Responses of damage and energy of sandwich and multilayer beams composed of metallic face sheets and aluminum foam core under bending loading

This present article deals with bending deformation and failure behavior of sandwich and multilayer beams composed of aluminum foam core and metallic face sheets analyzed byin-situ surface displacement analysis (SDA). The effect of beam structure on the failure mode of beam and the energy absorbed by beam failure were investigated and discussed. The SDA results revealed that collapse of the sandwich beams is by two basic modes, indentation (ID) and core shear (CS). The ID is localized deformation on the beam adjacent to the inner or outer roller in four-point bending, where displacement and compressive strains are at the maximum. As for CS mode, failure occurs in the core between inner and outer rollers, which corresponds to the maximum shear strain; discontinuous displacements in both the vertical and horizontal directions are the primary factors for shear crack initiation, growth, and broadening. The failure of the multilayer beams depends on whether the face sheets show ID mode or otherwise. If a single layer core sandwich fails in ID mode, the multilayer beams with similar face sheets show mixed ID + CS modes. If a single layer core sandwich fails fully in CS mode, no ID characteristic appears in the similar face sheet multilayer beams. The deformation energy of the beams relates strongly to the structure and geometry of beam. The predication of the bending fracture workW _x of a beam is given by

Dislocation damage in tantalum after cyclic loading

The dislocation distribution in tantalum foil after fatigue testing in the region of the yield point has been determined quantitatively using thin film electron microscopy. Changes in the measured microstructural parameters as a function of stress level, test temperature, and number of loading cycles have been related to a microscopic model of the fatigue process. The model leads to a consistent criterion for fatigue failure based on the gradual growth of impenetrable subboundary groups of immobile dislocation. Formerly, Instructor, Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa, Israel     

我国金属板带成形性能测试国家标准体系建设研究

介绍了我国金属板带成形性能试验方法系列国家标准体系的基本构成、每个标准主要特点,并与国内外同类标准进行了对比,最后指出了该国家标准体系有待进一步补充完善的地方.     

超细晶铜的高温循环变形及损伤行为

为揭示超细晶材料在高于室温条件下的动态疲劳变形行为,在室温到300℃温度范围内以及在恒应力幅为200 MPa条件下研究等通道转角挤压(ECAP)法制备的超细晶铜的循环形变行为及表面变形特征.结果表明:随着温度升高,循环软化更趋明显,相应的疲劳寿命显著降低,断口形貌特征由脆性逐步转变为韧性;表面变形特征及损伤行为也与温度密切相关,例如,作为主要变形方式的大尺度剪切带在室温疲劳下大量形成:随着温度升高,晶粒长大及位错滑移增强,晶界数量和体积分数明显降低,导致剪切带变得更细且不连续,当温度高于其再结晶温度时剪切带几乎消失,晶粒内的位错滑移成为材料的主要变形方式.     

循环加卸载下闪长玢岩蠕变特性及损伤本构模型

分级加载压缩蠕变试验未能充分考虑稳定蠕变中的黏塑性应变,故采用三轴循环加卸载压缩蠕变试验来实现岩石的黏弹、塑性应变分离,从而使岩石黏弹、塑性应变在岩石蠕变的各个阶段得以充分考虑。以某水电站闪长玢岩为例,探讨该类岩石蠕变特性。在破坏前,岩石的瞬时弹性应变以及瞬时塑性应变随着偏应力逐级增大呈线性增长;随着偏应力的增加,黏弹性应变和黏塑性应变呈非线性增长。引入一个分数阶Abel黏壶与Kelvin模型串联形成新型黏弹性模型;用分数阶Abel黏壶代替传统的黏塑性模型中的线性牛顿体并基于损伤建立黏塑性损伤模型。然后将新型黏弹性模型和黏塑性损伤模型与瞬时弹性模型和瞬时塑性模型串联组成一个新的岩石蠕变损伤模型。最后将该模型与岩石蠕变曲线进行拟合,从而证明该模型的适用性。      

Strength,damage and failure of iron-carbon alloys subiected to cyclic plasticity

Abstract

The cyclic deformation behaviour, fracture mechanisms and damage accumulation in pure iron and mild steel have been studied in strain control at various temperatures and deformation rates. Explanations are offered for the effects of prior cold working, cyclic deformation rate and temperature, including combinations of these parameters, on the cyclic stress response, dislocation substructure, surface damage and fatigue fracture.

It has been found that steady state cyclic deformation may be achieved by a critical amount of prior monotonic strain and the equivalence of monotonic and cyclic hardening processes is considered and supporting microstructural work is provided.

The influence of cyclic deformation rate on the material behaviour is considered. At room temperature the stress response increases with increasing deformation rate and it is shown that the effective stress component is lower than the equivalent value for monotonic deformation, however, the dislocation velocity exponent is independent of the deformation mode. The substructure is also considered. As the temperature is increased to 370°C changes in the internal stress component occur due to an increase in the rate of dislocation multiplication under dynamic strain aging conditions which enhance inhomogeneous deformation. At higher temperatures, the internal stress component decreases to accelerate thermal recovery.

Studies carried out on pure iron have shown that the fatigue life and the fracture mechanism are influenced by the homogeneity of plastic deformation. The dependence of fatigue life on the degree of dynamic strain aging is shown to be a single valued function of a dynamic strain aging probability element which is discussed. Under such conditions, restricted plasticity retards blunting of the propagating cracks, resulting in a high crack growth rate and a correspondingly decreased life.

On carrying out static strain aging at 125° and 250°C prior to strain cycling, it is observed that the stress response increases with the degree of aging, however, this does not influence the amount of cyclic softening which takes place. As opposed to the effect of dynamic strain aging, it is shown that the fatigue life increases with static strain aging.

Résumé

Les auteurs ont étudié le comportement en déformation cyclique, les mécanismes de rupture et l'accumulation des dommages dans du fer pur et de l'acier doux à différentes températures et taux de déformation. Les expériences ont été conduites en contrôlant la déformation. Ils proposent des explications pour l'influence de l'écrouissage préalable, du taux de déformation cyclique et de la température, ainsi que pour l'effet combiné de ces paramètres, sur la contrainte cyclique, la sous-structure des dislocations, les dégats de surface et la rupture lors de la fatigue.

Ils ont trouvé qu'un état stationnaire de déformation cyclique peut être atteint par une valeur critique de la déformation monotone antérieure. Ils ont étudié l'équivalence éntré les mécanismes de consolidation cyclique et monotone et ils apportent des preuves expérimentales par une analyse de la microstructure.

Ils ont aussi étudié l'influence du taux de déformation cyclique sur le comportement de ces matériaux. A la température ambiante, la contrainte cyclique augmente avec le taux de déformation. Ils montrent que la composante effective de la contrainte est plus basse en déformation cyclique que celle pour des déformations monotones. Cependant l'exposant de l'expression d.e la vitesse des dislocations en fonction de la contrainte est indépendant du mode de déformation. Ils ont aussi étudié la sous-structure. Lorsque la température augmente à 370°C, des changements dans la composante de la contrainte interne ont lieu. La cause en est une augmentation de la vitesse de multiplication des dislocations lorsqu'il y a vieillissement dynamique après écrouissage, ce qui favorise les déformations inhomogènes. A plus hautes températures, la composante de la contrainte interne diminue ce qui accélère la restauration.

Ils montrent que le temps de vie et les mécanismes de rupture dans le fer pur sont influencés par l'homogénéité de la déformation plastique. L'influence du degré de vieillissement dynamique après écrouissage sur le temps de vie en fatigue se caractérise par une fonction monotone de la probabilité de vieillissement dynamique après écrouissage, que les auteurs discutent. Dans de telles conditions, la plasticité roouite rétarde l'émoussement des fissures se propageant, ce qui entraïne une vitesse élevée de propagation ayant pour conséquence une diminution du temps de vie.

Des essais de vieillissement statique à 125° et 250°C avant la déformation cyclique entraïnent une augmentation de la contrainte cyclique qui dépend du degré de vieillissement; cependant, ce vieillissement n'influence pas l'amplitude de l'adoucissement cyclique. A l'opposé de l'effet du vieillissement dynamique après écrouissage, le temps de vie augmente avec Ie vieillissement statiq ue après écrouissage.     

Monotonic Lateral Loading of Piles in Calcareous Sand

This paper describes the results of a centrifuge modeling study of the response of piles embedded in calcareous sand under monotonic lateral loading. A number of features have been explored, including method of installation, rate of loading, and pile head restraint. The study has led to recommendations for load-transfer curves with the magnitude of lateral resistance linked to the soil strength through the cone resistance. Modification factors have been developed to allow for different methods of installation and for different rates of loading. The proposed load-transfer curves and resulting pile response are shown to provide an excellent match with the experimental data, and are compared with results derived using existing guidelines for terrigenous sands. Significant differences are demonstrated, confirming the need to treat calcareous sediments separately from other soil types with respect to lateral pile response.     

Behavior of Railroad Ballast under Monotonic and Cyclic Loading

A relatively new method for mechanized maintenance of railroad ballast (stoneblowing) puts a layer of single size stone between the ballast and each tie creating a two-layer gravel support. To get a better understanding of the behavior of this arrangement series of large diameter, triaxial tests have been carried out on single size and layered specimens. A new method of quantifying particle breakage during testing has been developed, and a conceptual model used to explain the combined effects of shearing and breakage on observed specimen behavior.     

CFRP-Strengthened and Corroded RC Beams under Monotonic and Fatigue Loads

An experimental program has been carried out to investigate the structural behavior of RC beams strengthened by carbon-fiber–reinforced polymer (CFRP) sheets and exposed to a corrosive environment. A total of eight specimens (120 × 175 × 2,000 mm) were tested. Six specimens were CFRP strengthened and corroded, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Two different strengthening schemes were applied: (1) wrapping the specimen with CFRP sheets; and (2) both specimen wrapping and flexural strengthening. Three specimens were tested under monotonic loading and five specimens were tested in fatigue. The results showed that the use of CFRP sheets for strengthening RC beams that are experiencing steel reinforcement corrosion is an efficient technique that can maintain the structural integrity and enhance the structural behavior of such beams. The ultimate monotonic strength of the CFRP strengthened-corroded specimens increased to a level between 37 and 87% above the predicted strength of a similar unstrengthened-uncorroded (virgin) specimen. The fatigue life of the CFRP strengthened-corroded specimens was increased within a range of 2.5–6.0 times that of a similar unstrengthened-corroded specimen but was lower than that of the uncorroded (virgin) specimen.     

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.     

Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part II. Damage modes

Zinc-based coatings are widely used for protection against corrosion of steel-sheet products in the automotive industry. The objective of the present article is to investigate the damage modes at work in three different microstructures of a zinc coating on an interstitial-free steel substrate under tension, planestrain tension, and expansion loading. Plastic-deformation mechanisms are addressed in the companion article. Two main fracture mechanisms, namely, intergranular cracking and transgranular cleavage fracture, were identified in an untempered cold-rolled coating, a tempered cold-rolled coating, and a recrystallized coating. No fracture at the interface between the steel and zinc coating was observed that could lead to spalling, in the studied zinc alloy. A complex network of cleavage cracks and their interaction with deformation twinning is shown to develop in the material. An extensive quantitative analysis based on systematic image analysis provides the number and cumulative length of cleavage cracks at different strain levels for the three investigated microstructures and three loading conditions. Grain refinement by recrystallization is shown to lead to an improved cracking resistance of the coating. A model for crystallographic cleavage combining the stress component normal to the basal plane and the amount of plastic slip on the basal slip systems is proposed and identified from equibiaxial tension tests and electron backscattered diffraction (EBSD) analysis of the cracked grains. This analysis requires the computation of the nonlinear stress-strain response of each grain using a crystal-plasticity constitutive model. The model is then applied successfully to other loading conditions and is shown to account for the preferred orientations of damaged grains observed in the case of plane-strain tension.     

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.     

Deformation and damage mechanisms of zinc coatings on hot-dip galvanized steel sheets: Part I. Deformation modes

Zinc-based coatings are widely used for protection against corrosion of steel-sheet products in the automotive industry. The objective of the present article is to investigate the deformation modes at work in three different microstructures of a thin (8 μm) zinc coating on an interstitial-free steel substrate under tension, plane-strain tension, and expansion loading. Damage mechanisms are addressed in a companion article. The plastic slip and twinning activity in the zinc grains of an untempered cold-rolled coating (labeled NSK), a tempered cold-rolled coating (labeled SK), and a recrystallized coating are compared with the response of the corresponding bulk low-alloyed zinc material. The in-plane grain size in the NSK and SK coatings ranges from 300 to 600 μm, vs about 30 μm in the recrystallized coating and bulk material. The coatings exhibit a strong crystallographic texture, with the c-axis generally normal to the sheet plane. Basal slip is shown to be the main deformation mechanism in bulk zinc and the recrystallized coating, whereas pyramidal π₂ slip and mechanical twinning are found to be major modes in the NSK and SK coatings. These results, obtained from an extensive, quantitative slip-line analysis combined with electron backscattered diffraction (EBSD) measurements, are explained by the constraining effect of the substrate. This effect is successfully modeled using a simple Taylor-like polycrystalline approach. The recrystallized coating behaves much like the bulk material. The interpretation of this grain-size effect between the NSK and SK coating, on the one hand, and the recrystallized coating, on the other hand, requires a full three-dimensional finite-element analysis of the multicrystalline coating provided in this work. The simulations show that strong strain gradients can develop in the recrystallized coating from the interface to the surface, which is not the case in the NSK and SK coatings.     

循环动力扰动下花岗岩细观损伤特性试验研究

为研究循环动力扰动下岩石细观损伤特性,本文选取花岗岩为研究对象,利用分离式霍普金森压杆(SHPB)进行循环冲击试验,并结合核磁共振系统(NMR),得到了循环动力扰动后花岗岩孔隙度、横向弛豫时间T₂谱分布以及核磁共振图像(MRI).试验结果表明:在岩石弹性极限范围内,动力扰动结束后,岩石材料孔隙度降低;初始与最终横向弛豫时间T₂缩短,循环扰动过程促使了小孔隙生成,大孔隙尺寸与数量减小;孔隙度降低不意味着所有类型孔隙的数量均减少,相反小孔隙数量是增加的,大孔隙数量减少是动力扰动后花岗岩孔隙度降低的根本原因.     

Temperature and strain rate dependence of cyclic deformation response and damage accumulation in ofhc copper and 304 stainless steel

The effects of temperature and strain rate on deformation behavior and dislocation structure were investigated for OFHC copper and type 304 stainless steel. It is shown that the cyclic stress response is inversely related to the cell size for copper cycled at different temperatures ranging from -75 to 650°C. Type 304 stainless steel underwent a change from a planar to a wavy slip character as the temperature was changed from room temperature to 760°C. At elevated temperatures, cells were observed and the size of the cells tended to increase with increase in temperature. The effects of temperature on the cyclic stress-strain parameters were investigated for copper, type 304 stainless steel and Ferrovac “E” iron. On studying the effects of temperature and strain rate on the fracture mechanisms it was found that a time dependent fracture mode was dominant at high temperature levels and low strain rates. However, at high strain rates the life was insensitive to temperature. The role of grain boundary migration on the fracture process was investigated. Grain boundary migration was found to be dependent on strain rate for copper. However, for type 304 stainless steel, the grain boundary migration was inhibited at high temperature (760°C) due to the presence of precipitates at the grain boundaries. In strain cycling of OFHC copper and type 304 stainless steel, it was found that the addition of creep-type damage to fatigue damage resulted in a total damage which was not equal to unity for failure when these different modes were imposed sequentially. The sense of the damage accumulation appeared to have no effect on this summation.     

An Evaluation of the summation of cyclic plastic strain damage in two high strength aluminum alloys

A damage equation based upon the integration of low cycle fatigue plastic strain ranges was verified experimentally for two high strength aluminum alloys 2024-T4 and 7075-T651. The damage equation which has been used extensively for many fatigue crack propagation theories assumes cyclic damage under increasing plastic strain ranges. In order to verify the damage equation, low cycle fatigue specimens were subjected to a fully reversed strain cycle in which the total strain-range was increased linearly by a constant amount Δ[Δε_d] per cycle. An excellent agreement was obtained between the predicted and observed fatigue lifetimes. The stress-strain response of these alloys was also measured. The experimental results showed that these two alloys cyclically harden substantially and that the single strain increment stress-strain curve is a fair lower bound approximation of the cyclic stress-strain curve.