含缺陷混凝土的弹性模量预测

提出一种新的预测含缺陷混凝土弹性模量的估算方法。将混凝土看作是由砂浆、骨料、界面和缺陷(包括圆形孔隙和币状裂隙)等组成的四相复合材料,建立随机骨料数值模型,运用细观力学数值均匀化方法预测混凝土的有效弹性模量,并与无缺陷情况下的计算结果以及试验结果进行对比,在此基础上进行各影响参数的敏感性分析。结果表明,缺陷的存在会减小混凝土的有效弹性模量;混凝土的有效弹性模量随着骨料或砂浆弹性模量的增大而增大;界面特性参数和骨料最大粒径对混凝土有效弹性模量的影响有限;任意多边形骨料模型比圆形骨料模型对混凝土有效弹性模量的预测更接近试验值。     

A multiscale and multiphysics numerical framework for modelling of hygrothermal ageing in laminated composites

In this work, a numerical framework for modelling of hygrothermal ageing in laminated composites is proposed. The model consists of a macroscopic diffusion analysis based on Fick's second law coupled with a multiscale FE² stress analysis in order to take microscopic degradation mechanisms into account. Macroscopic material points are modelled with a representative volume element with random fibre distribution. The resin is modelled as elasto‐plastic with damage, and cohesive elements are included at the fibre/matrix interfaces. The model formulations and the calibration of the epoxy model using experimental results are presented in detail. A study into the representative volume element size is conducted, and the framework is demonstrated by simulating the ageing process of a unidirectional specimen immersed in water. The influence of transient swelling stresses on microscopic failure is investigated, and failure envelopes of dry and saturated micromodels are compared. Copyright © 2017 John Wiley & Sons, Ltd.     

纤维束增强复合材料的双层次随机扩大临界核模型

针对纤维束增强复合材料, 提出了双层次随机扩大临界核模型。以复合材料制造工艺为基础建立了纤维间距和纤维束中纤维根数的计算模型, 将复合材料中的纤维分为2 个层次: 纤维束和纤维束群, 并提出父核和子核的概念对临界核的构成进行了区分, 用Beyerlein 公式计算纤维束群中纤维束相继失效引起的纤维束的平均应力集中因子, 用Sivasambu 公式来计算纤维束中纤维相继断裂造成的纤维的应力集中因子。然后, 以纤维断裂蔓延的主要模式为基础, 将逐渐增大的无效长度引入纤维束内部, 根据统计学理论推导相应的复合材料破坏概率计算公式。编制了相关程序, 通过该程序分别预测了S 玻纤、E 玻纤、玄武岩纤维无捻单向纤维布增强复合材料试件的拉伸强度。对3 种复合材料板进行了拉伸强度及基体的拉伸和剪切实验, 并对比了预测结果与实验结果。研究结果显示, 直接将实验对象的材料、几何参数代入就能得到与实验结果吻合的预测结果。      

界面相性态对纤维增强复合材料内应力传递的影响

本文用有限元法研究了具有基体裂纹的纤维增强复合材料内的应力传递问题.假设纤维与基体的界面为非理想的,文中运用“弹簧层”模型首先分析了在不同的组分弹性模量比、纤维体积含量与边界约束条件下,界面相性态对复合材料内的应力传递的影响,然后进一步考察了在几种典型的损伤模式下界面附近的应力分布情况.     

A new push‐pull sample design for microscale mode 1 fracture toughness measurements under uniaxial tension

A sample geometry is proposed for performing microscale tensile experiments based on a push‐pull design. It allows measuring mode 1 fracture toughness under uniform far‐field loading. Finite element simulations were performed to determine the geometry factor, which was nearly constant for Young's moduli spanning 2 orders of magnitude. It was further verified that mode 1 stress intensity factor K_I is nearly constant over the width of the tension rods and an order of magnitude higher than K_II and K_III. Notched samples with different a/w ratios were prepared in (100)‐oriented Si by a combination of reactive ion etching and focused ion beam milling. The mode 1 fracture toughness K_I,q was constant with a/w and in average 1.02 ± 0.06 MPa√m in good agreement with existing literature. The geometry was characterized and experimentally validated and may be used for fracture toughness measurements of all material classes. It is especially interesting when a uniaxial, homogeneous stress field is desired, if crack tip plasticity is important, or when positioning of the indenter is difficult.     

弹塑性颗粒物质准静态变形的细观力学行为

本文介绍了对由弹塑性圆球组成的粒度多分散系统进行的轴向准静态加载和卸载的数值模拟成果。首先介绍了基于理论接触力学导出的颗粒弹塑性接触模型,进而阐明了基于颗粒离散元法对颗粒物质准静态变形模拟的基本方法。研究发现:弹塑性球颗粒系统在加载时其轴向刚度对颗粒间产生的塑性变形量及颗粒间的摩擦均很敏感。然而,颗粒间的摩擦对轴向加载时系统产生的水平应力没有很强的影响。数值模拟还提供了颗粒量级上的微力学内部变量的信息。这里引入并介绍了结构异性张量、滑动接触比率以及平均配位数。通过对这些参数的研究表明:在卸载的全过程中,颗粒间相对滑移出现,相互作用的颗粒数减少,系统内部结构产生不可恢复变形。因此,尽管在卸载应力应变曲线的初始段约为线性,但这并不意味着颗粒系统的响应如传统连续力学中假定的那样为弹性。     

Parallel implementation of implicit finite element model with cohesive zones and collision response using CUDA

The aim of this work is to efficiently implement the Park‐Paulino‐Roesler cohesive zone model with the objective of creating realistic high‐resolution simulations of material deformation, fracture, and postfracture behavior. Intrinsically, unstructured meshes can create more realistic fracture patterns in bulk material than structured meshes. Implicit methods, stable for much larger time steps, have greater potential to model both fracture and postfracture behavior without sacrificing speed of execution. Several technical contributions are presented, including (i) GPU‐accelerated implementation of the Park‐Paulino‐Roesler cohesive zone model, (ii) efficient creation of sparse matrix structure, and (iii) comparison of different unloading/reloading relations when using an implicit scheme. A potential‐based collision response scheme was implemented that allows one to model the interaction of fragmented material. Several test simulations are carried out to demonstrate the flexibility of the model and its ability to reproduce different materials under various loading conditions. Benchmarking results show that most of the computational time is spent by the third‐party solver library, meaning that other operations do not require optimization. The library is made available as open source.     

Stress Intensity Factor and Plastic Zone of Auxetic Materials: A Fracture Mechanics Approach to a Chiral Structure Having Negative Poisson’s Ratio

This article summarizes the method of analytical formulation and computational approach of stress intensity factor and plastic zone calculation for auxetic materials, which have negative Poisson’s ratio. A chiral structure-based material is selected as an object of the study due to its popularity. The stress intensity factor is used in combination with the von Mises yielding condition to estimate the plastic zone’s shape and size. The results show that macroscopically the shape of the plastic zone for auxetic material is the same with that of ordinary materials. However, its size is smaller due to the reduction in its Young’s modulus from the solid material of which the auxetic material is made. Microscopically, an auxetic material has its plastic zone shape that is unique to its microstructure. Homogenization theory was convenient to use to bridge between the microscopic and macroscopic models.     

Upscaling of viscoelastic properties of highly-filled composites: Investigation of matrix–inclusion-type morphologies with power-law viscoelastic material response

In this paper, homogenization schemes for upscaling of elastic properties in the framework of continuum micromechanics are extended towards upscaling of viscoelastic material properties. Hereby, the Laplace–Carson transform method is applied to the Mori–Tanaka scheme, the self-consistent scheme, and the generalized self-consistent scheme and solved numerically by the Gaver–Stehfest algorithm. The performance of the so-obtained upscaling schemes is: (i) illustrated for an academic example (a 2-phase composite with Maxwellian-type creep response of the phases) and (ii) assessed considering a polyester matrix/marble dust filler composite with respective experimental data taken from the literature. Hereby, for the investigated range of volume fractions of inclusions, ranging from 29 vol.% to 55 vol.%, and a matrix/inclusion-type morphology, the transformed generalized self-consistent scheme emerged as the most suitable scheme for determination of the effective viscoelastic properties of this highly-filled composite material, resulting in a sound representation of the experimental data.     

Three-dimensional cell model analyses of void growth in ductile materials

Three-dimensional micromechanical models were developed to study the damage by void growth in ductile materials. Special emphasis is given to the influence of the spatial arrangement of the voids. Therefore, periodical void arrays of cubic primitive, body centered cubic and hexagonal structure are investigated by analyzing representative unit cells. The isotropic behaviour of the matrix material is modelled using either v. Mises plasticity or the modified Gurson-Tvergaard constitutive law. The cell models are analyzed by the large strain finite element method under monotonic loading while keeping the stress triaxiality constant. The obtained mesoscopic deformation response and the void growth of the unit cells show a high dependence on the value of triaxiality. The spatial arrangement has only a weak influence on the deformation behaviour, whereas the type and onset of the plastic collapse behaviour are strongly affected. The parameters of the Gurson-Tvergaard model can be calibrated to the cell model results even for large porosity, emphasizing its usefulness and justifying its broad applicability.