Interlaminar stress analysis of general composite laminates

In this study, based on the reduced from of elasticity displacement field for a long laminate, an analytical method is established to exactly obtain the interlaminar stresses near the free edges of generally laminated composite plates under the extension and bending. The constant parameters, which describe the global deformation of a laminate, are properly computed by means of the improved first-order shear deformation theory. Reddy's layerwise theory is subsequently utilized for analytical and numerical examinations of the boundary layer stresses within arbitrary laminated composite plates. A variety of numerical results are obtained for the interlaminar normal and shear stresses along the interfaces and through the thickness of laminates near the free edges. Finally the effects of end conditions of laminates on the boundary-layer stress are examined.     

On thermal edge effects in composite laminates

The results of a finite element investigation of the combined influence of edge effects due to mechanical and thermal mismatch in composite laminates with free edges is presented. Results are presented for laminates of unidirectional boron/epoxy symmetrically bonded to sheets of aluminum and titanium. It is shown that interlaminar thermal stresses may be more significant than the interlaminar stresses due to loading only. In addition the stresses due to thermal mismatch may be of the same sign as those due to Poisson's mismatch or they may be of opposite sign depending upon material properties, stacking sequence and direction of loading.The paper concludes with a brief discussion of thermal stresses in all composite laminates.     

Prestressed composite laminates featuring interlaminar imperfection

The third-order zigzag displacement model is improved to include inplane displacement jumps across each layer interface of composite laminates to enable interlaminar imperfection to be incorporated. The imperfection is characterized by a linear spring-layer model which includes perfect bonding as a special case. The principle of virtual work is used to derive a boundary value formulation for laminated composite plates initially in a prestress state. Bending, buckling and vibration problems are studied for the case of rectangular cross-ply laminated plates for illustrative purpose.     

Investigation on delamination morphology during drilling composite laminates

Drilling-induced delamination of composite materials is a key factor that affects the quality of subsequent machining. To investigate the developing process of delamination, experiments with different drilling depth are conducted. In order to observe the delamination of different cross-sections in radial direction of the hole, the grinding method is adopted. Three-dimensional morphology of delamination at the exit of hole is obtained. The regularity of delamination with the change of drilling depth is analyzed, and the existence of “hidden delamination zone” is obtained finally. Due to the rebound effect of hole diameter and the inverted cone of drill guide section, the “hidden delamination zone” will be generated under the condition that the edge of delamination area is compressed tightly again. The critical thrust force of delamination is also studied, and it is proved to be correct.     

Ultimate strength of composite laminates with free-edge delamination

The present paper deals with the free edge effect of composite laminates by using a generalized quasi-three dimensional analysis and experimental verification of an analysis performed for laminates with Teflon inserted on interfaces to simulate initial free-edge delamination. We performed tensile tests for laminates [30₂/?30₂/90]_s carbon-epoxy laminates with free-edge delamination under uniaxial tension. The experiment reveals that extensional stiffness of the laminate decreases by the initiation of the delamination, and that strength of the laminate without delamination is smaller than that of the laminates with delamination. Generalized quasi-three dimensional finite element technique, which employs energy release rate and maximum stress criteria, is developed to estimate behavior of the laminate after initial delamination. The numerical result by use of this technique predicts the ultimate strength of the laminates with sufficient accuracy according as the comparison with an experimental stress-strain curve. In the experiment conducted both for the laminate with initial delamination and the laminate without initial delamination, an unexpected results were obtained that is the ultimate load of the laminate without initial delamination was lower than that of the laminate with initial delamination. We presented clear explanation on the phenomenon occurred and developed the method to predict the nonlinear behavior of the laminate with or without initial delamination.     

Some observations on the interlaminar strength of composite laminates

Definition of the influence of interlaminar stresses on the failure characteristics of composite laminates may require the development of novel experimental characterization procedures. The so-called free edge problem in laminate elasticity offers a cnovenient mechanism to accomplish this purpose because of the high interlaminar stresses in the neighbourhood of a free boundary. The detailed design of a laminate specimen which can exhibit catastrophic delamination induced by interlaminar tension is presented, along with a preliminary failure hypothesis to characterize this mode of rupture and the associated experimental demonstration of the phenomena.     

A genetic algorithm for the optimization of fiber angles in composite laminates

A genetic algorithm for the optimization of composite laminates is proposed in this work. The well-known roulette selection criterion, one-point crossover operator, and uniform mutation operator are used in this genetic algorithm to create the next population. To improve the hill-climbing capability of the algorithm, adaptive mechanisms designed to adjust the probabilities of the crossover and mutation operators are included, and the elite strategy is enforced to ensure the quality of the optimum solution. The proposed algorithm includes a new operator called the elite comparison, which compares and uses the differences in the design variables of the two best solutions to find possible combinations. This genetic algorithm is tested in four optimization problems of composite laminates. Specifically, the effect of the elite comparison operator is evaluated. Results indicate that the elite comparison operator significantly accelerates the convergence of the algorithm, which thus becomes a good candidate for the optimization of composite laminates.     

Fatigue damage of quasi-isotropic composite laminates under tensile loading in different directions

The purpose of this work is to investigate fatigue damage of quasi-isotropic laminates under tensile loading in different directions. Low cycle fatigue tests of [0/−60/60]_s laminates and [30/−30/90]_s laminates were carried out. Material systems used are AS4/Epoxy and AS4/PEEK. The fatigue damage of [30/−30/90]_s is very different from that of [0/−60/60]_s. The experimental results are compared with the result obtained from the method for determining strain energy release rate components proposed by the authors. The analytical results were in good agreement with the experimental results. It is proved that the failure criterion based on the strain energy release rate is an appropriate approach to predict the initiation and growth of delaminations under cyclic loading.     

Terahertz scanning techniques for paint thickness on CFRP composite solid laminates

Terahertz ray (T-ray) scanning applications are promising tools. The use of T-ray for nondestructive evaluation was investigated on composite materials. In this characterization procedure, electromagnetic properties, such as the refractive index, were analyzed. The estimates of properties are in good agreement with known data. We successfully demonstrated the characteristics of T-ray propagating through Carbon fiber-reinforced plastic (CFRP) composites to acquire the refractive index by using the characterized material properties. A T-ray technique was developed for measuring paint thickness on CFRP laminates. Good results were obtained from tests performed on standard paint samples with thickness starting from approximately 100 μm. The method was based on reflection mode measurement with time of flight. Another method developed for measuring paint thickness using resonance frequencies was utilized to determine paint thickness on composite laminates. The paint thickness deduced from resonance frequencies agrees well with the result obtained directly from time-domain echoes.     

Post-buckling and delamination propagation behavior of composite laminates with embedded delamination

Delamination occurred due to poor manufacturing process or in-service actions significantly affects the mechanical and failure behavior of laminated composite structures. In this study, the buckling and post-buckling delamination behavior of laminated composite with an embedded initial delamination under in-plane compression was studied experimentally and numerically. First, compression tests for laminated composite specimens with embeded initial delamination were performed and the buckling and delamination responses were obtained. Then the experimental test was numerically simulated using finite element methods with the progressive failure accounted for by using cohesive zone modeling. The load-displacement curve, strain behavior and delamination shapes of experimental specimens obtained from load cells, strain gages installed at different locations, and C scan images, respectively, were compared with the FEM results, and good agreements were attained. The effect of the buckling modes, laminate stacking sequence and shape of initial delamination on the buckling load and propagation behavior was studied by considering different ply stacking and shapes of initial delaminations. It was found that the buckling mode determined the growth direction of the delamination propagation, and the stacking sequence influenced the extent of the propagation area, while the orientation of the delamination affected the buckling loads.     

Characteristics of thermo-acoustic emission from composite laminates during thermal load cycles

The thermo-acoustic emission (AE) technique has been applied for nondestructive characterization of composite laminates subjected to cryogenic cooling. Thermo-AE events during heating and cooling cycles showed a Kaiser effect. An analysis of the thermo-AE behavior obtained during the 1st heating period suggested a method for determining the stress-free temperature of the composite laminates. Three different thermo-AE types classified by a short-time Fourier transform of AE signals enabled to offer a nondestructive estimation of the cryogenic damages of the composites, in that the different thermo-AE types corresponded to secondary microfracturing in the matrix contacting between crack surfaces and some abrasive contact between broken fiber ends during thermal load cycles.     

Measurement of residual stress in thick section composite laminates using the deep-hole method

The deep-hole method is a method of measuring residual stress in large metallic components. In this paper, an extension to the deep-hole method is described to allow the residual stresses in thick section composite laminated plates to be evaluated. The method involves first drilling a small hole through the laminate perpendicular to the surface. The material around the hole is then machined away, resulting in a change in diameter of the hole due to the release of residual stress. This change in diameter is measured and used to calculate the residual stress. The calculation requires the evaluation of coefficients that depend on the properties of the composite. In this work, the finite element method is used to evaluate these coefficients. Using this method, the residual stresses in a 22 mm thick carbon/epoxy composite plate are measured and reported.     

Modelling of cross-ply piezoelectric composite laminates in cylindrical bending with interfacial shear slip

An approximate theory for cross-ply piezoelectric composite laminates in cylindrical bending with interfacial shear slip is developed. This theory uses only 4 displacement and potential variables, the number of which is independent of the number of layers involved. The displacement and electric potential fields are depicted by the displacement and electric potential distribution functions through thickness, respectively. The two functions are formulated according to particular solutions to the three-dimensional elasticity equilibrium equations and electrostatics charge equation. In this shear slip modelling interfacial opening is neglected. The interfacial bonding conditions are characterised by a linear slip law and an electrically permeable assumption. A corresponding finite element is also developed to deal with piezoelectric laminates with local shear slip. The accuracy and effectiveness of the present theory are demonstrated in numerical examples.     

Drilling-induced damage in uni-directional glass fiber reinforced plastic (UD-GFRP) composite laminates

Uni-directional glass fiber reinforced plastic (UD-FRP) composite materials are a feasible alternative to structural members that bear loads in only one direction. FRP composite materials have excellent properties in the direction of the fibers. Drilling- induced damage acts as an inhibitor to their application, as the holes act as stress concentration sites for failure under loading. The present study is an attempt to study the influence of drilling-induced damage on the residual tensile strength of uni-directional composite laminates and to propose a mathematical model correlating the residual strength with the drilling parameters. A finite element model (FEM) is also developed to study the drilling-induced damage in composite laminates.     

Elasto-plastic damage model considering cohesive matrix interface layers for composite laminates

A three-dimensional (3D) Finite element (FE)-based progressive damage model, which considers the interface matrix layer between two neighboring laminae as a layer of cohesive elements, is proposed to analyze laminated composite plates. An elasto-plastic damage model is integrated with the FE-based program ABAQUS that uses user-defined material subroutine. The present damage model includes fiber failure, matrix failure, and delamination effects. A cohesive zone model, which is available in ABAQUS and uses cohesive elements, is combined with the proposed model to address the delamination damage in the interface layers. 3D solid brick elements are used to model composite layers, and cohesive zone elements are used in between two composite layers to model the adhesive layers. The proposed model has been applied for the progressive damage simulation of AS4/PEEK composite laminates under in-plane and uniaxial tensile loading.     

Pitch-catch ultrasonic study on unidirectional CFRP composite laminates using Rayleigh wave transducers

The importance of carbon fiber reinforced plastics (CFRP) has been generally recognized, and CFRP composite laminates have become widely used. Thus, a nondestructive technique would be very useful for evaluating CF/epoxy composite laminates. A pitch-catch UT signal is more sensitive than is a normal incidence backwall echo of a longitudinal wave in composites. The depth of the sampling volume where the pitch-catch UT signal came from is relatively shallow, but the depth can be increased by increasing the separation distance of the transmitting and receiving probes. Moreover, a method is utilized to determine the porosity content of a composite lay-up by processing micrograph images of the laminate. The porosity content of a composite structure is critical to the overall strength and performance of the structure. The image processing method developed utilizes software to process micrograph images of the test sample. The results from the image processing method are compared with existing data. Beam profile is characterized in unidirectional CFRP using pitch-catch Rayleigh probes. The one-sided and two-sided pitch-catch techniques are utilized to produce C-scan images with the aid of an automatic scanner. The pitch-catch ultrasonic signal corresponds with the simulated results of unidirectional CFRP composites.     

Drilling of uni-directional glass fiber reinforced plastic (UD-GFRP) composite laminates

Drilling of fiber reinforced plastic (FRP) composite materials is a field open to a plethora of questions. Drilling-induced damage is a research area that has not been explored exhaustively. The present research endeavor is an effort to correlate drilling-induced damage with drilling parameters. Tool point geometry is considered a major factor that influences drilling-induced damage. Experiments were conducted and drilling-induced damage was quantified using the digital image processing technique. The results also reestablished the cutting speed to feed ratio as an important variable that influences drilling-induced damage. Mathematical models for thrust, torque, and damage are proposed that agree well with the experiments.     

复合材料层合板首末层失效强度的预测方法研究

建立复合材料的三维有限元模型,该模型能有效计算自由边界区的应力以及层间应力,与二维模型相比提高了求解精度;采用Tsai-Wu张量理论作为单元失效的判断依据,引入安全系数计算首层失效,然后使用增量法求解复合材料最终失效强度,使得求解速度加快的同时又不影响求解精度;使用正交实验方法研究不同刚度缩减系数( SCR)对首末层失效强度的影响,发现在进行渐进失效分析时,有限元模型是否合理应该根据首层失效强度与实验值进行比较,不能只考虑最终失效强度与实验值的差异.     

Stress field near transverse cracks under extension or in-plane shear in cross-ply composite laminates

Based upon the method of eigenfunction expansion and Leknitskii’s complex-variable potentials for the generalized plane deformation, the singular stress field near transverse cracks is examined for cross-ply composite laminates under two types of deformation, extension and in-plane shear. The stress singularity for each deformation is obtained from the eigenvalue equation resulting from appropriate near field conditions. It is found that the stress singularity for in-plane shear deformation is much stronger than for extension. To compute the stress intensity, use is made of the asymptotic representation for the stress and displacement field in conjunction with the singular hybrid finite element method. The numerical results are confirmed through comparison to those from other numerical techniques such as the boundary collocation method.     

Multi-objective design optimization of composite laminates using discrete shuffled frog leaping algorithm

This paper deals with the optimization of composite laminates using discrete shuffled frog leaping method. This approach is used to find optimal values of three design variables including fiber orientations in matrices, thickness of each ply and number of layers in order to minimize the weights and costs of the laminates under various loading cases. Three well-known failure criteria are employed to investigate the capability of the plate in bearing the applied loads. Moreover, the comparison of the obtained results with previously reported results is performed to prove the applicability of the algorithm in optimizing composite materials.