117,770 research outputs found
Notch and defect sensitivity under any kind of fatigue loading: an unifying approach
No full textThe present paper summarises on an attempt to link together two different engineering tools in order to propose an unifying approach suitable for predicting the material notch- as well as the material defect-sensitivity under multiaxial fatigue loading. The proposed approach takes as its starting point the assumption that the multiaxial fatigue limit of the parent material can initially be predicted by using the Modified Wöhler Curve Method recently proposed by Susmel and Lazzarin [1, 2]. This criterion postulates that the plane of maximum shear stress amplitude is coincident with the micro-crack initiation plane and its application requires the calculation of both the maximum shear stress amplitude and the maximum normal stress relative to the critical plane. According to the unifying diagram proposed by Atzori and Lazzarin [3, 4], the predicted multiaxial plain fatigue limit must then be corrected using both some LEFM concepts and the classical stress concentration factor, Ktg. The a..
The mean stress effect on the high-cycle fatigue strength from a multiaxial fatigue point of view
No full textIn the presence of non-proportional multiaxial fatigue loadings, principal stress directions at critical points rotate during the application of the cyclic load. The complexity of the stress field distribution can also be increased by the presence of non-zero mean stress components. In this case, one of the main challenges for researchers engaged in fatigue problems is to propose simple rules capable of accounting for these complex situations. In this study, by starting from the assumption that uniaxial fatigue is a simpler sub-set of the more complex multiaxial fatigue reality, both classical expressions accounting for the mean stress effect in uniaxial fatigue and three different critical plane approaches have been critically reviewed. In particular, the expressions due to Gerber, Dietman, Goodman, Soderberg and the so-called 'elliptical relationship' have been reformulated in terms of the critical plane approach in order to extend them to multiaxial fatigue conditions. In parallel, reliability and accuracy of the multiaxial fatigue criteria proposed by Matake and McDiarmid, respectively, have been checked in the presence of high values of the ratio between the maximum normal stresses and the shear stress amplitudes relative to the critical plane. All the performed analyses suggest that an upper value exists, depending on the material fatigue properties, beyond which the critical plane approach no longer has a physical sense, because the material failure is no longer governed only by the shear stress amplitude. The limit value of this approach has been calculated by using Susmel and Lazzarin's criterion, which has an intrinsic threshold value over which its use is not justified. The correspondence between this mathematical limit and the experimental reality has been investigated by using a number of data sets taken from the literature and generated under uniaxial as well as under multiaxial fatigue loadings. Susmel and Lazzarin's criterion is seen to be a useful tool for fatigue strength assessments in most situations of practical interes
High-cycle fatigue crack paths in specimens having different stress concentration features
No full textThis paper summarises an attempt to study the high-cycle fatigue cracking behaviour in specimens of low carbon steel
weakened by U-notches. The specimens were tested under uniaxial fatigue loading with a load ratio equal to 0.1, and the
considered Kt values, calculated with respect to the gross area, ranged from 3.8 up to about 25. The generated crack paths
were quite irregular showing a propagation occurring in alternate trans- and intra-crystalline mode: in many cases, this
made difficult to unambiguously measure orientation and length of Stage 1 planes. In spite of these experimental difficulties,
the observed material cracking behaviour seemed to suggest that a Stage 1-like process could always be assumed to be
representative of the crack initiation phenomenon, and this held true independently of the notch sharpness. In light of the
fact that, at a mesoscopic level, crack initiations never occurred on material planes parallel to the notch bisector, we
attempted to investigate whether it was possible to use a critical plane approach to estimate high-cycle fatigue damage
in notched components under uniaxial fatigue loading. In more detail, the generated results have initially been re-analysed
by using the Modified Wo ̈ hler Curve Method re-interpreted in terms of the Theory of Critical Distances [Susmel L. A unifying
approach to estimate the high-cycle fatigue strength of notched components subjected to both uniaxial and multiaxial
cyclic loadings. Fatigue Fract Eng Mater Struct 2004;27:391–411]. The accuracy in predicting the high-cycle fatigue behaviour
of the considered multiaxial fatigue method was then compared to the accuracy of two other uniaxial approaches: the
classical one by Smith and Miller [Smith RA, Miller KJ. Prediction of fatigue regimes in notched components. Int J Mech
Sci 1978;20:201–206] and the one recently proposed by Atzori and co-workers [Atzori B, Lazzarin P, Meneghetti G. A
unified treatment of the mode I fatigue limit of components containing notches or defects. Int J Fract 2005;133:61–87]
and based on the use of some classic LEFM concepts. In particular, this comparison was performed considering virtual
specimens having the same geometries as the ones investigated in the present study, but assuming that they were made
of materials having mechanical properties known from the literature. This exercise allowed us to see that the high-cycle
fatigue damage in notched specimens under uniaxial fatigue loading can satisfactorily be predicted not only using Mode
I-crack based methods, but also using multiaxial fatigue criteria modelling the crack initiation phenomenon
On the use of nominal stresses to predict the fatigue strength of welded joints under biaxial cyclic loading
No full textIn this paper, the modified Wöhler curve method proposed by Susmel and Lazzarin is employed to predict the fatigue life of welded connections subjected to biaxial cyclic loading. This criterion is reformulated here in order not to take into account the mean stress effect, as suggested by several design codes (at least when welded connections are not completely stress relieved). The accuracy of the proposed method in fatigue lifetime estimation was evaluated by using a number of data sets taken from the literature. The modified Wöhler curve method was applied in terms of nominal stresses and was calibrated using the uniaxial and torsional fatigue curve determined by reanalysing the experimental data, as well as using the standard fatigue curves of the Eurocode 3. The proposed approach was seen to be successful, giving multiaxial fatigue life predictions located within the widest scatter band related either to uniaxial or to torsional data, independently of both out-of-phase angle and load ratio value. Finally, the accuracy of the modified Wöhler curve method was compared to the one obtained by applying the procedure suggested by the Eurocode 3: the proposed criterion is demonstrated to be much more accurate and reliable than the standard one
Multiaxial fatigue life estimations for 6082-T6 cylindrical specimens under in-phase and out-of-phase biaxial loadings
No full textFully reversed bending/torsion fatigue tests were conducted on 6082-T6 solid cylindrical specimens under force control. Specimens were subjected to pure bending, pure torsion, in-phase and out-of-phase bending/torsion loadings and the investigated fatigue lives ranged between 104 and 2.106 cycles to failure. The actual strains were measured by means of strain gauges positioned in correspondence of critical points. Experimental strain measurements highlighted that all the tests were conduced in pure elastic stress conditions. The material fatigue behaviour was studied by analysing the cracks pattern due to the considered biaxial loadings. All the tests showed that crack initiation was always MODE II dominated (that is, it occurred on the plain of maximum shear stress amplitude), whereas the crack propagation was MODE I governed. Just in the presence of pure torsional loadings cracks grew under MODE II loadings. A good correlation with measured fatigue lives was obtained by applying the Susmel and Lazzarin's criterion valid for homogeneous and isotropic materials, despite the slight degree of anisotropy showed by the material
Modified Wöhler Curve Method, Theory of Critical Distances and EUROCODE 3: a novel engineering procedure to predict the lifetime of steel welded joints subjected to both uniaxial and multiaxial fatigue loading
No full textIn order to propose a procedure suitable for assessing steel welded joints by post-processing simple linear-elastic finite element (FE) models, this paper summarises an attempt to formalise a novel approach based on the use of the modified Wöhler curve method (MWCM) applied along with the theory of critical distances (TCD). This engineering procedure was initially calibrated by using the standard curves supplied by Eurocode 3. Subsequently, an unifying value for the multiaxial critical distance was calculated by taking full advantage from the notch-stress intensity factor (N-SIF) approach. The accuracy and reliability of the devised method was systematically checked by using a large amount of data taken from the literature and generated by testing steel welded details both under uniaxial and multiaxial nominal loading. In particular, both standard cruciform joints and structural details having complex geometries were considered and the devised procedure was applied to specimens tested both in as-welded and in stress-relieved condition. This approach was seen to be highly accurate, giving estimates falling within the widest standard scatter band between the two used to calibrate the approach itself. In particular, our method proved to be capable of correctly taking into account the scale effect as well as the degree of non-proportionality of the stress field damaging the fatigue process zone. Such results are very interesting and promising, especially in light of the fact that the numerical effort which has to be made to apply the proposed approach is the same as the one needed to apply other existing linear-elastic approaches
A simple and efficient reformulation of the classical manson-coffin curve to predict lifetime under multiaxial fatigue loading. Part I: plain materials.
No full textThis paper summarises an attempt to devise an engineering method suitable for predicting fatigue lifetime of metallic materials subjected to both proportional and non-proportional multiaxial cyclic loading. The proposed approach takes as a starting point the assumption that the plane experiencing the maximum shear strain amplitude (the so-called “critical plane”) is coincident with the micro/meso-crack initiation plane. In order to correctly account for the presence of both non-zero mean stresses and non-zero out-of-phase angles, the degree of multiaxiality/non-proportionality of the stress state damaging crack initiation sites is suggested here to be evaluated in terms of the ratio between maximum normal stress and shear stress amplitude relative to the critical plane. Such a ratio is used then to define non-conventional Manson-Coffin curves, whose calibration is done through two strain-life curves generated under fully-reversed uniaxial and under fully-reversed torsional fatigue loading, respectively. The accuracy and reliability of our approach was systematically checked by using approximately 350 experimental data taken from the technical literature and generated by testing 13 different materials under both in-phase and out-of-phase loading. Moreover, the accuracy of our criterion in estimating lifetime in the presence of non-zero mean stresses was also investigated. Such an extensive validation exercise allowed us to prove that the fatigue life estimation technique formalised in the present paper is a reliable tool capable of correctly evaluating fatigue damage in engineering materials subjected to multiaxial cyclic loading paths
Multiaxial fatigue behaviour of composite laminates
No full textThe behaviour of composite laminates and tubes subjected to multiaxial fatigue loads is analysed and discussed. Experimental data taken from literature are compared to investigate the influence of the main design parameters on the multiaxial fatigue strength. The effects of biaxiality ratio, off-axis angle, phase angle between load components as well as the presence of notches have been considered. The biaxiality ratio turned out to be the more influencing design parameter, for both smooth and notched component
Biaxial Non-Proportional Variable Amplitude Testing and Analysis on Bicycle Welded Components for the Definition of a Safety Standard
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