15 research outputs found
A General Damage Accumulation Model for Multiaxial, Proportional High Cycle Fatigue Loadings With Sines, Crossland and Dang Van Criteria
In this paper, a key differential equation is proposed to formulate fatigue damage evolution in metallic alloys under multiaxial, multiblock, proportional loadings in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. This differential equation possesses two main components: one is a stress function to accommodate the adopted fatigue criterion and the other one is a characteristic damage function that serves to capture the HCF response of alloys. Two distinct characteristic damage functions with three different multiaxial fatigue criteria, namely Sines, Crossland, and Dang Van criteria, are examined to develop six (out of many possible) variants of the presented damage accumulation model. As a validation measure, Chabocheâs HCF damage model is retrieved as a specific case of the developed formalism. For model parameters identification, an ad hoc two-level identification scheme is designed and numerically verified. It is demonstrated that endurance limit, which is determined from fully reversed HCF tests (i.e., R = â1), can be identified from fatigue tests with positive stress ratio (R > 0), thus making our development quite suitable for specimens prone to buckling under compression. Another salient feature of the devised identification scheme is its capability in extracting model parameters from noisy data
Modélisation et simulation du comportement micromécanique du polyéthylÚne semi-cristallin : effet de l'interphase
Elastic characterization of the interphase layer in polyethylene is implemented by applying the relationships of two micromechanical approaches, âExtended Composite Inclusion Modelâ and âDouble-Inclusion Methodâ, to the Monte Carlo molecular simulation data for the interlamellar domain. The results of the two approaches match perfectly. The interphase stiffness lacks the common feature of positive definiteness, which indicates its mechanical instability. Comparison with experimental results endorses the proposed methodology. For the hyperelastic characterization of the interlamellar domain and the interphase layer, the proposed hybrid algorithm consists in applying the constitutive equations of an isotropic, compressible, hyperelastic continuum to the molecular dynamics simulation results of a polyethylene stack. Evolution of the interphase boundaries are introduced as auxiliary variables and the notion of minimizing a set of nonnegative objective functions is employed for parameter identification. The identified hyperelastic parameters for the interlamellar domain arein good agreement with the ones that have been estimated experimentally. Finally, the large, viscoplastic deformation of an aggregate of polyethylene is reexamined. The Gent model adopted for the back stress of the noncrystalline phase, correcting the projection tensor for the modified Taylor approach, and the idea of multilevel optimization are among the contributions made.Dans ce travail, la caractĂ©risation mĂ©canique de lâinterphase entre les zones amorphes et cristallines dans le polyĂ©thylĂšne a Ă©tĂ© abordĂ©e. La caractĂ©risation Ă©lastique est effectuĂ©e en appliquant deux approches micromĂ©caniques Ă partir des donnĂ©es de la simulation molĂ©culaire pour la zone interlamellaire. Ces approches micromĂ©caniques sont dâune part le modĂšle Ă©tendu dâinclusion composite, et dâautre part la mĂ©thode de double inclusion. Les rĂ©sultats des deux approches sâaccordent parfaitement. Il a Ă©tĂ© mis en Ă©vidence que le tenseur de rigiditĂ© de lâinterphase nâest pas dĂ©fini positif, lâinterphase est donc mĂ©caniquement instable. La comparaison avec les rĂ©sultats expĂ©rimentaux valide la mĂ©thodologie proposĂ©e. Pour la caractĂ©risation hyperĂ©lastique, lâalgorithme hybride proposĂ© consiste Ă appliquer la loi de comportement dâun milieu continu isotrope, compressible et hyperĂ©lastique aux rĂ©sultats de la simulation de la dynamique molĂ©culaire dâun Ă©lĂ©ment unitaire de polyĂ©thylĂšne. La notion dâoptimisation dâun ensemble de fonctions coĂ»ts non nĂ©gatives est lâidĂ©e clĂ© de cette partie. Les paramĂštres hyperĂ©lastiques identifiĂ©s sont en bon accord avec ceux qui ont Ă©tĂ© estimĂ©s expĂ©rimentalement. LâĂ©volution des frontiĂšres de lâinterphase avec la dĂ©formation est le second rĂ©sultat de cette analyse. La fin du travail est dĂ©diĂ©e Ă la simulation numĂ©rique de la grande dĂ©formation viscoplastique dâun agrĂ©gat de polyĂ©thylĂšne. Le modĂšle de Gent adoptĂ© pour la contrainte de rappel, le tenseur de projection proposĂ© pour lâapproche modifiĂ©e de Taylor, et lâoptimisation multiniveau font parties des contributions apportĂ©es
On the parameters influencing the effective properties of piezoelectric nanocomposite film employing FEM
Characterization of the effective elasto-piezoelectric properties of a special class of nanocomposites is the subject of this article. The piezonanocomposites of interest are thin films of non-piezoelectric polymer reinforced with a row of piezoelectric boron nitride nanotubes. The boron nitride nanotubes are unidirectionally aligned but randomly distributed in the piezonanocomposite films. The characterization methodology relies primarily on the finite element simulations of the representative volume elements from the piezonanocomposites. Decoupling the coupled constitutive elasto-piezoelectric equations and double checking criteria for the evaluation of the effective properties are two highlights of the methodology. Sensitivity analyses are carried out to achieve the appropriate representative volume elements size. As influencing parameters, the effects of the boron nitride nanotubes volume fraction and aspect ratio on the overall effective properties are investigated. Additionally, different physical properties are assigned to the space inside nanotubes and their impacts on the effective properties are examined. The calculated effective properties are compared to the ones output by three different analytical approaches as well as the results of similar studies on conventional piezocomposites. The agreements observed corroborate the final results as well as the proposed methodology to be applied to similar problems
3D-Printable Unit Cell Design for Cubic and Orthotropic Porous Microstructures Using Topology Optimization Based on Optimality Criteria Algorithm
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