铺设角度与铺层顺序对层合板稳定性的影响

以层合板结构的临界屈曲载荷系数最大化为优化目标,基于改进型模拟退火算法对层合板结构铺设角度和铺层顺序进行优化。由于层合板结构的铺层角度是离散变量,模拟退火算法适合求解离散变量的优化问题。利用模拟退火算法优化层合板铺层,在算法内采用并行计算、引入记忆功能同时设置双阈值终止准则,有效地提高了优化过程的收敛速度,同时避免优化过程中出现局部最优解。以临界屈曲载荷系数作为目标函数,选取复合材料层合板的铺设角度顺序为设计变量,采用改进的模拟退火算法得出复合材料层合板的最优铺设角度以及铺层顺序。     

Stiffness and fracture analysis of laminated composites with off-axis ply matrix cracking

Matrix cracks parallel to the fibres in the off-axis plies is the first intralaminar damage mode observed in laminated composites subjected to static or fatigue in-plane tensile loading. They reduce laminate stiffness and strength and trigger development of other damage modes, such as delaminations. This paper is concerned with theoretical modelling of unbalanced symmetric laminates with off-axis ply cracks. Closed-form analytical expressions are derived for Mode I, Mode II and the total strain energy release rates associated with off-axis ply cracking in [0/θ]_s laminates. Stiffness reduction due to matrix cracking is also predicted analytically using the Equivalent Constraint Model (ECM) of the damaged laminate. Dependence of the degraded stiffness properties and strain energy release rates on the crack density and ply orientation angle is examined for glass/epoxy and carbon/epoxy laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is also discussed.     

Free-edge and ply cracking effect in cross-ply laminated composites under uniform extension and thermal loading

The interlaminar stresses and displacements near the free-edges and ply cracks are investigated by using the state space equation method for general cross-ply laminates subjected to extension and/or thermal loading. By this approach, a laminated plate may be composed of an arbitrary number of orthotropic layers, each of which may have different material properties and thickness. The method takes into account all independent material constants and guarantees continuous fields of all interlaminar stresses across interfaces between material layers. Numerical solutions are compared with results obtained from other methods. It is found that the theory provides a satisfactory approximation to the stress singularity occurring in the vicinity of the free-edges and ply cracks.     

Effect of ply angle misalignment on out-of-plane deformation of symmetrical cross-ply CFRP laminates: Accuracy of the ply angle alignment

This paper discusses the accuracy of ply angle alignment and how it relates to out-of-plane deformation in carbon fiber reinforced plastics (CFRP) laminates. We investigated the deformation of symmetrical cross-ply laminates under hot and humid conditions. In spite of the symmetrically stacked laminates, unpredictable out-of-plane deformation occurred over time due to ply angle misalignment. The deformation was unstable and disproportionate to the absorbed moisture. A Monte Carlo simulation based on laminate theory was performed to quantify the deformation induced by the ply angle misalignment. Symmetrical cross-ply laminates were found to twist as they absorbed water when they underwent ply angle misalignments. By comparing the analytical results with experimental results, we concluded that a standard deviation of approximately 0.4° exists as ply angle misalignment in the laminates used in this study and that this slight ply angle misalignment can be a significant factor in out-of-plane deformation of cross-ply laminates.     

Design guidelines for ply drop-off in laminated composite structures

The present investigation aims at developing a few guidelines for the design of tapered laminated composites. The tapering in laminated composites is introduced by terminating (dropping-off) plies at different locations. The main objective in designing a drop-off is to reduce stress concentration. At present some thumb rules are used to design the drop-off. In this paper, guidelines have been developed by studying the effect of important parameters that determine the strength of the laminate. The numerical study shows that some of the thumb rules used at present are rather conservative and may be relaxed to an extent.     

瓦楞型竹束单板复合材料的制备及力学性能表征

探索了一种新型竹质工程构件--瓦楞型竹束单板复合材料(CBLC)的制备工艺, 研究了三种典型铺装类型(Ⅰ型: (0°)₆; Ⅱ型: (0°/90°)₃; Ⅲ型: (90°)₆)对其拉伸、 双向弯曲、 三维压缩性能的影响, 同时利用数字散斑相关方法(DSCM)对其弯曲应变场信息进行了表征。结果表明: 铺装类型对各项力学性能有明显影响, 对于拉伸、 纵向弯曲性能, Ⅰ型> Ⅱ型> Ⅲ型, 对于横向弯曲, Ⅱ型> Ⅲ型> Ⅰ型。不同铺装类型下CBLC的拉伸断裂机制亦各不相同: Ⅰ型为延性断裂, Ⅱ型为逐渐分层断裂, Ⅲ型为脆性断裂。 x和y方向应变场集中分布在试样底端最外层瓦楞波形连接处, 且E_xx< E_yy。多重比较分析表明: 抗压缩性能在三维方向上存在明显差异, 且y>x>z; 铺装类型对CBLC的抗压载荷、 抗压强度有很大影响, 而抗压模量差异不明显。     

瓦楞型竹束单板复合材料的制备及力学性能表征

探索了一种新型竹质工程构件——瓦楞型竹束单板复合材料(CBLC)的制备工艺,研究了三种典型铺装类型(Ⅰ型:(0°)6;Ⅱ型:(0°/90°)3;Ⅲ型:(90°)6)对其拉伸、双向弯曲、三维压缩性能的影响,同时利用数字散斑相关方法(DSCM)对其弯曲应变场信息进行了表征.结果表明:铺装类型对各项力学性能有明显影响,对于拉伸、纵向弯曲性能,Ⅰ型＞Ⅱ型＞Ⅲ型,对于横向弯曲,Ⅱ型＞Ⅲ型＞Ⅰ型.不同铺装类型下CBLC的拉伸断裂机制亦各不相同:Ⅰ型为延性断裂,Ⅱ型为逐渐分层断裂,Ⅲ型为脆性断裂.x和y方向应变场集中分布在试样底端最外层瓦楞波形连接处,且Exx＜Eyy.多重比较分析表明:抗压缩性能在三维方向上存在明显差异,且y＞x＞z;铺装类型对CBLC的抗压载荷、抗压强度有很大影响,而抗压模量差异不明显.     

Exact analysis for static response of cross ply laminated smart shells

Models and analytical solutions are formulated and developed for the static behavior of cross ply smart laminated shells with extension piezoelectric laminae. The models are based on a rigorous first order shell theory. The state space approach is used to find exact solutions for the static response of cross ply spherical, cylindrical and doubly curved shells with various boundary conditions. The smart shells possess two parallel edges simply supported and the remaining ones having any possible combination of boundary conditions: free, clamped or simply supported. Deflections of cross ply laminated shells incorporating piezoelectric layers are determined. Numerical results of six layer laminates are generated to investigate the shell static behavior. The exact solutions for deflections can be used as benchmarks for approximate solutions such as Rayleigh–Ritz and finite element methods.     

On the angle ply higher order beam vibrations

Vibrations of angle ply laminated beams are studied using the higher order theory and isoparametric 1d finite element formulations through proper constitution of elasticity matrix. Subsequent to the validation of the formulation, deep sandwich and composite beams are critically analyzed for various boundary conditions. Frequencies classified based on their spectrum are presented along with those of first order theories for comparison.     

Mechanical properties of Al2O3/Ni laminated composites

Al₂O₃/Ni laminated composites were prepared by aqueous tape casting and hot pressing with intent to study mechanical properties including the fracture strength and toughness. The residual stress was evaluated and proved. The relations of mechanical properties with the thermal residual stress, the ductility of metal layers and the layer thickness ratio were studied, respectively. It was found that the toughness and work of fracture of Al₂O₃/Ni laminar reached to 12.56 MPa m^1/2 and 12 450 J m^− 2, which are 3.6 and 478.8 times that of pure Al₂O₃.     

Exact analysis for static response of cross ply laminated smart shells

Models and analytical solutions are formulated and developed for the static behavior of cross ply smart laminated shells with extension piezoelectric laminae. The models are based on a rigorous first order shell theory. The state space approach is used to find exact solutions for the static response of cross ply spherical, cylindrical and doubly curved shells with various boundary conditions. The smart shells possess two parallel edges simply supported and the remaining ones having any possible combination of boundary conditions: free, clamped or simply supported. Deflections of cross ply laminated shells incorporating piezoelectric layers are determined. Numerical results of six layer laminates are generated to investigate the shell static behavior. The exact solutions for deflections can be used as benchmarks for approximate solutions such as Rayleigh–Ritz and finite element methods.     

Analytical modelling of laminated composites

A significant number of research papers has been published on the analytical modelling of composite laminates over the past 20 years. The drive for more accurate analysis has led us to techniques which have become computationally more and more burdensome, while the engineering world continues to use simple, first-older shear deformable plate theory as its primary tool. This paper presents a unique approach to the analysis of thick laminated composites by presenting two simple finite element methods. The first uses the Predictor Corrector technique to extend the simple Mindlin type element to achieve greater accuracy, and the second develops a new Least Squares element which can approximate a C¹ continuous element. The Least Squares element has the capability to incorporate a simplified higher order basis into a piecewise continuous displacement field creating an accurate, yet computationally simple, element. These two methods have the potential to upgrade analysis methods significantly with little additional computational cost. It is hoped that this work can instigate further research into efficient modelling of composite laminates.     

Structural optimization of rotating tapered laminated thick composite plates with ply drop-offs

In this study, structural optimization of rotating tapered thick laminated composite plates with ply drop-offs has been investigated numerically. The governing differential equations of motion of the tapered composite plate have been presented including the energy associated with the inertia force, coriolis force, displacement dependent centrifugal force and initial stress resultants due to steady state rotation. Four noded quadrilateral finite element has been formulated based on the first order shear deformation theory. Finite element analysis results are validated with experimental results for natural frequencies of the tapered plate with various configurations. Various cases of optimization problems are formulated with different objective functions in terms of maximization of natural frequencies and damping factors (individually and combined) and solved using genetic algorithm in order to obtain optimal ply sequence and ply orientation. It is shown that the optimization problem with maximization of fundamental modal damping factor without rotating condition yields the optimal layout as 90° for all the layers in the plate. It is also observed that maximization of the fundamental modal damping factor yields identical optimal orientation for uniform and all the configurations of a tapered composite plate.     

Mechanical properties of laminated bamboo strips from Gigantochloa Scortechinii/polyester composites

In this study, the mechanical properties of culm fiber composites with various thicknesses from the inner through the outer layer of bamboo strips were investigated. This study utilized a specific type of bamboo species named Gigantochloa Scortechinii (Buluh Semantan), which was collected from the Bukit Larang village in Melaka, Malaysia. In these experiments, unsaturated polyester (UP) and bamboo fiber (BF) strips were prepared through the hand lay-up technique using 3 mm thick aluminum mould. The composite bamboo strips were prepared in 1.5–2.5 mm thicknesses. The weight of the inner, middle and outer bamboo parts varied from a minimum of 0.742 g to a maximum of 2.600 g. The specimens were then characterized using several techniques including tensile, flexural, hardness, and impact tests. The results indicated that the properties of the middle part of the bamboo strips improve as the bamboo strip thickness increases due to the incorporation of unsaturated polyesters. However, the hardness properties increase for the outer layer of the laminate. These findings suggest that bamboo strips, based on unsaturated polyesters, yield excellent mechanical properties and are a viable alternative to composite-based reinforcing fibers.     

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.     

Matrix cracking and delamination in laminated composites. Part I: Ply constitutive law, first ply failure and onset of delamination

Matrix cracking and delamination are the main initial forms of damage in advanced laminated composites manufactured by stacking unidirectional plies of fiber reinforced polymers. In this paper, the onset of matrix cracking is determined for in-plane stress states; in addition, delamination promoted by matrix cracks is analyzed. Taking into account that under in-plane shear stresses composite laminates show a non-linear response prior to the formation of a macro-crack, a plastic-damage model is proposed and implemented. The models predictions correlate well with published experimental data.     

The influence of ply sequence and thermoelastic stress field on asymmetric delamination crack growth behavior of embedded elliptical delaminations in laminated FRP composites

The influence of ply lay up and the interaction of residual thermal stresses and mechanical loading on the interlaminar asymmetric embedded delamination crack growth behavior have been investigated. Two sets of full three-dimensional thermo-elastic finite element analyses have been performed for the interlaminar elliptical delaminations, which may be due to manufacturing defects or other reasons and are located symmetrically with respect to the midplane in a quasi-isotropic FRP composite laminate lay up. Depending upon the through-the-thickness location of the embedded elliptical delaminations, four different laminate configurations have been considered. Strain energy release rate (SERR) procedures have been employed to assess the delamination crack growth characteristics at the interfaces. It is found that the individual fracture modes exhibit asymmetric and non self-similar crack growth behavior along the delamination front depending upon the location of the interfacial delaminations; ply sequence and orientation and thermo-elastic anisotropy of the laminae.