Gruppo Italiano Frattura (IGF)
Not a member yet
215 research outputs found
Sort by
Complexity: a new paradigm for fracture mechanics
No full textThe so-called Complexity Sciences are a topic of fast growing interest inside the scientific community. Actually, researchers did not come to a definition of complexity, since it manifests itself in so many different ways [1]. This field itself is not a single discipline, but rather a heterogeneous amalgam of different techniques of mathematics and science. In fact, under the label of Complexity Sciences we comprehend a large variety of approaches: nonlinear dynamics, deterministic chaos theory, nonequilibrium thermodynamics, fractal geometry, intermediate asymptotics, complete and incomplete similarity, renormalization group theory, catastrophe theory, self-organized criticality, neural networks, cellular automata, fuzzy logic, etc.
Aim of this paper is at providing insight into the role of complexity in the field of Materials Science and Fracture Mechanics [2-3]. The presented examples will be concerned with the snap-back instabilities in the structural behaviour of composite structures (Carpinteri [4-6]), the occurrence of fractal patterns and self-similarity in material damage and deformation of heterogeneous materials, and the apparent scaling on the nominal mechanical properties of disordered materials (Carpinteri [7,8]). Further examples will deal with criticality in the acoustic emissions of damaged structures and with scaling in the time-to-failure (Carpinteri et al. [9]). Eventually, results on the transition towards chaos in the dynamics of cracked beams will be reported (Carpinteri and Pugno [10,11])
Damages to stent stabilized left ventricular pacemaker electrodes during simulated lead extraction
No full textDuring biventricular pacemaker implantation stents can be applied for coronary sinus lead stabilization to prevent lead dislocations. A lot of issues have been raised in connection with the use of the stent. In some cases the implanted left ventricular lead must be explanted. It is crucial to avoid any injury to the heart when the electrode is removed. Another very important question concerns the type of injuries the electrode may cause during the removal process. An extraction model has been prepared using a special curve and a polymer tube. After the pacemaker leads were extracted, various microscopic examinations were executed. The findings may to make such intervention methods more successful, helping to better stabilize the electrode and to keep injuries during interventions to a minimum
Caratterizzazione microstrutturale e prove di resilienza su giunti Friction Stir Welding e Linear Friction Welding di compositi a matrice metallica
No full textIn questo studio sono stati caratterizzati giunti Friction Stir Welding e Linear Friction Welding sucompositi a matrice in lega di alluminio e rinforzo particellare ceramico. Il processo FSW è stato applicato a duecompositi ottenuti con processo fusorio, quindi estrusi e trattati termicamente T6: AA6061/20%vol.Al2O3p eAA7005/10%vol.Al2O3p. I giunti LFW sono stati invece realizzati su un composito con matrice in lega dialluminio e rinforzo particellare in carburo di silicio, ottenuto mediante metallurgia delle polveri, quindi forgiatoe trattato termicamente T4: AA2124/25%vol.SiCp. Sono stati esaminati gli effetti della saldatura sullecaratteristiche microstrutturali dei giunti, avvalendosi di tecniche di microscopia ottica con analisi di immagine edi microscopia elettronica in scansione (SEM) con microsonda a dispersione di energia (EDS). Sono statequindi condotte prove di resilienza con pendolo strumentato Charpy. Lo studio dei meccanismi didanneggiamento è stato effettuato mediante analisi al SEM delle superfici di frattura. Entrambi i processi disaldatura hanno portato a giunti sostanzialmente esenti da difetti. La microstruttura dei cordoni è risultatadipendente sia dalle caratteristiche microstrutturali iniziali dei compositi considerati, sia dalla tipologia diprocesso di saldatura. Nel caso dei compositi AA6061/20%Al2O3p e AA7005/10%Al2O3p saldati FSW si èosservato un sostanziale incremento di resilienza, rispetto al materiale base, in conseguenza dell’affinamento deigrani della matrice, della riduzione della dimensione media delle particelle di rinforzo e della loro spigolosità,indotte dal processo di saldatura. Il composito AA2124/25%SiCp saldato LFW ha presentato valori di resilienzaconfrontabili con quelli del materiale base, in conseguenza, soprattutto, dei limitati effetti della saldatura sudimensione e distribuzione delle particelle di rinforzo
Role of plasticity-induced crack closure in fatigue crack growth
No full textThe premature contact of crack surfaces attributable to the near-tip plastic deformations undercyclic loading, which is commonly referred to as plasticity induced crack closure (PICC), has long been focusedas supposedly controlling factor of fatigue crack growth (FCG). Nevertheless, when the plane-strain near-tipconstraint is approached, PICC lacks of straightforward evidence, so that its significance in FCG, and even thevery existence, remain debatable. To add insights into this matter, large-deformation elastoplastic simulations ofplane-strain crack under constant amplitude load cycling at different load ranges and ratios, as well as with anoverload, have been performed. Modeling visualizes the Laird-Smith conceptual mechanism of FCG by plasticblunting and re-sharpening. Simulation reproduces the experimental trends of FCG concerning the roles ofstress intensity factor range and overload, but PICC has never been detected. Near-tip deformation patternsdiscard the filling-in a crack with material stretched out of the crack plane in the wake behind the tip assupposed PICC origin. Despite the absence of closure, load-deformation curves appear bent, which raisesdoubts about the trustworthiness of closure assessment from the compliance variation. This demonstratesambiguities of PICC as a supposedly intrinsic factor of FCG and, by implication, favors the stresses and strainsin front of the crack tip as genuine fatigue drivers
Subsurface metals fatigue cracking without and with crack tip
No full textVery-High-Cycle-Fatigue regime for metals was considered and mechanisms of the subsurfacecrack origination were introduced. In many metals first step of crack origination takes place with specific areaformation because of material pressing and rotation that directed to transition in any volume to material ultrahigh-plasticity with nano-structure appearing. Then by the border of the nano-structure takes place volumerotation and fracture surface creates with spherical particles which usually named Fine-Granular-Area. Inanother case there takes place First-Smooth-Facet occurring in area of origin due to whirls appearing by the oneof the slip systems under discussed the same stress-state conditions. Around Fine-Granular-Area or First-Smooth-Facet there plastic zone appeared and, then, subsurface cracking develops by the same manner as forthrough cracks. In was discussed quantum-mechanical nature of fatigue crack growth in accordance with Yang’smodulus quantization for low level of deformations. New simply equation was considered for describingsubsurface cracking in metals out of Fine-Granular-Area or Fist-Smooth-Facet
Comparison between dog-bone and Gaussian specimens for size effect evaluation in gigacycle fatigue
No full textGigacycle fatigue properties of materials are strongly affected by the specimen risk volume (volumeof material subjected to a stress amplitude larger than the 90% of the maximum stress). Gigacycle fatigue tests,performed with ultrasonic fatigue testing machines, are commonly carried out by using hourglass shapedspecimens with a small risk volume. The adoption of traditional dog-bone specimens allows for increasing therisk volume, even if the increment is quite limited. In order to obtain larger risk volumes, a new specimen shapeis proposed (Gaussian specimen). The dog-bone and the Gaussian specimens are compared through FiniteElement Analyses and the numerical results are validated experimentally by means of strain gagesmeasurements. The range of applicability of the two different specimens in terms of available risk volume andstress concentration effects due to the cross section variation is determined
Previsione della resistenza a fatica in saldature per punti attraverso modellazione solida
No full textNel presente lavoro vengono messi a confronto due metodi idonei per la verifica di giunzionisaldate per punti: il metodo del raggio di raccordo fittizio ed il metodo del gradiente implicito. Il primo imponeun raggio di raccordo diverso da zero al piede o alla radice del cordone di saldatura, il secondo, invece,considera più semplicemente la saldatura come un intaglio acuto. Il confronto è fatto sulla capacità di prevederel’affidabilità di giunti saldati per punti in acciaio aventi spessore variabile da 0.8 a 1.5 mm. Tali giunti sonosollecitati a taglio (giunti a semplice sovrapposizione) o a trazione (giunti a tazza). Infine, è discussa lacondizione di convergenza delle analisi numeriche necessarie, in entrambi i metodi, per il calcolo di una tensioneequivalente da porre a confronto direttamente con la di resistenza a fatica del materiale
Effect of weld parameter on mechanical and metallurgical properties of dissimilar joints AA6082–AA6061 in T6 condition produced by FSW
No full textThe effect of processing parameters on the mechanical and metallurgical properties of dissimilarjoints of AA6082–AA6061 produced by friction stir welding was analysed in this study. Different FSW sampleswere produced by varying the welding speeds of the tool as 50 and 62 mm/min and by varying the alloypositioned on the advancing side of the tool. In all the experiments the rotating speed is fixed at 1600rpm. Allthe welds were produced perpendicularly to the rolling direction for both the alloys. Microhardness (HV) andtensile tests performed at room temperature were used to evaluate the mechanical properties of the joints. Inorder to analyse the microstructural evolution of the material, the weld’s cross-sections were observed opticallyand SEM observations were made of the fracture surfaces. The corrosion tests of base alloy and welded jointswere carried out in 3.5%NaCl solution at a room temperature. Corrosion current and potential were determinedusing potentiostatic polarization measurements. It was found that the corrosion rates of welded joints werehigher than that of base alloy
Optimization of fractional composition of the excipient in the elastomeric covering for asphalt highways
No full textThe computational method of optimum fractional composition of a dispersible filler of polymericcomposite on the basis of three-dimensionally linked elastomer is developed according to non-linearprogramming. The coefficient of dynamic viscosity of polymeric suspension or the initial module of aviscoelasticity of the join solidification low-molecular rubbers with the final functional groups, filled by manyfractional dioxide of silicon are considered as criteria of optimization. Influence of the limiting volume filling onenergy of mechanical destruction was investigated. The elastomeric material is offered for use as a covering ofasphalt highways in the form of a frost-proof waterproofing layer, which allowing multiply to increase operatingproperties
Il modello della fessura coesiva in trazione e compressione per la valutazione della duttilità degli elementi strutturali in calcestruzzo armato
No full textIl problema della valutazione della duttilità degli elementi in calcestruzzo armato soggetti a flessione o presso-flessione è stato largamente studiato negli ultimi decenni, sia da un punto di vista sperimentale che analitico. Data l’influenza di numerosi parametri di progetto sulla duttilità, tuttavia, è difficile sviluppare un modello in grado di descrivere completamente la risposta meccanica di elementi strutturali, tenendo conto di tutti gli effetti dovuti alla non-linearità dei materiali. Nel passato, in particolare, si è studiato in maniera approfondita l’effetto della classe di duttilità dell’acciaio, mentre il ruolo degli effetti di scala, evidenziato da più campagne sperimentali, non è stato ancora del tutto chiarito. Una delle ragioni principali è l’inadeguatezza dei modelli tradizionali, basati su leggi costitutive tra tensioni e deformazioni. Nel presente lavoro, si propone un nuovo modello basato sul concetto della localizzazione delle deformazioni, capace di descrivere la propagazione della fessura e l’avanzamento del crushing durante il processo di carico. In tale contesto, il comportamento non-lineare del calcestruzzo in compressione è modellato attraverso l’Overlapping Crack Model, modello analogo a quello coesivo valido per la trazione, che descrive la localizzazione delle deformazioni dovuta al danneggiamento del calcestruzzo mediante una compenetrazione del materiale. Con questo nuovo algoritmo è possibile cogliere l’effettiva risposta flessionale di elementi strutturali in calcestruzzo armato al variare della percentuale di armatura e della scala dimensionale. Applicazioni numeriche riguardano l’analisi della risposta post-picco di provini in calcestruzzo soggetti a compressione e la valutazione delle rotazioni plastiche di travi in calcestruzzo armato soggette a flessione su tre punti. Si propone infine un ampio confronto con i risultati di prove sperimentali, con lo scopo di dimostrare la validità del nuovo approccio