Gruppo Italiano Frattura (IGF)
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Structural integrity of hierarchical composites
No full textInterface mechanical problems are of paramount importance in engineering and materials science.Traditionally, due to the complexity of modelling their mechanical behaviour, interfaces are often treated asdefects and their features are not explored. In this study, a different approach is illustrated, where the interfacesplay an active role in the design of innovative hierarchical composites and are fundamental for their structuralintegrity. Numerical examples regarding cutting tools made of hierarchical cellular polycrystalline materials areproposed, showing that tailoring of interface properties at the different scales is the way to achieve superio
Damage localization and rupture with gradient damage models
No full textWe propose a method of construction of non homogeneous solutions to the problem of traction ofa bar made of an elastic-damaging material whose softening behavior is regularized by a gradient damage model.We show that, for sufficiently long bars, localization arises on sets whose length is proportional to the materialinternal length and with a profile which is also characteristic of the material. The rupture of the bar occurs at thecenter of the localization zone when the damage reaches there the critical value corresponding to the loss ofrigidity of the material. The dissipated energy during all the damage process up to rupture is a quantity c G whichcan be expressed in terms of the material parameters. Accordingly, c G can be considered as the usual surfaceenergy density appearing in the Griffith theory of brittle fracture. All these theoretical considerations areillustrated by numerical examples
Micro void coalescence of ductile fracture in mild steel during tensile straining
No full textThe ductile fracture occurs mainly in three stages i.e. void nucleation, void growth and the voidcoalescence. The present work focuses on the study the coalescence of existing micro void in a ductile material,mild steel. The specimen with holes in square array at various angle to load axis have been tested. The holeswere machined in the specimen and assuming those hole as the voids. The growth and coalescence behavioursduring tensile straining were observed both in macro and micro levels. Since the existing facility is not adequateto make hole size in micron, this work has been carried out by making hole upto 500 micron. The results arecompared with other specimen with bigger size hole and without any hole. Also the effects of micro voids(present in the material) on the progress of crack have been studied. It is found that with same amount of voids,present in different positions, the mechanical properties of the material are altered
Characterization of fracture properties of thin aluminuminclusions embedded in anisotropic laminate composites
No full textoai:ojs.www.gruppofrattura.it:article/126The fracture properties of thin aluminum inclusions embedded in anisotropic paperboardcomposites, of interest for food and beverage packaging industry, can be determined by performing tensile testson non-conventional heterogeneous specimens. The region of interest of the investigated material samples ismonitored all along the experiment by digital image correlation techniques, which allow to recover qualitativeand quantitative information about the metal deformation and about the evolution of the damaging processesleading to the detachment of the inclusion from the surrounding laminate composite. The interpretation of thelaboratory results is supported by the numerical simulation of the tests
Sulla stima di macro, micro e nano-durezza di materiali metallici mediante analisi elasto-plastiche agli elementi finiti
No full textIl lavoro sintetizzato nel presente articolo si pone come obiettivo primario quello di investigare lapossibilita di stimare, mediante un approccio elasto-plastico agli elementi finiti, la durezza dei materiali metalliciconvenzionali, e questo sia a livello macroscopico, che a livello microscopico, che, infine, a livello nanoscopico.Per verificare validita e accuratezza della metodologia FEM sviluppata, sono state condotte una serie di analisisperimentali su tre materiali metallici aventi caratteristiche metallurgiche estremamente diverse: una legadfalluminio (Al 7075-T6), un acciaio a basso tenore di carbonio (BS970-En3B) e, infine, un acciaio austenitico(AISI 316L). Lfindentazione Vickers e stata simulata con analisi elasto.plastiche agli elementi finiti considerandocarichi di prova nellfintervallo tra 490 N e 490 ƒÊN e calibrando le simulazioni numeriche mediante curvemonotone tensione.deformazione ottenute da prove di trazione eseguite utilizzando provini sia di dimensioneconvenzionale che aventi larghezza della zona calibrata dellfordine dei 100 ƒÊm.La sistematica comparazione tra risultati sperimentali e simulazioni numeriche ha posto in evidenza comelfaumentare del valore della durezza misurata al diminuire della dimensione dellfimpronta possa essere imputataal ruolo giocato dalla reale morfologia del materiale, ruolo che diventa predominante sulla plasticitaconvenzionale quando le dimensioni della superficie indentata diventano comparabili con le dimensioni mediedella grana cristallina delle leghe esaminate. Tali fenomeni, pertanto, non hanno consentito di estenderelfutilizzo della meccanica del continuo fino ad un livello nanoscopico per determinare correttamente i valoridella durezza. Alla luce di questi risultati e stata, pero, proposta una semplice metodologia di correzione dellestime eseguite mediante gli elementi finiti che si e dimostrata un valido strumento da utilizzarsi in situazioni diinteresse pratico per stimare la durezza dei materiali metallici, indipendentemente dalla dimensione dellasuperficie indentata
Mechanical properties of copper processed by Equal Channel Angular Pressing – a review
No full textThe Equal Channel Angular Pressing is a hardening treatment with which ductile metals can beprocessed to refine their grain and sub-grain structure. This process enhances the mechanical strength of metalsin terms of tensile strength, stress-controlled fatigue strength, and fatigue crack growth resistance. In this paperthe authors draw a review of the major results of a wide research activity they carried out on a coppermicrostructure processed by Equal Channel Angular Pressing. The essential results are that tensile and fatiguestrengths of the so obtained refined structure are improved by a factor of two with respect to the originalcoarse-grained metal. The fatigue crack initiation mechanism and the stability of the refined microstructureunder cyclic loading are topics also discussed, evidencing the essential role of the process and of the materialparameter, as the content of impurities in the microstructure. In this review, the authors also underline somecritical aspects that have to be more investigated
Experimental and numerical study of cemented bone-implant interface behavior
No full textAlthough the total hip replacement (THR) is a long-proven method of surgical treatment of diseases and disorders of the human hip, the surgery brings some risk of long-term instability of the joint. The aim of the research was to investigate the cemented bone-implant interface behavior. The main problems (cement layer degradation and bone-cement interface debonding) during physiological loading conditions have been investigated using a custom hip simulator. The experimental setup was designed to allow cyclic loading of the sample of pelvic bone with implanted cemented acetabular component. The hip contact force of required direction and magnitude was applied to the implant using a spherical femoral component head. The most unfavorable activity (downstairs walking) was simulated. The process of damage accumulation in the fixation was monitored by repeated scanning using high resolution micro Computed Tomography (µCT). Use of micro-focus source and large high-resolution flat panel detector allows investigation of structural changes and crack propagation both in the cement layer and the trabecular bone
On the application of the Theory of Critical Distances for prediction of fracture in fibre composites
No full textThis paper is concerned with the fracture of composite materials containing stress concentrationfeatures such as notches and holes. In particular, it addresses the question of the use of the Theory of CriticalDistances (TCD) – a method which is widely used for predicting notch effects in fatigue and fracture. The TCDmakes use of a length constant, L, known as the critical distance, which is normally assumed to be a materialproperty. However, many workers in the field of composite materials have suggested that the critical distance isnot a constant, but rather is a function of notch size. I examined the evidence for this assertion, and concludedthat it arises for four different reasons, two of which (process zone size and constraint) are real material effectswhilst the other two (choice of test specimen and estimation of the stress field) arise due to errors in making theassessments. From a practical point of view, the assumption of a constant value for L leads to only small errors,so it is recommended for engineering design purposes
Analisi basata sugli sforzi locali della resistenza a fatica di giunzioni incollate di materiali compositi
No full textIl lavoro prende spunto dai risultati di un’analisi sperimentale del comportamento a fatica di giunzioni incollate di materiali compositi laminati di elevato spessore formati da strati di unidirezionale e di tessuto di fibra di carbonio. I giunti sono stati realizzati in modo tale da saggiare l’influenza della lunghezza di sovrapposizione (da 25,4 mm a 110,8 mm), della forma del giunto (con e senza rastremazione), e della composizione degli aderendi (sostituzione di uno degli aderendi in composito con uno in acciaio). Mediante analisi 2D elastiche con il metodo degli elementi finiti sono state ricavate le distribuzioni degli sforzi all’interno dello strato di adesivo, al fine di individuare un parametro utile alla descrizione del comportamento a fatica in termini di sforzi locali - numero di cicli a rottura. Il ruolo della fase di propagazione viene discusso alla luce di osservazioni dell’avanzamento della frattura, condotta su alcuni dei giunti testati
Effetto delle dimensioni del cordone di saldatura sulla resistenza a fatica dei giunti a croce
No full textLa scelta della dimensione da assegnare al cordone di saldatura nel caso di giunzioni saldate a croce può presentarsi problematica, specie quando gli spessori delle lamiere che formano il giunto sono differenti tra loro. Le normative di tipo tecnologico suggeriscono in genere di prevedere uno spessore del cordone di saldatura inferiore al minimo spessore della lamiera da collegare, mentre le normative di tipo strutturale non prevedono una dipendenza della resistenza del giunto, sia statica che a fatica, dalle dimensioni del cordone di saldatura, a meno che queste non siano tanto ridotte da portare a rotture che si inneschino e si propaghino non più nella lamiera base ma nel cordone di saldatura. Scopo del presente lavoro è la verifica, teorica e sperimentale, della variazione della resistenza a fatica al variare del rapporto tra lo spessore del cordone di saldatura e quello minimo delle lamiere da saldare