1,721,114 research outputs found
Progettazione a Fatica di Giunzioni Saldate (... e non)
No full textScopo del Workshop
Il Gruppo Italiano della Frattura (IGF) in collaborazione con i gruppi di Costruzione di Macchine dell’Università di Ferrara, dell’Università di Padova e dell’Università di Udine ha organizzato questo Workshop sulle problematiche legate alla progettazione a fatica di giunzioni saldate (... e non), con l’intento di stimolare il dibattito su questo attuale e importante tema. In particolare, il presente Workshop vuole rappresentare una occasione di approfondimento sia sui meccanismi di danneggiamento sia sulle metodologie di progettazione di giunti soggetti a carichi di fatica. Con l’intento di dare ampia possibilità di presentare in modo adeguato le attività scientifiche svolte in tale ambito dai diversi Gruppi di Ricerca, si vorrebbero raccogliere memorie che sintetizzino il lavoro svolto nel recente passato, le problematiche aperte, nonché i possibili sviluppi futuri. Le memorie presentate saranno raccolte in un CD, nonché direttamente scaricabili dal sito internet IGF. In più, si vorrebbe realizzare un numero speciale di “Frattura ed Integrità Strutturale", rivista ufficiale del Gruppo Italiano Frattura, con l’obiettivo di dare una sintetica testimonianza sullo stato dell’arte in Italia riguardo alle problematiche legate alla progettazione di giunzioni sollecitate da carichi affaticanti.
Tra le attività del Workshop sono state inoltre previste tre Plenary Lectures che verranno tenute, rispettivamente, dal Prof. Paolo Lazzarin (Universtà di Padova, I), dal Prof. Neil James (Università di Plymouth, UK) e dal Prof. Morris Sonsino (LBF, Darmstadt, G).
Infine, con l’intento di favorire sia lo scambio di informazioni sia il dibattito tra i Partecipanti, la lingua ufficiale del presente Workshop sarà l’Italiano, e questo sia per le presentazioni sia per la stesura delle memorie, dando, possibilmente, ampio spazio ai giovani ricercatori.
Argomenti del Workshop
Il Workshop vorrebbe essere principalmente incentrato sulle problematiche legate alla fatica delle giunzioni saldate. Tuttavia, con l’intento di favorire una più ampia partecipazione, sarà organizzata anche una speciale sessione dove verranno trattati gli aspetti inerenti alla fatica di altri tipi di giunzioni
The Modified Wöhler Curve Method calibrated by using standard fatigue curves and applied in conjunction with the Theory of Critical Distances to estimate fatigue lifetime of aluminium weldments
No full textThis paper is concerned with the application of a novel engineering method we have recently devised to estimate fatigue lifetime of aluminium welded joints subjected to constant-amplitude uniaxial and multiaxial fatigue loading. The assessment technique employed in the present study is based on the use of the so-called Modified Wöhler Curve Method (MWCM), a conventional critical plane approach, applied in conjunction with the theory of critical distances (TCD). In more detail, the MWCM was initially calibrated by using two standard curves: the first one, stated by Eurocode 9, suitable for assessing ground butt welds subjected to uniaxial loading, whereas the second one, suggested by the International Institute of Welding (IIW), suitable for estimating fatigue strength of aluminium welded details loaded in torsion. Subsequently, a unifying critical distance value to be used to assess aluminium welded joints was calculated
by taking full advantage of the master curve supplied by the notch-stress intensity factor (N-SIF) approach and obtained by summarising the uniaxial fatigue strength of cruciform aluminium welded details characterised by different absolute dimensions. Finally, the accuracy and reliability of the devised method was systematically checked by means of several experimental results taken from the literature and generated by testing a variety of welded geometries subjected to uniaxial as well as to multiaxial fatigue loading. Such an extensive validation exercise allowed us to prove that our approach is successful in estimating fatigue damage in aluminium welded details, resulting in predictions mainly falling within the two reference scatter bands adopted to calibrate the method itself. Such a high accuracy level is very
promising, especially in light of the fact that our engineering approach can be applied to assess real aluminium welded components by directly post-processing simple linear-elastic finite element (FE) models
Multiaxial Notch Fatigue: from nominal to local stress-strain quantities
No full textReal mechanical components not only contain geometrical features resulting in stress concentration phenomena, but they are also subjected to in-service multiaxial fatigue loading. It is evident that, to efficiently address the above problem, structural engineers need sound and reliable methodologies which allow an adequate margin of safety to always be reached.
The present book then attempts to summarise the methods we have devised to design real components against multiaxial fatigue by taking full advantage not only of nominal but also of local stress/strain quantities.
This book begins by reviewing those definitions suitable for calculating the stress/strain quantities commonly used to perform the fatigue assessment. Subsequently, those pivotal concepts which should always be kept in mind when designing components against fatigue are briefly summarised (mainly considering uniaxial and torsional situations). By taking as a starting point the above concepts, the way of using the Modified Wöhler Curve Method is then explained in great detail, by focusing attention on both the high- and the medium-cycle fatigue regime. The existing links between the above multiaxial fatigue criterion and physical reality are also critically discussed. Subsequently, the procedure suitable for employing our method to estimate fatigue damage both in notched and in welded components is explained, by focusing attention on the most efficient way to estimate fatigue strength by post-processing Finite Element results. In light of the fact that the Modified Wöhler Curve Method as it stands can not be applied to estimate lifetime in the low-cycle fatigue regime, the main features of the so-called Modified Manson-Coffin Curve method are investigated in depth, by also reviewing those concepts playing a fundamental role in the so-called strain based approach. Lastly, the problem of performing the fatigue assessment of composite material is addressed by considering not only those design parameters influencing composites’ behaviour under complex cyclic loading paths, but also those criteria suitable for designing real components against multiaxial fatigue.
The present book contains also two Appendices summarising several experimental results taken from the technical literature. In particular, about 4500 experimental fatigue results generated by testing plain, notched and welded specimens under constant-amplitude proportional and non-proportional multiaxial fatigue loading are listed in a systematic and organic way
A simple and efficient numerical algorithm to determine the orientation of the critical plane in multiaxial fatigue problems
No full textThe present paper reports on an attempt of reformulating the Maximum Variance Method (MVM) to make it suitable for being used to perform the multiaxial fatigue assessment by numerically post-processing the complex load histories damaging real mechanical components. In more detail, the MVM takes as its starting point the assumption that the material plane where the crack initiation phenomenon takes place is the one containing the direction along which the variance of the resolved shear stress reaches its maximum value. From an engineering point of view, the most interesting implication of the above assumption is that the MVM can successfully be used to address problems involving not only constant but also variable amplitude problems. Moreover, thanks to its particular features, from a computational point of view, after calculating the variance and covariance terms relative to the considered load history, the effective time needed to locate the critical plane does not depend on the length of the load history itself. The above peculiar features make the MVM suitable for being used in situations of practical interests by easily taking full advantage not only of numerical results calculated by using commercial Finite-Element software packages but also of in-field acquisitions done through strain gages attached to the components under investigation
On the use of the Modified Wöhler Curve Method to Estimate Notch Fatigue Limits
No full textThis paper reviews on the use of the Modified Wöhler Curve Method (re-interpreted in terms of the Theory of Critical Distances) to predict the high-cycle fatigue strength of notched components subjected to fatigue loading. This method takes as its starting point the assumption that fatigue damage depends on all the physical phenomena taking place within the fatigue process zone (the so-called structural volume). The material volume controlling all the fatigue processes has a size which does not depend on the degree of multiaxiality of the stress field damaging critical sites, but its size is different for different materials. Assuming that crack initiation in the high-cycle fatigue regime is Mode II governed, the Modified Wöhler Curve Method must be applied by considering the linear-elastic stress state at the centre of the structural volume. The main advantage of this method is that it is based on the use of linear-elastic stresses: this aspect makes it suitable for being used to post-process results from linear-elastic Finite-Element models. Finally, accuracy and reliability of the proposed method was checked by using data taken from the technical literature: the Modified Wöhler Curve Method was seen to be a useful tool suitable for assessing mechanical components in situations of practical interest
The combined effect of non-zero mean stresses, out-of-phase angles and stress concentrators in metallic materials subjected to multiaxial fatigue loading
No full textFour stress analysis strategies to use the Modified Wöhler Curve Method to perform the fatigue assessment of weldments subjected to constant and variable amplitude multiaxial fatigue loading
No full textThe present paper investigates the different ways of using the Modified Wöhler Curve Method (MWCM) to perform the fatigue assessment of steel and aluminium welded joints subjected to in-service variable amplitude (VA) multiaxial load histories. Thanks to its specific features, the above critical plane approach can efficiently be applied in terms of both nominal, hot-spot, and local quantities, that is, by using any of the stress analysis strategies suggested by the Design Recommendations of the International Institute of Welding (IIW). The MWCM can efficiently be used also along with the so-called Theory of Critical Distances applied in the form of the Point Method (PM). The accuracy of the different formalisations of the MWCM investigated in the present paper was systematically checked against a large number of experimental results taken from the literature and generated by testing, under VA biaxial nominal loading, welded samples having different geometries. Such a systematic validation exercise allowed us to prove that our multiaxial fatigue criterion is successful in designing welded joints against VA multiaxial fatigue, this holding true independently from both definition adopted to calculate the necessary stress quantities and complexity of the assessed load history
The Theory of Critical Distances: a review of its applications in fatigue
No full textThis paper attempts to review the most interesting findings in the use of the theory of critical distances (TCD) to predict fatigue strength of notched mechanical components. Initially, the most modern formalisations of the TCD are considered, showing their peculiarities and differences. An ad-hoc section is then focused on the multiaxial high-cycle fatigue problem, considering all the open questions arising in the presence of complex stress fields damaging the fatigue process zone in the vicinity of the stress concentrator apex. Subsequently, the physical idea on the structural volume concept is briefly investigated showing some peculiar results generated in the high-cycle fatigue regime under both uniaxial and biaxial fatigue loading. Finally, our idea to extend the use of the TCD down to the low-medium cycle fatigue regime is briefly explained.
Working in collaboration with Prof. David Taylor, we have spent the last five years investigating this theory both to better understand its physical meaning and to systematically check its accuracy in predicting notch fatigue strength under different loading conditions. After so much work done in this area we feel so confident to proudly and loudly say that the TCD is a powerful engineering tool suitable for assessing real mechanical components in situations of practical interest. Finally, it can be highlighted also that the best TCD formalisations were seen to be those based on the use of linear-elastic stresses. This suggests that such a theory can successfully be used to post-process simple linear-elastic finite element (FE) models reducing time and costs of the design process
Estimating fatigue lifetime of steel weldments locally damaged by variable amplitude multiaxial stress fields
No full textThe present investigation aims to prove that the so-called Modified Wöhler Curve Method applied along with the Theory of Critical Distances (formalised in the form of the Point Method) can successfully be used also to estimate fatigue lifetime of steel welded joints subjected to both uniaxial and proportional/non-proportional multiaxial variable amplitude load histories. The accuracy and reliability of our fatigue life estimation technique was systematically checked by reanalysing approximately 500 experimental results taken from the literature and generated by testing welded samples having not only different geometries but also different absolute dimensions. Such an extensive validation exercise allowed us to fully confirm the robustness of our design methodology as well as to show that, when the method itself is formalised for a probability of survival equal to 97.7%, the obtained estimates fully complies, from a statistical point of view, with the recommendations of the available standard codes
Three different ways of using the Modified Wöhler Curve Method to perform the multiaxial fatigue assessment of steel and aluminium welded joints
No full textThe present paper reviews the different procedures to be followed to use the Modified Wöhler Curve Method (MWCM) to estimate fatigue lifetime of steel and aluminium welded joints subjected to multiaxial cyclic loading. In more detail, the MWCM is a conventional critical plane approach which postulates that fatigue damage reaches its maximum value on the plane experiencing the maximum shear stress range. According to the above assumption, the complexity of the stress field acting on the fatigue process zone is directly taken into account through the ratio between the ranges of the normal and shear stress relative to the critical plane, being such a ratio sensitive to the degree of non-proportionality of the applied loading but not to the presence of superimposed static stresses. Thanks to its peculiar features, our multiaxial fatigue criterion can be used to estimate fatigue damage in weldments by applying it in terms of either nominal, hot-spot or local stresses. The accuracy of the above three ways of applying the MWCM was checked by using several experimental results taken from the literature and generated by testing steel and aluminium welded details characterised by different geometries and subjected to different cyclic loading paths. Such a validation exercise allowed us to prove that the MWCM is a powerful engineering tool capable of highly accurate estimates, and it holds true independently of the definition adopted to calculate the stress quantities needed for its in-field application. This suggests that our fatigue life estimation technique can be considered as an efficient alternative to other existing linear-elastic approaches, allowing an adequate margin of safety to always be reached when designing welded components against fatigue
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