一个特殊单滑移取向铜单晶疲劳位错结构研究

为了更细致地揭示面心立方金属单晶体的循环变形机制,利用扫描电镜电子通道衬度(SEM-ECC)技术观察研究了Schmid因子为0.5的[41841]单滑移取向铜单晶体的循环饱和位错结构.实验表明,在单滑移铜单晶体中,胞结构除了在高应变幅下的循环变形中出现外,还可能出现在循环应力-应变(CSS)曲线平台区的较低塑性应变幅下.驻留滑移带(PSBs)会随应变幅的增大而在试样表面聚集成内部含有位错胞的粗滑移带,带内的位错胞结构被认为是由于带内滑移阻力增大引起的应变集中所致形成的.此外,CSS曲线高应变幅区起始部分对应的循环饱和位错结构观察揭示出迷宫结构和胞结构是由PSBs逐渐演变而成的.     

Cyclic Saturation Dislocation Pattern Observation in the Vicinity of Grain Boundaries by Electron Channeling Contrast Technique

State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Chinese Academy of Sciences,Shenyang, 110015, China)Abstract:The cyclic saturation dislocation patterns within grains and in the vicinity of low-angle grain boundaries in fatigued copper crystal were successfully observed by electron channeling contrast technique in SEM. The results show that the dislocation patterns within grains consisted of typical two-phase structure, i.e. persistent slip bands (PSB) and veins. With increasing plastic strain amplitude (γp1 ≥1.7×10-3), large amount of PSBs and regufar dislocation walls were observed.The dislocation walls and PSBs could cross through the low-angle grain boundaries continuously except that the dislocation-free zone (DFZs) appeared at some local regions. Combining with the cyclic stress-strain response and dislocation patterns, the effect of low-angle grain boundaries on cyclic deformation behavior was discussed.     

Half-cycle slip activity of persistent slip bands at different stages of fatigue life of polycrystalline nickel

The slip activity of persistent slip bands (PSBs) in polycrystalline nickel was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The half-cycle slip activity as well as the local shear strain amplitudes was investigated after half-cycle deformation at different numbers of cycles in the domain of stress saturation. Moreover, the fraction of grains containing cumulated PSBs and the accumulated volume fraction of PSBs was estimated depending on the number of cycles during fatigue life. The volume fraction of active PSBs during half-cycle deformation is significantly lower than the cumulated PSB volume and decreases with increasing number of cycles. Additionally, an increasing localization of cyclic plastic strain within the PSBs was observed. However, with increasing number of cycles the average local shear strain amplitude remains almost unchanged. Thus, PSBs in polycrystals are subjected to a life history which is characterized by active and inactive periods of their half-cycle slip activity during cyclic deformation at different stages of the saturation state.     

Effect of grain size on grain boundary strengthening of copper bicrystals under cyclic loading

Abstract

To clarify the strengthening effect of grain boundaries (GB), cyclic deformation behaviour of really grown [4¯79] ∥ [1¯25] copper bicrystals with different widths (4, 6, and 8 mm, denoted RB-4, RB-6, and RB-8) of com-ponent crystals and a combined copper bicrystal (denoted CB-6), obtained by sticking component single crystals G1 [4¯79] and G2 [1¯25] together, was investigated. The results showed that the cyclic saturation stresses increased in the order of bicrystals of CB-6 < RB-8 < RB-6 < RB-4. It is indicated that the GB effect caused different degrees of strengthening, which increased with the decreasing width of the RB bicrystals. By surface observation, it was found that only the primary slip system was activated in the combined bicrystal during cyclic deformation. However, an additional slip system appeared near the GB within the crystal G2 [1¯25] in the RB bicrystals (except in the primary slip system), and formed a GB affected zone (GBAZ). The width of the GBAZ was about 400 and 600 μm at plastic strain amplitudes of 0·1% and 0·2% respectively. Meanwhile, using an electron channelling contrast (ECC) technique in the SEM, the dislocation patterns near the GB and within the component crystals were observed. It was found that a two phase structure of persistent slip bands (PSBs) and matrix (or veins) can form in these bicrystals, similar to that in copper single crystals. But these PSBs cannot transfer through the GB during cyclic deformation. Based on the results above, the effect of grain size on GB strengthening of copper bicrystals was discussed.     

Similitude invariants and scaling laws for buckling experiments on anti-symmetrically laminated plates subjected to biaxial loading

Establishing the similitude between the model and prototype rigorously is a necessary step in designing an experiment efficiently. So far, to the best of the authors’ knowledge, no one has ever derived the similitude invariant for anti-symmetric cross- and angle-ply laminated plates subjected to biaxial loading before. This research paper is the first to establish the similitude invariant of anti-symmetric cross- and angle-ply laminated plates by applying the similitude transformation to the governing differential equations of buckling directly. Then the scaling laws for buckling loads of laminated plates subjected to biaxial loads are derived. But in reality, either due to the complexity of the scaling laws or to economize on costly experiments, it may not be feasible to construct the model conforming to the scaling laws completely, therefore partial similitude is investigated theoretically and approximate scaling laws are recommended. The buckling loads of the prototype predicted from the scaling laws are then compared with the available theoretical values. The complete similitude cases show exact agreement between results predicted from the scaling laws and the available analytical solutions. For partial similitude cases, the models distorted in stacking sequences, number of plies, and material properties are studied and the approximate scaling laws which yield good agreement are recommended.     

Grain boundary effects on cyclic deformation and fatigue damage

The cyclic deformation behaviors of single- and polycrystals were well documented in the past three decades. It has been recognized that there existed great difference in the fatigue damage mechanisms between single- and polycrystals, which can be mainly attributed to the effects of grain boundaries (GBs) and the crystallographic orientations. In the present work, a series of research work mainly on copper bicrystals with various GBs and different component crystals have been systematically investigated, including the macroscopic cyclic stress-strain responses and fatigue damage mechanisms in micro-scale. Firstly, direct evidence is offered to show the obvious strengthening effect caused by the large-angle GBs during cyclic deformation. The data of cyclic stress-strain responses will be presented to show the effects of the GBs and the crystallographic orientations in the macro-scale. Next, the influence of various GBs on fatigue cracking behavior will be considered for the crystalline materials. Clear evidences are shown that the interactions of persistent slip bands (PSBs) with various GBs play a decisive role in the intergranular fatigue cracking during cyclic deformation. It is suggested that the intergranular fatigue cracking strongly depends on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. The underlying fatigue damage mechanisms were further discussed in terms of the different interactions between PSBs and three types of GBs.     

"EFFECT OF LOADING MODE ON THE CYCLIC RESPONSE AND THE ASSOCIATED SUBSTRUCTURE OF POLYCRYSTALLINE COPPER IN THE HIGH-CYCLE REGIME"

Studies on the influence of loading mode on the cyclic response of small-grained polycrystalline copper and the associated dislocation structures have been carried out in the high-cycle regime. It is found that the saturation behaviour under constant load control, for two sets of specimens, with and without initial ramp-loading, exhibits strong differences in the “intermediate” range of stress amplitudes, i.e., between 70 and 98 MPa. Within this range the ramp-loading mode promotes a gradual substructure evolution which leads to localization of slip in primary systems and formation of persistent slip bands (PSBs), whereas conventional loading leads to the formation of elongated cells and multiple sets of wall structures (e.g., labyrinth structure), both intimately associated with multiple slip conditions. At low stress amplitudes observed differences in the plastic strain amplitudes obtained at saturation, as an effect of different loading modes, are relatively small and related to equally small differences in the uniformity and homogeneity, from grain to grain, of the dislocation structures associated with that stage. At high stress amplitudes equiaxed cell structures are promoted under both loading modes, the deformation is homogenized, and the cyclic response as a function of loading mode shows no differences.     

Ultrasonic back reflection evaluation of crack growth from PSBs in low-cycle fatigue of stainless steel under constant load amplitude

The objective of this study is to develop a method for evaluating crack growth from persistent slip bands (PSBs) in low-cycle fatigue of stainless steel, using an ultrasonic back reflection wave during the early stages of its fatigue life. Changes in the back reflection intensity from surface of the material under cyclic loading are measured. Back reflection intensity decreased due to the evolution of PSBs before the start of fatigue crack growth from the crack initiated along PSBs with increase in the number of cyclic loads. The average dislocation density in a grain including PSBs corresponds to the attenuation change measured during the fatigue test, from the initial state to the nucleation and growth of the fatigue crack. The attenuation is caused by the movement of dislocation due to ultrasonic waves, whose mechanism was considered quantitatively. In this study, micromechanical modeling was conducted as a prediction method for remaining fatigue life to start crack growth from PSBs based on the changes in ultrasonic back reflection intensity.     

铜单晶体在循环形变中形成的形变带

系统研究了循环形变铜单昌体中宏观形变带的产生规律和特征及相应的位错结构,并对其形成机制进行了综合探讨。结果表明,在不同取向铜单昌体的循环形变中,形变带ＤＢＩ近似沿主滑移面发展。而ＤＢⅡ的惯习面接近传统的扭折面｛１０１｝,两者成严格的正交分布。     

Wechselverformung und Gefügeänderungen von Ck 15 und Ck 45

Microstructural Changes and Cyclic Deformation The crack initiation starts due to weakening and strengthening process during rotating bending. However a smaller plastic deformation amplitude is noticed at the same nominal stress compared to tension-compression stressed specimens. This results in a higher fatigue life. The different cyclic deformation behaviour was proofed by SEM (rotating bending specimens showed a lower slip line density compared to tension-compression specimens at the same nominal stress) and TEM investigations (the rotating bending specimens showed a smaller dislocation density at the same nominal stress). Furthermore it is showed, a correlation of cyclic stress strain data σ(ε_pls) between tension-compression and rotating bending specimens exists. This is also valid for the Manson-Coffin-relationship. the relation between lg ε_pls and lg N_B depends on the material (Ck 15, Ck 45) but not on the state of stress.     

Cyclic deformation response and dislocation structure of Cu single crystals oriented for double slip

Cyclic deformation behavior of double-slip oriented Cu single crystals with a stress axis in the [034] direction was investigated under plastic strain control mode for a shear strain amplitude range of 1 × 10⁻⁴ to 5 × 10⁻³. Dislocation structures in the tested samples were observed using a transmission electronic microscope. It has been found that the effect of the operation of critical slip in these [034] crystals on cyclic responses and dislocation structures is nearly the same as that of increase in strain amplitude. The nucleation stress and number of cycles for PSB formation at each specific strain amplitude in the double-slip oriented crystals were found to be both considerably lower than those observed in single-slip oriented crystals. This observation is in a good agreement with the Kuhlmann-Wilsdorf and Laird analysis, in that the formation of PSBs is associated with glide behavior on the secondary slip system. A dislocation “cord” structure has also been observed and is believed to be caused by the operation of the cross-slip system during cyclic deformation. Labyrinth wall structures were found to form with increase in strain amplitude by the operation of critical slip and cross-slip systems. However, the formation of labyrinth structure was suppressed by the coplanar slip at high strain amplitudes.     

HIGH TEMPERATURE CYCLIC DEFORMATION OF NICKEL

Abstract— High temperature cyclic deformation in nickel is investigated by comparing its cyclic hardening curves at constant strain amplitudes at four temperatures with the monotonic hardening curves and by detailed electron microscopic examination of the bulk. Just as at ambient temperature, there are broad similarities between the cyclic and monotonic hardening of nickel at high temperatures, but there are also significant differences in detail. It is shown that, as in monotonic deformation, increased dynamic recovery with increasing temperature is the main cause for the reduction in flow stress at high temperatures. However, the increased vacancy concentration produced by cyclic straining causes a linear reduction in flow stress (as distinct from the "athermal" behavior observed in monotonic deformation over a limited temperature range). Also the tendency for dislocations to agglomerate into sign-balanced, low energy configurations requires a temperature higher than that observed for monotonic deformation before high temperature softening is observed.     

Use of scaled-down models for predicting vibration response of laminated plates

An analytical investigation was undertaken to show how and to what extent similitude theory can be applied in the design of scaled-down models. Through similitude the necessary similarity conditions, referred to herein as scaling laws, pertaining to free vibrations of laminated plates are derived. Establishment of scaling laws, based on the direct use of the governing equations is discussed and their use in the design of models is presented. These scaling laws provide the relationships between structural geometric parameters and frequency response parameters of a scaled-down model and its prototype. Later, these conditions are used to design a model, the experimental data of which can be projected in order to predict the behavior of the prototype. Attention is focused on the use of models for the analysis of free vibrations of multilayered composite rectangular plates. Angle-ply and cross-ply symmetric configurations are chosen for investigation. This analytical study indicates that distorted models with a different number of layers, material properties, and geometries than those of the prototype can predict the behavior of the prototype with good accuracy.     

A fractal approach to indentation size effect

In this paper, we propose an original interpretation of indentation size effect (ISE) in both single crystal and polycrystalline metals, which is based on the experimental evidence of the formation of fractal cellular dislocation patterns during the later stages of plastic deformation, in strain hardening metals, both under tensile loading and in compression. The proposed approach is a generalization of the arguments already proposed by the senior author in order to derive multifractal scaling laws (MFSL), which apply to tensile strength, fracture energy and the critical strain of brittle and quasi-brittle materials.This approach is thus in the mainstream of the geometrical interpretation of size-scale effects on the strength of solids, which has been counterposed, in recent years, to the mechanical interpretation. The proposed fractal approach aims at strengthening this view, which provides ease of interpretation, and at stimulating discussion on the central key role of dislocation evolution in the plastic deformation of metals, the fractal characteristics of which have not been adequately considered in literature.The obtained equation for ISE, based on the fractal approach, very closely resembles MFSL for tensile strength in quasi-brittle materials and the scaling equation already proposed by other authors, but it is based on a very different underlying physical model. Some experimental hardness data, obtained from microindentation on copper, have been fitted with MFSL, and show a very good agreement.     

Fatigue of Severely Deformed Metals

In this brief communication, we would like to review present data on fatigue performance of ultra‐fine grain materials fabricated by severe plastic deformation (SPD) and to discuss the possible mechanisms of their plastic deformation and degradation in light of currently available experimental data. The most prominent effect of SPD is often associated with significant grain refinement down to the nanoscopic scale. The other evident effect, which accompanies intensive plastic straining, is the dislocation accumulation up to limiting densities of 10¹⁶ m^–2. Since namely these two factors, the grain size and the dislocation density, govern the strengthening of polycrystalline materials, we shall primarily confine ourselves to their role in cyclic deformation of severely pre‐deformed metals.     

Modeling of nonlinear viscoelasticity at large deformations

A constitutive model of finite strain viscoelasticity, based on the multiplicative decomposition of the deformation gradient tensor into elastic and inelastic parts, is presented. The nonlinear response of rubbers, manifested by the rate effect, cycling loading and stress relaxation tests was captured through the introduction of two internal variables, namely the constitutive spin and the back stress tensor. These parameters, widely used in plasticity, are applied in this work to model the nonlinear viscoelastic behaviour of rubbers. The experimental results, obtained elsewhere, related with shear deformation in monotonic and cyclic loading, as well as stress-relaxation, were simulated with a good accuracy.     

Cyclic Deformation and Fatigue Damage Behavior of Cu Monocrystals, Bicrystals and Tricrystals

1.IntroductionMostofmechanisticstudiesoncyclicdeformationhavebeencarriedoutonCucrystalsinsingle-slipori-entations.ThereareseveralexcellentreviewarticleswhichemphasizeddifferentaspectsofthesubjectI1~6].Whenawell-annea1edCucrystalsuitablyorientedforsingleslipissubjectedtocyclicdeformationbetweenconstantplasticstrainamplitudelimits7platroomtemperatureinair,itwillhardenundertheactionofcyclicstraining.Thepeakstressincreasesrapidlyatfirstandthenmoreslowly,approachingavalue,thesaturationstressTs.Ap…     

An experimental methodology for characterizing fracture of hard thin films under cyclic contact loading

Hard thin films are often employed as protective coatings for metal substrates and their fatigue/fracture property (especially that under contact) needs to be sufficiently understood. In this study, we present a combined experimental/computational framework for exploring the fracture characteristics of hard thin films upon both monotonic and cyclic contacts. The techniques of acoustic emission and corrosion potential fluctuation are combined to monitor the mode and initiation of crack, and numerical simulations based on the finite element method provide further information on the criterion of film fracture. For a model system of a TiN film physical vapor deposited onto a stainless steel substrate, ring cracks are produced when the stress in the film exceeds critical — such a critical moment arrives when the substrate undergoes excessive deformation, which makes the curvature of film bending critical just outside the contact zone. Since cyclic contact loading encourages large plastic deformation of substrate due to ratcheting plasticity, it is found that the critical contact force degrades, compared with monotonic loading. The present experimental/computational methodology is useful for obtaining the information of film fracture property under both monotonic and cyclic contact loads.     

Fractographic and numerical study of hydrogen–plasticity interactions near a crack tip

This paper offers a fractographic and numerical study of hydrogen–plasticity interactions in the vicinity of a crack tip in a high-strength pearlitic steel subjected to previous cyclic (fatigue) precracking and posterior hydrogen-assisted cracking (HAC) under rising (monotonic) loading conditions. Experiments demonstrate that heavier cyclic preloading improves the HAC behaviour of the steel. Fractographic analysis shows that the microdamage produced by hydrogen is detectable through a specific microscopic topography: tearing topography surface or TTS. A high resolution numerical modelling is performed to reveal the elastoplastic stress–strain field in the vicinity of the crack tip subjected to cyclic preloading and subsequent monotonic loading up to the fracture instant in the HAC tests, and the calculated plastic zone extent is compared with the hydrogen-assisted microdamage region (TTS). Results demonstrate that the TTS depth has no relation with the active plastic zone dimension, i.e., with the size of the only region in which there is dislocation movement, so hydrogen transport cannot be attributed to dislocation dragging, but rather to random-walk lattice diffusion. It is, however, stress-assisted diffusion in which the hydrostatic stress field plays a relevant role. The beneficial effect of crack-tip plastic straining on HAC behaviour might be produced by the delay of hydrogen entry caused by residual compressive stresses and by the enhanced trapping of hydrogen as a consequence of the increase of dislocation density after cyclic plastic straining.     

A neural network approach for predicting steel properties characterizing cyclic Ramberg–Osgood equation

This paper attempts to demonstrate the applicability of artificial neural networks to the estimation of steel properties, cyclic strain‐hardening exponent and cyclic strength coefficient, characterizing cyclic Ramberg–Osgood equation on the basis of monotonic tensile test properties. For this purpose, steel tensile data were extracted from the literature and two separate neural networks were constructed. One set of data was used for training the two networks and the remaining for testing purposes. Regression analysis and mean relative error calculation were used to check the accuracy of the system in the training and testing phases. Comparison of the results obtained from the neural networks and the values obtained from direct fitting of experimental data, indicated the reasonable prediction of cyclic strain‐hardening exponent and cyclic strength coefficient, which are often used to characterize the cyclic deformation curve by a Ramberg–Osgood type equation.