钢筋混凝土开裂角软化薄膜模型

开裂角介于固定角和转动角之间,是裂缝实际的开展方向角.通过建立平衡方程、协调方程、钢筋和混凝土本构方程、泊松比效应系数方程,提出了钢筋混凝土开裂角软化薄膜模型,该模型考虑了拉应变与压应变的相互作用.本文还给出从应力到应变的计算方法,与传统从应变到应力的计算方法相比,该方法无须进行反复试算,计算简便.通过与8块钢筋混凝土薄板试验结果的比较,验证了所提出开裂角软化薄膜模型及其算法的正确性.     

一种从离散模拟到连续介质弹性模拟的过渡方法

提出了一种从离散分子动力学模拟(MD)到连续介质弹性有限元计算分析(FEA)的过渡方法, 简称MD-FEA方法. 首先通过MD计算获得晶体材料原子的移动位置, 然后根据晶体结构的周期性特征构造连续介质假设下的有限单元变形模型, 进一步结合材料的力学行为本构关系获得应变和应力场. 为了检验MD-FEA方法的有效性, 将该方法应用于详细分析Al-Ni软硬组合两相材料纳米柱体的拉伸变形问题和基底材料为Al球形压头材料为金刚石的纳米压痕问题. 采用MD-FEA方法获得了上述两种问题的应力?应变场, 并将计算结果分别与传统MD方法中通过变形梯度计算的原子应变以及原子的位力应力进行了比较, 详细讨论了用MD-FEA方法计算的应力?应变场与传统MD原子应变和位力应力的区别, 并对MD-FEA方法的有效性及其相较于传统MD方法所具有的优势进行了探讨. 结论显示, MD-FEA方法与传统MD方法在应力?应变变化平缓的区域得到的结果接近, 但在变化剧烈的区域以及材料的表/界面区域, MD-FEA方法能够得到更加精确的结果. 同时, MD-FEA方法避免了传统MD方法中, 需要人为选取截断半径以及加权函数所导致的误差. 另外, 当应变较大时, MD-FEA方法计算的小应变与传统MD方法计算的格林应变存在一定差异, 因此, MD-FEA方法更适合应变较小的情形.      

波动问题的一个逆问题从测得的应变波形建立材料的应力应变应变率关系

这篇文章导出了从应变波形计算材料的应力、应变和应变率关系的方程式。利用这些方程,本文建议了一个鉴别应变率无关材料的方法,指出了仅从测应变波形来得到其它物理量的基本步骤及算得的物理量可能具有的误差。作为应用的例子,我们得到了石灰岩在低压区的应力应变关系。 本上还指出,应力峰值的衰减指数不仅依赖于材料性质,也依赖于加载波形。因而不能用化爆来模拟核爆炸时的应力峰值衰减。     

稳定节点积分伽辽金无网格法的应力计算方法研究

应力计算是基于稳定节点积分的伽辽金无网格法的重要组成部分.该文着重研究稳定节点积分伽辽金无网格法的应力计算方法,对稳定节点积分方法的变分一致条件进行了讨论.证明当节点代表域内的应变采用非局郎光滑应变时,相应的应力在节点代表域内为常数,稳定节点积分伽辽金无网格离散方程是变分一致的.文中提出了三种节点应力计算方法,研究表明,基于位移梯度的节点应力计算方法不满足变分一致性要求,而采用光滑应变的节点应力计算方法和一致形心应力计算方法满足变分一致性要求.典型数值算例的误差分析表明,满足变分一致性不一定确保得到更为精确的结果.而基于光滑应变的一致形心应力计算方法总是较其它两种方法更为精确.     

一个基于塑性的损伤本构模型的参数辨识

研究了一个用于混凝土的基于塑性的损伤模型本构参数辨识问题。把从实验获得的应力-应变曲线与数值计算中获得的应力-应变曲线的差别,作为局部水平上最小二乘法的目标函数。为了求解这个反问题,局部水平上求解损伤弹塑性正问题的子程序被嵌入到本文的反问题的迭代格式之中。灵敏度系数矩阵是通过有限差分方法近似计算得到的。给出的数值计算例子计算了单轴压缩试验结果的参数辨识问题。采用反分析得到的模型参数值,对单向拉伸及三种不同侧压作用下的压缩试验进行了数值模拟。数值结果表明:本文采用的应力反分析计算格式稳定,且具有合理的准确性,数值计算得到的应力-应变结果可以较准确地拟合实验曲线。     

非比例循环载荷下内时本构理论的修正

非比例加载下，应力、应变的求解是个困难的问题。通过修正内蕴时间尺度，本文用内时本构理论给出了计算循环稳定应力、应变滞环的简便方法。这对于低周疲劳寿命的预测是非常必要的     

高应变率拉伸SHB试验设备的研制和测试技术

本文介绍了自行研制的用于高应变率试验的分离式霍布金生拉杆设备，从一维应力波理论出发，详细推导了拉伸和压缩ＳＨＢ方法测量动态应力应变关系的计算公式；并利用本设备对硬铝合金ＬＣ进行了不同高应变率拉伸试验，得到相应的应力应变关系。     

塑性成型弹塑性有限元模拟的静水压力间接计算法

在单元内偏应力导数精度分析的基础上，了偏应力导数佳点逐层积分建立了修正的静水压力间接算法，将该方法应用于金属成型过程的弹塑性有限元模拟中，较好了解决了大步长时应力计算精度低的问题，提高了计算效率，算例表明，在变形总量和应变步长都较大的情况下。用庐方法计算得到的应力和变形载荷仍具有较好的精度。     

非平稳温度场影响下混凝土结构的随机徐变应力

大体积混凝土结构的徐变应力受多种随机因素的影响。文中视混凝土结构内部温度场为非平稳随机过程，对此非平稳温度场采用了时域上简化处理的方法。同时，为进一步考虑材料参数（如徐变度、弹模等）随机性的影响，文中将随机有限元法引入到混凝土结构随机徐变应力的计算中，提出了求解随机徐变应力的初应变随机有限元隐式解法。     

一种计算疲劳寿命的新方法--Neuber局部应力-应变分析法

引言传统的疲劳寿命计算方法是根据构件的名义应力历史和构件的S-N曲线计算寿命。这种方法需要做构件的S-N曲线,且不能考虑构件应力集中部位的真实应力-应变状况。1961年,Neuber提出一个在局部屈服之下计算缺口应力-应变响应的半经验公式,后来这个公式用到疲 ...     

Improvement of the Contour Method for Measurement of Near-Surface Residual Stresses from Laser Peening

A study was conducted to develop a methodology to obtain near-surface residual stresses for laser-peened aluminium alloy samples using the contour method. After cutting trials to determine the optimal cut parameters, surface contours were obtained and a new data analysis method based on spline smoothing was applied. A new criterion for determining the optimal smoothing parameters is introduced. Near-surface residual stresses obtained from the contour method were compared with X-ray diffraction and incremental hole drilling results. It is concluded that with optimal cutting parameters and data analysis, reliable near-surface residual stresses can be obtained by the contour method.     

Separation of principal stresses in dynamic photoelasticity

A new and effective method used to separate the transient principal stresses for dynamic photoelasticity is proposed. This is a hybrid method combining the optical method of dynamic caustics and the boundary element numerical method. Firstly, a modified Cranz-Schardin spark camera is used to record simultaneously the isochromatic fringe patterns of photoelasticity and the shadow spot patterns in the dynamic process. By means of the isochromatic fringe patterns, the difference between transient principal stresses in the whole domain and the principal stresses along the free boundary can be solved. In addition, the method of caustics is a very powerful technique for measuring the concentrative load. Then, the sum of the principal stresses is calculated by the boundary integral equation obtained from the Laplace integral transform of the wave equation. So, the transient principal stresses can be determined from the experimental and numerical results. As an example, the transient principal stresses in a polycarbonate disk under an impact load are resolved. Concurrently published in the Chinese Edition of Acta Mechanica Sinica, Vol. 26, No. 1, 1994     

Analysis of axially symmetrical residual stresses in bars and tubes

A new method has been developed for the determination of residual stresses in a cylinder. Boring out or removing layers from outside induces changes in the length and diameter of the remaining bar. The initial distributions of the stresses are derived from measurements of the length change only. Details of equations required for the calculations are described. The method rests on an assumption that the radial displacement just below a surface is equal to the radial displacement at a new surface after removal of the surface layer of material. This assumption leads to relations between the three residual-stress components. The numerical calculations of these relations agree well with the experimental data for quenched cylinders obtained by using the Sachs method in other investigations. A brief general discussion is given on the equilibrium conditions of the residual stresses determined.     

动态光弹性方法的主应力分离的研究

本文提出了动态光弹性、动态焦散线实验方法同边界元法结合的混合法，并用这种方法解决了动态光弹性主应力分离的问题，首先对现有的多火花高速摄影系统进行了改造，在动态实验过程中，成功地得到了不同瞬时的清晰的动态光弹性的等差线条纹和动态焦散线系列图像，这样，便可提取不同瞬时的边界应力值、全场主应力差值及边界上的外载。继而提出用Ｌａｐｌａｃｅ变换域上的边界元法来求解在冲击载荷作用下二维弹性模型全场的主应力和。最后，以受冲击载荷作用的圆盘为例，进行实验及边界元法计算，得到了分离的主应力场。     

A new analytical-numerical method for calculating interacting stresses of a multi-hole problem under both remote and arbitrary surface stresses

Based on the elementary solutions and new integral equations, a new analytical-numerical method is proposed to calculate the interacting stresses of multiple circular holes in an infinite elastic plate under both remote stresses and arbitrarily distributed stresses applied to the circular boundaries. The validity of this new analytical-numerical method is verified by the analytical solution of the bi-harmonic stress function method, the numerical solution of the finite element method, and the an...     

Analysis of residual stresses at shakedown: A hybrid approach

A new effective method for determining residual stresses in elastic perfectly plastic material has been developed using Melan's shakedown theorem and a modified form of the Haar-Karman principle. The method is validated on test problems where results exist from other sources. The capability of the approach is then demonstrated on the complex problem of residual stresses in railroad rails where both crack nucleation and crack propagation life are affected by the magnitude of these stresses.     

Phase Shifting Full-Field Interferometric Methods for Determination of In-Plane Tensorial Stress

A new method that combines phase shifting photoelasticity and transmission Coherent Gradient Sensing (CGS) is developed to determine the tensorial stress field in thin plates of photoelastic materials. A six step phase shifting photoelasticity method determines principal stress directions and the difference of principal stresses. The transmission CGS method utilizes a standard four step phase shifting method to measure the x and y first derivatives of the sum of principal stresses. These stress derivatives are numerically integrated using a weighted preconditioned conjugate gradient (PCG) algorithm, which is also used for the phase unwrapping of the photoelastic and CGS phases. With full-field measurement of the sum and difference of principal stresses, the principal stresses may be separated, followed by the Cartesian and polar coordinate stresses using the principal stress directions. The method is demonstrated for a compressed polycarbonate plate with a side V-shaped notch. The experimental stress fields compare well with theoretical stress fields derived from Williams solution for a thin plate with an angular corner.     

Consistent Finite Element Calculations of Boundary and Internal Fluxes

The purpose of this paper is to develop a consistent method to calculate the fluxes or the stresses in an incompressible flow, particularly a turbulent flow. A post-processing technique has been developed such that fluxes or stresses are treated as unknown values, which are solved for, weakly, using the finite element method.

The method has been applied to calculate the Reynolds stresses in a turbulent channel flow. As a result of the new consistent method, the Reynolds stresses have shown good agreement with the DNS data confirming the momentum conservation of the finite element method.     

弹性力学问题的一阶有限元解法

求解弹性力学问题的应力时,如果采用常规的位移有限元法,需要先求得单元的节点位移,再经过求导运算得到。为了解决这种求解方式引起的应力精度下降的问题,提出了弹性力学问题的一阶多变量形式,使得应力与位移精度同阶,并推导了弱形式。采用有限元方法,对弹性力学问题给出了一阶解法的二维、三维数值算例,并且将一阶解法的结果与常规位移有限元法的解进行了比较。数值计算结果表明,一阶解法有效提高了应力的精度,并且应力的误差和节点位移的误差具有相同的收敛阶,验证了本文方法的有效性,为提高有限元法的应力精度提供了新的思路。     

A Dynamic Method for the Residual Stress Measurement During Polymer Crystallization

A new dynamic method based on bilayer system is proposed to characterize the residual stress formation during the crystallization of semi-crystalline polyethylene glycol 10000 (PEG10000). The resin is coated on a solid polymeric film to create a filmsubstrate compound. Its temperature field and dynamic deflection are monitored by synchronized optical and thermography cameras. The crystallization kinetics is first characterized from the former information. Then a simple dynamic model is proposed to relate the dynamic deflection with crystallization process. Residual stresses are established and in the range of 0–2.1 MPa. The generation of residual stresses is due to the edge constraints of the cantilever beam and to the increase of viscosity during solidification that allows the polymer to carry tensile loadings. The spherulite impingement is found to be important for this period from a microscopic view. Boundary condition should be well controlled to steer residual stresses. Such method is promising to measure residual stresses at the micro-scale for polymers to be spread on a flexible substrate and can mimic different mechanical situations of interest.