Notched strength prediction of centre-hole carbon/carbon composites

The present work investigates the behaviour of 2D carbon/carbon (C/C) composites containing centre holes and proposes methodologies for the prediction of the residual strength of such C/C laminates. Two different concepts were adopted in order to understand the fracture behaviour of C/C composites in the presence of holes and to evaluate their strength. The first approach is based on the hypothesis of the existence of a damage zone around the hole, which originates during tensile loading. The second uses monitoring of the acoustic emission (AE) activity of the centre-hole specimen, loaded up to a stress level well below its failure strength, as an indication of damage formation, and combines it with a defined calibration curve. Both models predict very accurately the strength of notched laminates for a number of specimens tested.     

Enhancing compressive strength of unidirectional polymeric composites using nanoclay

Morphology of nanoclay dispersed in resin and suspended in acetone was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images show intercalation of resin in the gallery spaces of nanoclay and regions of exfoliated clay with random orientation. A vacuum assisted wet lay-up (VAWL) process was used for the inclusion of nanoclay in conventional fiber reinforced composites. The VAWL specimen displayed improvement in off-axis compressive strength for nanoclay enhanced fiber composites. Addition of nanoclay produced a substantial increase in longitudinal compressive strengths (extracted from off-axis tests) of glass fiber reinforced composites. An elastic–plastic model was used to predict the compressive strength of fiber reinforced composites based on the matrix properties. The model predictions matched well with the experimental results.     

A statistical theory of the tensile strength of short-fiber-reinforced composites

This paper is concerned with the prediction of the tensile strength of short-fiber-reinforced composites by following the statistical theory of the strength of fiber bundles. Unidirectional, misaligned, and random-fiber directions are all treated. Depending on the fiber length, two failure modes can be distinguished. When the fiber length is smaller than 2L_c, where L_c is the critical transfer length, the composite should fail by direct fiber pull-out. By contrast, it the fiber length is longer than 2L_c, most fibers break first and are then pulled out. At small fiber lengths, the prediction of the present theory is one-half the value predicted by the conventional model. However, the prediction of the present theory is in close agreement with that of the conventional approach if the fiber length is long.     

基于单胞解析模型的单向复合材料强度预报方法

基于单胞解析模型,建立一种从复合材料细观组分到宏观单向板的强度预报方法。根据连续介质力学和均匀化方法构建细-宏观关联矩阵,通过该关联矩阵将细观组分材料的弹性和损伤性能传递到宏观单向板中。考虑复合材料细观损伤状态,当纤维和基体满足各自强度准则时失效,并通过失效因子折算成刚度的衰减。在此基础上,结合有限元分析,实现复合材料单向板纵横向拉伸模拟,从而预报单向板的拉伸强度。结果表明:该方法预报的模量和强度与实验值基本一致,验证了该方法的有效性与高效性。     

应用桥联模型预测复合材料吸湿老化剩余强度

采用试验与理论相结合的方法, 研究了玻璃纤维/环氧树脂复合材料的吸湿老化剩余强度, 测试了基体的吸湿性能, 对老化前后基体、 纤维及其复合材料的拉伸性能进行了试验研究。根据纤维和基体性能测试数据, 应用桥联模型计算了复合材料的吸湿老化剩余强度, 计算结果与试验结果一致。结果显示: 在小吸湿率条件下, 利用桥联模型能够准确预测复合材料的破坏演化规律及其最大剩余强度。当主承力层破坏由纤维控制时, 由湿老化引起的纤维强度的下降直接导致复合材料最大拉伸强度的下降; 当破坏由基体控制时, 影响复合材料吸湿剩余强度的主要因素是基体硬化及强度的下降。较小吸湿率对纤维与基体的界面粘结强度影响有限, 界面破坏不是层合板破坏的主要因素。      

三维编织复合材料渐进损伤模拟及强度预测

采用考虑纤维束相互挤压的纤维束截面八边形单胞模型, 引入周期性边界条件, 对三维编织复合材料的渐进损伤过程进行数值模拟, 并预测了材料的拉伸强度。通过在应变能密度函数中引入损伤状态变量, 建立了含损伤材料的刚度矩阵, 运用基于不同失效模式下损伤状态变量的刚度渐进折减法表征材料积分点损伤, 通过数值结果与试验结果的对比, 分析了Hashin和Tsai-Wu两种准则作为判定纤维束起始损伤的适用性。分析表明: 基于引入不同失效模式的Tsai-Wu准则的数值模拟和试验结果吻合良好; Hashin准则不适合作为编织纤维束的损伤判据; 不同编织角材料的失效机制不同。      

基于细观结构的2.5维机织复合材料强度预测模型

采用经纱矩形截面及纬纱六边形截面假设,将经纱的屈曲轨迹简化为折线形式,建立了2.5维机织复合材料单胞几何模型。以单胞为研究对象,引入改进的三维Hashin失效准则和Mises准则作为组分材料的失效判据,采用不同的刚度退化方式来表征不同的失效模式,建立了基于逐渐损伤理论的强度预测模型。利用有限元分析（FEA）技术,开发了相应的参数化2.5维机织复合材料逐渐损伤分析程序,预测了浅交弯联结构不同机织参数2.5维机织复合材料的拉伸强度,并模拟了经向拉伸和纬向拉伸的损伤扩展过程。与静拉伸试验结果相比,拉伸强度的预测误差在10%以内;模拟的失效模式与试验结果吻合较好。     

三维六向编织复合材料渐进损伤模拟及强度预测

基于三维六向编织复合材料的细观结构,假设第六向纱线的截面形状为菱形,建立了三维六向编织复合材料的渐进损伤有限元模型。采用Linde等提出的失效准则,引入周期性位移边界条件,对三维六向编织复合材料的纵向拉伸应力-应变行为进行了渐进损伤数值模拟,讨论了单胞模型在纵向拉伸载荷作用下的细观损伤起始、扩展和最终失效的演化过程,并预测了材料的拉伸强度。在此基础上,进一步研究了编织角、纤维体积分数和编织纱水平取向角等参数对材料纵向拉伸力学性能的影响规律。研究结果表明,三维六向编织复合材料的轴向纱线拉伸断裂是导致其破坏的最主要因素。所得数值结果与现有试验值吻合较好,验证了该模型的有效性,为更深入研究此类材料的力学性能奠定了基础。     

Incorporation of statistical length scale into Weibull strength theory for composites

In this paper an extension of Weibull theory by the introduction of a statistical length scale is presented. The classical Weibull strength theory is self-similar; a feature that can be illustrated by the fact that the strength dependence on structural size is a power law (a straight line on a double logarithmic graph). Therefore, the theory predicts unlimited strength for extremely small structures. In the paper, it is shown that such a behavior is a direct implication of the assumption that structural elements have independent random strengths. By the introduction of statistical dependence in the form of spatial autocorrelation, the size dependent strength becomes bounded at the small size extreme. The local random strength is phenomenologically modeled as a random field with a certain autocorrelation function. In such a model, the autocorrelation length plays the role of a statistical length scale. The focus is on small failure probabilities and the related probabilistic distributions of the strength of composites. The theoretical part is followed by applications in fiber bundle models, chains of fiber bundle models and the stochastic finite element method in the context of quasibrittle failure.     

Interfacial debonding in laminated titanium matrix composites

The results of an experimental program in which multiaxial loads were applied to [0₄] and [±45]_s silicon carbide/titanium (SiC/Ti) tubes are reviewed showing that stress coupling, matrix viscoplasticity (including room temperature creep) and fiber/matrix interfacial damage all contribute to nonlinear response and permanent strains in titanium matrix composites (TMC). A micromechanical model that explicitly considers the aforementioned phenomena is presented herein. The model assumes a periodic microstructure and uses finite elements to analyze a representative volume element. The composite is assumed to be in a state of generalized plane strain making it possible to discretize only a generic transverse plane while still being able to apply three-dimensional loading through appropriate boundary conditions. The response of laminated composites is predicted by incorporating the micromechanical results into nonlinear lamination theory. Predictions are presented to show the influence of the model parameters on the effective composite response of unidirectional [0₄] and angle-ply [±45]_s TMC laminates.     

Probabilistic failure prediction for FRP composites

Results are presented from a theoretical study on failure-locus prediction for unidirectional FRP laminae under complex in-plane loading, taking into account statistical aspects of the basic strengths of the material. The purpose of the analysis is to establish simple rules and methodologies for failure prediction under specific reliability requirements. Therefore, the problem one is faced with is the definition of the cumulative distribution function (CDF) of the failure condition if the respective CDFs of the basic material strengths are known by experiment. To this end, two analytical approaches, namely a functional expansion technique and the introduction of Pearson’s semi-empirical distribution function, were developed and implemented in software. Both methods were shown to predict satisfactory results compared with Monte Carlo simulated ones and experimental data, wherever available. Finally, a semi-deterministic approach was examined according to which the failure criterion is used in the usual deterministic way but with basic strength values of a certain reliability level. Results from this simple and fast method were found in good agreement with those derived by expensive pure statistical methods or numerically simulated ones and experimental data.     

Analysis of mechanical properties and SEM for laminated SiC/W composites

In this letter, it is reported that a laminated SiC/W composite has been developed using the hot pressing method. It is found that a chemical reaction between W and SiC occurs during the preparation process. Making use of SEM, the components within the sandwiched-in metal and the fracturing crack for the laminated SiC/W composite are determined. Testing mechanical properties of the laminated SiC/W composite indicates that fracture toughness increases while bending strength reduces, with an increase of the thickness of the sandwiched-in metal ranging from 10–50-μm thickness.     

Residual compressive strength of laminated plates with delamination

A study of residual compressive strength in delaminated laminates is presented. A methodology is proposed for simulating the whole compressive failure responses, such as initial buckling, postbuckling, contact of delamination front region, delamination propagation, fiber breakage, and matrix cracking etc. An finite element analysis (FEA) of the residual compressive strength is conducted on the basis of the Von Karman's nonlinearity assumption and the first-order shear deformation plate theory, combined with a stiffness degradation scheme. The numerical analysis models and methods are briefly introduced in this paper and some numerical examples are presented to illustrate it. From numerical results and discussion, it is clear that the compressive failure response involves complex multi-failure modes during compressive process. The method and numerical conclusions provide in this paper should of great value to engineers dealing with composite structures.     

缝纫泡沫夹芯复合材料失效强度的理论预测与试验验证

基于经典层板理论和细观力学桥联模型, 提出了缝纫泡沫夹芯复合材料失效强度的理论预测方法, 并进行了失效强度的相关试验验证。其中, 将缝纫复合材料面板看作单层组成的准层状结构, 采用经典层板理论进行逐层失效分析, 并同时考虑了局部皱曲的面板失效模式; 而对缝纫泡沫夹芯, 引入桥联模型计算其各组分材料中的应力, 并通过对各组分材料选取适当失效准则来建立失效判据; 对于缝纫泡沫夹芯复合材料采取逐级加载方式, 当面板或者夹芯失效时, 则认为其发生整体失效, 由此可以确定其在不同载荷形式下的失效强度。此外, 通过试验得到了缝纫泡沫夹芯复合材料板试件在平压、 侧压、 横向剪切及三点弯曲载荷形式下的失效模式及其失效强度, 并利用本文方法对缝纫泡沫夹芯复合材料的失效强度进行了理论预测, 所得结果与试验吻合, 证明了本文方法的有效性。      

Bearing strength prediction for reinforced composite laminates for cryogenic and aerospace applications

The procedures of bearing strength prediction for composite laminates reinforced with high-strength unidirectional fibers at room and cryogenic temperatures are discussed. The best agreement between the calculated and experimental data is observed when the temperature dependence of effective mechanical parameters of plies is allowed for within the framework of the elastic theory of laminated heterogeneous bodies. __________ Translated from Problemy Prochnosti, No. 2, pp. 11–25, March–April, 2008.     

湿-热-力耦合环境下复合材料结构损伤分析与性能研究

将考虑湿热效应的经典层合板理论引入有限元方法中, 通过精确考察单层板等效参数衰减、追踪初始破坏后的卸载以及对最终破坏进行判定, 在精确预测湿热环境对复合材料性能影响的同时追踪了损伤演化过程。通过建立湿-热-力耦合下复合材料宏观性能的定量表达式, 发展湿-热-力耦合环境下复合材料性能演化表征与预测方法, 实现在有限元分析中考虑湿热效应的影响。对3 种不同铺层方式复合材料层合板进行湿热环境试验研究, 并利用本文中分析方法进行损伤模拟, 分析结果与实验结果符合得较好。      

Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete

This paper summarizes the results of an experimental investigation into the bond behavior between recycled aggregate concrete (RAC) and deformed steel rebars, with the main variables being the recycled coarse aggregate replacement ratio (RCAr) and water-to-cement ratio of the concrete mixture. The investigation into splitting cracking strength indicates that the degradation of the bond splitting tensile stress of the cover concrete was affected by not only the roundness of the coarse aggregate particles but also the weak interfacial transition zone (ITZ) between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete. In this study, a linear relationship between the bond strength and the density of the RCA was found, but the high compressive strength reduced the effects of the parameters. To predict the bond strength of RAC using the main parameters, a multivariable model was developed using nonlinear regression analysis. It can be inferred from this study that the degradation characteristic of the bond strength of RAC can be predicted well, whereas other empirical equations and code provisions are very conservative.     

Deformation of laminated epoxy composites reinforced with high-strength fibers

We study the elastic deformation behavior of laminated epoxy composites reinforced with unidirectional carbon fibers and satin-woven glass fabric. The efficiency of various experimental procedures of determination of averaged elastic characteristics of laminates is analyzed. The effect of decreasing the test temperature to 77 K on the mechanical behavior of the above-mentioned materials is studied. The possibility of predicting stiffness and compliance parameters of unidirectional or satin-woven fabric composites is discussed. __________ Translated from Problemy Prochnosti, No. 1, pp. 41–57, January–February, 2006.     

Cohesive models for damage evolution in laminated composites

A trend in the last decade towards models in which nonlinear crack tip processes are represented explicitly, rather than being assigned to a point process at the crack tip (as in linear elastic fracture mechanics), is reviewed by a survey of the literature. A good compromise between computational efficiency and physical reality seems to be the cohesive zone formulation, which collapses the effect of the nonlinear crack process zone onto a surface of displacement discontinuity (generalized crack). Damage mechanisms that can be represented by cohesive models include delamination of plies, large splitting (shear) cracks within plies, multiple matrix cracking within plies, fiber rupture or microbuckling (kink band formation), friction acting between delaminated plies, process zones at crack tips representing crazing or other nonlinearity, and large scale bridging by through-thickness reinforcement or oblique crack-bridging fibers. The power of the technique is illustrated here for delamination and splitting cracks in laminates. A cohesive element is presented for simulating three-dimensional, mode-dependent process zones. An essential feature of the formulation is that the delamination crack shape can follow its natural evolution, according to the evolving mode conditions calculated within the simulation. But in numerical work, care must be taken that element sizes are defined consistently with the characteristic lengths of cohesive zones that are implied by the chosen cohesive laws. Qualitatively successful applications are reported to some practical problems in composite engineering, which cannot be adequately analyzed by conventional tools such as linear elastic fracture mechanics and the virtual crack closure technique. The simulations successfully reproduce experimentally measured crack shapes that have been reported in the literature over a decade ago, but have not been reproduced by prior models.     

A higher-order polynomial shear deformation theory for geometrically nonlinear free vibration response of laminated composite plate

In this paper, a nonlinear analysis for large amplitude free vibration of laminated composite plates is developed using higher-order shear deformation theory. The effect of all higher-order terms arising from nonlinear strain-displacement relations are included in the formulation and present plate theory exhibits traction-free surface of the laminated plate in von-Karman sense. A finite element procedure considering a C° continuous isoparametric nine-node rectangular element is implemented for nonlinear model. The accuracy of the theory is validated with some available theory for different aspect ratio, modular ratio, number of layers, ply orientations, etc. through some numerical examples.