Multiaxial Kitagawa analysis of A356-T6

Experimental Kitagawa analysis has been performed on A356-T6 containing natural and artificial defects. Results are obtained with a load ratio of R = -1 for three different loadings: tension, torsion and combined tension-torsion. The critical defect size determined is 400 \pm 100 \mum in A356-T6 under multiaxial loading. Below this value, the microstructure governs the endurance limit mainly through Secondary Dendrite Arm Spacing (SDAS). Four theoretical approaches are used to simulate the endurance limit characterized by a Kitagawa relationship are compared: Murakami relationships [Y Murakami, Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, Elsevier, 2002.], defect-crack equivalency via Linear Elastic Fracture Mechanics (LEFM), the Critical Distance Method (CDM) proposed by Susmel and Taylor [L. Susmel, D. Taylor. Eng. Fract. Mech. 75 (2008) 15.] and the gradient approach proposed by Nadot [Y. Nadot, T. ~Billaudeau. Eng. Fract. Mech. 73 (2006) 1.]. It is shown that the CDM and gradient methods are accurate; however fatigue data for three loading conditions is necessary to allow accurate identification of an endurance limit.Comment: 27 pages, 11 figure

Influence of casting defects on the fatigue behavior of cast aluminum AS7G06-T6

cited By 23International audienceAS7G06-T6 cast aluminum alloy is tested under tension fatigue loading for two load ratios. After the quantification of the Secondary Dendrite Arm Spacing (SDAS) and grain size of the material, fatigue tests are analyzed through fractographic Scanning Electron Microscope (SEM) observations in order to reveal the type of defects at the origin of the failure. The quantification of the defect size is performed for each defect and Kitagawa type diagrams are produced for each load ratio. It is shown that the critical defect size that does not lower the fatigue strength is close to the grain size of the material. The Defect Stress Gradient (DSG) approach that aims to simulate defect influence on the fatigue strength is presented in a multiaxial context. DSG approach is finally implemented in a finite element simulation of a structural component in order to show that such an approach can provide a defect size map. The latter can be used to define allowable defect size for industrial components. © 2014 Elsevier Ltd. All rights reserved

Micromechanical Modeling of Effective Orthotropic Elastic and Viscoelastic Properties of Parallel Strand Lumber Using the Morphological Approach

International audienceStrand-based wood and bamboo composites are a class of structural materials which are increasingly being used in the construction industry. These bio-based composites consist of orthotropic wood or bamboo strands bonded together with small amount of resin (glue). In order to estimate the effective properties of such composites, a specific micromechanical approach from the literature, called the “morphological approach” (MA), is employed. The accuracy of the MA in estimating the effective elastic and viscoelastic properties of an idealized unit cell of a special strand-based wood composite product, a parallel strand lumber (PSL) beam, is investigated using numerical reference solutions obtained by full-field finite-element (FE) simulations. The MA is also compared to analytical micromechanics equations previously proposed by the authors for predicting the effective properties of strand-based composites. MA results are shown to be closer to the numerical reference solutions than previous analytical estimates. Thus, the MA can be used as a valuable alternative for computing the effective properties of strand-based composites with rectangular shaped orthotropic strands. More generally, it could be used in efficient multiscale analysis of large structural composites made from various wood and bamboo strands using less restrictive unit cells. From a fundamental viewpoint, this paper shows a novel application of the MA to orthotropic reinforcements in a Prony series–based viscoelastic matrix

Fatigue life assessment in notched injection-molded specimens of a short-glass fiber reinforced Polyamide 6 with different injection gate locations

Based on a previously published Through Process Modeling (TPM) for multiaxial fatigue life assessment of injection-molded components, this work aimed to capture the difference of fiber orientation distribution ahead of notch tips in flat notched samples of PA6GF30 tested in tension at ambient temperature. The fiber orientation originated from different locations of the injection gate for an unchanged macroscopic geometry. Good lifetime estimations were obtained in Side- (resp. Top-) injected samples, regardless the choice of the sample type from which the fatigue criterion was identified. The experimentally observed differences of crack initiation zones were correlated to numerical simulations

Micro-mechanical analysis of the confined amorphous phase in semi-crystalline polymers

AbstractThe mechanical modelling of semi-crystalline polymers is highly complex due to their different micro-structural scales and to some lacks regarding the understanding of the respective contributions of crystalline and amorphous phases on the macroscopic behaviour. In particular, the important role of the confined amorphous phase has to be investigated. To this aim, this work proposes to combine full-field simulations with kinematics field measurements at the same scale. The material is the linear low density polyethylene (LLDPE), chosen for its “simple” microstructure. Displacement fields at the micro-scale are measured by using the digital image correlation technique in the SEM. It is scheduled to apply the experimental strain as periodic boundary conditions to a unit cell whose morphological content will be built from the observed real microstructure and to analyse both the resulting macroscopic average response and local fields. To this aim, specific numerical tools are developed notably for statistical field analysis. The results on a simple stack model under various loading cases/paths are presented as a preliminary stage before the complete analysis for the LLDPE under tensile tests. The long term aim is to better understand the strain mechanisms in the confined amorphous phase, to determine its mechanical behaviour and conclude on its influence on the macroscopic behaviour

A non-classical micro-macro transition towards the anisotropic damage behaviour of particulate composites

The complex problem of micromechanically based constitutive description of composite materials for which damage consists in grain-matrix debonding is treated employing the homogenization method initially proposed by Christoffersen [1] for elastic bonded granulates. The localization relations (microscopic level) as well as the homogenized stress are established and discussed for a fixed state of damage (i.e. for fixed numbers of open and closed cracks) and using the hypothesis of no sliding on closed crack lips. Finally, a complementary, thermodynamically motivated approach is performed in order to express the crack opening effect as a function of macroscopic strain and of two damage variables involving granular aspects and emerging from the scale transition

Through process modeling for the fatigue life assessment of notched injection-molded specimens

NoThe study is based on a previously proposed methodology for multiaxial fatigue life assessment of injection-molded components (called ‘Through Process Modeling’ (TPM)). The present contribution focuses on stress concentration effects induced in notched samples. Purely macroscopic approaches are unable to capture the different mechanical responses of variably injected parts with the same shape. A high interest of the present method is to take into account the difference of fiber orientation resulting from the process. After briefly reminding the TPM method, it will be shown that good lifetime estimations are obtained for laterally injected samples, from a fatigue criterion identification based on longitudinally injected ones