Simplified modelling of the behaviour of 3D-periodic structures such as aircraft heat exchangers

In this paper, experimental, analytical and numerical analysis are used to study and model the mechanical behaviour of a heat exchanger core consisting of a 3D-periodic structure. The purpose of the present investigation is not only to acquire knowledge on the mechanical behaviour of a given heat exchanger core but also to propose a simplified approach to model this behaviour. An experimental study is carried out in order to get an insight on the mechanical behaviour of this structure. Global static characteristics are obtained via analytical and finite element analysis of a unit cell of the core. Dynamic behaviour is studied by means of finite element calculations based on the results of the static modelling. The proposed approach is validated by comparison with experimental tests results

A predictive fatigue life model for anodized 7050 aluminium alloy

The objective of this study is to predict fatigue life of anodized 7050 aluminum alloy specimens. In the case of anodized 7050-T7451 alloy, fractographic observations of fatigue tested specimens showed that pickling pits were the predominant sites for crack nucleation and subsequent failure. It has been shown that fatigue failure was favored by the presence of multiple cracks. From these experimental results, a fatigue life predictive model has been developed including multi-site crack consideration, coalescence between neighboring cracks, a short crack growth stage and a long crack propagation stage. In this model, all pickling pits are considered as potential initial flaws from which short cracks could nucleate if stress conditions allow. This model is built from experimental topography measurements of pickled surfaces which allowed to detect the pits and to characterize their sizes (depth, length, width). From depth crack propagation point of view, the pickling pits are considered as stress concentrator during the only short crack growth stage. From surface crack propagation point of view, machining roughness is equally considered as stress concentrator and its influence is taken into account during the all propagation stage. The predictive model results have been compared to experimental fatigue data obtained for anodized 7050-T7451 specimens. Predictions and experimental results are in good agreement

Surface characterization and influence of anodizing process on fatigue life of Al 7050 alloy

The present study investigates the influence of anodizing process on fatigue life of aluminium alloy 7050-T7451 by performing axial fatigue tests at stress ratio ‘R’ of 0.1. Effects of pre-treatments like degreasing and pickling employed prior to anodizing on fatigue life were studied. The post-exposure surface observations were made by scanning electron microscope (SEM) to characterize the effect of each treatment before fatigue testing. The surface observations have revealed that degreasing did not change the surface topography while pickling solution resulted in the formation of pits at the surface. Energy dispersive spectroscopy (EDS) was used to identify those constituent particles which were responsible for the pits formation. These pits are of primary concern with respect to accelerated fatigue crack initiation and subsequent anodic coating formation. The fatigue test results have shown that pickling process was detrimental in reducing the fatigue life significantly while less decrease has been observed for anodized specimens. Analyses of fracture surfaces of pickled specimens have revealed that the process completely changed the crack initiation mechanisms as compared to non-treated specimens and the crack initiation started at the pits. For most of the anodized specimens, fatigue cracks still initiated at the pits with very few cracks initiated from anodic coating. The decrease in fatigue life for pickled and anodized specimens as compared to bare condition has been attributed to decrease in initiation period and multi-site crack initiations. Multi-site crack initiation has resulted in rougher fractured surfaces for the pickled and anodized specimens as compare to bare specimens tested at same stress levels

Influence de gammes d'usinage sur la tenue en fatigue d'un alliage léger aéronautique

Les gammes d'usinage en tournage et étau limeur, pratiquées respectivement sur des éprouvettes de flexion rotative et plane,ont permis de mettre en évidence les facteurs d'influence de l'état de surface sur la tenue en fatigue d'un alliage léger aéronautique. Un modèle mécanique a été développé pour établir l'origine physique de cette influence

Propagation de fissure sur X38CrMoV5 sur éprouvettes à l'échelle réduite

Cette étude concerne la caractérisation et la modélisation de l’endommagement des surfaces d’outillages en service (type outillages de fonderie). La littérature montre que l’endommagement des surfaces d’outillages (ex. faïençage) peut présenter une profondeur comprise entre 50 et quelques centaines de micromètres. Des travaux sur l’endommagement de matériaux d’épaisseur mince utilisés en électronique énoncent que les propriétés mécaniques sont dépendantes de l’épaisseur des échantillons. En effet, les propriétés mécaniques (vitesse de propagation de fissures, limite élastique,...) d’un matériau de faible épaisseur (éprouvette mince) peuvent être différentes des propriétés du même matériau de forte épaisseur

Influence du décapage sur la tenue en fatigue de l'alliage Al7010-T7451 traité par oxydation anodique chromique

L’oxydation anodique chromique (OAC) est un procédé de traitement des alliages d’aluminium pour améliorer leur tenue à la corrosion. L’influence de ce traitement sur la durée de vie de l’alliage 7010-T7451 est étudiée à partir d’essais de fatigue en flexion rotative. Une réduction de la durée de vie par rapport à l’état usiné est observée. Il est montré que cette diminution de la durée de vie est imputable en grande partie à la dégradation de la rugosité de la surface occasionnée par la phase de décapage. Il est également montré que la correction des résultats de fatigue par le coefficient de concentration de contrainte local, déterminé par la méthode des éléments finis à partir du relevé du profil de rugosité, permet de rationaliser les courbes de fatigue de cet alliage : l’ensemble des résultats d’essais obtenus sur des éprouvettes n’ayant rien subi après l’usinage ou n’ayant subi que les phases de dégraissage et de décapage s’alignent sur une même courbe de fatigue maîtresse. Il est également montré que, traités par cette méthode du Ktlocal, les résultats de la durée de vie obtenus sur des éprouvettes décapées puis anodisées s’alignent sur une deuxième courbe décalée par rapport à cette courbe maîtresse

Static and dynamic behaviour of a 3D periodic structure

This contribution deals with the assessment of static and dynamic behavior a 3D-periodic structure. Equivalent Young and shear moduli are evaluated respectively via a spring-network calculation performed on a representative volume element (RVE)of the structure and an energy approach. To assess the accuracy of the two methods, FE simulations are performed, and tensile experiments are conducted on specimens cut out from 3D-structure panels. Results show disagreement for stiffness values along y-direction. This is thought to be due to stiffness variation between RVE's due to boundary conditions. A new analytical/numerical approach is proposed taking into consideration the presence of different types of boundary conditions on RVE's. Static results are further used as an input for FE investigations of the dynamic behavior of the structure. Two models are proposed: a) homogeneous and b) laminated models. Vibration tests are performed on different specimens and fundamental frequencies are noted. Results for both models correlate well with experimental data. However, the homogeneous model is limited to low frequencies

Modélisation de la multi-fissuration d’un alliage d’aluminium 7010 traité par oxydation anodique chromique

L’oxydation anodique chromique (OAC) est un procédé de traitement des alliages d’aluminium pour améliorer leur tenue à la corrosion mais il entraîne une très forte réduction de la durée de vie par rapport à l’état usiné. Cette diminution de la durée de vie est largement imputable à la dégradation de la rugosité de la surface occasionnée par la phase de décapage. La présente étude est consacrée à la modélisation de la fissuration à partir des piqûres de décapage observées à la surface d’échantillon en alliage 7010 traités puis testés en flexion rotative. Ces piqûres de décapage sont caractérisées par mesures topographiques. Ces topographies servent à élaborer des modèles éléments finis qui permettent de déterminer la contrainte locale maximale au droit de ces piqûres. Dans le modèle de calcul en fatigue, ces piqûres sont considérées comme des microfissures qui, en se propageant aussi bien en surface qu’en profondeur, peuvent coalescer avant de poursuivre leur propagation. Le calcul en fatigue est stoppé à la fin du stade II de la propagation

An investigation of the crack propagation in tool steel X38CrMoV5(AISI H11) in SET specimens

An approach is proposed for the evaluation of surface fatigue damage of hot forming tools that undergo severe thermo mechanical loading. Fatigue crack propagation in a hot work tool steel X38CrMoV5-47HRC is investigated using single-edge cracked tension specimens with 3 different thicknesses (2.5, 1, 0.6 mm) and two R-values. The stress intensity factor is evaluated with ABAQUS®. Paris curves are established for the crack propagation experiments

Crack propagation in tool steel X38CrMoV5 (AISI H11) in SET specimens

An approach is proposed for the evaluation of surface fatigue damage of hot forming tools that undergo severe thermo mechanical loading. Fatigue crack propagation in a hot work tool steel X38CrMoV5-47HRC is investigated using single-edge cracked tension specimens with 3 different thicknesses (2.5, 1, 0.6 mm) and two R-values. The stress intensity factor is evaluated with ABAQUS®. Paris curves are established for the crack propagation experiments