Mechanical properties of henequen fibre/epoxy resin composites

By using surface-treated and untreated henequen fibres and an epoxy resin, composites were made by compression moulding, and their mechanical properties and failure modes were determined experimentally in tension, bending, and impact loading. The results obtained show that the treatment of fibre surface does not improve the bond between the fibres and the resin matrix, and the general mechanical properties of the composites are similar.     

Testing of screw joint hybrid composite materials

Laminate composite materials have excellent coefficient of stiffness to weight and because of this laminates are very widely applied often replacing raditional materials. A very interesting group of such a type materials are the hybrid ones. A main disadvantage of hybrid composite materials is the weak impact strength. The paper presents results of testing handmade screw joint hybrid composite materials. Results of testing are compared with numerical analysis based on the Finite Elements Method. Investigations were made on designed for this purpose the statical stresses laboratory stand. The testing specimens were made by a contact method (handmade laminating) and laminar mats or woven fabrics spreading (resin supersaturated). (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

An accurate method to predict the stress concentration in composite laminates with a circular hole under tensile loading

The paper presents a theoretical-numerical hybrid method for determining the stresses distribution in composite laminates containing a circular hole and subjected to uniaxial tensile loading. The method is based upon an appropriate corrective function allowing a simple and rapid evaluation of stress distributions in a generic plate of finite width with a hole based on the theoretical stresses distribution in an infinite plate with the same hole geometry and material. In order to verify the accuracy of the method proposed, various numerical and experimental tests have been performed by considering different laminate lay-ups; in particular, the experimental results have shown that a combined use of the method proposed and the well-know point-stress criterion leads to reliable strength predictions for GFRP or CFRP laminates with a circular hole. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 4, pp. 531–570, July–August, 2007.     

新型轻质复合材料夹芯结构振动阻尼性能研究进展

作为新一代先进轻质超强韧结构材料,复合材料格栅和点阵夹芯结构受到了国内外学者的广泛关注.目前关于该类结构材料的设计制备以及相关力学性能研究已取得了大量的研究成果.然而对该类结构振动阻尼性能的研究则处于起步阶段.该文综述了纤维增强树脂基复合材料简单层合结构以及各类夹芯结构振动阻尼性能的研究现状.首先阐述其阻尼机理, 然后分别概述了复合材料简单层合板的微观和宏观阻尼模型、复合材料粘弹性阻尼夹层结构和新型夹层结构的阻尼预报工作,最后总结归纳现有关于该类结构阻尼特性研究工作中已取得的成果和不足之处,并对其未来发展进行了展望.     

Effect of elevated temperatures on the strength of cured resins and resin-based materials

An analysis of the relationship between the relative changes of compressive strength and weight loss for glass laminates has shown that at elevated temperatures (to 600° C) the changes in strength are primarily caused by processes of mechanical destruction of the resin phase. The process of thermo-oxidative degradation of the resin in the laminate is described by the same stages as were detected in studying cured resins.Moscow Aviation-Engineering Institute. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 832–838, September–October, 1968.     

The Sensitivity of Residual Stresses of Cross-Ply Laminates to Manufacturing and Material Parameters

By using a finite-element model elaborated, the sensitivity of residual stresses of polyester/glass cross-ply laminates to manufacturing and material parameters is investigated. The development of residual stresses in the laminates and the significance of the parameters for the problem are discussed. It is found that the main attention in calculating residual stresses should be focused on the properties of resin, which must be measured with care. The most important parameters related to the resin are, of course, its stiffness, thermal expansion, and chemical shrinkage, while the properties of fibers can be obtained from material handbooks with a sufficient accuracy. In curing a thin laminate in an autoclave, the simulation of chemical reactions and the parameters needed in thermal analysis are quite insignificant, because, in practice, the autoclave temperature and the properties of the mold determine the laminate temperature history.     

Braided reinforced composite rods for the internal reinforcement of concrete

This paper reports on the development of braided reinforced composite rods as a substitute for the steel reinforcement in concrete. The research work aims at understanding the mechanical behaviour of core-reinforced braided fabrics and braided reinforced composite rods, namely concerning the influence of the braiding angle, the type of core reinforcement fibre, and preloading and postloading conditions. The core-reinforced braided fabrics were made from polyester fibres for producing braided structures, and E-glass, carbon, HT polyethylene, and sisal fibres were used for the core reinforcement. The braided reinforced composite rods were obtained by impregnating the core-reinforced braided fabric with a vinyl ester resin. The preloading of the core-reinforced braided fabrics and the postloading of the braided reinforced composite rods were performed in three and two stages, respectively. The results of tensile tests carried out on different samples of core-reinforced braided fabrics are presented and discussed. The tensile and bending properties of the braided reinforced composite rods have been evaluated, and the results obtained are presented, discussed, and compared with those of conventional materials, such as steel. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 327–338, May–June, 2008.     

Optimum stacking sequence of composite laminates for maximum strength

A method is presented for maximum strength optimum design of symmetric composite laminates subjected to in-plane and transverse loadings. The finite element method based on shear deformation theory is used for the analysis of composite laminates. Ply orientation angles are chosen as design variables. The quadratic failure criterion which is meant to predict fracture, is used as an object function for optimum stacking sequence design of a laminated plate. The Broydon-Fletcher-Goldfarb-Shanno optimization technique is employed to solve the optimization problem effectively. Numerical results are given for various loading conditions, boundary conditions, and aspect ratios. The results show that the quadratic failure criterion such as Tsai-Hill theory is effective for the optimum structural design of composite laminates.Presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October 1995).Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 3, pp. 393–404, May–June, 1995.     

Evaluation of porosity in composite aircraft structures

Conclusion The use of an analytical model for determining the moduli of elasticity of composite laminates made of woven or unidirectional plies with different porosity levels was described. The analysis of aircraft composite parts with different levels of voids (porosity) was based on a method which utilizes the results of state-of-the-art nondestructive testing methods (ultrasonic through transmission, loading, or pulse/echo) as the starting data for the analytical model. The porosity distribution over the volume of the material and correlations for the nondestructive testing methods were determined for epoxy-carbon laminates with standard and stiffened binders and corroborating experiments were conducted. It was shown that the moduli of elasticity of composite laminates decrease with an increase in the porosity levels. The type, thickness, and layup of the laminate are the basic factors that affect the decrease in the elastic properties of porous composite laminates.Translated from Mekhanika Kompozitnykh Materialov, Vol. 30, No. 6, pp. 813–830, November–December, 1994.     

Choice of optimal structure of glass laminates with random reinforcement. I

The effect of structural parameters — length, diameter, and distribution of the reinforcing elements — on the mechanical characteristics of glass-reinforced plastics is investigated with reference to the case of glass laminates with randomly distributed, straight, uncut glass fibers in parallel planes. It is shown that the reduced strength of these laminates as compared with unidirectional material is associated with the redistribution of the load between the fibers and the resin and the relative reduction in the number of fibers in the cross section. A formula is proposed for estimating the strength of glass-reinforced plastics with a random distribution of the fibers in parallel planes.All-Union Scientific-Research Institute of Glass-Reinforced Plastics and Glass Fiber, Moscow Region. Moscow Bauman Higher Technical College. Translated from Mekhanika Polimerov, Vol. 4, No. 6, pp. 1043–1050, November–December, 1968.     

Heat conduction in transversally graded laminates with non-uniform microstructure

Two-component laminates made of conductors distributed non-periodically as laminas along one direction are considered in this note. The macroscopic properties of these laminates are described by continuous slowly-varying functions across laminas. In order to analyse heat conduction, the approach, called the tolerance modelling, is used. The aim of this note is to use averaged equations of the tolerance model for transversally graded laminates to analyse stationary heat conduction across laminas. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

带裂纹层合板复应力强度分析

本文用经典板理论求得了复合材料层合板的界面应力强度因子.利用裂纹尖端能量释放率和复应力强度因子间的关系,给出最一般的复合材料层合板,在相应外荷载和模型混合参数下的复应力强度因子的一个封闭形式的解.然后提出确定这一模型混合参数的步骤,给出某些层合板的数值结果.并给出在相应外荷载下的小范围接触条件.特别证明了界面韧度曲线的对称性质.最后讨论振荡指数消失后,预计的断裂荷载的精确性.还通过一个实例表明β=0法的有效性和局限性.     

Non-linear oscillations of a thin-walled composite beam with shear deformation

A geometrically non-linear theory is used to study the dynamic behavior of a thin-walled composite beam. The model is based on a small strain and large rotation and displacements theory, which is formulated through the adoption of a higher-order displacement field and takes into account shear flexibility (bending and warping shear). In the analysis of a weakly nonlinear continuous system, the Ritz’s method is employed to express the problem in terms of generalized coordinates. Then, perturbation method of multiple scales is applied to the reduced system in order to obtain the equations of amplitude and modulation. In this paper, the non-linear 3D oscillations of a simply-supported beam are examined, considering a cross-section having one symmetry axis. Composite is assumed to be made of symmetric balanced laminates and especially orthotropic laminates. The model, which contains both quadratic and cubic non-linearities, is assumed to be in internal resonance condition. Steady-state solution and their stability are investigated by means of the eigenvalues of the Jacobian matrix. The equilibrium solution is governed by the modal coupling and experience a complex behavior composed by saddle noddle, Hopf and double period bifurcations.     

Design rules for the laminate stiffness

This paper aims at promoting laminate designs with specific stiffness properties: quasi-homogeneous, quasi-isotropic quasi-homogeneous, and thermally stable laminates. For this, the paper first discusses the concept of design, reviews the classical laminated plate theory and the usual design rules for laminates, and introduces the polar method for plane anisotropy. Then it develops rules and methods to design the stiffness properties that are looked for. In conclusion, it suggests that the classical design rules are too restrictive and that innovative designs should be explored for a better use of composites.     

Estimation of the Tensile Strength Reduction of a Composite Laminate with a Hole

The strength problem for a body with an artificial defect in the form of a hole is considered. To solve this problem, an approach is suggested according to which the local strength of a material in the area of stress concentration is assumed to depend on the size of this area. The scale of the problem is introduced via the ratio between the characteristic sizes of the deformed area and the characteristic length of the material. This approach is used to estimate the strength of composite laminates weakened by holes and notches. Expressions for the failure stress are obtained, which can be applied to isotropic and orthotropic laminates both in quasi-brittle failure and in failure associated with significant inelastic deformations. A comparison between the calculated _c values and the known experimental data for glass/epoxy and graphite/epoxy laminates is presented.     

A consistent second-order plate theory for monotropic material

Mathematical homogenization (or averaging) of composite materials, such as fibre laminates, often leads to non-isotropic homogenized (averaged) materials. Especially the upcoming importance of these materials increases the need for accurate mechanical models of non-isotropic shell-like structures. We present a second-order (or: Reissner-type) theory for the elastic deformation of a plate with constant thickness for a homogeneous monotropic material. It is equivalent to Kirchhoff's plate theory as a first-order theory for the special case of isotropy and, furthermore, shear-deformable and equivalent to R. Kienzler's theory as a second-order theory for isotropy, which implies further equivalences to established shear-deformable theories, especially the Reissner-Mindlin theory and Zhilin's plate theory. Details of the derivation of the theory will be published in a forthcoming paper [3]. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the plane deformation of fibre-reinforced viscoelastic plates

The realization method of elastic solutions is used to solve the problem of bending of a viscoelastic plate reinforced unidirectionally by elastic fibres. Numerical computations are carried out for three kinds of external load. The plane deformation of this plate is discussed.     

Weight‐Reduction of Sandwich Structures by an Efficient Topology Optimization Technique

A heuristic algorithm for the weight minimization of sandwich structures by a specific kind of topology optimization is presented. The method employs a preexisting algorithm for the layerwise topology optimization of symmetric laminates under in‐plane loads and expands this method for the case of bending. During the optimization procedure the algorithm adds or subtracts material from the layers of the face sheets and the core of the sandwich plate in regions of high or low stresses respectively. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multicriterion optimization of hybrid composite plates with various supports under transverse load

In this paper applications of multicriteria optimization to design hybrid laminates is shown. In the considered numerical example, the obtained preferred solution is 30% heavier than Carbon/Epoxy laminate and use a third of the more expensive material. Moreover, a small number of layers is needed to build an optimal solution. In the case of scalar optimization three layers are required. In the case of bicriteria optimization one E-Glass/Epoxy layer placed between four Carbon/Epoxy layers is enough to build an optimal laminate. Two criteria may lead to a set of preferred equal solutions. Thus, connection of Pareto concept with a lexicographic order by introduction of an additional criterion is useful for ordering them. A similar approach may be used for designing hybrid laminates with a requirement of a compromise between the stiffness and cost of a plate. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Asymptotic Behavior of a Structure Made by a Plate and a Straight Rod

This paper is devoted to describing the asymptotic behavior of a structure made by a thin plate and a thin perpendicular rod in the framework of nonlinear elasticity. The authors scale the applied forces in such a way that the level of the total elastic energy leads to the Von-K′arm′an’s equations (or the linear model for smaller forces) in the plate and to a one-dimensional rod-model at the limit. The junction conditions include in particular the continuity of the bending in the plate and the stretching in the rod at the junction.