孔隙结构特征及其对岩石力学性能的影响分析

利用三维孔隙模型以及对应的三维网络模型获得的孔隙结构特征,构建孔隙圆盘模型,通过研究岩石孔隙结构对圆盘模型力学性能的影响来研究对岩石力学性能产生的影响。研究结果表明,在孔隙率出现变化时,会对模型的破坏产生影响,其孔隙率的大小,会对破坏力产生的原因有所影响;当孔隙密度出现变化时,参数的大小会对模型拉伸强度产生影响;而孔隙空间位置的变化对模型拉伸强度的影响较小。     

发泡硅橡胶本构模型与力学性能的研究

基于Neo-Hookean与Yoeh超弹性材料本构模型和发泡硅橡胶细观结构,将发泡硅橡胶的孔隙率作为参数引入应变能密度函数和本构方程中,建立了发泡硅橡胶力学性能预测模型,同时采用有限元分析和试验方法,对发泡硅橡胶单轴压缩和简单剪切两种工况下的力学性能进行研究。通过比较理论模型、有限元模拟和试验结果,发现采用基于发泡硅橡胶细观结构建立的理论模型可以有效地描述发泡硅橡胶力学性能。试验研究了加载速率和环境温度对发泡硅橡胶力学性能的影响,为发泡硅橡胶的应用提供依据。     

稀土电解槽石墨内衬氧化腐蚀-应力损伤耦合本构模型

利用Weibull分布函数表征石墨内衬微元强度分布,对石墨内衬氧化腐蚀劣化的力学性质进行研究,建立石墨内衬单轴压缩下力学损伤本构关系。考虑石墨内衬氧化腐蚀作用下孔隙率变化,引入石墨内衬化学损伤变量,得到氧化腐蚀损伤本构关系。根据推广的lemaitre应变等效原理,综合考虑氧化腐蚀-应力损伤耦合作用影响,建立石墨内衬氧化腐蚀-应力损伤耦合本构模型。利用CT显微技术获取受到氧化腐蚀的试样孔隙结构演化图像,通过石墨内衬在不同氧化腐蚀时间下的单轴压缩实验得到应力-应变曲线,检验氧化腐蚀-应力损伤耦合本构模型的合理性。结果表明:模型预测曲线和实验结果基本吻合,能较客观地反映石墨内衬氧化腐蚀下损伤演化特性。     

炭黑聚集体填充橡胶的力学性能分析

采用Digimat软件建立5种不同炭黑聚集形态的多颗粒炭黑聚集体随机分布的三维代表体积单元(RVE)模型,研究橡胶形变、应力集中和炭黑聚集体形态对橡胶复合材料力学性能的影响。结果表明:RVE模型预测结果与试验数据吻合较好;在单轴拉伸的变形场下,炭黑聚集体形变较小,应力集中点主要位于炭黑聚集体与炭黑聚集体相接近的区域;当炭黑体积分数较小且颗粒处于相对理想分散状态下,采用接近三棱柱或二十面体形状的聚集体模型描述炭黑聚集体形态更能真实反映填充橡胶的力学行为。     

碳纤维/环氧树脂层压板孔隙率及力学性能的试验表征

 采用超声C扫描、显微镜分析及图像分析软件对层压板的孔隙含量、分布、形状及尺寸进行了定量表征,分析了孔隙率对纵向拉伸强度、压缩强度和层间剪切强度的影响规律。结果表明,铺层方式影响孔隙的分布;对于两种铺层形式的层压板,孔隙等效直径和纵横比都随孔隙率的增加而增加;随着固化压力的不断减小,孔隙率不断增大,层间剪切强度、纵向拉伸强度和压缩强度不断下降,且铺层方式影响纵向拉伸模量变化。     

静水压下白砂岩的动态力学性能研究

深部岩石在地下高地应力环境的动态力学性能对深部矿山工程的开展有重要影响,因此,其动态力学性能研究具有重要意义。本文以平顶山某煤矿下白砂岩为研究对象,利用三维霍普金森杆装置开展动态力学试验并采用HJC本构模型的白砂岩进行三维霍普金森杆有限元仿真,研究静水压对白砂岩动态力学性能、能量吸收和损伤的影响。结果表明：白砂岩的峰值应力随着静水压的增加而增加,峰值应变随着静水压的增加而降低;静水压可抑制裂纹的扩展,增强白砂岩的强度;HJC本构模型可以较好地模拟白砂岩的损伤失效。预期结果可为深部岩石动态力学性能的研究提供参考,并为深部岩石工程的实施提供理论基础。     

基于PAM-RTM的CFRP平板RTM工艺及孔隙缺陷仿真

树脂传递模塑(RTM)成型过程中不同工艺参数对碳纤维增强树脂基复合材料(CFRP)构件质量和性能有极大影响。为了精准设计RTM成型参数,降低最终构件的孔隙缺陷含量,分析充模过程中两种尺度孔隙率的变化及在构件中的分布规律,基于PAM-RTM软件对CFRP平板模型RTM成型的充模及双尺度孔隙形成进行了仿真模拟。分析了不同树脂黏度、浇注压力对填充时间及宏观和微观孔隙分布、孔隙率的影响规律;在构件中选取了不同位置的横向轴线并对比轴上宏观/微观孔隙的含量及分布特征,分析随着距离浇注口位置变化孔隙率的变化规律。结果表明,在恒压浇注条件下,浇注压力越大,树脂黏度越小,填充时间越短;树脂黏度为0.1 Pa·s、浇注压力为1.5 MPa时,充模时间最小,为13.11 s。宏观孔隙率随浇注压力升高而减小,微观孔隙率则相反。宏观孔隙在开始填充一段时间后形成,孔隙率最终在出胶口达到最大值;微观孔隙则在填充初始阶段开始形成,孔隙率随着填充距离增加逐渐减小。3条横向轴上两种尺度孔隙率的变化趋势相近;浇注压力影响宏观孔隙开始形成的位置及微观孔隙结束形成的位置,并影响填充结束时的最大宏观孔隙率和填充开始时的最大微观孔...     

三元乙丙橡胶/聚丙烯热塑性硫化胶细观力学数值模拟分析(Ⅰ)三元乙丙橡胶含量对细观力学性能的影响

通过建立三元乙丙橡胶(EPDM)/聚丙烯(PP)热塑性硫化胶(TPV)的细观尺寸多颗粒随机分布的RVE模型,研究了分散相EPDM含量对TPV的细观力学性能以及细观尺度应力分布的影响,并对有限元计算的弹性模量(E)与细观力学理论mixing module、Halpine-Tsai计算的E值进行了比较。结果表明,细观尺寸上TPV的应力分布不均匀,在分散相EPDM与连续相PP界面Mises应力值大,其赤道区域处Mises应力值最大;增加分散相EPDM含量能够降低TPV的应力值和E值,尤其当分散相EPDM体积分数增加至40%~50%时,TPV的E值显著降低,有限元计算的E值介于细观力学理论mixing module和Halpine-Tsai计算的E值之间。     

随机短纤维增强复合材料弹性性能数值模拟

能够准确地预测短纤维增强复合材料(SFRP)的等效弹性性能对于实际应用具有重要意义,为了获得SFRP的等效弹性性能,通常采用有限元的方法对SFRP的代表体积单元(RVE)进行均匀化。在本研究中,首先采用随机序列吸附算法数值生成了SFRP的RVE模型,为了实现单胞模型的自由网格划分,选择对单胞模型施加了一般性周期边界条件,得到了SFRP的RVE模型的等效弹性参数。探究了短纤维体积分数对SFRP的等效弹性模量的影响并与Halpin-Tsai方程计算的结果进行了验证,结果表明预测的杨氏模量的的误差很小。通过施加一般性周期性边界条件,可准确的预测出SFRP的等效弹性常数。     

孔隙率对纤维复合材料电学性能和力学性能的影响

复合材料中存在着不同程度的孔隙,严重时对其性能产生很大的影响。本文研究了孔隙率对单向玻璃纤维增强环氧树脂复合材料的电气性能和力学性能的影响。结果表明,单向纤维复合材料的孔隙率对其介电损耗因数体积电阻率(ρυ)、电击穿强度(E_(BD))、层间击穿电压以及45°拉伸强度、剪切强度都有明显影响。     

GF/PET混纤纱复合材料成型工艺条件与材料性能关系的探讨

复合材料的成型参数可影响复合材料的空隙率,而空隙主又对复合材料的层间剪切强度、弯曲强度等力学性能产生显著影响。本文针对这一点探讨连续玻璃纤维增强涤纤轮混纤纱复合材料的成型工艺条件与复合材料的空隙率及力学性能的关系。     

Mechanical behavior of interlocking multi-stepped double scarf adhesive joints including void and disbond effects

Surface topographical effects on the mechanical behavior of interlocking multi-stepped double scarf adhesive joints under tensile load were studied. For this purpose, finite element analysis (FEA) of the joint geometry at 10 different step angles was carried out. In the second stage, the effects of substrate voids and adhesive delaminations on the interfacial strength were studied for the scarf angle of 32.2° by FEA simulation as well as experimentally. For the cases of the missing steps (voids) and delamination (absence of bonding induced by release agent) the ratios of maximum stresses (principal, von Mises, normal, shear and transverse) between the completely bonded and altered (void or delaminated) joints were compared with the failure load ratios for the same joints to interpret the mechanism of failure. The results revealed that except for the normal stress, the maximum stress ratios reach a maximum value and then decrease with increasing scarf angle. FEA analysis with the voids showed that the strength of the joint not only depends on their size, but also on their location in the joint. When the experimental results were compared with the FEA using the stress ratio between the unmodified (completely bonded) and modified (void or disbond) cases, the results indicated that the normal stress dominates the failure behavior of the 32.2° scarf angle joint. Comparison of the experimental results for the void, and disbond cases revealed that the disbond cases can possess higher joint strength in comparison to the void cases. This finding could not be predicted by FEA, and was attributed to the presence of friction at the interface subsequent to delamination.     

Prediction of orthotropic mechanical properties of plain‐weave composites with matrix voids using unit cells at multi‐scales

A numerical procedure for predicting the orthotropic mechanical properties of plain‐weave composites with matrix voids through a combined approach of the representative volume element method and finite element analyses is presented. The representative volume element method was implemented using two unit cells established at different length scales with equation boundary conditions. By considering the presence of randomly scattered voids throughout the matrix induced during the manufacturing process, it was assumed in the simulation that the spatial distribution of matrix voids is completely random. The procedure was exemplified with a glass fiber‐reinforced (plain‐weave fabric) epoxy composite with matrix voids. Sensitivity studies were conducted to quantify the influence of fiber volume fraction and mechanical properties of the constituent phases on the orthotropic mechanical properties of the composite. The numerical procedure, which can be implemented in ABAQUS, is an efficient tool for guiding the design of plain‐weave composites at materials level and also provides effective properties of such composites for the design optimization of engineering structures made of such composite materials. © 2012 Society of Chemical Industry     

Effect of compression process on void behavior in structural resin transfer molding

To realize low void content molded parts, we propose structural resin transfer molding (SRTM) with the compression process. Resin was injected with a gap height between the upper and lower halves of matched metal dies. Mold filling was performed by a squeezing motion of the upper mold die. The usefulness of the compression process was examined by comparison to a conventional SRTM method. Void distribution and bending properties in both processes were compared. The SRTM method with compression was better at driving out voids. The effects of the gap height between the upper and the lower halves of the molds and of the gap closure rate during mold closing after injection on the void distribution were examined. The gap height did not affect the void distribution. When the gap closure rate was high, the voids were expelled towards the end of cavity, whereas voids remained on the top surface of the molded parts using a small gap closure rate. The difference in void behavior due to the gap closure rate was determined by a balance of the permeability between the flow and the transverse directions.     

Statistical properties of interparticle/void distance for amorphous plastics toughening

Statistical properties of interparticle/void distance (ID) for various particle/void and dispersion types are studied in relation with toughening of plastics using computer‐generated three‐dimensional models. Particle/void size groups adopted were either of constant diameter or of log‐normal distribution. Particles/voids were dispersed at uniform‐random or flocculated with multiple clusters. It was found that IDs are (a) of approximately Gaussian distribution for particles/voids of either a constant diameter or a log‐normal distribution, when they are dispersed at uniform‐random, but (b) not of Gaussian distribution for particle/void sizes of bimodal log‐normal distribution, nor for flocculated log‐normal distribution of particles/voids dispersed with multiple clusters. It was also found that the degree of ID uniformity for a single group of log‐normally sized particles/voids is not sensitive to standard deviation of particle/void size. Mixing effect on ID properties using two groups of log‐normally distributed particles/voids with similar mean particle/void diameters was simulated. It was found that, when a significant amount (36 vol %) of particles/voids of a small mean and standard deviation of ID was mixed with a group of particles/voids of a large mean and standard deviation of ID, mean and standard deviation of ID for the mixture were not substantially lower than those of the group of particles/voids of the large mean and standard deviation of ID. It was also found that the degree of ID uniformity for the mixture of the two groups was lower than those of individual groups, indicating that the mixing has deleterious effect on toughening. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4256–4262, 2006     

热压成型工艺对纬编衬经衬纬热塑性复合材料性能影响

通过实验研究了四种不同的热压成型工艺对纬编衬经衬纬热塑性复合材料的空隙率、结晶度及拉伸性能的影响；研究结果表明，随着保温时间的增加，复合材料的空隙率降低，结晶度和强力增加；在压机内冷却的方式比自然冷却方式制成的复合材料空隙率低、结晶度高、拉伸强力大。     

A multiscale modeling for predicting the thermal expansion behaviors of 3D C/SiC composites considering porosity and fiber volume fraction

Coefficients of thermal expansion (CTEs) are an essential design criterion of the three-dimensional carbon fiber reinforced SiC matrix composites (3D C/SiC). Representative volume element (RVE) models of microscale, void/matrix, and mesoscale developed in this work were used to investigate the CTEs of these composites. A coupled temp-displacement steady-state analysis step was created for assessing the thermal expansions behaviors of the composites by applying periodic displacement and temperature boundary conditions. Three RVE models of cuboid, hexagonal and fiber random distribution were respectively established to comparatively study the influence of fiber package pattern on the CTEs at microscale. Similarly, the effects of different void size, locations, and shapes on the CTEs of the matrix are comparatively analyzed by the void/matrix models. The prediction results at mesoscale corresponded closely to the experimental results. The effect of the porosities on the CTEs was studied by the void/matrix RVE models. The voids were effective in lowering the CTE of the 3D C/SiC composites. Furthermore, the effect of fiber volume fractions on the CTE were also taken into consideration. Equal in-plane and out-of-plane CTEs were realized by selecting appropriate fiber volume fractions for the different directions. The multiscale models developed in this work can be used to predict the thermal expansion behaviors of other complex structure composites.     

Processing/performance relationships considering voids in structural reaction injection molding

The causes of void content and its resulting effect on the material properties of structural reaction injection molded (SRIM) composites were investigated. Plaques were molded under different combinations of five two-level factors, and the resulting effect of each factor on void content was determined. The geometry of the glass reinforcement had the largest effect, as woven mats caused double the void content of random mats. Evacuating the mold cavity proved to greatly reduce void content as well. Varying the injection rate and shot size had little effect. Material property testing was conducted on groups of specimens with similar void content. Tension testing revealed no decrease in strength with increasing void content. One million cycles of tension-tension fatigue at 20% of the ultimate strength caused no decrease in strength at any void content, while fatigue at 30% caused premature failure. Impact testing showed that only very high void contents had a substantial effect on performance. Environmental conditioning showed moisture absorption proportional to void content and furthermore significantly reduced the residual tensile strength after a single thermal cycle to 0°C. These results suggest that void content in conjunction with a weak interface may have been responsible for limiting material properties.     

Air‐void characterisation of foam concrete

The pore structure of cementitious material, predetermined by its porosity, permeability and pore size distribution, is a very important characteristic as it influence the properties of the material such as strength and durability. The pore parameter could therefore be a primary factor influencing the material properties of foam concrete and an in depth look into this aspect is required to establish relationships between this and material properties. In order to evaluate these relationships it was necessary to develop parameters to explain and quantify the air‐void structure of foam concrete. This paper discusses the investigations done to characterise the air‐void structure of foam concrete by identifying few parameters and influence of these parameters on density and strength. A camera connected to an optical microscope and computer with image analysis software were used to develop these parameters. It is found that out of the air‐void parameters investigated, volume, size and spacing of air voids have influence on strength and density. Mixes with a narrower air‐void size distribution showed higher strength. At higher foam volume merging of bubbles seems to produce larger voids, results in wide distribution of void sizes and lower strength. Air‐void shape has no influence on the properties of foam concrete.     

Transverse properties of three-phase composites comprising continuous glass fibre,glass microspheres,and epoxy resin

Some transverse mechanical properties of composites comprising uniaxially aligned continuous glass fibres embedded in a glass microsphere-reinforced expoxy resin matrix have been determined from three-point bending experiments. For a constant fibre volume fraction, the transverse modulus, strength, fracture surface energy for crack initiation and work of fracture are investigated as a function of microsphere content and compared with the equivalent two-phase composites without beads. Modification of composite transverse properties are attributed to corresponding changes in matrix properties and comparisons are made with boundary expressions established for two-phase particulate systems. The effects of voids, microsphere size and surface treatment on the properties are also examined.