Gruppo Italiano Frattura (IGF)
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A brittle fracture criterion for PMMA V-notches tensile specimens based on a length-enriched eXtended Finite Element approach
No full textA criterion for the prediction of the static failure loads in tensile PMMA specimens with sharp notches is presented. The proposed criterion is based on a regularized version of the eXtended Finite Element Method (XFEM), which has been previously applied to concrete-like materials. The main feature of the proposed approach is that the cracking process is not treated as a local process, but it is modeled by assuming that macro-cracks stem from the interaction of micro-cracks within a finite width process zone. The case of a brittle materials with thin process zone is tackled by assuming one layer of enriched finite elements. Preliminary results concerning PMMA specimens subjected to mode-one loading are presented
Coupled FEM-DBEM method to assess crack growth in magnet system of Wendelstein 7-X
No full textThe fivefold symmetric modular stellarator Wendelstein 7-X (W7-X) is currently underconstruction in Greifswald, Germany. The superconducting coils of the magnet system are bolted onto a centralsupport ring and interconnected with five so-called lateral support elements (LSEs) per half module. Afterwelding of the LSE hollow boxes to the coil cases, cracks were found in the vicinity of the welds that couldpotentially limit the allowed number N of electromagnetic (EM) load cycles of the machine. In response to theappearance of first cracks during assembly, the Stress Intensity Factors (SIFs) were calculated andcorresponding crack growth rates of theoretical semi-circular cracks of measured sizes in potentially criticalposition and orientation were predicted using Paris’ law, whose parameters were calibrated in fatigue tests atcryogenic temperature. In this paper the Dual Boundary Element Method (DBEM) is applied in a coupledFEM-DBEM approach to analyze the propagation of multiple cracks with different shapes. For this purpose,the crack path is assessed with the Minimum Strain Energy density criterion and SIFs are calculated by the Jintegralapproach. The Finite Element Method (FEM) is adopted to model, using the commercial codes Ansysor Abaqus;, the overall component whereas the submodel analysis, in the volume surrounding the cracked area,is performed by FEM (“FEM-FEM approach”) or alternatively by DBEM (“FEM-DBEM approach”). The“FEM-FEM approach” considers a FEM submodel, that is extracted from the FEM global model; the latterprovide the boundary conditions for the submodel. Such approach is affected by some restrictions in the crackpropagation phase, whereas, with the “FEM-DBEM approach”, the crack propagation simulation isstraightforward. In this case the submodel is created in a DBEM environment with boundary conditionsprovided by the global FEM analysis; then the crack is introduced and a crack propagation analysis has beenperformed to evaluate the effects of the crack shape and of the presence of nearby cracks on the allowednumber of EM load cycles
Studio delle prestazioni di giunzioni incollate ibride (Strength evaluation of structural hybrid bonded joints)
No full textThe idea of “hybrid joints” arises from the need of joining together materials, producing a tough, stiff and lightweight joint. The core concept is: use two different joining techniques, with the aim to increase the strength and gather the advantages of each technique, reducing the drawbacks. In this work hybrid joints consist in joints produced with two different kinds of joining techniques: one of them being adhesive bonding and the other being one among Resistance Spot Welding, Pop Riveting, Clinching, Self Piercing Riveting. The performance of hybrid joints are compared with those of traditional joints for the single lap joint geometry, for different geometric (plates thickness, pitch of the mechanical fastening) and environmental condition (temperature and ageing). The experimental tests are planned and therefore their results analyzed by means of Design of Experiment techniques.
Results show that hybrid joints allow a significant performances enhancement with respect to simple joints when the two simple joining techniques give similar performances, while the performance of hybrid joints are only equal or lower than the those of the strongest joint when the performances of the two joining techniques are significantly different
Studio del danneggiamento mediante tomografia in luce del sincrotrone: impatto di un cono d’ombra sulla qualità finale delle ricostruzioni
No full textUn’accurata osservazione della geometria tridimensionale di cricche e difetti è necessaria per lostudio dei meccanismi alla base del processo di danneggiamento. I metodi convenzionali utilizzati a questoscopo sono distruttivi o non possiedono una sufficiente risoluzione. Le tecniche di imaging che utilizzano laluce di sincrotrone, ed in particolare la microtomografia (micro-CT) a raggi X, invece, uniscono i vantaggi di unatecnica non distruttiva ad un’elevata risoluzione spaziale e risultano quindi particolarmente interessanti. Unlimite all’applicazione di questa tecnica è costituito dalla propensione della cricca a richiudersi una volta rimossoil carico che ha provocato il danneggiamento, superabile attraverso l’impiego di un dispositivo in grado diesercitare un carico di trazione durante l’acquisizione dei dati. Facendo riferimento al set-up sperimentale dellalinea SYRMEP di Elettra, il sincrotrone di Trieste, e tralasciando per il momento i vincoli legati a pesi eingombri, è possibile pensare di inserire tra camera di ionizzazione e CCD una macchina per prove di trazionemono-colonna commerciale, in grado di mantenere aperto il difetto per tutta la durata della tomografia. Inquesto lavoro viene valutato l’impatto di questo vincolo sulla qualità finale delle ricostruzioni
The numerical simulation of ceramic composites failure at axial compression
No full textOn the basis of the developed model of quasibrittle medium the brittle and quasibrittle failure ofcomposite ceramic materials with zirconium dioxide matrix and various percentage of hardening particles ofcorundum are studied numerically. The theory of the damaged media is applied in the calculations. It is shownthat failure process educes in 2 stages - a relatively slow quasistationary phase of accumulation of inelasticdeformations and damages in all hierarchy of scales and superfast catastrophic phase - a blow-up regime whenthe failure process reaches the macro-scale and there is a macro-crack formation. The failure process in theproposed model assumes to be completed when the damage function reaches its maximum value and thestrength of the composite evolve to zero
An approach for the modeling of interface-body coupled nonlocal damage
No full textFiber Reinforced Plastic (FRP) can be used for strengthening concrete or masonry constructions.One of the main problem in the use of FRP is the possible detachment of the reinforcement from the supportmaterial. This paper deals with the modeling of the FRP-concrete or masonry damage interface, accounting forthe coupling occurring between the degradation of the cohesive material and the FRP detachment. To this end,a damage model is considered for the quasi-brittle material. In order to prevent strain localization and strongmesh sensitivity of the solution, an integral-type of nonlocal model based on the weighted spatial averaging of astrain-like quantity is developed. Regarding the interface, the damage is governed by the relative displacementoccurring at bond. A suitable interface model which accounts for the mode I, mode II and mixed mode ofdamage is developed. The coupling between the body damage and the interface damage is performedcomputing the body damage on the bond surface. Numerical examples are presented
Dalla Frattura alla Scienza dei Metalli
No full textLa frattura inizia ad essere considerata oggetto di studio da Leonardo da Vinci e trova in Vannoccio Biringucci ed in Galileo Galilei i precursori di un approccio scientifico al come e perché si produce una rottura. Nell'arco di poco più di un secolo, il sedicesimo, si trovano i primi riferimenti scritti di esperienze e interpretazioni su cui lavorare: i limiti tecnologici e di conoscenza scientifica dei materiali allora disponibili hanno impedito una interpretazione soddisfacente del fenomeno della rottura. Sarebbero occorsi altri due secoli per collegare con una relazione lineare, prima forza e deformazione, e poi tensione e deformazione
Computed Tomography analysis of damage in composites subjected to impact loading
No full textThe composites, used in the transportation engineering, include different classes with a wide range of materials and properties within each type. The following different typologies of composites have been investigated: laminated composites, PVC foam sandwiches, aluminium foam and honeycomb sandwiches. Aim of this paper was the analysis of low-velocity impact response of such composites and the investigation of their collapse modes. Low velocity impact tests were carried out by a drop test machine in order to investigate and compare their structural response in terms of energy absorption capacity. The failure mode and the internal damage of the impacted composites have been, also, investigated using 3D Computed Tomography
La modellazione microstrutturale di materiali a struttura eterogenea: princìpi ed applicazioni
No full textMolti problemi della Meccanica e Fisica dei Solidi e della Scienza dei Materiali, non sonofacilmente risolvibili con gli approcci tradizionali. Oltre allo studio delle proprietà effettive dei solidi eterogenei,vi è la crescente necessità di incorporare un maggiore numero di informazioni sui meccanismi di deformazione edanneggiamento generati alla microscala, anche per i materiali abitualmente considerati omogenei.Micromeccanismi di cavitazione e concentrazioni locali di tensione e deformazione, sono indispensabili perspiegare fenomeni non-lineari come la rottura di fatica o il cedimento duttile, altrimenti non inquadrabili conapprocci classici di tensioni e deformazioni medie. La micromeccanica si occupa della determinazione precisa, odi una stima accurata, di grandezze di campo microstrutturali locali. In questo lavoro sono illustrati i princìpiche sono alla base dell’approccio micromeccanico, come i concetti di multiscala, di distribuzione statistica dellefasi, di descrizione mediante volumi di riferimento e di omogeneizzazione e localizzazione, e, attraverso alcuneapplicazioni pratiche delle principali tecniche di modellazione, sono illustrati e discussi criticamente i risultatidella ricerca effettuata su varie strutture di ghisa nodulare
Analysis of crack-tip plastic zone in a Compact Tensile Shear (CTS) Specimen
No full textThe minimum plastic zone radius (MPZR) criterion is one of the recent criterions to estimate crack initiation direction under mixed mode loading. The MPZR theory is based on the theoretical computations of plastic zone size (PZS). In this investigation, the shape and size of crack-tip plastic zones have been estimated by analytical and the elastic finite element computations in a Compact Tensile Shear (CTS) specimen under mixed mode (I/II) loading according to von Mises yield criteria. The theoretical and the finite element analysis results are compared to analyze the minimum plastic zone radius (MPZR) criterion for crack-initiation angle with reference to the loading angle and stress intensity factor