The micromechanics and microstructure of CO2 crazes in polystyrene

Although CO₂ at 1 atmosphere pressure is not a crazing and/or cracking agent for polystyrene (PS), we have established that it becomes one at higher pressure. Crazes grown from cracks in PS thin films in high pressure CO₂ are investigated using transmission electron microscopy (TEM). The fact that broken craze fibrils retract strongly upon exposure to high pressure CO₂ gas suggests that the primary effect of the CO₂ is plasticization, not surface energy reduction. Quantitative analyses of TEM micrographs of crazes grown at CO₂ pressures in the range 5 to 100 MPa at 34°C and 45°C have been carried out to find the craze fibril volume fractions v_f(x) and the surface displacements w(x) along each craze. From the fibril volume fraction profile along the craze, the dominant craze thickening mechanism of CO₂ crazes is shown to be the same as that for air crazes, i.e. the surface drawing mechanism, and not the fibril creep mechanism. The craze surface stress profile is computed from the craze surface displacements using a distributed dislocation analysis. These profiles all show a stress concentration at the craze tip which falls to a roughly constant value σ_b, over the rest of the craze. The fracture toughness G_Ic (and critical stress intensity factor K_Ic) for propagation of a crack in PS at these CO₂ pressures can also be computed. All these quantities (V_f, σ_b, G_Ic and K_Ic) show pronounced minima as a function of CO₂ pressure at 20 MPa, the same CO₂ pressure at which T_g of the polymer also reaches a minimum. These minima are more pronounced at 45°C than at 34°C. The G_Ic's and K_Ic's are depressed by orders of magnitude at the minimum, which corresponds to the qualitative observation that CO₂ becomes a severe cracking agent at these pressures. These observations provide additional confirmation that the major mechanism for the environmental crazing and cracking of PS by CO₂ is plasticization of the craze fibrils and surfaces.     

On the Barzilai-Borwein basic scheme in FFT-based computational homogenization

Building upon the equivalence of the basic scheme in the work of Moulinec and Suquet with gradient descent methods, we investigate the effect of using the celebrated Barzilai-Borwein step size selection technique in this context. We provide an overview of recent convergence theory and present efficient implementations in the context of computational micromechanics, with and without globalization. In contrast to polarization schemes and fast gradient methods, no lower bound on the eigenvalues of the material tangent is necessary for the Barzilai-Borwein scheme. We demonstrate the power of the proposed method for linear elastic and inelastic large scale problems with finite and infinite material contrast.     

Lippmann-Schwinger solvers for the computational homogenization of materials with pores

We show that under suitable hypotheses on the nonporous material law and a geometric regularity condition on the pore space, Moulinec-Suquet's basic solution scheme converges linearly. We also discuss for which derived solvers a (super)linear convergence behavior may be obtained, and for which such results do not hold, in general. The key technical argument relies on a specific subspace on which the homogenization problem is nondegenerate, and which is preserved by iterations of the basic scheme. Our line of argument is based in the nondiscretized setting, and we draw conclusions on the convergence behavior for discretized solution schemes in FFT-based computational homogenization. Also, we see how the geometry of the pores' interface enters the convergence estimates. We provide computational experiments underlining our claims.     

Raman microscopy of fibres and composites

Some of the progress made in using Raman microscopy to examine the structure and micromechanics of polymer and inorganic fibres is reviewed. Examples of how the technique has been used to verify some of the theories of composite reinforcement are then given. Finally, a specific example of how Raman microscopy has been used, in conjuction with other approaches, to understand the micromechanics of the microbond test, which is often used to measure interfacial shear strength in composite systems, is presented.     

金属芯压电纤维/聚合物复合材料压电-黏弹-塑性行为的细观力学模型

为了表征金属芯压电纤维增强聚合物基（MPF/PM）复合材料非线性、时变的压电-黏弹-塑性行为,基于变分渐近理论建立MPF/PM增量形式的细观力学模型。首先分别导出聚合物和MPF增量型本构方程,基于汉密尔顿扩展原理推导出MPF/PM压电-黏弹-塑性变分原理的能量泛函。考虑材料的时变和非线性特征,建立与求解瞬时有效机-电耦合矩阵有关的增量过程,并通过有限元技术实现模型的数值模拟。利用构建模型研究了不同铝芯体积分数、电场变化率和加载条件对MPF/PM有效全局应力-应变和单轴纵向拉伸性能的影响。结果表明,构建的模型能准确模拟MPF/PM多场耦合作用下的非线性、时变行为,为该新型智能材料的实际工程应用奠定理论基础。     

短纤维增强SiC／Al复合材料在高温下粘弹性拉伸性能的细观力学分析

本文采用细观力学模型和有限元法研究短纤维增强ＳｉＣ／Ａｌ复合材料在高温下的粘弹性行为，着重讨论了纤维体分比和长径比对复合材料总体蠕变性能的影响。结果表明，随着纤维体分比和长径比的增加，纤维能显著抑制复合材料沿轴向蠕变行为。     

真三轴条件下的岩石细观损伤力学模型

本文提出了真三轴条件下的岩石细观损伤力学模型，建立了岩石的损伤演化方程，给出了损伤柔度的求解公式。数值分析表明中主应力对岩石应力－应变关系有明显的影响，一般表现为随中主应力的增加，最大主应力方向的变形减小，最小主应力方向的变形增大；但当最大主应力很大时，大的中主应力反而使最大主应力方向的变形增加，最小主应力方向的变形减小。     

BCC金属物理型动态本构关系及在钽中的应用

构建了一种新的、基于塑性变形物理机制的BCC金属动态本构关系,然后将其应用于军用材料钽,通过采用新的约束条件下的多变量非线性规划方法,结合有关流应力实验数据确定出了多晶钽的本构模型。计算结果表明,该模型与实验数据吻合较好,而与Z-A本构模型相比其准确性得到提高,与NN-I本构模型相比预测能力接近,但实用性更强。另外,验证可知该模型的适用条件可以有效外推至很宽的温度及应变率范围。     

考虑界面影响的混凝土弹性模量的数值预测

提出了一种考虑界面过渡层影响的混凝土弹性模量的数值预测方法。将球形骨料与包裹它的界面过渡层作为二相复合球结构的等效颗粒,由广义自洽方法计算不同粒径骨料与界面过渡层组成复合球的有效模量。然后由等效颗粒生成的随机骨料模型建立体积表征单元,施加均匀位移边界条件,通过数值方法计算该体积表征单元中的应力和应变场,由细观力学数值均匀化方法预测体积表征单元的有效弹性模量。计算结果表明:对于不同骨料含量的混凝土,有效弹性模量的预测值与试验值非常接近,界面过渡层的厚度对混凝土的整体弹性性质有较大影响。     

非圆截面纤维增强复合材料的弹塑性性能

基于Tandon和Weng(1988)建立的理论框架,应用细观力学方法,本文提出了非圆截面纤维增强复合材料的弹塑性理论。对于两种复合材料,即单向排列纤维形成的正交各向异性材料和在截面内随机排列纤维形成的横观各向同性材料,作出了割线模量,基体应力增大系数及屈服条件曲线。通过各种截面纤维之间的比较,得出结论:在横观各向同性材料情况下,片状纤维增强材料的力学性能优于其它材料。