Gruppo Italiano Frattura (IGF)
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A model for steady state stage III creep regime at low-high stress/temperature range
No full textAlthough diffusional flow creep is often considered out of practical engineering applications, the need for a model capable to account for the resulting action of both diffusional and dislocation type creep is justified by the increasing demands of reliable creep design for very long lives (exceeding lOO.OOOh), high stress-low temperatures and high temperature-low stress regimes. In this paper, a creep model formulation, in which the change of the creep mechanism has been accounted for through an explicit dependence of the creep exponent n on stress and temperature, has been proposed. An application example of the proposed approach to high purity aluminum is given
Notiziario IGF n.19
No full text1) Editoriale del Segretario IGF;2) Verbale dell'Assemblea Annuale dei Soci 2007;3) Verbali dei Consigli di Presidenza;4) Cariche sociali biennio 2007-2009;5) Resoconti delle attività TC-ESIS nel 2007;6) Summer School IGF 2008;7) Convocazione Assemblea Annuale dei Soci 2008;8) Calendario congressi internazionali
Effetto supporto, tensione locale e gradiente relativo di tensione nella predizione della vita a fatica
No full textObiettivo ultimo delle ricerche nelle quali si inquadra questo studio è superare, o almeno limitare, la necessità di complicate e costose prove sperimentali a favore di altri metodi predittivi della vita a fatica. Scopo specifico del presente lavoro è, dopo una analisi critica delle diverse soluzioni proposte in letteratura, la valutazione di una formulazione atta a predire, tramite analisi agli Elementi Finiti (FEM), la vita a fatica locale, nella regione ad alto numero di cicli, di componenti di forma complessa con una distribuzione irregolare di tensione. Concetto base di questa teoria è che il fenomeno della vita a fatica sia governato dal gradiente relativo di tensione nella zona maggiormente sollecitata del componente. Le simulazioni proposte per il calcolo della pendenza delle curve S-N e del limite di fatica sono verificate tramite il confronto con dati sperimentali tratti da letteratura. Mentre la formulazione per il calcolo del limite di fatica mostra un buon accordo con i dati sperimentali (errore medio percentuale del 7%), la pendenza valutata mediante l'equazione proposta risulta in alcuni casi sovrastimata (errore medio percentuale oltre il 100%). Nel presente lavoro si propone quindi una nuova formula che permette di ottenere un errore medio percentuale sulla pendenza dell'ordine del 20% nel confronto con i dati sperimentali
Modello coesivo per l’avanzamento di fratture mediante rilascio nodale di strutture discretizzate con elementi finiti
No full textLa simulazione numerica della propagazione di una frattura in MODO I, viaggiante ad elevatavelocità in un acciaio a comportamento duttile è realizzata attraverso un modello coesivo che governa ladistribuzione delle forze di rilascio nodale. Come noto, la ricerca di un valore di tensione all’apice non ha alcunsenso nel caso elastico; infatti, la tensione può essere valutata solo mediante fattori di campo. Nel caso elasto-plastico, incrudimento e softening di origine geometrica o legato al progressivo danneggiamento influisconodecisamente sull’andamento esponenziale del campo tensionale. È possibile comunque individuare un valore diriferimento, di entità finita, mediante estrapolazione delle tensioni elasto-plastiche nella zona di inizio softeningdella frattura. Tale grandezza può essere presa come fattore di riferimento per il calcolo delle forze di rilasciocoesive e quindi dell’energia dissipata. Nel lavoro viene discusso come determinare, dal campo di tensioneelasto-plastico locale, il valore che governa la zona coesiva al variare del T-stress
Design and experimental characterization of a 350 W High Temperature PEM fuel cell stack
No full textHigh Temperature Proton Exchange Membrane (HT PEM) fuel cell based on polybenzimidazole (PBI) polymer and phosphoric acid, can be operated at temperature between 120 °C and 180 °C. Reactants humidification is not required and CO content up to 2% in the fuel can be tolerated, affecting only marginally performance. This is what makes HT PEM very attractive, as low quality reformed hydrogen can be used and water management problems are avoided. Till nowadays, from experimental point of view, only few studies relate to the development and characterization of high temperature stacks. The aim of this work is to present the main design features and the performance curves of a 25 cells HT PEM stack based on PBI and phosphoric acid membranes. Performance curves refer to the stack operating with two type of fuels: pure hydrogen and a gas mixture simulating a typical steam reformer output. The stack voltage distribution analysis and the stack temperature distribution analysis suggest that cathode air could be used as coolant leading to a better thermal management. This could simplify stack design and system BOP, thus increasing system performanc
Mechanical characterization of metal-ceramic composites
No full textMetal-ceramic composites represent a class of quasi-brittle materials for advanced structural applications that require adequate mechanical characterization. Difficulties and costs associated with material production and specimen extraction prevent the execution of a statistically meaningful number of standard laboratory tests. Parameter calibration methodologies based on instrumented indentation and inverse analysis represent fast and reliable identification procedures in the present context, as shown by the present contribution, based on some experience achieved in the framework of the European Network of Excellence on ‘Knowledge-based Multi-component Materials for durable and safe performance’ (KMM-NoE)
Temporal peculiarities of brittle fracture of rocks and concrete
No full textWhen we want to compare the strength of two materials, we compare the table values of quasistaticstrength. However results of some experiments show that the strength ratio of the materials can changewith increase in rate of loading. Such "substitution effect" of material strength at different strain rates is studiedin this paper. It is shown that one material can have a lower dynamic strength for a higher static strengthcompared to the other material. Tests of two different materials, tests of mortar and concrete, and tests ofconcrete with different water content are considered. It is shown that load-carrying capacity of materials and thesubstitution effect of material strength in a wide range of loading rates can be predicted by the incubation timecriterion
Relay-race deformation mechanism during uniaxial tension of cylindrical samples of carbon steel: using digital image correlation technique
No full textThe work deals with experimental study of macro localization of plastic yielding occurrences of structural carbon steel, research of singularity of deformation wave processes by complex use of contemporary test equipment and high effective digital image correlation method. Evolution of nonuniform axial strain fields on surface of cylindrical samples during uniaxial tension was registered, time dependences were drawn, and a ‘relay-race’ mechanism of material deformation was found out at the stage of yield plateau forming. Strain concentration ratio was estimated for several material deformation stages
Analysis of the fatigue strength under two load levels of a stainless steel based on energy dissipation
No full textIn this paper the fatigue behaviour of a stainless steel AISI 304L is analysed. In the first part of the work the results obtained under constant amplitude fatigue are presented and synthesised in terms of both stress amplitude and energy released to the surroundings as heat by a unit volume of material per cycle, Q. Then some specimens have been fatigued in variable amplitude, two different load level tests: the first level was set higher while the second was lower than the constant amplitude fatigue limit. The Q values, evaluated during the second part of the fatigue test, have been compared with those calculated under constant amplitude fatigue at the same load level. The comparison allowed us to notice that the Q parameter is sensitive to the fatigue damage accumulated by the material during the first part of the fatigue test
On a kinked crack model to describe the influence of material microstructure on fatigue crack growth
No full textThreshold condition and rate of fatigue crack growth in both short and long crack regime appear tobe significantly affected by the degree of crack deflection. In the present paper, a theoretical model of aperiodically-kinked crack is presented to describe the influence of the degree of crack deflection on the fatiguebehavior. The kinking of the crack is due to a periodic self-balanced microstress field having a length scale, d.By correlating the parameter d with a characteristic material length (e.g. average grain size in metals, maximumaggregate dimension in concrete), the possibility of using the present model to describe some experimentalfindings related to crack size effects in fatigue of materials is explored. Well-known experimental resultsconcerning two different situations (fatigue threshold and fatigue crack growth in the Paris regime) are brieflyanalysed