采用显微网格法研究韧性材料平面应力状态下裂尖附近的应变场

用显微网格数字图象处理方法，测量了韧性材料平面应力条件下Ｉ型单边裂纹尖端附近的应变场。实验结果表明：在裂尖两侧存在与裂纹方向夹角约为５０°的大变形带。最大应变位于离裂尖一定距离的大变形带上，随着载荷增加，最大应变的位置逐渐靠近裂纹尖端。应变峰值轨迹线在裂纹顶端呈放射状，载荷增加时，应变峰值的轨迹线位置并不改变。     

氢对体心立方金属裂纹尖端变形过程的影响

利用扫描电镜(SEM)原位拉伸,观察了 Fe-3％Si 单晶体裂尖的变形和断裂过程。用网格法加计算机处理系统测出了裂纹尖端的应变场及氢对应变分布的影响。发现在Ⅰ型载荷下,主裂纹尖端应变呈不对称分布,形变带内应变梯度很大。形变带内应变分布为: ε_(yy)=f(x)exp(-Ar).氢不改变裂纹尖端塑性变形特征,但能明显促进裂纹尖端局部塑性变形的非均匀增加。     

Experimental analysis of crack tip fields in rubber materials under large deformation

A three-nested-deformation model is proposed to describe crack-tip fields in rubber-like materials with large deformation.The model is inspired by the distribution of the measured in-plane and out-of-plane deformation.The inplane displacement of crack-tip fields under both Mode I and mixed-mode(Mode I-II) fracture conditions is measured by using the digital Moire’ method.The deformation characteristics and experimental sector division mode are investigated by comparing the measured displacement fields under different fracture modes.The out-of-plane displacement field near the crack tip is measured using the three-dimensional digital speckle correlation method.     

Three-dimensional crack tip deformation measurement using combined Moiré-Sagnac interferometry

A combined Moiré-Sagnac interferometry method is developed for in-plane (u andv) and out-of-plane (w) surface deformation measurement. The combined optical setup is used to measure three-dimensional crack tip deformations of AI 2024-0 and AI 2024-T4 specimens at room temperature and an inconel 909 specimen at 570°C. Measured displacements near the crack tip region of the AI 2024-T4 specimen are used as input nodal displacements to determine stress intensity factors based on two-dimensional and three-dimensional Jacobian derivative method. The values compare favorably with theoretical calculations. The extent of the three-dimensional crack tip deformation zone is also discussed.     

阶跃载荷作用下弹塑性悬臂梁的变形机制与能量耗散

基于大挠度动力控制方程,应用有限差分离散求解,研究了阶跃载荷作用下弹塑性悬臂梁的动力行为。通过对动力响应早期内力、变形以及能量分布规律的分析,考察了悬臂梁的弹塑性响应模式和变形机制,并与已有的刚塑性分析进行了系统的比较。数值计算表明,阶跃载荷的不同幅值使得梁的响应模式存在较大差异,弹塑性分析肯定了刚塑性理论在处理中载情形的准确性,同时也指出了其在处理低载和高载情形时的缺陷。通过与小变形理论计算结果的比较,指出了考虑大变形效应的必要性,为今后的大变形刚塑性动力分析提出了建议。     

Special rotational deformation as a toughening mechanism in nanocrystalline solids

A theoretical model is suggested which describes the effect of special rotational deformation on crack growth in deformed nanocrystalline ceramics and metals. Within the model, the special rotational deformation (driven by the external stress concentrated near the tip of a mode I crack) occurs in a nanograin through formation of immobile disclinations whose strengths gradually increase during the formation process conducted by grain boundary sliding and diffusion. The special rotational deformation releases, in part, local stresses near the crack tip, thus serving as a toughening mechanism in nanocrystalline materials. The effects of the special rotational deformation on the growth of pre-existent, comparatively large cracks in nanocrystalline metals and ceramics are estimated.     

集中力拉伸楔体大变形理论分析及数值计算

对任意向集中力拉伸用下的橡胶类材料楔体尖端应力应变场进行了渐近分析,并利用有限元与无限元耦合作了数值计算,结果表明大应变情况下,楔体尖端场处于单向拉伸状态,应力主奇异项为ｒ＾－λ,ｒ为变形后至顶点距离,λ依赖于材料本构常数,数值计算与理论解吻合很好。     

Mechanics and fracture of crack tip deformable bi-material interface

Novel interface deformable bi-layer beam theory is developed to account for local effects at crack tip of bi-material interface by modeling a bi-layer composite beam as two separate shear deformable sub-layers with consideration of crack tip deformation. Unlike the sub-layer model in the literature in which the crack tip deformations under the interface peel and shear stresses are ignored and thus a “rigid” joint is used, the present study introduces two interface compliances to account for the effect of interface stresses on the crack tip deformation which is referred to as the elastic foundation effect; thus a flexible condition along the interface is considered. Closed-form solutions of resultant forces, deformations, and interface stresses are obtained for each sub-layer in the bi-layer beam, of which the local effects at the crack tip are demonstrated. In this study, an elastic deformable crack tip model is presented for the first time which can improve the split beam solution. The present model is in excellent agreements with analytical 2-D continuum solutions and finite element analyses. The resulting crack tip rotation is then used to calculate the energy release rate (ERR) and stress intensity factor (SIF) of interface fracture in bi-layer materials. Explicit closed-form solutions for ERR and SIF are obtained for which both the transverse shear and crack tip deformation effects are accounted. Compared to the full continuum elasticity analysis, such as finite element analysis, the present solutions are much explicit, more applicable, while comparable in accuracy. Further, the concept of deformable crack tip model can be applied to other bi-layer beam analyses (e.g., delamination buckling and vibration, etc.).     

Stress-strain field near the notch tip of a rubber sheet

Analized in this paper is the elastostatic field near a notch tip in a rubber-like thin sheet. The asymptotic equations for the notch tip field are derived based on the constitutive relation given by Ref. [1]. Near field solutions are obtained in regions that decreases and increases in size as the notch tip is approached. Their singular character depends on the constitutive parameters as well as the angle of notch that is evaluated numerically. The project supported by the National Natural Science Foundation of China     

橡胶楔体与刚性缺口接触大变形分析

利用Ｋｕｏｗｌｅｓ与Ｓｔｅｒｎｂｅｒｇ提出的非线性弹性大变形应变能函数,对橡胶楔体与刚性缺口接触问题进行大变形渐近分析,推导了楔体尖端场的渐近方程,得到楔体尖端附近的应力应变场及应力的奇异性指数与橡胶楔体角度、刚性缺口角度及材料常数有关的表达式;楔尖附近同一半径上应力分量为常数,同时,利用非线性有限元理论编制了大变形有限元程序,考虑楔体尖端与缺口接触边界条件,计算得到了与分析解一致的结论,当缺口角     

Application of microline/grid methods to temperature dependent microscopic deformation and damage in CFRP laminates

Tensile tests of CFRP symmetric cross-ply laminates are carried out in a scanning electron microscope (SEM), and microscopic interlaminar deformation and damage near the transverse crack tip are visualized by microlines or microgrids printed on the specimen edge surface. The local deformation around the transverse crack tip is observed at 20°C, 80°C, 120°C and 160°C to evaluate the effect of thermal residual stress on the microscopic deformation and damage in the interlaminar region near the transverse crack tip. Temperature dependence of the axial crack opening displacement (COD) is also measured. The displacement field of the specimen edge surface obtained from these experimental results is compared with the theoretical model proposed by Lee, Allen and Harris. The analysis is modified to consider the temperature effect, and a good agreement is obtained between the modified theoretical predictions and experimental results.     

The near tip stress and strain fields in cracked single crystals

The numerical analyses of stationary mathematically sharp Mode I crack in FCC and BCC crystals with elastic-ideally plastic (EIP) and fast hardening saturation (FHS) law are carried out in the present paper. From the calculated results, it is shown that: for the cases of small strain, EIP crystal cracks, the features of concentrated deformation patterns and the stress state in near-crack tip deformation fields are identical to the earlier analytical solutions, but along the angular sector boundaries, there exist narrow complex stress zones. The overall characteristics of deformation patterns for the cases of EIP and FHS are similar. The behaviours of crack tip opening can be characterized by crack-tip-opening-displacement (CTOD). For the case of FHS, finite deformation BCC crystal crack, our calculations are qualitatively in agreement with recent experimental observations. The project supported by National Natural Science Foundation of China     

刚-柔耦合动力学系统的建模理论研究

刚－柔耦合动力学系统的传统的混合坐标方法是零次近似方法,在建模过程中,直接套用的结构动力学的小变形假设,忽略了变形位移的高次耦合变形量．本文对柔性梁建立较零次近似更精确的高次耦合动力学模型,从连续介质力学理论出发,在变形位移中,计及横向位移引起的轴向缩短,导出变形位移的二次耦合量．用一致质量有限元方法对梁进行离散,基于Jourdain速度变分原理导出大范围运动为自由的柔性梁的刚－柔耦合动力学方程．计算了柔性重力摆的角速度和摆端点的横向变形,揭示零次近似模型和耦合模型的刚－柔耦合动力学性质的根本差异．     

橡皮薄膜缺口顶端场的大变形分析

分析了橡皮类薄膜缺口顶端的弹性场，借助于文［１］引入的本构关系得到了缺口顶端场的渐近方程．通过将缺日顶端场划分为收缩角区和扩张角区，得到了顶端场的解．该解表明，奇异特性与本构参数和缺口角度有关，这一结论得到了数值解的证实．     

Fracture investigation at V-notch tip using coherent gradient sensing (CGS)

Local deformation field and fracture characterization of mode I V-notch tip are studied using coherent gradient sensing (CGS). First, the governing equations that relate to the CGS measurements and the elastic solution at mode I V-notch tip are derived in terms of the stress intensity factor, material constant, notch angle and fringe order. Then, a series of CGS fringe patterns of mode I V-notch are simulated, and the effects of the notch angle on the shape and size of CGS fringe pattern are analyzed. Finally, the local deformation field and fracture characterization of mode I V-notch tip with different V-notch angles are experimentally investigated using three-point-bending specimen via CGS method. The CGS interference fringe patterns obtained from experiments and simulations show a good agreement. The stress intensity factor obtained from CGS measurements shows a good agreement with finite element results under K-dominant assumption.     

A combination optical method of lateral shearing interferometry and caustics

A novel experimental technique is presented that combines the optical method of caustics and the full-field lateral shearing interferometer of coherent gradient sensing (CGS) in one setup. Collimated light transmitted through a deformed specimen is separated into two legs. One is processed by the lateral shearing interferometer and is recombined with the other on a single image plane. By adjusting the optical path length of each leg, a combination lateral shearing interferogram/caustic image is generated. When this technique is applied to a fracture specimen, information about the stress intensity factor can be obtained from the caustic and the CGS interference fringes for exactly the same deformation field. This allows for a direct comparison between the two methods when investigating phenomena such asK-dominance, three-dimensional deformation and so on. In this paper, the technique is presented in detail, and its feasibility is tested by application to mode I and mixed-mode crack tip deformation fields.     

Nanoscale rotational deformation effect on dislocation emission from an elliptically blunted crack tip in nanocrystalline materials

A grain size-dependent model is theoretically established to describe the effect of a special physical micromechanism of plastic flow on the dislocation emission from an elliptical blunt crack tip in nanocrystalline solids. The micromechanism represents the fast nanoscale rotational deformation (NRD) occurring through collective events of ideal nanoscale shear near crack tips, which as a stress source is approximately equivalent to a quadrupole of wedge disclinations. By the complex variable method, the grain size-dependent criterion for the dislocation emission from an elliptical blunt crack tip is derived. The influence of the grain size and the features of NRD on the critical stress intensity factors for dislocation emission is evaluated. The results indicate that NRD releases the high stresses near the crack tip region and thereby enhances the critical stress intensity factor for dislocation emission. The NRD has great influence on the most probable angle for dislocation emission. The critical stress intensity factor will increase with the increment of the grain size, which means the emission of the dislocation becomes more difficult for larger grain size due to the effect of NRD.     

阴极充氢对结构钢裂尖形变行为的影响

采用微机控制激光散斑干涉技术，原位研究了结构钢单边缺口预裂纹试样在３．５％Ｎａｄ水溶液中，－１４００ｍＶ（ＳＣＥ）电位下阴极完氢６小时前后，裂尖材料形变行为的变化。同时，还对充氢与未充氢条件下光滑圆柱试样的应力－应变行为进行了对比研究，用扫描电镜观察了断口形貌。结果表明：由于氢的进入，使材料的加工硬化程度加剧，从而导致了变形困难。     

非定常热传导对弹-粘/塑性变形和断裂的红外图像分析

本文使用CCD像机、热电偶和红外摄像机对有裂纹平板试样,在单轴拉伸实验中对裂纹尖端附近弹-粘/塑性变形,裂纹的发生、进展,裂纹开口位移和试样表面温度进行了测量.并且用非定常热传导理论对弹-粘/塑性变形和断裂过程中的温度效应进行了解析(FEM).结果显示对于铁合金在不同的应变速率下,用非定常热传导理论解析得到的裂纹尖端附近温度分布与红外图像非常接近.     

地磁环境下结构变形引起的扰动磁场

基于磁弹性相互作用的线性化理论，针对地磁环境特点，忽略磁场对结构位移的影 响而仅考虑位移对磁场的耦合作用，通过将位移梯度引入变形后界面外法线矢量方 法，给出了变形引起的扰动磁场计算的定解场方程和界面衔接条件，并计算了符拉 芒问题的变形扰动磁场. 结果表明，变形扰动磁场完全取决于位移梯度在界面外法 向上的投影；扰动场强与外力成正比；空气中扰动磁场的法向强度和切向强度分布 特征明显不同，法向强度在力作用点对称分布，并且在 力作用点处存在奇点；切向强度则呈反对称分布，在力作用点处发生突变.