Multiaxial creep–fatigue under anisothermal conditions

Because creep–fatigue is mainly studied in uniaxial tension, it is shown here how to proceed to perform both experiments and calculations under multiaxial loading and when the temperature varies both in time and space. The constitutive equations used are those of elasto‐visco‐plasticity coupled or not, to damage, with isotropic and kinematic hardening. It is shown that the unified damage law first proposed for ductile failure and then for fatigue may also be applied to multiaxial creep–fatigue interactions with a new expression for the damage threshold. The procedure for the identification of material parameters is described in detail. Finally, it is shown that the uncoupled calculation procedure, where damage is calculated as a post‐processing of an elasto‐visco‐plastic computation, gives satisfactory results in comparison to the fully coupled analysis; the latter being more accurate but very expensive in computer time.     

多轴低周载荷下钛合金BT9疲劳损伤的微观机理

应变控制比例及非比例载荷低周疲劳试验结果表明,非比例载荷下钛合金BT9附加强化程度很小,而疲劳寿命降低明显.采用透射电镜(TEM)对钛合金BT9的疲劳位错亚结构进行了观测和分析,结果表明,钛合金BT9比例、非比例载荷下出现的多滑移位错亚结构都呈条块状;位错密度随等效应变强度、相位角的增加而增加,且分布极不均匀;非比例载荷下钛合金BT9中的局部高密度位错是其低周疲劳损伤程度加剧及寿命降低的主要因素.     

Multiaxial thermo‐mechanical fatigue on material systems for gas turbines

Material systems made from nickel based superalloys with protective coatings have been tested in thermo‐mechanical fatigue with superposed thermal gradients, which generated multiaxial stress states. The testing conditions were selected for simulating the fatigue loading in the wall of an internally cooled gas turbine blade of an aircraft engine. After thermo‐mechanical testing the damage behaviour of the materials has been investigated by means of microscopic methods. The laboratory experiments have been accompanied by numerical simulations. Based on the results of the simulations and observed damage features the test parameters in subsequent laboratory tests have been controlled to facilitate the validation of models describing the initiation and propagation of damages. This contribution gives an overview over results on the influence of multiaxial stress states on (i) oriented deformation and coagulation of γ’‐precipitates (‘rafting’) in the substrate, (ii) on morphological instabilities of the surface of metallic oxidation protection coatings (‘rumpling’), and (iii) on crack initiation and growth in material systems with additional ceramic thermal barrier coating.     

多轴疲劳破坏准则现状及评估

简要回顾了材料多轴疲劳破坏准则,引用了5种材料在单轴、比例、纯扭和90°非比例加载情况的疲劳试验结果,对基于应力、应变和能量的有关多轴疲劳破坏准则进行了分析评估.分析结果表明:应力准则中Lee准则在各种材料、各种载荷情况下均可得到误差在5%以内的较好的预测结果,但是,Lee准则中含有物理意义不明确的材料常数,需要一定量的试验数据来拟合,从而限制了该准则的应用,比例加载下,Gough准则计算简单并可适用于大部分材料;应变准则中Kandil,Brown和Miller准则和Borodii和Strizhalo准则在各种材料及各种载荷情况下都能较好的预测疲劳寿命,但Borodii和Strizhalo准则需要大量的试验数据支持,从而限制了它的应用.     

Multiaxial creep–fatigue life prediction for cruciform specimen

This paper describes the high temperature multiaxial creep–fatigue life prediction for type 304 stainless steel. Finite element analyses were performed for determining the stress–strain state in the gage part of a cruciform specimen subjected to creep–fatigue loading under four strain waves at three principal strain ratios. Creep–fatigue lives of cruciform specimens were discussed in relation to the principal stress amplitude calculated by finite element analysis. Creep–fatigue damage was evaluated by linear damage rule and the suitability of three low cycle fatigue and three creep damage parameters was discussed.     

Étude des effets d'extrémité et du champ magnétique induit, dans un générateur MHD

In this work, we consider the expansion to infinity of an electro-conducting fluid in a jet tube of constant rectangular section having two of the electrodes applied in the region x 0 of the horizontal boundaries (the x-axis is the axis of the jet tube) and the vertical isolating boundaries. Flow takes place in the presence of an exterior magnetic field applied only in the electrode region.

The mathematical problem, presented in the terms of the distributions, contains two parameters R_M and λ = R_MR_h. By using three Green functions, a repeating process is given from which can be determined all the approximations (in the series according to the power of λ) of the functions of current, electric potential and magnetic field.     

Le froid, élément de la sécurité alimentaire dans le monde

Improving food security is now a basic and one of the most difficult problems of world food policy and economy. The problem exists because of the need for faster growth in food resources than population levels, especially in the developing countries where food consumption per capita is very low. If refrigeration processes can be constantly adjusted in form to meet the needs of food economy, they can play an important role in the following areas: food reserves; reducing losses in food production and turn-over; reducing seasonal variations; international food trade development; increasing the possibilities for the world fish economy; and frozen food production. The building up of foof resources and consequent stabilizing of the world's food situation became possible mainly through the existence of cold stores. The total volume of all multi- and one-branch cold stores can be estimated as ap; 1.5 × 10⁸ m³. The USA, Japan, USSR, Canada, UK, France and FRG are among the major shareholders of this global potential. This volume could provide storage for 4 × 10⁷ tons of food requiring refrigeration. This is the equivalent to 10 days food production in the world. Further development of cold storage is, therefore, necessary. Although the exact estimation of losses from food production and turn-over is very difficult, many experts evaluate these losses as 20–30%. Some of these experts estimate tthat wide-spread use of refrigeration could save 2.5 × 10⁸ tons of food from deterioration per year. This is equivalent to 80 kg for each person in the world. A decrease in seasonal variation in food production is necessary to adapt seasonal food production to constant consumption. Cold storage can allow a gap of several months between food production and its consumption. Refrigeration has enabled the development of large-scale international and intercontinental food trade. At present > 3 × 10⁷ tons of food per year is involved in world trade (i.e. meat, butter, bananas). After World War 2 the total weight of fish caught increased very quickly from 1.9 × 10⁷ tons in 1948 to 7.2 × 10⁷ tons in 1980. This was possible only because of the application of refrigeration, especially at sea in large oceans. Frozen food production is seen by some as the biggest achievement of the twentieth century in food technology. Production of frozen food has been estimated as ap; 3.0 × 10⁷ tons per year, exclusive of the 1.0 × 10⁷ tons of ice-cream. Increased world food security depends on refrigeration techniques developing at a faster rate than population growth, as well as the growth rate of world food production.     

Multiaxial strain-softening of concrete

The present paper deals with an experimental investigation of the strain-softening behaviour of concrete subjected to multiaxial loading conditions. A multiaxial apparatus for testing cubical specimens was developed and built at Eindhoven University of Technology. Servocontrols in combination with stiff loading frames allowed for measuring stable post-peak response. Loading was applied to the specimens using brush bearing platens. The failure modes of uni- and triaxially loaded specimens are discussed in the first part of this paper. Important in this context were the results of uniaxial compression tests on prisms with varying height. The experiments revealed a localised fracture mode in uniaxial compression. A constant fracture energy was measured, irrespective of the specimen’s size. Furthermore, from the multiaxial tests it was found that the slope of the descending branch is dependent on the applied symmetric or non-symmetric confinement. It was concluded that strain-softening is the response of the structure formed by the specimen and the complete loading system. In Part II [15] attention will be paid to the behaviour of this “structure” subjected to multiaxial load histories.

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Résumé On expose ici une étude expérimentale du phénomène d’adoucissement pour un béton soumis à une sollicitation pluriaxiale. Un équipement d’essai pluriaxial pour éprouvettes cubiques a été mis au point et construit à l’Université de Technologie d’Eindhoven. Un système de servo-commandes et de batis de chargement rigide a rendu possible la mesure de réponses post-crêtes stables. Les charges ont été appliquées aux éprouvettes avec utilisation de balais montés sur les plateaux. On considère dans la première partie de cet article les modes de rupture sous charge uni- et tri-axiale. En regard de cette étude, les résultats d’essais de contrainte uniaxiale sur prismes de différentes hauteurs ont pris de l’importance. On a mis en évidence un mode de rupture localisée sous compression uniaxiale. Quelle que soit la dimension de l’éprouvette, on mesure une énergie de rupture constante. En outre, les essais pluriaxiaux montrent que la pente décroissante de la courbe dépend du choix du confinement symétrique ou pas. La conclusion est que l’adoucissement est la réponse de la structure que composent l’éprouvette et l’ensemble du système de chargement.

     

Multiaxial fatigue criterion for a high‐density polyethylene thermoplastic

The multiaxial fatigue behaviour of a high‐density polyethylene was investigated at room temperature and constant frequency. As a consequence of the mode of failure, an end‐of‐life criterion for fatigue tests is discussed in the first part of the work, in order to define the number of cycles to failure. Based on force controlled fatigue tests under tension, compression and torsion at two stress ratio, a multiaxial fatigue criterion including the stress‐ratio effect is proposed for the fatigue design of this polymer. This criterion is based on the maximum and mean values of the second invariant of the stress tensor.     

Multiaxial fatigue behaviour of A356‐T6

Aluminum alloy A356‐T6 was subjected to fully reversed cyclic loading under tension, torsion and combined loading. Results indicate that endurance limits are governed by maximum principal stress. Fractography demonstrates long shear mode III propagation with multiple initiation sites under torsion. Under other loadings, fracture surfaces show unique initiation sites coincidental to defects and mode I crack propagation. Using the replica technique, it has been shown that the initiation life is negligible for fatigue lives close to 10⁶ cycles for combined loading. The natural crack growth rate has also been shown to be comparable to long cracks in similar materials.     

Energetic efficiency of cogeneration systems for combined heat, cold and power productionEfficacité énergétique des systèmes de cogénération utilisés pour produire simultanément de la chaleur, du froid et de la puissance électrique

This paper deals with the problem of energetic efficiency evaluation of cogeneration systems for combined heat, cold and power production. Cogeneration systems have a large potential for energy saving, especially when they simultaneously produce heat, cold and power as useful energy flows. Various cogeneration systems for combined heat, cold and power production are designed by means of computer simulation to minimize consumption of the primary energy. Equations of energetic efficiency of this combined cogeneration systems are presented, that relate the primary energy rate (PER) and comparative primary energy saving (Δq_p) to energy parameters of designed systems. Comparison of energetic efficiency of combined cogeneration systems with contemporary conventional separate production of heat, cold and power shows a large potential for energy saving by designed combined cogeneration systems.     

Multiaxial fatigue — An overview

Many engineering components are subjected to multiaxial type of fatigue loading and the fatigue life relation based on uniaxial testing needs modifications before it could be used for multiaxial condition. In this paper a brief review of the different approaches based on stress and strain is presented for the correlation of bulk parameters with the fatigue life and the merits and limitations are discussed.     

Propriétés des films étain-azote amorphes réalisés par pulvérisation cathodique réactive-nature de la liaison métal-azote

Les propriétés électriques, optiques et structurales des films étain-azote amorphes réalisés par pulvérisation cathodique réactive sont étudiées. La variation de la conductivité électrique en fonction de la température indique un comportement semiconducteur et permet de distinguer deux processus de conduction. Le caractère diélectrique et la largeur de bande interdite (3,5–3,6 eV) du nitrure d'étain Sn₃N₄ amorphe, principal constituant des films, sont déduits de l'étude de l'absorption optique. La longueur de la liaison Sn---N (2,25±0.04 Å) est calculée à partir des données de spectroscopie infrarouge. La considération des longueurs de liaison et des largeurs de bande interdite de Ge₃N₄, Sn₃N₄ et Pb₃N₄ permet de conclure que la liaison métal-azote est de type monoélectronique.     

Evaluation du coût énergétique du steak haché surgelé

To analyse the total energy consumption for quick-freezing, an examination has been made of the sequence of operations to which minced steak is subjected. Non-proportional ‘general energy expenses’ (heating, cleaning, lighting, services, etc) must be related to the quantity of foodstuffs produced annually in a given manufacturing plant. Another factor which arises is an ‘energy amortisation’ to take into account the equipment employed and where manufacture involves certain energy costs.The paper identifies the operations which are necessary in the manufacture of quick-frozen minced steaks: mincing, mixing, forming; quick-freezing proper (blast air tunnel); packagings (unitary packages, secondary packaging, a 10-steak carton, bulk packages, cartons containing 5 secondary boxes); storage (accepted duration: 1 month); transport (long distance: 500 km in 10 h); retail sales (glass-fronted wall cabinets, 7 days); home preservation (3 days); home preparation (electrical cookers). The study demonstrates the predominating importance, as regards energy, of packagings, retail sales and cooking.     

Effet Hall et propriétés magnétiques des films de nickel déposés et étudiés sous ultra-vide

Experimental data on thin nickel films from 100 Å to 800 Å thick, evaporated under ultra high vacuum conditions are reported. It is shown that the films exhibit a residual magnetization normal to the plane of the film and that the susceptibility at the residual magnetism can take two different values. These values depend upon the direction of the previously applied induction which saturates the sample. These properties are attributed to a uniaxial anisotropy induced by internal stresses, whose easy axis is out of the plane of the film.     

Multiaxial low‐cycle fatigue life evaluation under different non‐proportional loading paths

This paper presents analytical and experimental investigations for fatigue lives of structures under uniaxial, torsional, multiaxial proportional, and non‐proportional loading conditions. It is known that the rotation of principal stress/strain axes and material additional hardening due to non‐proportionality of cycle loading are the 2 main causes resulting in shorter fatigue lives compared with those under proportional loading. This paper treats these 2 causes as independent factors influencing multiaxial fatigue damage and proposes a new non‐proportional influencing parameter to consider their combined effects on the fatigue lives of structures. A critical plane model for multiaxial fatigue lives prediction is also proposed by using the proposed non‐proportional influencing factor to modify the Fatemi‐Socie model. The comparison between experiment results and theoretical evaluation shows that the proposed model can effectively predict the fatigue life due to multiaxial non‐proportional loading.     

Confined Assembly of Asymmetric Block‐Copolymer Nanofibers via Multiaxial Jet Electrospinning

Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene‐b‐isoprene), PS‐b‐PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post‐fabrication annealing of the fibers to obtain equilibrium self‐assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self‐assembled structures. Various confined assemblies such as co‐existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self‐assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self‐assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block‐copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS‐b‐PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS‐b‐PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self‐assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high‐end applications such as drug delivery.