materiali diversi è ottenuta intermini di rateo di rilascio energetico totale G ed in termini degli angoli di fase e che caratterizzano il mix tra i modi di frattura e taglio e di apertura. In particolare, ipotizzando che l'influenza della discontinuità elastica all'interfaccia possa essere ritenuta trascurabile quando introdotta solo da una diversa orientazione delle fibre delle lamine contenenti la delaminazione, gli angoli di fase sono definiti come la tangente inversa della radice dei rapporti tra le componenti di rateo di rilascio energetico GII e GIII rispetto a quella associata al modo di apertura GI. L'analisi di una double cantilever beam (DCB) per varie lunghezze di delaminazione e diversi materiali ortotropi è presentata per validare l'approccio BEM e le ipotesi proposte e mostrare la versatilità degli strumenti numerici proposti studiando l'influenza della sequenza di laminazione sulla distribuzione tridimensionale dei parametri di frattura. Infine, dopo aver derivato un modello per sensori piezoelettrici prismatici, si studia una configurazione drop-ply, rappresentativa di una analisi Global/Local di delaminazione tra skin e stiffener, con un array di sensori incollati alla flangia dell'irrigidimento. L'obiettivo è quello di investigare la sensibilità del sensore piezoelettrico alla delaminazione e quindi la sua efficacia in un sistema aeronautico di monitoraggio della salute strutturale. L'obiettivo di questa tesi è quindi dare una approfondita conoscenza sull'efficacia di un nuovo strumento di analisi numerica di strutture aeronautiche in composito in presenza di delaminazioni. La ricerca bibliografica ha inoltre evidenziato come lo studio numerico dei problemi di frattura all'interfaccia tra materiali anisotropi sia condotto esclusivamente per mezzo del metodo degli elementi finiti, mentre il metodo degli elementi al contorno è usato solo per problemi piani e in ambito 3D solo per cricche all'interfaccia tra materiali isotropi o trasversalmente isotropi. Quindi, il presente lavoro è anche sviluppato per 'colmare la lacuna' nell'ambito della ricerca BEM sulla caratterizzazione 3D di cricche all'interfaccia tra materiali anisotropi. Inoltre, l'uso dell'integrale di chiusura di Irwin unitamente ad una soluzione agli elementi al contorno permette di modellare i problemi di frattura in mezzi omogenei usando semplicemente elementi al contorno regolari, poichè il prodotto tra tensioni e spostamenti risulta finito anche al fronte di cricca. Quanto detto, unitamente all'ipotesi che il carattere oscillatorio dei campi di tensione e spostamento al fronte di cricca possa ritenersi trascurabile quando la discontinuità elastica è solo dovuta al diverso orientamento delle fibre suggerisce la possibilità di usare elementi regolari anche per modellare le delaminazioni. Ulteriore obiettivo del lavoro è quindi quello di verificare l'attendibilità dei risultati forniti usando il BEM 3D ad elementi regolari congiuntamente all'integrale di Irwin per il calcolo dei parametri di frattura per delaminazioni in strutture composite Abstract The high stiffness-to-weight and strength-to-weight ratios of composite materials as well as the path-loads management capability make these materials very suitable in the framework of aerospace structures. The increasing interest in using composite materials also finds its rationale on the advantages derived in manufacturing large size panels with less riveted joints, which leads to a reduction of the overall structural complexity and of the manufacturing and inspection times and costs. However, despite the aforementioned advantages, extreme caution must be taken in employing composite materials in highly loaded structures, especially if cutouts, holes or other geometric discontinuities are present, because these sites are prone to delamination onset and show high notch sensitivity. The behavior of such materials appears very complex, particularly in presence of damage, and therefore accurate and efficient numerical modeling strategies are needed to catch their structural response close to the discontinuities. It is to be stressed, in fact, that unlike fracture problems in homogeneous material, delamination cracks in laminate composites usually involve all three modes of fracture, and the crack tip stress and strain fields also exhibits oscillatory singular behavior which leads to complex stress and displacement fields representation as well as to complex valued stress intensity factors (SIFs). The complex nature of the stress and displacement fields close to the crack front is a mathematical artifact introduced by the linear elastic fracture mechanics theory. Moreover, it gives rise to the problem of the fracture mode phase angles Psi and Phi estimation, since the stresses, displacements and SIFs components based definition of the mode-mix generally lead to non-converging results depending on the entity of the bi-material interface discontinuity. This thesis deals with the modeling of three-dimensional fracture between two anisotropic layers using the boundary element method. The multi-domain technique is implemented to model the layered configuration and cracks occurring at the bi-material interface. For the purpose of characterizing the fracture behavior of the delamination, the crack closure integral is implemented, taking full advantage of the boundary element representation of the stress and displacement fields close to the crack front. The fracture mechanics behavior of bi-material cracks is characterized in terms of the total Strain Energy Release Rate G and the phase angles and , which provide information about the mix between the opening and the sliding and tearing modes of fracture, respectively. The mode mix phase angles are computed as the inverse tangent of the square root of the energy release rates opening GI and shearing, GII and GIII, components ratio, because the effect of the material discontinuity at the interface is considered negligible when it is only due to fibers orientation mismatch. A double cantilever beam (DCB) specimen is then modeled and studied for various orthotropic material and delamination length in order to validate the proposed approach and to investigate the influence of interface lay-up on the three-dimensional distribution of the fracture mechanic parameters. Last, a piezoelectric patch sensor bonded on a drop-ply configuration, which is representative of a skin-stiffener debonding in a Global/Local fashion, is analyzed. The study has the aim of assessing the piezoelectric device sensitiveness to delamination and its effectiveness for a typical aeronautical structural health monitoring system implementation. The main objective of this thesis is to develop a new effective numerical tool for obtaining a deeper insight in the fracture behavior of delaminated aircraft composite structures. Moreover numerical studies in the framework of interface crack between dissimilar anisotropic materials have been mostly addressed by using the finite element method. Contributions to this research using the boundary element method remains rare, particularly in the area of three-dimensional problem simulation. The BEM has been used to model 2D interface crack problems between distinct anisotropic material, whereas 3D BEM has been used to analyze interface crack by distinct isotropic materials or transversely isotropic materials. Thus, the present work is also developed to 'fill the gap' in the BEM research area on the subject of three-dimensional characterization of interface crack between distinct anisotropic materials. It is proved that the use of the Irwin's Crack Closure Integral in conjunction with the BEM solutions allows to model the crack front in a homogeneous medium simply by means of regular elements, since the product of stress and displacements represents energy which is finite everywhere in the body including the region next to the crack front. Moreover, the oscillatory behavior characterizing stress and displacements fields near the front of an interface crack can be assumed to be negligible when interface material mismatch is introduced by ply orientation only. Hence, another objective of the work is to validate as effective the proposed regular 3D Crack Closure BEM to compute fracture parameters for a delamination occurring in laminated composite structures

Computational Analysis of the Active Control of Incompressible Airfoil Flutter Vibration Using a Piezoelectric V-Stack Actuator

The flutter phenomenon is a potentially destructive aeroelastic vibration studied for the design of aircraft structures as it limits the flight envelope of the aircraft. The aim of this work is to propose a heuristic design of a piezoelectric actuator-based controller for flutter vibration suppression in order to extend the allowable speed range of the structure. Based on the numerical model of a three degrees of freedom (3DOF) airfoil and taking into account the FEM model of a V-stack piezoelectric actuator, a filtered PID controller is tuned using the population decline swarm optimizer PDSO algorithm, and gain scheduling (GS) of the controller parameters is used to make the control adaptive in velocity. Numerical simulations are discussed to study the performance of the controller in the presence of external disturbances

MRI Insights in Breast Imaging

In the world, breast cancer is the most commonly diagnosed cancer among women. Currently, MRI is the most sensitive breast imaging method for detecting breast cancer, although false positive rates are still an issue. To date, the accuracy of breast MRI is widely recognized across various clinical scenarios, in particular, staging of known cancer, screening for breast cancer in high-risk women, and evaluation of response to neoadjuvant chemotherapy. Since technical development and further clinical indications have expanded over recent years, dedicated breast radiologists need to constantly update their knowledge and expertise to remain confident and maintain high levels of diagnostic performance in breast MRI. This review aims to detail current and future applications of breast MRI, from technological requirements and advances to new multiparametric and abbreviated protocols, and ultrafast imaging, as well as current and future indications

Exact Solution of the Quantum Calogero-Gaudin System and of its q-Deformation

A complete set of commuting observables for the Calogero-Gaudin system is diagonalized, and the explicit form of the corresponding eigenvalues and eigenfunctions is derived. We use a purely algebraic procedure exploiting the co-algebra invariance of the model; with the proper technical modifications this procedure can be applied to the $q-$deformed version of the model, which is then also exactly solved.Comment: 20 pages Late

Breast multiparametric ultrasound: a single-center experience

Purpose: To evaluate the role of multiparametric ultrasound (mpUS) in the characterization of focal breast lesions (FBLs). Methods: This prospective study enrolled patients undergoing multiparametric breast ultrasound for FBLs. An experienced breast radiologist evaluated the following ultrasound features: US BI-RADS category, vascularization pattern (internal, vessels in rim and combined) and presence of penetrating vessels with each Doppler method (Color-Doppler, Power-Doppler, Microvascular imaging), strain ratio (SR) and Tsukuba score (TS) with Strain Elastography (SE), Emax, Emean, Emin and Eratio with 2D-shear wave elastography (2D-SWE). Core biopsy for all BI-RADS 4-5 FBLs and 24-month follow-up for all BI-RADS 2-3 FBLs were considered for standard of reference. The diagnostic performance was assessed with the area under curve (AUCs) and cut-off values were determined according to the Youden's index. Results: A total of 139 FBLs were included with 75/139 (53.9%) benign and 64/139 (46.1%) malignant FBLs. Internal vascularization patterns (p < 0.001), penetrating vessels (p < 0.001), TS 4-5 (p < 0.001) and all 2D-SWE parameters (p < 0.001) were significantly different between benign and malignant FBLs. The BI-RADS score provided an AUC of 0.876 (95% CI 0.810-0.926) for the diagnosis of malignant FBLs. Among the 2D-SWE measurements, an excellent diagnostic performance was observed for Emax with an AUC of 0.915 (95% CI 0.856-0.956) and Emean of 0.908 (95% CI 0.847-0.951). Optimal cutoff for the diagnosis of malignant FBLs were US BI-RADS > 3, Strain Ratio > 2.52, Tsukuba Score > 3, Emax > 82.6 kPa, Emean > 66.0 kPa, Emin > 54.4 kPa and Eratio > 330.8. Multiparametric ultrasound, particularly SWE, can improve specificity in the characterization of FBLs

Artificial intelligence-based, semi-automated segmentation for the extraction of ultrasound-derived radiomics features in breast cancer: a prospective multicenter study

Purpose: To investigate the feasibility of an artificial intelligence (AI)-based semi-automated segmentation for the extraction of ultrasound (US)-derived radiomics features in the characterization of focal breast lesions (FBLs). Material and methods: Two expert radiologists classified according to US BI-RADS criteria 352 FBLs detected in 352 patients (237 at Center A and 115 at Center B). An AI-based semi-automated segmentation was used to build a machine learning (ML) model on the basis of B-mode US of 237 images (center A) and then validated on an external cohort of B-mode US images of 115 patients (Center B). Results: A total of 202 of 352 (57.4%) FBLs were benign, and 150 of 352 (42.6%) were malignant. The AI-based semi-automated segmentation achieved a success rate of 95.7% for one reviewer and 96% for the other, without significant difference (p = 0.839). A total of 15 (4.3%) and 14 (4%) of 352 semi-automated segmentations were not accepted due to posterior acoustic shadowing at B-Mode US and 13 and 10 of them corresponded to malignant lesions, respectively. In the validation cohort, the characterization made by the expert radiologist yielded values of sensitivity, specificity, PPV and NPV of 0.933, 0.9, 0.857, 0.955, respectively. The ML model obtained values of sensitivity, specificity, PPV and NPV of 0.544, 0.6, 0.416, 0.628, respectively. The combined assessment of radiologists and ML model yielded values of sensitivity, specificity, PPV and NPV of 0.756, 0.928, 0.872, 0.855, respectively. Conclusion: AI-based semi-automated segmentation is feasible, allowing an instantaneous and reproducible extraction of US-derived radiomics features of FBLs. The combination of radiomics and US BI-RADS classification led to a potential decrease of unnecessary biopsy but at the expense of a not negligible increase of potentially missed cancers

PARLOMA – A Novel Human-Robot Interaction System for Deaf-blind Remote Communication

Deaf-blindness forces people to live in isolation. Up to now there is no existing technological solution enabling two (or many) Deaf-blind persons to communicate remotely among them in tactile Sign Language (t-SL). When resorting to t-SL, Deaf-blind persons can communicate only with persons physically present in the same place, because they are required to reciprocally explore their hands to exchange messages. We present a preliminary version of PARLOMA, a novel system to enable remote communication between Deaf-blind persons. It is composed of a low-cost depth sensor as the only input device, paired with a robotic hand as output device. Essentially, any user can perform handshapes in front of the depth sensor. The system is able to recognize a set of handshapes that are sent over the web and reproduced by an anthropomorphic robotic hand. PARLOMA can work as a “telephone” for Deaf-blind people. Hence, it will dramatically improve life quality of Deaf-blind persons. PARLOMA has been designed in strict collaboration with the main Italian Deaf-blind associations, in order to include end-users in the design phase

Predictors of Central Compartment Involvement in Patients with Positive Lateral Cervical Lymph Nodes According to Clinical and/or Ultrasound Evaluation

Lymph node neck metastases are frequent in papillary thyroid carcinoma (PTC). Current guidelines state, on a weak level of evidence, that level VI dissection is mandatory in the presence of latero-cervical metastases. The aim of our study is to evaluate predictive factors for the absence of level VI involvement despite the presence of metastases to the lateral cervical stations in PTC. Eighty-eight patients operated for PTC with level II-V metastases were retrospectively enrolled in the study. Demographics, thyroid function, autoimmunity, nodule size and site, cancer variant, multifocality, Bethesda and EU-TIRADS, number of central and lateral lymph nodes removed, number of positive lymph nodes and outcome were recorded. At univariate analysis, PTC location and number of positive lateral lymph nodes were risk criteria for failure to cure. ROC curves demonstrated the association of the number of positive lateral lymph nodes and failure to cure. On multivariate analysis, the protective factors were PTC located in lobe center and number of positive lateral lymph nodes < 4. Kaplan-Meier curves confirmed the absence of central lymph nodes as a positive prognostic factor. In the selected cases, Central Neck Dissection (CND) could be avoided even in the presence of positive Lateralcervical Lymph Nodes (LLN+)