外应力场下NiAl合金微裂纹动态扩展的分子动力学模拟

目的 对NiAl合金中不同晶体取向的裂纹扩展动力学行为进行原子尺度研究,明晰在塑性变形过程     

A modeling approach for the complete ductile–brittle transition region: cohesive zone in combination with a non-local Gurson-model

The present study deals with the simulation of crack propagation in the ductile–brittle transition region on the macro-scale. In contrast to most studies in the literature, not only the ductile softening by void growth and coalescence is incorporated but also the particular material degradation by cleavage. A non-local Gurson-type model is employed together with a cohesive zone to simulate both failure mechanisms simultaneously. This consistent formulation of a boundary value problem allows arbitrary high mesh resolutions. The results show that the model captures qualitative effects of corresponding experiments such as the cleavage initiation in front of a stretch zone, the formation of secondary cracks and possible crack arrest. The influence of the temperature on the predicted toughness is reproduced in the whole ductile–brittle transition region without introducing temperature-dependent fit parameters. A comparison with experimental data shows that the shift of the ductile–brittle transition temperature associated with a lower crack-tip constraint can be predicted quantitatively.     

Competing risks models for fracture in the ductile to brittle transition temperature region

When fracture toughness testing is carried out over the ductile to brittle transition temperature region cleavage instability may be observed at the initiation of cracking or after some prior ductile crack growth. The amount of precleavage ductile crack growth increases with increasing temperature. At the lower test temperatures, it is possible to assume that all tests will result in cleavage instability. However, as the test temperature increases, at some limiting temperature, the failure mode during the final instability changes from cleavage to ductile. These two different types of behaviour can be accommodated in a statistical analysis which is based on the method of competing risks. A statistical approach is presented for the analysis of data by competing risks and a procedure is given for the estimation of the probability of cleavage failure.     

Effect of lattice orientation on crack tip constraint in ductile single crystals

In this work, the effect of lattice orientation on the fields prevailing near a notch tip is investigated pertaining to various constraint levels in FCC single crystals. A modified boundary layer formulation is employed and numerical solutions under mode I, plane strain conditions are generated by assuming an elastic–perfectly plastic FCC single crystal. The analysis is carried out corresponding to different lattice orientations with respect to the notch line. It is found that the near‐tip deformation field, especially the development of kink or slip shear bands is sensitive to the constraint level. The stress distribution and the size and shape of the plastic zone near the notch tip are also strongly influenced by the level of T ‐stress. The present results clearly establish that ductile single crystal fracture geometries would progressively lose crack tip constraint as the T ‐stress becomes more negative irrespective of lattice orientation. Also, the near‐tip field for a range of constraint levels can be characterized by two‐parameters such as K–T or J–Q as in isotropic plastic solids.     

Evaluation of temperature‐sensitive fatigue crack propagation of a high‐speed railway wheel rim material

The fatigue crack growth rate (FCGR) of ER8C high‐speed railway wheel rim material was tested at various service temperatures. The temperature sensitivity of fatigue crack propagation was evaluated, and the effect of temperature on the crack propagation mechanism was analyzed. The obtained results indicate a fatigue ductile‐to‐brittle transition (FDBT) point at ?20°C for the ER8C wheel rim materials. A reverse relationship was found between FCGR and temperature for the near threshold and Paris regimes when the temperature was below the FDBT point. However, no evident changing rule was found when the temperature was above this transition point. An evident fatigue crack propagation mode transition was found from lamellar tearing to intergranular cracks, which was related to the FDBT for the near‐threshold regime.     

Fatigue Damage Evolution in Ultrafine‐Grained Interstitial‐Free Steel

The current work presents the crack propagation behavior in ultrafine‐grained (UFG) interstitial‐free (IF) steel, and in particular, focuses on the damage evolution in UFG IF steel under cyclic loading. The current results indicate that equal‐channel angular pressing (ECAP) has a major influence on the cyclic deformation response of the UFG IF steel, such that the failure and the crack path depend on the inclination plane during ECAP. Furthermore, the UFG IF steel demonstrates significant notch sensitivity in comparison to its coarse‐grained counterpart. This is attributed to the ultrafine grains with a large volume fraction of high‐angle grain boundaries, where glide of dislocations is hindered and the resulting internal stresses increase the stress concentration further in the presence of a pre‐existing notch.     

Two‐scale diffusion–deformation coupling model for material deterioration involving micro‐crack propagation

In this paper, we introduce a two‐scale diffusion–deformation coupled model that represents the aging material deterioration of two‐phase materials involving micro‐crack propagations. The mathematical homogenization method is applied to relate the micro‐ and macroscopic field variables, and a weak coupling solution method is employed to solve the two‐way coupling phenomena between the diffusion of scalar fields and the deformation of quasi‐brittle solids. The macroscopic mechanical behavior represented by the derived two‐scale two‐way coupled model reveals material nonlinearity due to micro‐scale cracking induced by the scalar‐field‐induced deformation, which can be simulated by the finite cover method. After verifying the fundamental validity of the proposed model and the analysis method, we perform a simple numerical example to demonstrate their ability to predict aging material deterioration. Copyright © 2010 John Wiley & Sons, Ltd.     

The brittle to ductile transition of single crystal materials—An indentation model

A model for the brittle to ductile transition of brittle single crystal materials under indentation has been investigated. Continuous dislocation pile-ups against the wedge tip are used to explain the plastic deformation. The indentation depth is attributed to the dislocation pile-ups. The critical indentation depth p _cof brittle to ductile transition is proposed. Thus, the single crystal material is in brittle mode during the indentation loading if the indentation depth is greater than p _c. Otherwise, it is ductile. Micrographs support this modeling. Indentation on the surfaces of (100) or (110) in fcc and bcc single crystal materials is compared. The parameter Sis proportional to the number of dislocations and to the reciprocal of wedge angle. The value of Sis smaller for (100) than for (110) in fcc structure, but the trend of bcc structure is opposite. The shape of indenter is similar to that of grinding particles consisting in cutting tools. In order to maintain cutting efficiency in ductile mode, the cutting tool must be replaced if the grinding particles are blunt.     

Fracture of Metal Foams: In‐situ Testing and Numerical Modeling

This paper is on a combined experimental/modeling study on the tensile fracture of open‐cell foams. In‐situ tensile tests show that individual struts can fail in a brittle or ductile mode, presumably depending on the presence of casting defects. In‐situ single strut tests were performed, enabling observation of deformation and fracture behavior and, in addition, serving as calibration for the proposed single‐strut model. The single strut model consists of beam elements to account for elasticity and plasticity, and of special‐purpose fracture elements to account for failure. The model is demonstrated for a characteristic loading configuration, combining stretching and bending.     

Micromechanical modeling of grain boundary resistance to cleavage crack propagation in ferritic steels

In ferritic steels a propagating cleavage microcrack changes its propagation direction as it advances from grain to grain. This is due to differences in the orientation of the cleavage planes of two neighboring grains. In order to reach a cleavage plane in a new grain, a microcrack must first penetrate the grain boundary. Grain boundaries therefore act as natural barriers in cleavage fracture. The influence of a grain boundary and the associated misorientation in cleavage planes on crack arrest is here examined using a 3D finite element model with axisymmetric periodicity, representing two grains whose cleavage planes are tilted and twisted relative to each other. The temperature dependent mechanical properties of ferrite are modeled using a temperature dependent viscoplastic response. The development of the crack front as the microcrack penetrates through a grain boundary is here presented. The influence of the twist misorientation on the critical grain size, defined as the largest grain size that can arrest a rapidly propagating microcrack, is examined in a temperature range corresponding to the ductile to brittle transition (DBT) region. It is shown that when both tilt and twist misorientation are present, the influence of tilt and twist, respectively, on crack growth resistance can be decoupled.     

A model of brittle fracture propagation part I—continuum aspects

The micromechanism of crack propagation in steel is described and analyzed in continuum terms and related to the macroscopic fracture behavior. It is proposed that propagation of cleavage microcracks through favorably oriented grains ahead of the main crack tip is the principal weakening mode in brittle fracture. This easy cleavage process proceeds in the Griffith manner and follows a continuous, multiply connected, nearly planar path with a very irregular front which spreads both forward and laterally and leaves behind disconnected links which span the prospective fracture surface. A discrete crack zone which extends over many grains thus exists at the tip of a running brittle crack. Final separation of the links is preceeded by plastic straining within the crack zone and occurs gradually with the increasing crack opening displacement. It is suggested that in low stress fracture, straining of the links is the only deformation mode. However, it is recognized that under certain conditions plastic enclaves may adjoin the crack zone. This deformation mode is associated with high stress fracture, energy transition and eventually with crack arrest.

Energy dissipation resulting from the two deformation mechanisms is related to crack velocity, applied load and temperature and the crack velocity in a given material is expressed as a function of the external conditions. Fracture initiation and crack arrest are then discussed in terms of the conditions which are necessary to maintain the propagation process. Finally, the dimensions of a small scale crack tip zone for a steady state, plane strain crack are evaluated as functions of material properties and the elastic stress intensity factor.

The microstructural aspects of brittle fracture will be discussed in a separate Part 2 [1].     

The effect of T‐stress on crack‐tip plastic zones under mixed‐mode loading conditions

In this paper, the influence of T‐stress on crack‐tip plastic zones under mixed‐mode I and II loading conditions is examined. The crack‐tip stress field is defined in terms of the mixed‐mode stress intensity factors and the T‐stress using William's series expansion. The crack‐tip stress field is incorporated into the Von Mises yield criteria to develop an expression that determines the crack‐tip plastic zone. Using the resultant expression, the plastic zone is plotted for various combinations of mode II to mode I stress intensity factor ratios and levels of T‐stress. The properties of the plastic zone affected by T‐stress and mixed‐mode phase angle are discussed. The observations obtained on plastic zones variations are important for further fatigue and fracture analyses for defects in engineering structures under mixed‐mode loading conditions.     

Anomalous Tensile Strength and Fracture Behavior of Polycrystalline Iridium from Room Temperature to 1600 °C

The nature of the brittleness of Iridium crystal is still unclear. The aim of this study is to explore the mechanism of ductile‐to‐brittle transition (DBT) and the fracture behavior in polycrystalline Iridium. Tensile tests are conducted from room temperature to 1600 °C. Furthermore, fracture morphology and deformation substructures are characterized by OM, SEM, and TEM. The results show that the tensile strength increases anomalously below 600 °C and then decreases with the increasing temperature. The elongation increases slowly from room temperature to 700 °C, and it then changes sharply from 9.88% at 700 °C to 31% at 800 °C. Below 700 °C, the polycrystalline Iridium exhibits intergranular and partial transgranular cleavage fracture pattern. In contrast, the ductile fracture morphologies associated with microvoids coalescence are observed between 800 and 1600 °C. Massive tangling screw dislocations form at 700 °C and less tangles appear when stretching at 900 °C, manifesting that the DBT is around 800 °C in polycrystalline Iridium.

     

超精密车削单晶硅刀具振动频谱分析

为了在线监测单晶硅超精密车削的脆塑转变现象及分析单晶硅车削过程中材料的微纳去除方式,采用圆弧刃金刚石刀具对单晶硅（100）晶面进行了超精密车削,研究了单晶硅超精密车削时刀具振动频谱分布与切削参数的关系,并对刀具振动频谱的变化规律及其演变机理进行了分析.结果表明,刀具振动频谱分布与刀具和单晶硅接触方式、单晶硅微纳去除模式密切相关.当单晶硅的去除模式从脆性域过渡到塑性域时,材料由崩碎状脆性去除方式转变为以剪切滑移变形为主的塑性去除方式,刀具振动频谱高频段信号增多,且振动总能量增大;塑性域车削时,切削速度越小、切削深度越大、进给量越大,材料微观剪切变形区内位错滑移数量越多,刀具振动频谱高频段信号越多,刀具振动主总能量越大.切削速度、进给量、切削深度对刀具振动频谱分布的影响依次减小,采用合理的切削参数,可以降低切削系统的总体振动.     

All‐Optical Control of Shape

Photoresponsive liquid crystal elastomers (LCEs) are a unique class of anisotropic materials capable of undergoing large‐scale, macroscopic deformations when exposed to light. Here, surface‐aligned, azobenzene‐functionalized LCEs are prepared via a radical‐mediated, thiol‐acrylate chain transfer reaction. A long‐lived, macroscopic shape deformation is realized in an LCE composed with an o‐fluorinated azobenzene (oF‐azo) monomer. Under UV irradiation, the oF‐azo LCE exhibits a persistent shape deformation for >72 h. By contrasting the photomechanical response of the oF‐azo LCE to analogs prepared from classical and m‐fluorinated azobenzene derivatives, the origin of the persistent deformation is clearly attributed to the underlying influence of positional functionalization on the kinetics of cis→trans isomerization. Informed by these studies and enabled by the salient features of light‐induced deformations, oF‐azo LCEs are demonstrated to undergo all‐optical control of shape deformation and shape restoration.     

Rupture crack branching in solids with structural hierarchy

Both growth and branching of sharp cracks in perfect single crystals are studied. Strength and deformation criteria of sharp crack branching are proposed. These criteria describe brittle, quasi-brittle, and ductile material behavior under fracture propagation. For inner cracks, simple relations have been obtained that describe crack branching when curves of Kolumb-Mohr type theoretical strength for the generalized deformation mode are known. Multiple crack branching, associated with a multiplicity of eigenvalues when the system is buckled, is discussed. It has been ascertained that the principle of local symmetry in the vicinity of a crack tip is valid for perfect single crystals if the symmetry axis of a crystal is in line with the crack axis. When asymmetric perturbations of an atomic lattice occur in the vicinity of a crack tip or the symmetry axis of a single crystal is inconsistent with the crack axis, the principle of local symmetry fails. Solids with a hierarchy of regular structures that are typical for Macro-, Micro- and Nano-scales are studied in the same way. The curves of theoretical strength (Kolumb-Mohr type) of every structural level of material are considered to be known for the generalized stress state.     

Rupture crack branching in solids with structural hierarchy

Both growth and branching of sharp cracks in perfect single crystals are studied. Strength and deformation criteria of sharp crack branching are proposed. These criteria describe brittle, quasi-brittle, and ductile material behavior under fracture propagation. For inner cracks, simple relations have been obtained that describe crack branching when curves of Kolumb-Mohr type theoretical strength for the generalized deformation mode are known. Multiple crack branching, associated with a multiplicity of eigenvalues when the system is buckled, is discussed. It has been ascertained that the principle of local symmetry in the vicinity of a crack tip is valid for perfect single crystals if the symmetry axis of a crystal is in line with the crack axis. When asymmetric perturbations of an atomic lattice occur in the vicinity of a crack tip or the symmetry axis of a single crystal is inconsistent with the crack axis, the principle of local symmetry fails. Solids with a hierarchy of regular structures that are typical for Macro-, Micro- and Nano-scales are studied in the same way. The curves of theoretical strength (Kolumb-Mohr type) of every structural level of material are considered to be known for the generalized stress state.     

A modified cohesive zone model for a high‐speed expanding crack

The present work studies a self‐similar high‐speed expanding crack of mode‐I in a ductile material with a modified cohesive zone model. Compared with existing Dugdale models for moving crack, the new features of the present model are that the normal stress parallel to crack faces is included in the yielding condition in the cohesive zone and traction force in the cohesive zone can be non‐uniform. For a ductile material defined by von Mises criterion without hardening, the present model confirms that the normal stress parallel to crack face increases with increasing crack speed and can be even larger than the normal traction in the cohesive zone, which justifies the necessity of including the normal stress parallel to the crack faces in the yielding condition at high crack speed. In addition, strain hardening effect is examined based on a non‐uniform traction distribution in the cohesive zone.     

Change of fracture mode in Charpy toughness transition temperature range

Abstract

A transition layer of width 5 - 10 μm was found on the boundary between ductile and brittle fracture for Charpy V notch specimens in the transition temperature range of a structural steel having a microstructure of polygonal ferrite -pearlite. The fracture mode in the transition layer was shearing with occasional submicrometre dimples. From tensile tests on notched specimens, the cleavage fracture stress and flow stress by ductile decohesion were determined. Based on the experimental data and the assumption that the volume of metal involved in the plastic deformation during fracture was related to the volume of the dimples, it was deduced that the transition layer width represents the size of the plastic zone immediately before cleavage initiation. The crack opening displacement and the crack tip radius for the change of fracture mode were calculated.