316L不锈钢非比例路径疲劳失效的微观机理

进行316L不锈钢在600℃圆路径下不同应变范围的低周疲劳实验,用透射电子显微镜(TEM)观察疲劳失效断口附近的显微组织,研究了位错结构的路径相关性、幅值相关性以及动态应变时效(Dynamic strain aging,DSA)的路径相关性。结果表明：在室温和600℃的单轴加载变形以平面滑移方式为主,生成了脉络状位错结构;而在圆路径下则生成等轴胞状位错结构,显著降低了材料抵抗变形的能力,在600℃等效应变范围为1.0%条件下的疲劳寿命比单轴路径降低了81%。同时,在圆路径下材料形成胞状位错结构所需最小等效应变范围比单轴的低。600℃圆路径下的DSA效应更为完全,在等效应变范围为1.0%的条件下最大应力跌幅比单轴路径增大了680%;同时,压缩阶段的DSA现象更为显著,锯齿类型由A型经过B型过渡到C型。     

Rolling–sliding contact fatigue of surfaces with sinusoidal roughness

Surfaces of mechanical components under combined rolling and sliding motions may be subjected to accelerated contact fatigue failure due to increased number of microscopic stress cycles and pressure peak heights caused by rough-surface asperity contacts. Available rolling contact fatigue (RCF) theories were developed mainly for rolling element bearings, for which the effect of sliding is usually insignificant. In various types of gears, however, considerable sliding exist in the critical tooth contact area below the pitch line, where excessive wear and severe pitting failures originate. Ignorance of sliding is most likely the reason why the conventional RCF models often overestimate gear fatigue life. This paper studies the effect of sliding motion on the contact fatigue life of surfaces with sinusoidal roughness that mimicks the topography from certain manufacturing processes. A set of simple equations for stress cycle counting is derived. Mixed elastohydrodynamic lubrication simulations are executed with the considerations of normal loading and frictional shear. Relative fatigue life evaluations based on a subsurface stress analysis is conducted, taking into account the two sliding-induced mechanisms, which are the greatly increased number of stress cycles and the pressure peak heights due to surface interactions. Obtained results indicate that sliding leads to a significant reduction of contact fatigue life, and rough surface asperity contacts result in accelerated pitting failure that needs to be considered in life predictions for various mechanical components.     

Partition of deformation and anelastic energies in multiple impacts in copper

Response of a copper specimen to a large number of impacts by a single indenter was investigated. Work hardening produced during the first few impacts was studied based on measurements of impact parameters. Analysis of the energy distribution during impacts indicated that a large portion of the energy is expended in anelastic effects. Study of the subsurface microstructure showed a dislocation cell structure similar to the cell structure developed in fatigued materials. Finite element analysis of the stress distribution during quasi-static indentation suggests that one of the possible mechanism's contribution to material removal is a fatigue-like tensile-compression cyclic loading.     

Numerical simulation of growth pattern of a fluid-filled subsurface crack under moving hertzian loading

Cracking of a fluid-filled subsurface crack is studied by means of the distributed dislocation technique within the framework of two-dimensional linear elastic fracture mechanics. The Griffith crack was initially opened by the application of hydrostatic pressure of an incompressible fluid within the crack. A moving Hertz line contact load distribution is applied at the surface of the half-plane in the presence of friction. The stress intensity factors at the tips of the fluid-filled crack are analyzed with the restriction that due to the fluid incompressibility there is no change of the crack-opening volume. When the crack starts to propagate/kink, numerical results show that the internal fluid pressure will be relieved, and as the ratio of the branched crack length to main crack length increases, the elastic strain energy release rate decreases. The crack growth is assumed to be arrested when the energy release rate is below a certain value. Based on the energy criterion, predictions are attempted for determining the load position where the crack propagation/kink commences as well as the growth increment of the branch crack before it is arrested. A step-by-step crack path is constructed to simulate the growth pattern of the fluid-filled crack under moving Hertzian loading.     

Crack propagation in a material with random toughness

An efficient Monte Carlo procedure is presented for characterizing the propagation of a crack in a material whose fracture toughness is a random field. The simulations rely on accurate approximate solutions of the integral equations that govern the dislocation densities, stress intensity factors, and energy release rates of curvilinear cracks. For a plate containing an edge crack that propagates towards a subsurface crack representing a traction-free boundary, results for the distributions of crack trajectories, critical applied far-field stresses, and nominal fracture toughness are presented for various parameters that quantify the randomness of the material's critical energy release rate. A demonstrative probabilistic model for crack trajectories is built and size effects are discussed.     

"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.     

Atomistic modeling of plastic deformation in B2-FeAl/Al nanolayered composites

In this work, the deformation response of the B2-FeAl/Al intermetallic composites, as a model material system for nanolayered composites comprised of intermetallic interfaces, has been explored. We use atomistic simulations to study the deformation mechanisms and the interface misfit dislocation structure of B2-FeAl/Al nanolayered composites. It is shown that two sets of dislocations are contained in the interface misfit dislocation network and are correlated with the initial dislocation nucleation from the interfaces. The effects of layer thickness on the uniaxial deformation response of the B2-FeAl/Al multilayers are investigated. We observed that under compressive loading the smaller proportion of the FeAl layers leads to the lower overall flow stress. Under tensile loading, the void formation mechanism is investigated, suggesting the interface structure and the dislocation activities in the FeAl layers playing a significant role to trigger the strain localization which leads to void nucleation commencing at the interface. It is also found that the deformation behavior in the “weak” Fe/Cu interface behaves substantially different than that of the “strong” FeAl/Al interface. The atomistic modeling study of the nanolayered composites here underpinned the mechanical response of “strong” intermetallic interface material systems. There is no void nucleation during the entire plastic deformations in the Fe/Cu simulations, which is attributed to much higher dislocation density, more slip systems activated, and relative uniformly distributed dislocation traces in the Fe phase of the Fe/Cu multilayers.

     

Influence of friction and adhesion on the onset of plasticity during normal loading of spherical contacts

Increasing contact loading causes early transformation from elastic to elastic–plastic deformations in many conventional systems as well as micro/nano-electro-mechanical systems. The load required for yielding and the location of the onset of plasticity is critical in the robustness of systems with contacts. For frictionless (such as fully-lubricated) contacts, inception of plastic yielding occurs beneath the contact surface. However, frictional slip (contact shear) and adhesion push the inception of plastic yielding toward the contact surface. The influence of elastic mismatch, shear tractions and adhesive normal tractions on the subsurface stress field is studied analytically by superposition of the Hertzian stress field and the stress field created by the shear and additional (due to adhesion) normal tractions. Specifically, three contact conditions have been studied in this work: (i) frictionless, (ii) finite friction, and (iii) infinite friction (full stick). Also, a finite-element model is developed to verify certain assumptions in the analytical solution for the contact with finite friction. The results obtained are applied to two sets of in situ nanoindentation experiments to explain the change in the yielding behavior of submicrometer polycrystalline aluminum grains.     

Elastic-plastic analysis of indentation damage: cyclic loading of copper

This is the second in a two-paper series examining the elastic-plastic deformation of a metallic target material under repeated impact. This paper examines the effect of statically cyclic loading on copper and numerical calculations have been performed up to ten cycles. The extent of the loading phase in each cycle is specified by a constant external work done by the indenter. The deformed configuration of the target material, the locus of material flow, and elastic-plastic boundaries due to cyclic loading are presented. There is a saturation of the stress field near the bottom of the indented crater after the first few cycles of loading. A residual tensile stress field in the direction normal to the target surface exists underneath the indenter in every cycle, which is responsible for the formation of subsurface layer cracks. Results of the coefficient of restitution obtained from the analysis and experiments are also presented.     

On dislocation-based artificial neural network modeling of flow stress

Computational design of materials processes has received great interests during the past few decades. Successful designs require accurate assessment of material properties, which can be influenced by the internal microstructure of materials. This work aim to develop a novel computational model based on dislocation structures to predict the flow stress properties of metallic materials. To create sufficient training data for the model, the flow stress of a precipitation–hardening aluminum alloy was measured by characterizing the dislocation structure of specimens from interrupted mechanical tests using a high resolution electron backscatter diffraction technique. The density of geometrically necessary dislocations was calculated based on analysis of the local lattice curvature evolution in the crystalline lattice. For three essential features of dislocation microstructures – substructure cell size, cell wall thickness, and density of geometrically necessary dislocations – statistical parameters of their distributions were used as the input variables of the predictive model. An artificial neural network (ANN) model was used to back-calculate the in situ non-linear material parameters for different dislocation microstructures. The model was able to accurately predict the flow stress of aluminum alloy 6022 as a function of its dislocation structure content. In addition, a sensitivity analysis was performed to establish the relative contribution of individual dislocation parameters in predicting the flow stress. The success of this approach motivates further use of ANNs and related methods to calibrate and predict inelastic material properties that are often too cumbersome to model with rigorous dislocation-based plasticity models.     

Low cycle fatigue behaviour of low carbon microalloyed steel: microstructural evolution and life assessment

Abstract

In this paper the cyclic stress–strain response, low cycle fatigue (LCF) behaviour, and evolution of dislocation structures under LCF loading in the case of a low carbon microalloyed steel are discussed. The cyclic stress response revealed cyclic softening resulting from the propagation of Lüders bands. The experimental LCF life was compared with the life predicted using Tomkins' model and the modified universal slopes (MUS) equation. While the life predicted by Tomkins' model showed good correlation with the experimental results, the life predicted using the MUS equation grossly overestimated the life. Inclusion induced delaminations under cyclic loading were thought to be responsible for the overestimation by the MUS equation. Low energy dislocation structures, i.e. cells, were observed near the fracture surfaces. Interrupted tests revealed cell formation after 10 cycles at a total strain amplitude of 0·3%.     

Computational modelling of irregular open‐cell foam behaviour under impact loading

Cellular structures represent an important class of engineering materials. Typical representative of such structures are metallic foams, which are being increasingly used in many advanced engineering applications due to their low specific weight, appropriate mechanical properties and excellent energy absorption capacity. For optimal design of cellular structures it is necessary to develop proper computational models for use in computational simulations of their behaviour under impact loading. The paper studies the effects of open‐cell metallic foam irregularity on deformation behaviour and impact energy absorption during impact loading by means of parametric computational simulations, using the lattice‐type modelling of open‐cell material structure. The 3D Voronoi technique is used for the reproduction of real, irregular open‐cell structure of metallic foams. The method uses as a reference a regular mesh structure and utilises an irregularity parameter to reproduce the irregularity of real open‐cell structure. A smoothing technique is introduced to assure proper stability and accuracy of explicit dynamic simulations using the produced lattice models. The effects of the smoothing technique were determined by comparative simulations of smoothed and unsmoothed lattices subjected to dynamic loading.     

Analysis of Critical Stress for Subsurface Rolling Contact Fatigue Damage Assessment Under Roll/Slide Contact

As one of the main failure modes of component operated under rolling contact loading, the rolling contact fatigue is classified into two types: subsurface initiated and surface initiated. Different stresses such as orthogonal shear stress, maximum principle shear stress, and octahedral shear stress have been applied as the critical stresses for the assessment of the subsurface cracks’ initiation due to rolling contact fatigue. The influences of friction on distributions of the ranges of orthogonal shear stress, maximum principle shear stress, and octahedral shear stress in subsurface were analyzed with reference to the results of the reference articles. The results show that friction does influence the subsurface distributions of these stresses to a certain extent. However, the upper limits of both the maximum principle shear stress and octahedral shear stress are smaller than that of range of orthogonal shear stresses under the rolling contact conditions of usual steel components. Hence, it is more appropriate that the orthogonal shear stress be selected as the critical stress for the assessment of subsurface rolling contact fatigue.     

Is a crack an effective stress riser for triggering of plasticity?

From the stress intensity factors induced by a circular loop attached to a crack front, the resulting image force acting on the dislocation was calculated. The relaxation of the elastic energy stored due to neighbouring surfaces is obtained through integration of the variation of the image force while shifting the loop towards the inside of the material. The amount of relaxation, as numerically determined in this paper under a given loading, is too small to sufficiently decrease the energy barrier for triggering of dislocation emission. Homogeneous nucleation on a perfect crack front is impossible. Since dislocation emission has been experimentally observed under the loading considered, it must come from defects along the crack front.     

Rolling contact fatigue mechanism of a plasma‐sprayed and laser‐remelted Ni alloy coating

The rolling contact fatigue behaviour of the plasma‐sprayed and laser‐remelted Ni‐Cr‐B‐Si alloy coatings under two different tribological conditions of contact pressure was investigated. Two sets of fatigue‐life data of coatings were characterized by Weibull distributions. The failure mode of the coatings was identified on the basis of worn morphologies as observed at the surfaces of the failed coatings. The tribological mechanism leading to the formation of the fatigue spall was discussed on the basis of the subsurface morphologies observed in the failed coating. Experimental results showed that, the mean life and characteristic life of the coating decreased with increasing the contact pressure. The failure of the coatings can be termed as spalling‐type failure. A refined ‘ring‐crack model' was proposed to explain the formation of the fatigue spall. In the refined model, it was postulated that the joining of the ring‐type cracks and subsurface branched cracks was directly responsible for the spall formation.     

A model of whisker crystal growth from a pentagonal small particle

We present a physical model of growth of whisker crystals from metal pentagonal small particles (PSPs). The model is based upon the notions of nucleation and slippage of prismatic dislocation loops in the elastic field of disclination defects that are inherent in PSPs. In the framework of this model, the escape of interstitial dislocation loops at the PSP surface leads to an increase in the whisker length relative to the base, while incorporation of the vacancy-type loops is accompanied by their accumulation on the internal surface. The model is illustrated by calculations that show a gain in the total PSP energy as a result of the formation of a pair of prismatic dislocation loops with opposite signs.     

Microcrack initiation and growth in heat-resistant 15Kh2MFA steel under cyclic deformation

This paper presents the results of investigation of a nuclear reactor pressure vessel steel 15Kh2MFA of two strength levels under cyclic loading. The mechanism of microcrack formation on the surface and in the bulk of 15Kh2MFA steel under cyclic deformation was investigated. Analysis of the specimen surfaces has shown that microcracks are caused by cyclic sliding in grains most favourably oriented with respect to the direction of the maximum shear stresses. Transmission electron microscope investigations show that microcracks in the material inside the grains are formed mainly along the band‐type dislocation structure parallel to the dislocation subboundary. During cyclic deformation, the dislocation density on the subboundaries increases, in the local areas the dislocation density becomes limiting and it reaches the plasticity limit and causes microcrack formation. The interrelation of the average length of microcracks and their surface density with the energy density of inelastic deformation has been found.     

MICROSTRUCTURAL STUDY OF CYCLIC STRAIN HARDENING BEHAVIOUR IN BIAXIAL STRESS STATES AT ELEVATED TEMPERATURE

This paper describes the cyclic strain hardening behaviour and dislocation structures of material in biaxial low cycle fatigue at elevated temperatures. In this study, push-pull, reversed torsion and combined push-pull/reversed torsion tests were carried out using a type 304 stainless steel in air. While there was no significant difference between the cyclic stress amplitudes in the push-pull and reversed torsion tests on a von Mises' base, combination tests exhibited a 40% increase in stress amplitude. Most of the dislocations in the first two types of test adopted ladder or maze structures, while in the later case cells were found. Changing the loading mode at a certain cycle, for example, from push-pull to reversed torsion, revealed that stress amplitude depended mainly on the concurrent applied strain mode and furthermore, that the strain mode before the interchange had little or no effect on the stress amplitude after the interchange. Tests were also performed in order to examine how prestrained material hardened in the three different loading modes, with the following results: prestrained material in push-pull or in reversed torsion exhibited an anisotropic stress response, while the material in the combined tests exhibited an isotropic response. These cyclic responses are discussed in connection with the dislocation structure.     

Modeling particulate self-healing materials and application to uni-axial compression

Using an advanced history dependent contact model for DEM simulations, including elasto-plasticity, viscosity, adhesion, and friction, pressure-sintered tablets are formed from primary particles. These tablets are subjected to unconfined uni-axial compression until and beyond failure. For fast and slow deformation we observe ductile-like and brittle softening, respectively. We propose a model for local self-healing that allows damage to heal during loading such that the material strength of the sample increases and failure/softening is delayed to larger strains. Local healing is achieved by increasing the (attractive) contact adhesion forces for those particles involved in a potentially breaking contact. We examine the dependence of the strength of the material on (a) the damage detection sensitivity, (b) the damage detection rate, and (c) the (increased) adhesion between healed contacts. The material strength is enhanced, i.e., the material fails at larger strains and reaches larger maximal stress values, when any of the parameters (a)–(c) is increased. For very large adhesion between the healed contacts an interesting instability with strong (brittle) fluctuations of the healed material’s strength is observed.     

Direct observation of frictional seizure of mild steel sliding on aluminum by X-ray imaging Part I Methods

Many of the mechanical or metallurgical failures originate from subsurface cracks or changes in microstructure of the materials. It would be quite informative to material scientists to view the changes below the surface while testing to get a direct evidence of the mechanisms claimed to be governing a particular process like dislocation movement/grain boundary sliding, crack opening displacement, wear, stress concentration etc. In this work, we have used an X-ray microscope for in-situ observation of interfacial features during the friction and wear testing of mild steel specimens sliding against Al 6061 disk. This technique enables the observation of interfacial features of the hidden contact. Multiple tests were conducted at different sliding speeds of 2, 4 and 5 m/s. The images obtained during the tests indicated the presence of various sub surface processes such as adhesive transfer, stress fields, island formation and roughening. Wear was also found to be concentrated over a certain specific area during the initial part of the test but later the contact developed into a conformal contact following a lumpy transfer of material.