纳观尺度裂纹扩展微观机理的晶体相场法研究

采用晶体相场方法模拟不同预变形量的样品在单轴拉应变作用下的纳观裂纹扩展行为.观察纳观尺度的裂纹演化过程,结果表明：对于无预变形的样品,在拉应变作用下,由于裂口处应变集中,当应变量达到临界值时,裂口开始起裂并伴随位错出现,随着裂尖的前进位错伴随着裂尖而滑移.对于预变形的样品,在较小的临界应变值裂口起裂,且预变形量越大,裂纹越易起裂和扩展.在裂纹扩展早期阶段呈现出扩展-转向-扩展的长大特征,其本质是裂纹扩展由于裂尖附近的位错滑移受阻引起应变集中,造成主次原子晶面方向的原子键交替断裂,使得裂尖扩展沿[√3,1]和[√3,-1]方向交替改变而前进,裂纹边缘呈锯齿形状.样品的裂纹扩展后期出现显著的类似解理裂纹扩展行为,解理方向沿[√3,1]和[√3,-1]方向,且预变形量大的样品裂纹解理扩展更明显且扩展较快.     

晶体相场法模拟微裂纹的扩展行为

用晶体相场模型模拟预变形条件下的单轴拉伸的裂纹扩展行为.展示出裂纹扩展过程的演化图。从裂纹演化图可以清楚的观察到,随着应变量的增加,裂纹长度增加,裂纹呈钝化-扩展-钝化的扩展模式,裂纹形状为锯齿形:随着预变形量的增加,解理方式的裂纹扩展减弱,转变成锯齿形状的裂纹扩展方式。     

α-Fe纯I型裂纹扩展中位错形成过程的三维分子动力学模拟

通过分子动力学方法模拟了三维 α-Fe I型裂纹的单向拉伸实验中的裂纹扩展过程。研究了在不同温度下裂纹扩展时位错的形成过程和断裂机理。计算结果表明,裂纹扩展过程是位错不断发射的过程。 裂纹尖端附近先形成无位错区和层错,当裂纹处应力增加到KI=0.566 MPam1/2时,裂纹尖端附近的某一层原子会逐渐分叉形成两层原子,分层后的原子层继续分离形成位错;当应力KI 达到0.669MPam1/2时第一个位错发射。随着温度的升高,临界应力强度因子逐渐降低,同时位错发射也相应地加快。     

α-Fe纯I型裂纹扩展中位错形成过程的三维分子动力学模拟

通过分子动力学方法模拟了三维 α-Fe I型裂纹的单向拉伸实验中的裂纹扩展过程。研究了在不同温度下裂纹扩展时位错的形成过程和断裂机理。计算结果表明,裂纹扩展过程是位错不断发射的过程。 裂纹尖端附近先形成无位错区和层错,当裂纹处应力增加到KI=0.566 MPam1/2时,裂纹尖端附近的某一层原子会逐渐分叉形成两层原子,分层后的原子层继续分离形成位错;当应力KI 达到0.669MPam1/2时第一个位错发射。随着温度的升高,临界应力强度因子逐渐降低,同时位错发射也相应地加快。     

体心立方Fe中<100>位错环对微裂纹扩展影响的分子动力学研究

在辐照环境下,载能粒子与材料相互作用导致材料中原子移位,造成辐照损伤.其中,由辐照形成的过饱和自间隙原子团簇形成的间隙型位错环,是体心立方Fe为基的材料中常见的辐照缺陷之一,其与材料中其他缺陷之间相互作用,是导致辐照硬化、脆化、肿胀及蠕变等辐照损伤的原因之一.除此相互作用外,在材料表面或内部沿晶界、沉积相、惰性气体形成的气泡所导致的微裂纹,是诱发辐照促进应力腐蚀开裂的重要原因.因此,理解辐照条件下间隙型位错环与微裂纹之间的相互作用,是理解辐照促进应力腐蚀开裂微观机制的重要一步.在本研究中,利用分子动力学方法,模拟了原子尺度微裂纹与间隙型位错环之间的相互作用,研究了位错环与微裂纹之间的距离、相对位置及位错环尺寸对二者相互作用的影响,揭示了位错环对微裂纹是否沿滑移面扩展的影响,发现当二者的相互作用起主导作用时(如在临界水平或垂直距离之内),形成的以100为主或高密度的1/2111位错网络可以抑制微裂纹沿滑移面的扩展.当位错环尺寸发生变化时,只有当位错环位错核与微裂纹尖端相互作用时,才能抑制微裂纹沿滑移面的扩展.这些结果为进一步理解辐照应力开裂提供了新的参考.     

沿〈111〉晶向冲击加载下铜中纳米孔洞增长的塑性机制

用分子动力学方法计算模拟了单晶铜中纳米孔洞（约φ1．3nm）在〈111〉晶向冲击加载过程中的演化及其周围区域发生塑性变形的过程。模拟结果的原子图像如图1所示，其中活塞速度为500m／s，图中所示为4族连续三层穿过孔洞中心的{111}晶面在4000个时间步时（处于拉伸应力状态）的原子排列图像。从面心立方铜晶体中位错成核及运动特点可知，当位错在{111}面上成核和运动后，将产生层错和部分位错结构，我们正是根据此特点来判断在某{111}晶面上是否有位错的成核和运动。从图1可以看到，沿〈111〉晶向冲击加载后，     

α-Fe裂纹的分子动力学研究

通过分子动力学方法，模拟了α-Fe裂纹的单轴拉伸实验中的形变过程.研究了不同晶体取向裂纹的形变特点和断裂机理，观察到各种形变现象，如位错形核和发射，位错运动，堆垛层错或孪晶的形成，纳米空洞的形成与连接等.计算结果表明，裂纹扩展是塑性过程和弹性过程相结合的过程，其中塑性过程表现为由裂尖发射的位错导致的原子切变行为，而弹性过程的发生则是由无位错区中的原子断键所导致.同时还研究了α-Fe裂纹的形变特点和断裂机理与温度场和应力场的依赖关系.     

韧性材料的微裂纹扩展与分叉的晶体相场模拟

采用晶体相场方法研究韧性单晶材料在双轴拉伸条件下微裂纹扩展与分叉的演化过程,分析应变、温度、初始裂口形状等因素对裂纹扩展和分叉的影响.结果表明:对于简单的单向拉伸,应变需要达到一定的临界值,裂纹扩展才会启动.对于二组相互垂直的双轴拉伸作用,当应变达到临界值后,裂纹扩展过程中会发生分叉现象.温度越高,裂纹扩展越快且分叉越多.裂纹在扩展过程中,体系能量不断降低,当裂纹出现分叉时,体系能量降低得更快.在裂纹扩展过程中,有时是会在裂尖处前方出现微小的空洞,类似在裂纹尖端前方出现位错发射情况,这些微小的空洞逐渐扩大连成裂纹.本文所得结果与相关模拟结果和实验结果符合.     

HMX晶体热致相变对损伤的影响

HMX基PBX炸药混合体系中炸药晶体在发生高温熔化和分解反应之前，会率先发生非均匀热膨胀和固相晶型转变，使材料的力学性能和安全性能发生突变。为探究HMX晶体的热致相变对材料内部损伤演化的影响机制，发展了考虑HMX晶体热膨胀和相变等变形机制的热力耦合晶体本构模型，从力学角度揭示了黏结剂包覆HMX晶体相变对体积变形、应力状态以及裂纹成核演化过程的影响机理，量化分析了升温速率对材料相变和裂纹损伤状态的影响规律。结果表明：随着加载温度升高，HMX晶体的热膨胀和β→δ相变导致体积增大，晶体内部形成拉伸应力状态，同时晶体与黏结剂相互挤压形成的局部压剪作用使晶体内部出现裂纹成核和扩展现象。相变温度附近HMX晶体内部裂纹成核和扩展数量显著增加，晶体内部发生不可逆损伤。外界升温速率对晶体内部裂纹形核扩展与损伤造成显著影响，较高的升温速率会加大晶体损伤程度，增加炸药内潜在热点源及意外点火风险。     

高应变率加载下FCC金属晶体取向对孔洞增长的影响

采用率相关的晶体塑性本构模型研究了冲击荷载作用下晶体取向对面心立方金属内部孔洞增长的影响。利用VUMAT子程序,将率相关晶体塑性本构模型嵌入ABAQUS有限元软件中,分析了单晶晶内孔洞、双晶晶界孔洞和三角晶界孔洞的增长行为,结果显示：孔洞的变形模式与晶体取向、晶界位置（冲击加载方向与晶界的相对方位）、加载方向相关,晶体的滑移线模型与晶界位置之间的关系可以反映孔洞增长方向。对于晶内孔洞,加载方向越接近[011],孔洞开始增长变形时间越晚,但孔洞的总体增长变形越大;加载方向越接近[111],孔洞开始增长变形时间越早,但孔洞的总体增长变形越小。对于晶界处孔洞,晶界位置影响孔洞的部分变形,但不会影响总体变形。晶体受冲击之后,若孔洞增长方向沿晶内,则晶界会促进孔洞沿晶内增长;若增长方向沿晶界,则晶界会促进孔洞沿晶界方向增长,抑制其向晶内增长。     

The effect of the location of stage-I fatigue crack across the persistent slip band on its growth rate – A 3D dislocation dynamics study

Abstract

A 3D dislocation dynamics study to ascertain the probable path of stage-I fatigue crack propagation across the persistent slip band (PSB) in austenitic stainless steel is presented. Cyclic plasticity and the resulting crack tip slip displacement (CTSD) are evaluated for cracks of varying length introduced at PSB-center and at two PSB-matrix interfaces. CTSD attains high value at either of the two interfaces irrespective of the proximity of crack front to the grain boundary. Further, a difference in microcrack propagation rate is also observed among the two interfaces. The present results assert microcrack propagation preferrentially along one of the two PSB-matrix interfaces rather than at the PSB-center. A pre-existing PSB dislocation structure localises the cyclic slip for crack lengths up to approximately half of the grain depth for an applied strain range of 2 × 10^?4.     

Crack growth on the basal plane in single crystal zinc: experiments and computations

Crack propagation on the basal planes in zinc was examined by means of in situ fracture testing of pre-cracked single crystals, with specific attention paid to the fracture mechanism. During quasistatic loading, crack propagation occurred in short bursts of dynamic crack extension followed by periods of arrests, the latter accompanied by plastic deformation and blunting of the crack-tip. In situ observations confirmed nucleation and propagation of microcracks on parallel basal planes and plastic deformation and failure of the linking ligaments. Pre-existing twins in the crack path serve as potent crack arrestors. The crystallographic orientation of the crack growth direction on the basal plane was found to influence both the fracture load as well as the deformation at the crack-tip, producing fracture surfaces of noticeably different appearances. Finite element analysis incorporating crystal plasticity was used to identify dominant slip systems and the stress distribution around the crack-tip in plane stress and plane strain. The computational results are helpful in rationalizing the experimental observations including the mechanism of crack propagation, the orientation dependence of crack-tip plasticity and the fracture surface morphology.     

Non-linear dynamical analysis of crack surface perturbations and their dependence on velocity and direction

The fracture surfaces of single crystal [1 0 0] silicon specimens, fractured under three-point bending (3PB) and subjected to a high strain energy upon cracking, revealed exceptional surface perturbations, generated during the unstable propagation. While macroscopically the crack is propagating on the (1 1 1) low energy cleavage plane, microscopic examination revealed small angled deviations from and fluctuations along that plane. Furthermore, while the crack is propagating at a velocity of nearly 3000 m/s in the direction, its velocity in the direction is two orders of magnitude lower, with distinctive surface perturbations. The amplitude and complexity of the perturbations increase as the normal velocity vector changes its direction and magnitude. These perturbations were recorded with a profilometer and analyzed using non-linear dynamical analysis tools. This study provides an opportunity to interpret surface phenomena of one of the most general cases of fracture and to study the effect of major variables on the nature of the perturbations involved, such as the local crack tip velocity and the crystallographic orientations. It is shown that the surface perturbations are chaotic deterministic in nature and can be described by high order non-linear differential equations; the order of the equation varying with the variations of the local velocity and direction.     

Effect of electric current on the migration of hydrogen interstitial atoms in the region of a crack tip in a crystal

The effect of a constant electric current on the migration of interstitial atoms dissolved in a crystal in the region of a tensile crack tip is estimated. The calculation takes into account plastic deformation that is produced in the vicinity of the crack tip in the loaded sample by dislocation motion in active slip planes of the crystal under the action of mechanically and electrically induced shear stresses, Joule heat release, the Thomson effect, and ponderomotive forces and allows for the effect of gas exchange near the crack edges on the evolution of the distribution of interstitial impurity atoms. The time dependence of the stress intensity factor is found for both the cases of the presence and absence of a constant electric current near the crack tip. Numerical calculations are performed for an α-Fe crystal.     

An extensive 3D dislocation dynamics investigation of stage-I fatigue crack propagation

Stage-I fatigue crack propagation is investigated using 3D discrete dislocation dynamics (DD) simulations. Slip-based propagation mechanisms and the role of the pre-existing slip band on the crack path are emphasized. Stage-I crack growth is found to be compatible with successive decohesion of the persistent slip band/matrix interface rather than a mere effect of plastic irreversibility. Corresponding crack tip slip displacement magnitude and the associated crack growth rate are evaluated quantitatively at various tip distances from the grain boundary. This shows that grain boundaries systematically amplify slip dispersion ahead of the crack tip and consequently, slow down the stage-I crack growth rate. The results help in developing an original crack propagation model, accounting for the boundary effects relevant to polycrystals. The crack growth trend is then evaluated from calculations of the energy changes due to crack length increments. It is shown that the crack necessarily propagates by increments smaller than 10 nm.     

Nonparaxial propagation of Hermite-Laguerre-Gaussian beams in uniaxial crystal orthogonal to the optical axis

Analytical expressions for the three components of the nonparaxial propagation of a Hermite-Laguerre-Gaussian （HLG） beam in uniaxial crystal orthogonal to the optical axis are derived. The intensity distribution of an HLG beam and its three components propagating in a uniaxial crystal orthogonal to the optical axis are demonstrated by numerical examples. Although the y and z components of an HLG beam in the incident plane are both equal to zero, they emerge upon propagation inside the uniaxial crystal. Moreover, the beam profile of the x component is relatively stable and the beam profiles of the y and z components have the same evolution law. If the ratio of the extraordinary refractive index to the ordinary refractive index is larger than unity, the beam profile of the HLG beam is elongated in the x direction and generally rotates clockwise. Otherwise, the beam profile of the HLG beam is elongated in the y direction and generally rotates anticlockwise. This research is beneficial to the optical trapping and nonlinear optics involved in the rotation of a beam profile.     

Characteristics of paraxial propagation of a super Lorentz-Gauss SLG₀₁ mode in uniaxial crystal orthogonal to the optical axis

Analytical propagation expression of a super Lorentz-Gauss(SLG) 01 mode in uniaxial crystal orthogonal to the optical axis is derived.The SLG 01 mode propagating in uniaxial crystal orthogonal to the optical axis mainly depends on the ratio of the extraordinary refractive index to the ordinary refractive index.The SLG 01 mode propagating in uniaxial crystals becomes an astigmatic beam.The beam spot of the SLG 01 mode in the uniaxial crystal is elongated in the x-or y-direction,which is determined by the ratio of the extraordinary refractive index to the ordinary refractive index.With the increase of the deviation of the ratio of the extraordinary refractive index to the ordinary refractive index from unity,the elongation of the beam spot also augments.In different observation planes,the phase distribution of an SLG 01 mode in the uniaxial crystal takes on different shapes.With the variation of the ratio of the extraordinary refractive index to the ordinary refractive index,the phase distribution is elongated in one transversal direction and is contracted in the other perpendicular direction.This research is beneficial to the practical applications of an SLG mode.     

Characteristics of paraxial propagation of a super Lorentz—Gauss SLG01 mode in uniaxial crystal orthogonal to the optical axis

Analytical propagation expression of a super Lorentz-Gauss（SLG） 01 mode in uniaxial crystal orthogonal to the optical axis is derived.The SLG 01 mode propagating in uniaxial crystal orthogonal to the optical axis mainly depends on the ratio of the extraordinary refractive index to the ordinary refractive index.The SLG 01 mode propagating in uniaxial crystals becomes an astigmatic beam.The beam spot of the SLG 01 mode in the uniaxial crystal is elongated in the x-or y-direction,which is determined by the ratio of the extraordinary refractive index to the ordinary refractive index.With the increase of the deviation of the ratio of the extraordinary refractive index to the ordinary refractive index from unity,the elongation of the beam spot also augments.In different observation planes,the phase distribution of an SLG 01 mode in the uniaxial crystal takes on different shapes.With the variation of the ratio of the extraordinary refractive index to the ordinary refractive index,the phase distribution is elongated in one transversal direction and is contracted in the other perpendicular direction.This research is beneficial to the practical applications of an SLG mode.     

Effect of plastic deformation at the crack tip in a crystal on the direction of the tip growth under a mixed load

Evolution of plastic deformation at the tip of a wedge-shaped crack in a crystal under planar strain (modes I and II) was calculated for different cleavage planes, easy-slip systems, angles at the wedge tip, and ratios of the external extension and shear loads. Time distributions are obtained for the plastic deformation, the effective shear stress, the stress intensity factor, and the crack growth direction under monotonic load of the crystal up to a specified limit and further relaxation to establishment of equilibrium distributions under a constant external load. Numerical calculations were performed for an α-Fe crystal.     

Transition of deformation mechanisms in nanotwinned single crystalline SiC

ABSTRACT

The ability to experimentally synthesise ceramic materials to incorporate nanotwinned microstructures can drastically affect the underlying deformation mechanisms and mechanics through the complex interaction between stress state, crystallographic orientation, and twin orientation. In this study, molecular dynamics simulations are used to examine the transition in deformation mechanisms and mechanical responses of nanotwinned zinc-blende SiC ceramics subjected to different stress states (uniaxial compressive, uniaxial tensile, and shear deformation) by employing various twin spacings and loading/crystallographic orientations in nanotwinned structures, as compared to their single crystal counterparts. The simulation results show that different combinations of stress states and crystal/twin orientation, and twin spacing trigger different deformation mechanisms: (i) shear localised deformation and shear-induced fracture, preceded by point defect formation and dislocation slip, in the vicinity of the twin lamellae, shear band formation, and dislocation (emission) avalanche; (ii) cleavage and fracture without dislocation plasticity, weakening the nanotwinned ceramics compared to their twin-free counterpart; (iii) severe localised deformation, generating a unique zigzag microstructure between twins without any structural phase transformations or amorphisation, and (iv) atomic disordering localised in the vicinity of coherent twin boundaries, triggering dislocation nucleation and low shearability compared to twin-free systems.