1,721,311 research outputs found
Local approaches applied to fracture and fatigue problems
Full text linkIntentionally designed or accidentally caused, notches, cracks or defects are inevitably present in engineering components and can induce high stress gradients when a far field loading is applied. Then, structural strength assessments are often based on the local stress and strain state in the close neighbourhood of the stress raisers.
The present PhD thesis is divided into six Chapters corresponding to different research topics, all related to new applications of important and widely employed local approaches to notched or cracked structural components.
In the first Chapter, the adopted local approaches, namely the Notch Stress Intensity Factor-based approach (NSIF), the averaged Strain Energy Density (SED) criterion and the Peak Stress Method (PSM), are briefly introduced and described along with their theoretical frameworks.
The second Chapter deals with brittle fracture under mixed mode static loading. A wide experimental campaign has been carried out on PMMA as well as on graphite cracked and notched specimens subjected to mixed mode I+II and I+III loading. Then, all experimental results have been reanalysed by means of the SED approach.
The third Chapter deals with multiaxial fatigue loadings. First, the fatigue strength of severely notched titanium grade 5 alloy, Ti-6Al-4V, has been investigated. Then, the SED criterion has been applied for the first time to an industrial case study, that is the multiaxial fatigue strength assessment of steel welded rollers produced by Rulmeca S.p.a. Finally, some remarks about the phase angle effect on sharp V-notched components under multiaxial fatigue have been drawn on the basis of a proposed analytical frame.
The fourth Chapter addresses the numerical study of 3D effects in notched and cracked components. Initially, the attention has been focused on coupled modes and on the effect of different boundary conditions in 3D cracked discs and plates subjected to nominal mode III or mode II loading. Then, the presence of three-dimensional effects has been investigated both theoretically and numerically in blunt notched components under cyclic plasticity conditions.
The fifth Chapter, instead, is related to the comparison between different fracture criteria. The SED approach and that based on the Finite Fracture Mechanics (FFM) have been compared considering sharp V-notches under pure mode I or mode II loading.
Finally the sixth Chapter address the link between the SED approach and the Peak Stress Method (PSM). Cracks under in-plane mixed mode I+II and out-of-plane mixed mode I+III loading have been investigated. A method to rapidly evaluate the averaged SED based on the peak stresses at the crack tip has been proposed. On the basis of the derived link, some practical applications related to the fatigue strength assessment of aluminium and steel butt welded joints and of tube-to-flange steel welded joints have been carried out
Tensile fracture analysis of blunt notched pmma specimens by means of the strain energy density
No full textIn this paper, a volume criterion based on a simple scalar quantity, the mean value of the strain energy (SED), has been used to assess the static strength of notched components made of Polymethylmethacrylate (PMMA). The local-strain-energy based approach has been applied to a well-documented set of experimental data recently reported in the literature. Data refer to blunt U-notched cylindrical specimens of commercial PMMA subjected to static loads and characterised by a large variability of notch tip radius (from 0.67 mm to 2.20 mm). Critical loads obtained experimentally have been compared with the theoretical ones, estimated by keeping constant the mean value of the strain energy in a well-defined small size volume. In addition, some new tests dealing with V-notched specimens with end holes have been carried out to investigate the effect of the notch opening angle
Three-dimensional cracked discs under anti-plane loading and effects of the boundary conditions A review of recent developments
No full textPurpose: Accordingly to the recent multi-scale model proposed by Sih and Tang, different orders of stress singularities are related to different material dependent boundary conditions associated with the interaction between the V-notch tip and the material under the remotely applied loading conditions. This induces complex three-dimensional stress and displacement fields in the proximity of the notch tip, which are worthy of investigation. The paper aims to discuss these issues. Design/methodology/approach: Starting from Sih and Tang's model, in the present contribution the authors propose some analytical expressions for the calculation of the strain energy density (SED) averaged over a control volume embracing the V-notch tip. The expressions vary as a function of the different boundary conditions. Dealing with the specific crack case, the results from the analytical frame are compared with those determined numerically under linear-elastic hypotheses, by applying different constraints to the through-the-thickness crack edges in three-dimensional discs subjected to Mode III loading. Free-free and free-clamped cases are considered. Findings: Due to three-dimensional effects, the application of a nominal Mode III loading condition automatically provokes coupled Modes (I and II). Not only the intensity of the induced modes but also their degree of singularity depend on the applied conditions on the crack flanks. The variability of local SED through the thickness of the disc is analysed by numerical analyses and compared with the theoretical trend. Originality/value: The capability of the SED to capture the combined three-dimensional effects is discussed in detail showing that this parameter is particularly useful when the definition of the stress intensity factors (SIFs) is ambiguous or the direct comparison between SIFs with odd dimensionalities is not possible
State-of-the-art review of peak stress method for fatigue strength assessment of welded joints
No full textIn the fatigue design framework of welded structures, the peak stress method (PSM) is a rapid engineering, FE-based technique for estimating notch stress intensity factors (NSIFs). This review provides a theoretical background and applies the PSM to assess the fatigue strength of arc-welded joints made of aluminium alloys or structural steels, which are subjected to uniaxial and multiaxial fatigue loads. Because two fatigue design curves are available for each class of materials, a criterion for selecting the appropriate curve depending on the local biaxiality of the stress state is established
Brittle Fracture of Rounded V-Notches in Isostatic Graphite under Static Multiaxial Loading
No full textWhile a large bulk of experimental results from cracked specimens of polycrystalline graphite under pure modes of loading, in particular under mode I loading, can be found in the literature, only a very limited number of tests have been carried out on notches. At the best of the author knowledge dealing with the specific case of V-notches under mixed mode loading (tension + torsion) no results can be found in the literature. With the aim to fill this lack, the problem of mixed mode (I + III) brittle fracture of polycrystalline graphite is investigated systematically here for the first time. The present study considers cylindrical specimens weakened by circumferential notches characterized by different acuities. A new complete set of experimental data is provided considering different geometrical configurations by varying the notch opening angle and the notch tip radius. The multiaxial static tests have been performed considering different values of the mode mixity ratio (i.e. the ratio between the nominal stress due to tension and that due to torsion loading). A criterion based on the local strain energy density previously applied by the same authors only to pure modes of loading is extended here to the case of tension and torsion loadings applied in combination. The proposed criterion allows a sound assessment of the fracture loads
Elastic-plastic fracture analysis of notched Al 7075-T6 plates by means of the local energy combined with the equivalent material concept
No full textThe main goal of the present research is to analyze tensile fracture in Al 7075-T6 thin plates weakened by blunt V-notches. For this purpose, first, 27 fracture tests are carried out on rectangular plates containing a central rhombic hole with two blunt V-shaped corners horizontally located. The experimental observations indicated that a plastic region initiates from the notch tip and grows as the tensile load monotonically increases, and finally, fracture happens suddenly with a significant opening of the notch tip. By showing significant plastic deformations around the notch tip and also inclined fracture planes, the specimens after fracture confirm well the ductile rupture in V-notched Al 7075-T6 plates. As the main experimental result, the load-carrying capacity of the notched plates corresponding to the onset of crack initiation from the notch tip is recorded. To theoretically predict the experimental results, the equivalent material concept is utilized together with the well-known brittle fracture criterion, namely the averaged strain energy density criterion. Without requiring elastic-plastic finite element analysis, it is shown that the combination of the averaged strain energy density and equivalent material concept is successful in predicting the load-carrying capacity of the V-notched Al 7075-T6 plates that fail by moderate-scale yielding regime
Tensile Fracture Analysis of Key-Hole Notches by Means of the Strain Energy Density
No full textThe aim of the present research is twofold. Firstly, to provide a new set of experimental results regarding tensile brittle fracture in key-hole notches and secondly, to check the suitability of the local strain energy density (SED) criterion in predicting the test results. For this purpose, 21 fracture tests were conducted at room temperature on a new version of the well-known Brazilian disk specimen containing a central dumbbell-shaped slit with two key-ends (Key-BD specimen) made of PMMA. The experimentally obtained fracture loads were theoretically predicted for different notch radii by means of the local SED averaged over a specified control volume which embraces the notch edge. It was shown that the experimental results could be well predicted by means of the SED criterio
Innesco di cricche di fatica in barre intagliate in titanio e acciaio soggette a carichi uniassiali e multiassiali: sintesi basata sul SED mediato
No full textIn questo contributo si è analizzato il comportamento a fatica di provini intagliati circonferenzialmente in lega di titanio grado 5, Ti-6Al-4V, e in acciaio inossidabile austenitico, AISI 304L. Sono stati eseguiti test a pura flessione, pura torsione e flesso-torsione combinate, sia in fase che fuori fase, su provini caratterizzati da diversi raggi all’apice dell’intaglio: da 0.1 a 4 mm. Le fasi di innesco e di propagazione della cricca di fatica sono state monitorate utilizzando la tecnica della caduta di potenziale elettrico. I risultati sperimentali sono stati rianalizzati per mezzo della densità di energia di deformazione (SED) mediata su un volume di controllo di raggio R0 che abbraccia l’apice dell’intaglio. Nella rianalisi, è stata considerata la vita ad innesco al fine di ridurre l’effetto dei meccanismi estrinseci agenti durante la propagazione, come il contatto strisciante e/o l’ingranamento tra le superfici di frattura. L’approccio SED ha consentito di correlare la vita ad innesco in una banda di dispersione piuttosto ristretta valida per ciascun materiale testato (Ti-6Al-4V e AISI 304L)
Assessment of root failures in tube-to-flange steel welded joints under torsional loading according to the Peak Stress Method
No full textIn the context of the local approaches to fatigue design of welded joints, the notch stress intensity factor
(NSIF) approach assumes the weld toe as well as the weld root profile as sharp V-shaped notches, the
V-notch opening angle being greater than zero at the toe side and equal to zero at the root side. The
Peak Stress Method (PSM) is an engineering, finite element (FE)-oriented method to rapidly estimate
the NSIF local stress parameters by means of linear elastic FE analyses with coarse meshes. By using
the averaged Strain Energy Density (SED, which can be expressed as a function of the relevant NSIFs)
as a fatigue strength criterion, a so-called equivalent peak stress can be defined and adopted to assess
either weld toe and weld root fatigue failures in conjunction with an existing design fatigue curve, that
was originally calibrated on experimental results relevant only to weld toe failures. In the present paper
new fatigue test results relevant to weld root failures have been generated by testing tube-to-flange
fillet-welded steel joints under torsional loading. Good agreement has been obtained between theoretical
estimations according to the PSM and experimental fatigue test results
- …
