Comportamento a fatica dei giunti saldati in funzione della densità di energia di deformazione locale: influenza dei campi di tensione singolari e non singolari

Abstract

RIASSUNTO.Il criterio della densità di energia di deformazione (SED) considera un preciso volume di controllo posizionato in corrispondenza del piede o della radice dei cordoni di saldatura, ossia delle zone di possibile innesco delle cricche di fatica. Modellati i cordoni come intagli a V non raccordati e con diverso angolo di apertura, il volume è riconducibile a un settore circolare nei casi di tensione o deformazione piana, e il raggio vale circa 0.3 mm per i giunti saldati in acciaio strutturale. Il valore medio della densità di energia di deformazione dipende essenzialmente dalle distribuzioni singolari nei giunti di medio ed elevato spessore, mentre importante diventa il contributo della T-stress nei giunti di spessore ridotto. Entrambi gli effetti sono correttamente computati utilizzando modelli agli elementi finiti, anche utilizzando mesh con un numero ridotto di gradi di libertà. Il fatto è di notevole interesse per una possibile applicazione del metodo a strutture saldate di geometria complessa. Agli effetti descritti, tipicamente riconducibili a una modellazione piana, si possono accompagnare campi singolari non convenzionali, legati a effetti tridimensionali indotti dalla geometria. L'effetto out-of-plane è qui evidenziato in relazione ai giunti a semplice sovrapposizione.ABSTRACT. In the Strain Energy Density (SED) approach for fatigue strength assessments of welded joints a well-defined control volume is considered. This volume surrounds the weld root or weld toe, both modelled like sharp (zero radius) V-notches with different opening angles. The volume becomes a circular sector under plane strain conditions, with the radius being about 0.3 mm for welded joints made of structural steel. The mean value of the SED mainly depends on the singular stress fields when the main plate thickness is large enough, whereas the influence of the T-stress component cannot be neglected in the case of thin-walled welded joints. Both contributions are directly accounted for by using finite element models, also when the relevant meshes are quite coarse. This fact makes the application of the SED approach easier than any stress-based approach in the case of complex structures. Due to three-dimensional effects, a non conventional out-of-plane singular mode can be present, in addition with respect to modes I and II of the Williams' solution. This out-of-plane mode, analogous to the Mode III, is discussed here with reference to welded (seam) lap joints under tensile-shear loads. PAROLE CHIAVE. Giunti saldati, resistenza a fatica, fattori di intensificazione delle tensioni, densità di energia di deformazione. INTRODUZIONEe verifiche a fatica delle unioni saldate possono essere condotte con criteri diversi, basati sulle tensioni nominali, sulle tensioni strutturali o di 'hot-spot', sulla Meccanica della Frattura lineare elastica [1]. Come criterio locale, le raccomandazioni dell'International Welding Institute e quelle dell'Ente FKM prevedono l'utilizzo del criterio di L

Notch stress intensity factors under mixed mode loadings: an overview of recent advanced methods for rapid calculation

“…ecause of the detrimental effects played by notches on mechanical behavior [1][2][3] , several criteria have been proposed in literature for the assessment of material strength of components [4,5]. Concerning brittle and quasibrittle materials, Notch stress intensity factor (NSIF) has been reported to be an effective criterion for the evaluation of static strength [6], and also for high cycle fatigue strength of structural materials weakened by sharp Vnotches [7] and of welded joints [8,9].…”

“…The main drawback exploiting NSIF criterion for the calculation of (1) and (2) is that very refined meshes are needed using finite element (FE) analysis, and this requirement holds also for cracked component [12][13][14][15][16][17]. However, an approach based on the determination of the total elastic strain energy density (SED) averaged over a control volume of radius R 0 surrounding the points of stress singularity has been developed [18] and it has broadly gained interest in the last years, since it allows to exploit course meshes overcoming the main drawback of NSIF criterion both for static static [19][20][21][22][23][24] and fatigue [1,[25][26][27] strength evaluation. Moreover, some approximate methods for the rapid calculation of the NSIFs, based on the averaged strain energy density, are available in literature and, referring to mixed mode loading, they required the solution of a system of two equations in two unknowns (K1 and K2).…”

The fatigue behavior of V-notches in presence of residual stresses: recent developments and future outcomes

“…Starting from Williams' research [6], several works were written about the effect of local stress field rising from singularities, especially on fatigue strength of welded joints, that can be quantified by means of local Strain Energy Density criterion (SED) averaged over a control volume of a critical radius RC [7][8][9][10][11] or Notch Stress Intensity Factor criterion (NSIFs) [12][13][14][15][16]. Intensity and distribution of residual stresses are difficult to be quantified near the weld toe, and, moreover, residual stresses are related to welding parameters, geometry, clamping conditions, applied stress and number of cycles.…”

Local strain energy density for the fatigue assessment of hot dip galvanized welded joints: some recent outcomes

“…In fact, at the best of author's knowledge the only data available in literature for notched components are due to Huhn and Valtinat [11], that examined S 235 JR G2 plates with holes and bearing-type connections with punched and drilled holes. Besides this lack of C experimental results on notched components, that represents a great gap since notches greatly affect the mechanical behavior [12][13][14][15] , the detrimental effect of the zinc layer on the fatigue strength cannot be quantified yet, neither in [11], since a direct comparison between uncoated and hot-dip galvanized notched specimens was not performed. Furthermore, though hot-dip galvanization is widely used to enhance the corrosion resistance of welded joints, none researchers have interested in assessing the effect of this surface treatment on their fatigue behavior.…”