高分子材料注塑固化阶段的残余应力分析

在结晶性高分子材料注塑过程的固化阶段，温度分布、材料细观结构和应力应变之间相互耦合，因而其变化规律非常复杂．本文在井上等人考虑材料细观结构变化的金属热加工工艺应力分析［３～６］的基础上，发展了一套用于高分子注塑固化阶段残余应力分析的本构描述和有限元分析方法．在本构模型中，同时考虑了温度变化、结晶和“冻结取向”对变形的贡献．     

Z-pin复合材料细观模型固化残余应力研究

针对植入Z-pin后碳纤维增强复合材料的微观结构，通过施加Z-pin周期性边界约束条件，建立了Z-pin复合材料单层板单胞细观模型．考虑固化过程中树脂体积收缩、弹性模量随固化度变化和纤维因Z-pin进入偏转因素，运用有限单元法计算了单胞结构在固化成型工艺过程中树脂和纤维应力发展和分布，并研究了Z-pin直径和分布密度对单层板面内残余应力的影响．结果表明：凝胶点之前，树脂模量和残余应力很小，凝胶点之后，树脂模量和残余应力增加较快；残余应力分布与纤维偏转有关；Z-pin直径和分布密度增加会使固化残余应力增大．     

Measurement of Applied Stresses and Residual Stresses on a Residual Stress Model by Applying Two Different Loads

Applied stresses on a residual stress model have previously been obtained by measuring the residual stresses and the resultant stresses generated by applying a load. The present paper reports that the applied stresses and the residual stresses on the residual stress model can be obtained by measuring two resultant stresses generated by applying loads of two different magnitudes. In the proposed method, the residual stresses need not be obtained from the residual stress model before applying a load. The residual stress model used to test the proposed method is a circular disk with frozen stresses that is subjected to a diametral compressive load at a certain angle. The applied stresses and the residual stresses on a residual stress model were experimentally and precisely obtained by digital photoelasticity using linearly polarized light.     

Dynamic buckling of stiffened plates under fluid-solid impact load

A simple solution of the dynamic buckling of stiffened plates under fluid-solid impact loading is presented. Based on large deflection theory, a discretely stiffened plate model has been used. The tangential stresses of stiffeners and in-plane displacement are neglected. Applying the Hamilton‘ s principle, the motion equations of stiffened plates are obtained. The deflection of the plate is taken as Fourier series, and using Galerkin method, the discrete equations can be deduced, which can be solved easily by Runge-Kutta method. The dynamic buckling loads of the stiffened plates are obtained from Budiansky-Roth ( B-R ) curves.     

Thermal stresses in one-sided bond repair: geometrically nonlinear analysis

The problem of thermal stresses in an infinite isotropic plate rigidly bonded with an unsymmetrical, polygon-shaped reinforcement is analyzed within large deflection plate theory, using the equivalent inclusion method [J. Thermal Stresses 26 (2003) 457]. For simplicity, the plate assumes to be cooled down uniformly from a stress free temperature. The earlier approach proposed in [J. Thermal Stresses 26 (2003) 457] has been extended in the present paper to include geometric nonlinearity. Extension of the method to address residual thermal stresses associated with curing of the adhesive is also discussed.     

A bending-to-stretching analysis of the blister test in the presence of tensile residual stress

The adhesion of films and coatings to rigid substrates is often measured using blister geometries, which are loaded either by an applied pressure or a central shaft. The measurement will be affected if there are residual stresses that make a contribution to the energy release rate. This effect is investigated using analytical solutions based on the principle of virtual displacements. A geometrically nonlinear finite element analysis is conducted for comparison. Furthermore, the relationships among strain energy release rate, load, deflection, and fracture radius are discussed in detail. Both analytical solutions and numerical results reveal that uniform tensile residual stresses reduce a specimen’s deflection if it experiences plate behavior under small loads. However, this effect becomes negligible when membrane stresses induced by the loading become dominant.     

受面内冲击载荷下加筋板的非线性动态屈曲

分析了受面内冲击载荷下加筋板的非线性弹性动态屈曲．考虑板与筋的膜力，忽略面内位移，运用Hamilton变分原理，得出非线性控制方程，采用双级数形式的挠度假设，由Galerkin方法得到离散方程组，根据B-R准则，判断加筋板的动态屈曲．     

Analysis of residual stresses in cylindrically anisotropic materials

Metal-forming operations leave residual stresses in formed parts due to nonuniform deformation occurring during the process. An exact method of determining the longitudinal, radial and circumferential (tangential) residual stresses in axisymmetric specimens was proposed by Mesnager¹ and further developed by Sachs². The boring-out technique can be complemented by a similar procedure in which strains are measured on the inner surface of the tube when material is removed from the outer surface.The work proposed in this paper extends previous analyses of residual stresses to the case where the material exhibits cylindrical elastic anisotropy, i.e., the principal axes of anisotropy correspond to the longitudinal, radial and circum-ferential directions of the tube. In addition, the present analysis considers the case in which a residual-shear stress, developed by twisting the tube about its axis, exists in the tube. When such shearing stresses are present, the principal axes of the residual-stress distribution are not parallel to the principal axes of the tube.     

A Novel Cutting Strategy for Measuring Two Components of Residual Stresses Using Slitting Method

An exact knowledge of residual stresses that exist within the engineering components is essential to maintain the structural integrity. All mechanical strain relief (MSR) techniques to measure residual stresses rely on removing a section of material that contains residual stresses. Therefore, these techniques are destructive as the integrity of the components is compromised. In slitting method, as a MSR technique, a slot with an increasing depth is introduced to the part incrementally that results in deformations. By measuring these deformations the residual stress component normal to the cut can be determined. Two orthogonal components of residual stresses were measured using the slitting method both experimentally and numerically. Different levels of residual stresses were induced into beam shaped specimens using quenching process at different temperatures. The experimental results were then compared with those numerically predicted. It was shown that while the first component of residual stress was being measured, its effect on the second direction that was normal to the first cut was inevitable. Finally, a new cutting configuration was proposed in which two components of residual stresses were measured simultaneously. The results of the proposed method indicated a good agreement with the conventional slitting.     

存在初应力时光弹性复合材料条纹值标定研究

详细研究了存在初应力时光弹性复合材料条纹值标定问题，提出了存在初应力时圆盘标定ｆＬＴ值的方法。分析用直条试件和圆盘试件进行了实例标定，两种标定结果一致，而且标定试验具有自检功能。     

An Improved Computational Strategy to Estimate Residual Stresses from Crack Compliance Data

The successive cracking (crack compliance) method is a destructive technique aimed at determination of residual stresses in various structural members. The laboratory measurements performed during extension of a crack are followed by a computational analysis. We propose a modification of the numerical approach in order to simplify the method and improve its accuracy. The basic idea of the proposed modification is to approximate plastic strains rather than the residual stresses directly. Furthermore, we use the goal oriented adaptive finite element method that generates optimal meshes for evaluation of strains at specific points.     

加筋板弹性大挠度的冲击响应分析

用半解析的方法分析了横向冲击载荷下加筋板的非线性瞬态响应。考虑膜力的存在 ,忽略筋截面上的剪切应力 ,引入板的应力函数 ,采用离散加筋板模型 ,运用能量原理建立加筋板的动响应控制方程。假设挠度为双级数形式 ,运用迦辽金法 ,将加筋板的动响应方程转化为一个多自由度的动力系统 ,采用数值方法来求解。最后给出了几个模型的计算结果。     

Residual stresses in evaporated AI-Si films

Residual stresses in vacuum evaporated thin Al-Si films were evaluated by measuring the deflection of a thin cantilevered substrate during removal of the film. Post-thermal treatment and thermal cyclings at temperatures between ?269° C and 560° C were also introduced to determine their effects on the residual stresses of the films.     

SEMI-ANALYTICAL AND SEMI-NUMERICAL METHOD FOR DYNAMIC ANALYSIS OF FOUNDATION

A semi-analytical and semi-numerical method is proposed for the dynamic analysis of foundations. The Lamb's solution and the approximate formulae were used to establish the relation of the contact force and deflection between the foundation and soil. Therefore, the foundation can be separated from soil and analyzed by FEM as for the static cases. The plate can be treated as that the known forces are acting on the upper surface, and the contact pressure from soil can be represented as the deflection. So that only the plate needs to be divided into elements in the analysis. By this method, a series of vibration problems, including various shapes and rigidities of foundations, different excitation frequencies, were analyzed. Furthermore, it can be used for the embedded foundation. The numerical examples show that this method has simplicity, highly accurate and versatile. It is an effective method for the dynamic analysis of foundations.     

Vibration analysis of a circular disk tensioned by rolling using finite element method

Summary The paper proposes a method in finite element analysis for estimating natural frequencies of a disk tensioned by rolling, without the use of eigenvalue analysis. The natural frequencies of a disk vary when the localized plastic deformation caused by roll-tensioning induces residual stresses. Tensioning is used for improving the dynamic stability of circular saws; the optimal condition of rolling can be predicted from natural frequency characteristics. In the proposed method, the natural frequencies after rolling are easily estimated from the mode shapes of the disk before rolling and the stress distribution after rolling. The method is based on ideas similar to thermal stress and sensitivity analysis rather than on eigenvalue analysis. The effectiveness of the method is shown by comparing the natural frequency characteristics obtained by this method with those by eigenvalue analysis. Received 18 June 1998; accepted for publication 8 April 1999     

Direct method of solving the boundary-value problem of relaxation of residual stresses in a hardened cylindrical specimen under creep conditions

A direct method of solving a boundary-value problem for a surface-hardened cylindrical specimen affected by a tensile load under creep conditions is proposed. Relations for estimating the kinetics of the stress-strain state in the hardened layer are obtained. The adequacy of the solution is verified by experimental data on relaxation of residual stresses in the hardened layer of a cylindrical specimen made of éI 691 steel at T = 400°C. The calculated and experimental residual stresses are demonstrated to be in good agreement.     

Stress intensity factors and crack initiation directions for ceramic/metal joint

Four points bending tests for Si₃N₄/Cu/S45C joint specimen showed that the bending strengths depend on the residual stresses that originated from joining process. The residual thermal stresses caused an edge sub-interface crack to initiate in the ceramic. The stress intensity factors (SIFs) of the edge sub-interface crack located at distance h from the interface with or without interlayer metal were calculated by the Green's function obtained from a finite element analysis. The crack path at the joint specimen under four points bending loading with the influence of residual stresses was also evaluated by the maximum tensile stress criterion. Finally the effect of residual stress on the crack path was found numerically; the interlayer metal decreases the deflection angle of crack from interface by reducing the residual stress.     

NUMERICAL PREDICTION OF PROCESS-INDUCED RESIDUAL STRESSES IN GLASS BULB PANEL

A numerical simulation model for predicting residual stresses which arise during the solidification process of pressed glass bulb panel was developed. The solidification of a molten layer of glass between cooled parallel plates was used to model the mechanics of the buildup of residual stresses in the forming process. A thermorheologically simple thermoviscoelastic model was assumed for the material. The finite element method employed was based on the theory of shells as an assembly of flat elements. This approach calculates residual stresses layer by layer like a truly three-dimensional calculation, which is well suited for thin pressed products of complex shape. An experimental comparison was employed to verify the proposed models and methods.     

Incremental Computation Technique for Residual Stress Calculations Using the Integral Method

The Integral Method for determining residual stresses involves making surface deformation measurements within a sequence of small increments of material removal depth. Typically, the associated matrix equation for solving the residual stresses within each depth increment is ill-conditioned. The resulting error sensitivity of the residual stress evaluation makes it essential that data measurement errors are minimized and that the residual stress solution method be as stable as possible. These two issues are addressed in this paper. The proposed method involves using incremental deformation data instead of the total deformation data that are conventionally used. The technique is illustrated using an example ESPI hole-drilling measurement.     

Measurement of residual-stress distribution by the incremental hole-drilling method

The hole-drilling method is widely used to measure residual stresses in mechanical components. Recent developments have shown that strains measured on the surface during an incremental drilling can be related to residual-stress distribution. Researchers throughout the world have proposed different calibration methods which lead to more or less accurate results.The present paper discusses different approaches used. A new calibration method is proposed. We also show how finite-element analysis can be used to determine the correlation coefficients. The variation of the strains measured on the surface for each increment is due to, first, the residual stresses in the layer and, second, the change of the hole geometry. Most authors do not consider the latter aspect. Our results show that this causes a significant error in the experimental data. The finite-element method has been used to compute the coefficients for all types of strain-gage rosettes when the hole diameter is predetermined.Another problem of the hole-drilling method is the selection of the drilling tool. Two systems have been studied: ultra-high-speed air turbine and conventional milling machine. The method has been applied on both shot-peened and water-quenched test specimens. The results are successfully compared with the bending-deflection and the X-ray method.