Synergistic effects of anisotropy and image force on TEM diffraction contrast of dislocation loops

Based on column approximation (CA) assumption, many-beam Schaeublin–Stadelmann diffraction equations are employed for simulating the transmission electron microscopy (TEM) diffraction image contrast of dislocation loops within thin TEM foil of finite thickness, and two beam and many beam diffraction conditions are compared. Moreover, the effects of materials anisotropy and free surface relaxation induced elastic fields distortion of dislocation loops on the black-white image contrast are specially focused. It is found that anisotropy has a remarkable impact on the TEM image contrast of dislocation loop, and free surface relaxation induced image forces can change the black-white contrast features when dislocation loops are near TEM foil free surfaces. Thus, in order to make reliable judgment on the nature of defects, effects of free surface and anisotropy should be included when analysing irradiation induced dislocation loops and other type of defects in in-situ electron, proton, heavy-ion irradiation experiments under TEM environments.     

Cyclic loading of beams based on the Prager and Frederick–Armstrong kinematic hardening models

The kinematic hardening theory of plasticity based on the Prager and Frederick–Armstrong models are used to evaluate the cyclic loading behavior of a beam under the axial, bending, and thermal loads. The beam material is assumed to follow non-linear strain hardening property. The material's strain hardening curves in tension and compression are assumed to be both identical for the isotropic material and different for the anisotropic material. A numerical iterative method is used to calculate the stresses and plastic strains in the beam due to cyclic loadings. The results of the analysis are checked with the known experimental tests. It is concluded that the Prager kinematic hardening theory under deformation controlled conditions, excluding creep, results into reversed plasticity. The load controlled cyclic loading under the Prager kinematic hardening model with isotropy assumption results into reversed plasticity. Under anisotropy assumption of tension/compression curve, this model predicts ratcheting. On the other hand, the Frederick–Armstrong model predicts ratcheting behavior of the beam under load controlled cyclic loading with non-zero mean load. This model predicts reversed plasticity under the load controlled cyclic loading with zero mean load, and deformation controlled cyclic loading.     

Identification of sheet metal plastic anisotropy using heterogeneous biaxial tensile tests

The plastic behavior of anisotropic steel and aluminum sheets is identified by combining the results of classical uniaxial tensile tests and heterogeneous biaxial tensile tests on non-standard cruciform specimens specifically designed for obtaining a high sensitivity of strain fields to material anisotropy. The strain fields are measured on the surface of the specimens by means of an image correlation method. The 8-parameter anisotropic yield function proposed by Ferron et al. [1] is adopted for identification. On the one hand, the results of uniaxial tensile tests are analyzed to determine the strain-hardening parameters and yield function parameters related to transverse strain-anisotropy (angular variation of the anisotropy coefficient R) and stress-anisotropy (angular variation of the yield stress σ). On the other hand, strain fields measured in the biaxial tests are used as input data in an optimization procedure that consists of fitting simulated fields with experimental ones in order to determine the material parameters describing the shape of the yield surface in the biaxial stretching range. The identified yield function is validated using experimental data issued from biaxial tests that were not considered during the optimization process.     

Limit strain predictions for strain-rate sensitive anisotropic sheets

The combined effects of material strain-rate sensitivity and anisotropy on necking or “limit” strain predictions are examined for thin sheets with transversely isotropic properties. Various rate dependent constitutive laws based on flow theory and deformation theory of plasticity are considered.The strong effect of material strain-rate sensitivity in increasing the amount of straining prior to localized necking is first emphasized. We then discuss the joint influence of rate dependence and anisotropy on the theoretical limit strains and forming limit curves. Both strain-rate sensitivity and the local shape of the anisotropic yield surface are shown to significantly affect the predicted limit strains.A necking-band bifurcation analysis is also carried out to reveal in an explicit manner the remarkable sensitivity of overall forming limit diagram shapes to the parameters in the anisotropic yield function.     

Implications of warping restraint on statics and dynamics of elastically tailored thin-walled composite beams

Static response and free vibration of elastically tailored thin-walled beams accounting for the warping restraint effect are investigated via an exact solution methodology within the context of a refined beam model. Analytical results obtained from the restrained warping model are compared with those based on its Saint-Venant model counterpart. It is revealed that the beam slenderness and thickness ratio, as well as the elastic anisotropy, considered in conjunction with the warping restraint have profound effects on the static and dynamic response characteristics. It is also shown that even for anisotropic composite thin-walled beams with high slenderness ratios, warping restraint can still be significant, implying the inadequacy of merely considering the geometric aspects in the modeling of anisotropic composite thin-walled beams.     

Prediction of anisotropic material behavior based on multiresolution continuum mechanics in consideration of a characteristic length scale

New advanced materials have received more attention from many scientists and engineers because of their outstanding chemical, electrical, thermal, optical, and mechanical properties. Since the design of advanced material by experiments requires high cost and time, numerical approaches have always been of great interest. In this paper, finite element analysis of anisotropic material behavior has been carried out based on a multiresolution continuum theory. Gurson-Tvergaard-Needleman (GTN) damage model has been applied as a constitutive model at macroscale. Effects of plastic anisotropy on deformation behavior are assessed using Hill??s 48 yield function for anisotropic material and von Mises yield function for isotropic material, respectively. The material parameters for both isotropic and anisotropic damage models have systematically been determined from microstructure through unit cell modeling. The newly proposed linear approximation of local velocity gradient resolved the underdetermined problem of the previous homogenization process. Anisotropic material behaviors of a tensile specimen have been investigated by the proposed multiresolution continuum theory.     

Estimation of surface area and length with the orientator

The orientator is a new technique for the estimation of length and surface density and other stereological parameters using isotropic sections. It is an unbiased, design-based approach to the quantitative study of anisotropic structures such as muscle, myocardium, bone and cartilage. A simple method for the practical generation of such isotropic planes in biological specimens is described. No special technical equipment is necessary. Knowledge of an axis of anisotropy can be exploited to optimize the efficiency. To randomize directions in space, points are selected with uniform probability in a square using various combinations of simple random, stratified random, and systematic random sampling. The point patterns thus produced are mapped onto the surface of a hemisphere. The mapped points define directions of sectional planes in space. The mapping algorithm ensures that these planes arc isotropic, hence unbiased estimates of surface and length density can be obtained via the classical stereological formulae. Various implementations of the orientator are outlined: the prototype version, the orientator-gencrated ortrip, two systematic versions, and the smooth version. Orientator sections can be generated without difficulty in large specimens; we investigated human skeletal muscle, myocardium, placenta, and gut tissue. Slight practical modifications extend the applicability of the method to smaller organs like rat hearts. At the ultrastructural level, a correction procedure for the loss of anisotropic mitochondrial membranes due to oblique orientation relative to the electron beam is suggested. Other potential applications of the orientator in anisotropic structures include the estimation of individual particle surface area with isotropic nucleators, the determination of the connectivity of branching networks with isotropic disectors, and generation of isotropic sections for second-order stereology (three-dimensional pattern analysis).     

机器人辅助激光超声检测系统及参量匹配方法

为解决飞行器复合材料构件的非接触、高精度无损检测问题,提出基于关节型机器人的激光超声检测系统及光声学参量匹配方法。采用有限元方法建立层状复合材料模型,计算分析材料层状各向异性导致激光超声的非对称分布、声束倾斜和畸变特征,结合实验分析得出利用激光超声表征分层的光声学参量匹配方法。在系统设计上,利用1 064 nm波长的Nd:YAG脉冲激光器激励超声波,利用基于光折变效应的双波混合干涉测量系统探测超声信号,激励和探测激光由光纤传导并投射至被检测工件表面,采用精密六轴关节型机器人作为C型扫描装置,建立系统的实验室原型,实现碳/环氧复合材料试样的C型扫描检测,得到试样中模拟缺陷的分布、形状和尺寸特征,验证了检测系统及参量匹配方法的有效性。研究结果表明,研制的机器人辅助激光超声检测系统可以实现碳/环氧复合材料内部直径1 mm以上分层的检测与成像,在飞行器复合材料构件的无损检测方面具有应用前景。     

基于阵元指向性的复合材料曲面结构超声表面契合法研究

采用相控阵超声结合表面契合法(Surface adaptive ultrasonic,SAUL)检测复合材料曲面结构过程中周向缺陷检测分辨力低,给缺陷定量带来困难。为提高SAUL的检测能力,以T700/环氧树脂T形长桁为研究对象,建立弹性各向异性有限元模型,并对照弹性各向同性情况分析其声传播规律,发现弹性各向异性和层间反射共同作用导致肋板处形成明显噪声。在此基础上,基于阵元指向性开展曲面结构成像检测研究,结果表明：引入阵元指向性校正系数对声源脉冲信号幅值进行优化,降低探头频率,可使阵元声场更适应曲面结构,从而减弱两侧肋板反射,提高成像质量。针对周向长度4.5 mm、深1.5 mm的分层缺陷,改进后的SAUL方法对应仿真和试验中周向缺陷长度定量误差较常规SAUL结果分别减小7.4%和13.1%,表明优化阵元指向性可有效改善SAUL周向检测分辨力,提高复合材料曲面结构超声检测缺陷定量水平,具有推广应用价值。     

A model of one-surface cyclic plasticity and its application to springback prediction

This paper presents an elasto-plastic constitutive model based on one-surface plasticity, which can capture the Bauschinger effect, transient behavior, permanent softening, and yield anisotropy. The combined isotropic-kinematic hardening law was used to model the hardening behavior, and the non-quadratic anisotropic yield function, Yld2000-2d, was chosen to describe the anisotropy. This model is closely related to the anisotropic non-linear kinematic hardening model of Chun et al. [2002. Modeling the Bauschinger effect for sheet metals, part I: theory. International Journal of Plasticity 18, 571-95.]. Different with the model, the current model captures in particular permanent softening with a constant stress offset as well as the Bauschinger effect and transient behavior under strain path reversal. Inverse identification was carried out to fit the material parameters of hardening model by using uni-axial tension/compression data. Springback predicted by the resulting material model was compared with experiments and with material models that do not account for permanent softening. The results show that the resulting material model has a good capability to predict springback.     

基于RSM的SiC单晶片表面粗糙度预测及参数优化

通过响应面分析法(RSM)对超声振动辅助金刚石线锯切割SiC单晶体的工艺参数进行分析和优化。采用中心组合设计实验,考察线锯速度、工件进给速度、工件转速和超声波振幅这4个因素对SiC单晶片表面粗糙度值的影响,建立了SiC单晶片表面粗糙度的响应模型,进行响应面分析,采用满意度函数(DFM)确定了切割SiC单晶体的最佳工艺参数,验证试验表明该模型能实现相应的硬脆材料切割过程的表面粗糙度预测。     

Probability of detection simulations to study the influence of surface roughness on the reliability of ultrasonic testing system

The influence of the material’s surface roughness on ultrasonic echo signals has been well studied and documented in the literature. However, these studies were mostly based on the monitoring of absolute value of the echo amplitude, which cannot quantify a defect’s detectability. In this work, the influence of the surface roughness on the Probability of Detection (POD) of a defect is studied using POD simulations to provide quantitative and insightful analysis. Furthermore, POD simulations are used to quantify the influence of surface roughness on the parameters of ultrasonic testing procedure. A steel block having non-metallic inclusions type of defects was inspected by simulations using CIVA software. The inspection used the ultrasonic immersion pulse-echo setup with straight beam and angle beam techniques. Comprehensive POD curves are obtained, discussed and analyzed.     

Applications of Hurst orientation transform to the characterization of surface anisotropy

Fractal-based methods have been used in surface characterization with increasing success over the past years. These methods have been employed to characterize isotropic surfaces but, as yet, little quantitative consideration has been given to the characterization of anisotropic surfaces. In this work, the Hurst orientation transform (HOT) is used to characterize the surface anisotropy and directionality. The calculation of the HOT involves searching all pairs of pixels in a circular region to build a table of maximum differences. From this table, the Hurst coefficients are calculated in many directions and plotted as a function of orientation to reveal surface anisotropy. In this work, two new surface texture parameters, i.e. texture aspect ratio and texture direction, obtained from rose plots of the Hurst coefficients were used in the characterization of surface anisotropy and directionality. Applications of the HOT method to stereo and interferometric images of wear particles and X-ray images of healthy and osteoarthritic joints are also demonstrated.     

Effect of planar anisotropy on wrinkle formation tendencies in curved sheets

Wrinkle formation tendencies in the form of short-wavelength shallow buckling modes are investigated for sheet materials exhibiting planar anisotropy. The critical state for the onset of these short-wavelength shallow modes are determined from plastic bifurcation theory. A local analysis is performed by considering the current deformed state of a sheet element in a doubly-curved, biaxial plane stress state. The planar anisotropy is prescribed using recently proposed anisotropic yield criteria. Parametric studies are performed to assess the effects of the various material and geometric parameters on wrinkling.     

A Parametric Study of Beam Steering for Ultrasonic Linear Phased Array Transducer

All the parameters of a linear phased array may affect the performance and cost of the ultrasonic inspection system. The characteristic of the acoustic field for an array transducer is the most important factor whether the echo information of the inspected area in a specimen can be obtained and utilized effectively, and it is the main basis of designing a phased array. A mathematical model was presented to simulate the characteristic of an acoustic field radiated from an ultrasonic linear phased array on the basis of Huygens' principle. Based on the model, the beam directivity function can be obtained, and the effects of inter-element spacing, number of elements, element width, and transducer center frequency on beam directivity and steerability in a linear phased array transducer are systematically discussed.     

Effects of plastic anisotropy and yield criteria on prediction of forming limit curves

The effects of yield criteria on predictions of the right-hand side of forming limit diagrams (FLDs) are investigated. Predictions of limit strains are determined from an initial imperfection model based on the early work of Marciniak and Kuczynski (1967). Particular attention is placed on the effect of normal plastic anisotropy on limit strains during biaxial sheet stretching. The anisotropic yield criteria investigated in this paper include Hill’s (1948) quadratic criterion, Hosford’s (1979) higher-order criterion, and case 4 of Hill’s (1979) non-quadratic criterion. Several important characteristics of the yield surface shape are discussed and a new parameter that quantifies some of these aspects is introduced. Similar to the work of Barlat (1987), this parameter is based on the relative position of plane strain on the yield surface and can be used to predict the various effects of yield criteria on limit strains. Results indicate that predictions of FLD are very sensitive to selection of yield criteria.     

Anisotropy Effects in Atomic-Scale Friction

The static and kinetic friction experienced by a point mass elastically driven at different angles on surface lattices with square, hexagonal, and honeycomb symmetries are estimated by analytical and numeric calculations based on the Prandtl?CTomlinson (PT) model. Assuming a strong surface coupling, the anisotropy of static friction increases from 3.7 up to 46.3% when the density of packing of the surface atoms is reduced, but this is not the case for kinetic friction, the anisotropy of which is maximal on a square lattice. Although these results have not been supported by accurate experimental verifications so far, the PT model was successfully applied to interpret anisotropy effects in the friction force profiles measured, among other surfaces, on rectangular lattices with complex unit cells and on stepped crystal surfaces.     

基于各向异性皮质骨的钻削温度研究

为了研究钻削用量与钻削温度之间的关系,利用ABAQUS软件建立医用麻花钻和皮质骨的钻削仿真模型。基于皮质骨各向异性的本质属性,分析比较各向同性与各向异性模型,证明各向异性模型更符合真实情况;利用各向异性模型研究钻削温度与转速、进给速度和背吃刀量之间的关系。     

Boundary and thermal non-equilibrium effects on convective instability in an anisotropic porous layer

The effects of boundary and local thermal non-equilibrium on the criterion for the onset of convection in a sparsely packed horizontal anisotropic porous layer are investigated. A two-field temperature model each representing the solid and fluid phases separately is used and the flow in the porous medium is described by the Brinkman extended-Darcy model. The lower boundary is rigid, while the upper boundary is considered to be either rigid or free with fixed temperature conditions at the boundaries. The stability equations are solved numerically using the Galerkin method to extract the critical stability parameters. The influence of local thermal non-equilibrium, mechanical and thermal anisotropy parameters representing the fluid and solid phases is assessed on the stability characteristics of the system. The existing results are obtained as limiting cases from the present study.     

Some aspects of surface roughness measurement

The connection between the profile and surface Power Spectral Densities of a rough surface is investigated, and explicit functional relations between the two are obtained for isotropic surfaces. These relations allow the surface Power Spectral Density (PSD) to be obtained from the PSD of a single profile, for isotropic surfaces. For anisotropic surfaces, it is shown how the surface PSD may be obtained from the cross-spectra of several parallel profiles. Techniques for obtaining these cross-spectra are briefly mentioned.A simple example of an isotropic surface is used to show that the profile PSD may seriously distort the spectral content of the surface roughness by giving undue weight to long wavelengths at the expense of short wavelengths.Questions of filtering of the surface and profile PSDs are discussed for isotropic surfaces, and it is shown that removal of all wavelengths smaller than λ₀ on the surface requires their removal on the profile, but in addition requires some attenuation of all wavelengths on the profile greater than λ₀. The question of which profile filters are admissible in the sense that they give rise to physically realizable surface filters (with 0 ? attenuation ? 1) is also examined, and it is shown that all profile filters involving an infinitely sharp cut-off at some wavelength are inadmissible.Based on an examination of the connection between the surface PSD and various surface statistics of interest, four indices of anisotropy involving the moments of this PSD are developed. It is shown how these indices may be evaluated by means of measurements on five nonparallel profiles.