缝合复合材料层板面内局部纤维弯曲模型及剪切强度预报

提出了面内局部纤维弯曲模型,基于有限元法和周期性边界条件建立了缝合层板面内剪切强度分析方法,采用桥联模型和最大应力判据分析损伤扩展并获得面内剪切强度,预报结果与试验吻合较好,探讨了缝合参数对层合板面内剪切强度的影响规律,结果表明缝合削弱了层合板的面内剪切强度,缝合针距和行距越大对面内剪切强度越有利,较细的缝合线对面内剪切强度有利.     

基体开裂的正交复合材料对称层合板的粘弹性力学行为

本文讨论了含有均匀基体裂纹的正交复合材料对称层合板的线性粘弹性力学行为.采用二维剪切滞后模型并对其层间剪应力在厚度方向进行线性假设分布,求得层合板的平均应力应变的线弹性解,利用等效约束模型和经典层合理论可得到层合板因为含有基体裂纹而所引起的刚度退化现象.在弹性-粘弹性对应原理的基础上对其层合板的线粘弹性进行了讨论和研究.结果表明:层合板的松弛模量和蠕变泊松比随着时间的增加而减少,到达稳态后其值基本上是恒值.并跟Zocher的解析解和有限元数值解作了比较,发现结果非常吻合.     

微粘结实验影响因素的边界元法分析

微粘结实验是一种新的简便的测量复合材料界面剪切强度的方法，本文运用数值方法－边界单元法，对一个实际试件进行了界面应力计算，讨论了材料常数，刀口宽度和新月尖角三种主要因素对界面应力分布可能产生的影响，从而为实验结果分析，实验方法改进和评估提供依据。     

Interlaminar hygrothermal stresses in laminated plates

Within the elasticity formulation the most general displacement field for hygrothermal problems of long laminated composite plates is presented. The equivalent single-layer theories are then employed to determine the global deformation parameters appearing in the displacement fields of general cross-ply, symmetric, and antisymmetric angle-ply laminates under thermal and hygroscopic loadings. Reddy’s layerwise theory is subsequently used to determine the local deformation parameters of various displacement fields. An elasticity solution is also developed in order to validate the efficiency and accuracy of the layerwise theory in predicting the interlaminar normal and shear stress distributions. Finally, various numerical results are presented for edge-effect problems of several cross-ply, symmetric, and antisymmetric angle-ply laminates subjected to uniform hygrothermal loads. All results indicate high stress gradients of interlaminar normal and shear stresses near the edges of laminates.     

Development of a portable shear test fixture for low modulus porous (foam) materials

The shear properties of brittle and highly porous carbon (graphitic) foam cannot be measured reliably with most standard test methods, such as single rail, double rail, Iosipescu shear, etc. A new testing device has been developed to accurately measure the shear stiffness and strength of carbon foam or other porous materials. Specimens of cylindrical cross section are used to reduce the high stress concentration that normally occurs in the vicinity of the grip section. Since strain gages could not be installed on the specimen surface (due to porosity), the shear strain is determined from the specimen end rotation. A high resolution in the rotational measurement is achieved by using a stepper motor with multiple gear reduction. In view of testing low modulus material, the load cell of the fixture was mounted onto an axial roller to relieve the axial constraint while twisting the specimens. The accuracy of the measurement and calibration of the test fixture has been demonstrated by measuring the shear modulus of two plastic (polyvinyl chloride (PVC) and urethane).     

Fracture of composite laminates containing cracks due to shear loading

The responses of graphite/epoxy [0/90/±45] _s , [±45]_2s , [0/90]_2s and [0/±45]_2s composite laminates with and without center cracks were studied under shear loading using the three-rail shear test. The shear stress/strain relationship, the failure mechanisms and the notched strength were analyzed. Substantial amounts of local buckling were observed in some of the laminates. The present paper shows that shear modulus can be determined accurately using the three-rail shear test with proper interpretation of data. Using the minimum strength model, only one characteristic length was needed to predict accurately the notched strength of a composite laminate under shear and tensile loadings.Paper was presented at the 1987 SEM Spring Conference on Experimental Mechanics held in Houston, TX on June 14–19.     

First-ply failure of laminated composite plates

Composite laminates offer superior load carrying capacity. Reliable application of structures requires a knowledge of their stress/strain and failure behavior. past treatments involved assumptions in both the stress and failure analyses; they become increasingly more difficult when the failure of the microstructure constituents is to be included in the continuum analysis of the laminates. Recognizing the conventional failure criteria used for composite material analyses, this work adopts the first-ply failure criterion by application of a polynomial function and the finite element procedure.The laminates are modeled by the Reissner-Mindlin plate theory that accounts for moderate rotation. This is because shear effects are more pronounced in composite laminates whose transverse shear modulus is low relative to the Young's modulus. Failure loads are obtained for different laminate thicknesses, stacking sequences and aspect ratios and different failure criteria. The results show that predictions made from the maximum stress criterion are nearly the same as the others, except for those obtained by the Hill criterion.     

Experimental study and analysis of RC beams strengthened with CFRP laminates under sustaining load

Six reinforced concrete beams strengthened in flexure using carbon fiber reinforced polymer (CFRP) laminates subjected to different sustaining loads were tested. The main goal of the test is to examine the effects of initial load and load history on the ultimate strength of strengthened reinforced concrete beams by externally bonded CFRP laminates. The main experimental parameters include different levels of sustaining load at the time of strengthening and load history. To explain the experimental results in quantitative terms, a theoretical model for flexural behavior of the strengthened reinforced concrete beam is also developed. Test results in the current study show that sustaining load levels at the time of strengthening have important influence on the ultimate strength of strengthened reinforced concrete beams. If the initial load is basically same, the ultimate strength of reinforced concrete beams strengthened with CFRP laminates is almost same regardless of load history at the time of strengthening.     

含铺层拼接层合板的剪切强度

对含铺层拼接的碳纤维增强树脂基复合材料层合板进行了剪切强度实验研究.从三种不同铺层拼接角层合板上切取含缺口的剪切试件,通过实验测定了其载荷-位移曲线,得到了剪切强度值.实验结果表明,三组试件的剪切强度基本相同,即拼接层角度改变几乎不会引起层合板的剪切强度发生明显变化.采用有限元软件ABAQUS6.5对实验过程进行模拟,得到拼接层角度改变将引起拼接层中0°层出现切应力集中,但沿缺口切应力的平均值几乎不变.这也说明拼接层角度的变化几乎不影响层合板的剪切强度.     

Assessment of composite failure and ultimate strength without experiment on composite

This paper attempts to estimate the ultimate strength of a laminated composite only based on its con- stituent properties measured independently. Three important issues involved have been systematically addressed, i.e., stress calculation for the constituent fiber and matrix materials, failure detection for the lamina and laminate upon the internal stresses in their constituents, and input data determination of the constituents from monolithic measurements. There are three important factors to influence the accuracy of the strength prediction. One is the stress concentration factor （SCF） in the matrix. Another is matrix plasticity. The third is thermal residual stresses in the constituents. It is these three factors, however, that have not been sufficiently well realized in the composite community. One can easily find out the elastic and strength parameters of a great many laminae and laminates in the current literature. Unfortunately, necessary information to determine the SCF, the matrix plasticity, and the thermal residual stresses of the composites is rare or incomplete. A useful design methodology is demonstrated in the paper.     

Dynamic compressive behavior of thick composite materials

The effect of strain rate on the compressive behavior of thick carbon/epoxy composite materials was investigated. Falling weight impact and split Hopkinson pressure bar systems were developed for dynamic characterization of composite materials in compression at strain rates up to 2000 s^–1. Strain rates below 10 s^–1 were generated using a servohydraulic testing machine. Strain rates between 10 s^–1 and 500 s^–1 were generated using the drop tower apparatus. Strain rates above 500 s^–1 were generated using the split Hopkinson pressure bar. Unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the longitudinal and transverse directions, and cross-ply laminates were characterized. The 90-deg properties, which are governed by the matrix, show an increase in modulus and strength over the static values but no significant change in ultimate strain. The 0-deg and cross-ply laminates show higher strength and ultimte strain values as the strain rate increases, whereas the modulus increnases only slightly over the static value. The increase in strength and ultimate strain observed may be related to the shear behavior of the composite and the change in failure modes. In all cases, the dynamic stress-strain curves stiffen as the strain rate increases. The stiffening is lowest in the longitudinal direction and highest in the transverse direction.     

缺口复合材料层合板疲劳性能的试验研究

对带各种缺口的复合材料层合板的疲劳性能进行了研究。设计了不同类型的制品试件,分别进行静态和疲劳试验。试验数据表明,缺口对复合材料的静强度和静刚度的影响不大,缺口试样的疲劳寿命带高于缺口试样,与金属材料相比,复合材料对缺口不敏感。采用条状试样获得的力学性能不参代表复合材料的真实性能,实现表明自由边的缺口对结构提高疲劳寿命是有利的。文中还对疲劳过程中复合材料的刚度变化进行了研究,对文中所研究的层合板,     

Mechanical Characterization of Angle-Ply Composite Laminates by Differential Moiré Method: The Influence of Specimens' Length-to-Width Ratio

The influence exerted by the specimen proportions on the measured performance of angle-ply [+45°/–45°]_s composite laminates is studied. Three kinds of specimens are considered, having different length-to-width ratios, made with glass/epoxy unidirectional or fabric laminae. The static tensile characteristics are measured by means of load cells, electric strain gauges and through the Moiré fringes method. The strain fields are analysed and the effects of stress concentration at the laminate-free edges are evaluated, as well as their influence on the strain distribution and the measured strength and stiffness. The results are summarized as diagrams of ultimate load and elastic modulus versus length-to-width ratio of the different specimens investigated. These diagrams allow the designer to evaluate the influence exerted by specimen proportions on the measured performance, as well as to compare characteristics which are heterogeneous, as obtained with different kinds of specimen.     

Interlaminar shear strength of cloth-reinforced composites

A test specimen to define the interlaminar shear strength of cloth-reinforced composite materials is developed. The specimen is a hollow circular cylinder which is subjected to torsion. The experiment is employed for the determination of the warp-normal shear strength of a graphite-fiber carbon-matrix composite material. It is demonstrated that the proper failure mode takes place, while the problems associated with the use of strain gages on this porous material, nonlinearity, difference in tension and compression properties, and the influence of clamping effects are all discussed. The proposed experiment appears to be ideally suited to study the interlaminar shear response of cloth-reinforced composites.     

缝纫复合材料层合板面内弹性模量分析

基于对缝纫孔附近局部细观结构的分析,提出了一种预测缝纫复合材料层合板面内力学性能的理论模型．从分析缝孔单胞的纤维弯曲几何特征入手,最终得到单向板及层合板的弹性常数．通过有限元分析研究了缝纫参数对复合材料层合板面内等效模量的影响．研究结果表明,缝纫造成单向板及层合板面内材料性质的不均匀,随着缝纫密度和缝纫线直径的增加,层合板的等效模量逐渐降低．     

复合材料面板夹杂粘弹性阻尼材料结构的弯曲实验分析

对单层板的轴向、横向拉伸强度及粘弹性材料的拉伸强度进行测定以确定其弹性模量及泊松比。为了同理论结果比较,制作了专用夹具,用于测量复合材料方板的实验数据。该实验装置模拟四边固支边界条件,用于测量复合材料夹杂粘弹性阻尼材料结构受均布载荷时的静态挠度和应变,并通过转换公式得到复合结构下表面中点的正应力。实验结果同理论分析基本吻合,其误差主要由单层板与粘弹性阻尼材料的粘结剂厚度引起。     

Analysis of progressive fiber debonding in elastic laminates

The evolution of fiber debonding, and sliding, in fibrous laminates is modeled by a coupled micro/macro-mechanical analysis scheme. The laminates under consideration have a symmetric layup, and are subjected to mechanical loads. The individual plies are elastic, have a unidirectional reinforcement, and can suffer local damage at the fiber/matrix interface when the resolved normal and shear stresses exceed their ultimate magnitudes. The local fields in the plies are assumed to be periodic, and are approximated by the finite element method for overall loads and local resolved stresses that are in excess of the interface strength. Local effects in the individual plies are scaled up to the laminate analysis through stress transformation factors, which are a function of the elastic properties of the plies and their stacking configuration.The proposed analysis was implemented for a periodic array model of the laminas, and for in-plane loading of the laminate. The model predictions for a unidirectional steel/epoxy system subjected to transverse loading compare remarkably well with experimental measurements. This result, and several other examples given for axial and off-axis loading of SiC/CAS laminates, illustrate the model capabilities in predicting the overall strains in the presence of simultaneous, progressive debonding in the individual plies.     

Experimental strain analysis of the losipescu shear test specimen

An experimental strain analysis of the losipescu shear test specimen was performed, utilizing a 20-ply AS4/3501-6 carbon/epoxy unidirectional composite. Using three-element strain-gage rosettes, it was shown that the presence of loading-point-induced transverse normal strains in the gage section do not affect the measured shear strain. Thus, the shear modulus determined using the standard notch specimen is not affected. Likewise, modulus determination is not influenced by cracking at the notch tips, since this occurs at strains beyond the range over which modulus is determined. To further evaluate the effect of notch-tip cracking, material was removed adjacent to the standard V-notches where these cracks initiate. The measured shear strength was unaffected by removing this material, although the shear modulus was reduced slightly (by as much as eight percent for the more grossly exaggerated geometries). E.Q. Lewis, former graduate student, is now Engineer, Lockheed Corporation.     

Effects of Environmental Degradation on Flexural Failure Strength of Fiber Reinforced Composites

Fiber-reinforced composite laminates are often used in harsh environments that may affect their long-term durability as well as residual strength. In general, environmental degradation is observed as matrix cracking and erosion that leads to deterioration of matrix-dominated properties. In this work, cross-ply laminates of carbon fiber reinforced epoxy were subjected to environmental degradation using controlled ultraviolet radiation (UV) and moisture condensation and the post-exposure mechanical properties were evaluated through elastic modulus and failure strength measurements. Additionally, both degraded and undegraded were subjected to cyclic fatigue loading to investigate possible synergistic effects between environmental degradation and mechanical fatigue. Experimental results show that the degradation results in reduced failure strength. Greater effects of degradation are observed when the materials are tested under flexural as opposed to uniaxial loading. Based on strength measurements and scanning electron microscopy, we identified various damage modes resulting from exposure to UV radiation and moisture condensation, and cyclic loading. The principal mechanisms that lead to reduction in mechanical properties are the loss of fiber confinement due to matrix erosion, due to UV radiation and moisture condensation, and weakened/cracked ply interfaces due to mechanical fatigue. An empirical relationship was established to quantify the specific influence of different damage mechanisms and to clarify the effects of various degradation conditions.     

基于双剪统一强度理论的复式钢管混凝土轴压承载力计算

考虑内、外钢管对混凝土的双重约束作用, 分析复式钢管混凝土的轴压应力状态, 采用双剪统一强度理论分析混凝土和钢管各自的极限轴压强度, 得出复式钢管混凝土轴压承载力公式; 通过推导钢管在极限状态的强度折减系数及钢管与混凝土之间的紧箍力值, 计算出的轴压承载力与试验数据进行对比, 吻合较好, 验证了双剪统一强度理论在复式钢管混凝土轴压承载力计算中的适用性; 并给出了内圆钢管的径厚比和直径与轴压承载力提高系数的关系, 为复式钢管混凝土的优化设计提供理论依据.