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1.
Negative size effects are commonly reported for advanced composite materials where the strength of the material decreases with increasing volume of the test specimen. In this work, the effect of increasing specimen volume on the mechanical properties of all-cellulose composites is examined by varying the laminate thickness. A positive size effect is observed in all-cellulose composite laminates as demonstrated by a 32.8% increase in tensile strength as the laminate thickness is increased by 7 times. The damage evolution in all-cellulose composite laminates was examined as a function of the tensile strain. Enhanced damage tolerance concomitant with increasing specimen volume is associated with damage accumulation due to transverse cracking and strain delocalisation. A transition from low-strain failure to tough and high-strain failure is observed as the laminate thickness is increased. Simultaneously, scale effects lead to an increase in the void content and cellulose crystallinity at the core, with increasing laminate thickness.  相似文献   

2.
The emergence of advanced computational methods and theoretical models for damage progression in composites has heralded the promise of virtual testing of composite structures with orthotropic lay-ups, complex geometries and multiple material systems. Recent studies have revealed that specimen size and material orthotropy has a major effect on the open hole tension (OHT) strength of composite laminates. The aim of this investigation is develop a progressive failure model for orthotropic composite laminates, employing stepwise discretization of the traction–separation relationship, to predict the effect of specimen size and laminate orthotropy on the OHT strength. The results show that a significant interaction exists between delamination and in-plane damage, so that models without considering delamination would over-predict strength. Furthermore, it is found that the increase in fracture toughness of blocked plies must be incorporated in the model to achieve good correlation with experimental results.  相似文献   

3.
A tension fatigue life prediction methodology for composite laminates is presented. Tension fatigue tests were conducted on quasi-isotropic and orthotropic glass epoxy, graphite epoxy, and glass/graphite epoxy hybrid laminates. Edge delamination onset data were used to generate plots of strain energy release rate as a function of cycles to delamination onset. These plots were then used along with strain energy release rate analyses of delaminations initiating at matrix cracks to predict local delamination onset. Stiffness loss was measured experimentally to account for the accumulation of matrix cracks and for delimination growth. Fatigue failure was predicted by comparing the increase in global strain resulting from stiffness loss to the decrease in laminate failure strain resulting from delaminations forming at matrix cracks through the laminate thickness. Good agreement between measured and predicted lives indicated that the through-thickness damage accumulation model can accurately describe fatigue failure for laminates where the delamination onset behaviour in fatigue is well characterized, and stiffness loss can be monitored in real time to account for damage growth.  相似文献   

4.
Most of the previously performed damage analyses in composite laminates have been restricted to model the plain laminates without geometry discontinuities. In this study, a micromechanical damage model is combined with the finite element formulation and is implemented in the integration points to perform progressive damage analyses of composite laminates. A micromechanical damage model based on the stress transfer method is used to find the degradation of mechanical properties of composite laminates. Crack density is also used as an only state variable representing the damage in each Gauss point of every layer of the laminate. The strain energy and critical energy release rate criterion is also used to predict the damage initiation and evolution in each layer. A finite element discretization is used in conjunction with the user element definition capability of ABAQUS commercial software. To verify the developed procedure, a single element is analysed, and the obtained results are compared with available results in the literature. Progressive damage analyses are also performed for several symmetric cross‐ply laminates with and without geometry discontinuity subjected to matrix cracking damage mechanism under in‐plane loading conditions. The obtained mechanical response and variations of matrix crack density versus the applied load are also discussed.  相似文献   

5.
This paper presents a new technique of neural network constitutive modelling for non‐linear characterization of anisotropic materials. The proposed technique, based on a recently developed energy‐based characterization framework, derives the variations of the external work applied to and the strain energy induced in a specimen. The error between the variations of the energies is subsequently applied to correct the neural network properties by using a modified backpropagation algorithm. Unlike the conventional techniques for neural network constitutive modelling, the proposed technique develops models by quantifying the deformation of the specimen on a continuum basis. This allows the neural network constitutive models to be constructed from a single load test of one specimen. Numerical examples first examine the effect of specimen geometries and loading conditions. The effect of noise in the experimental measurements is subsequently investigated while having the applicability for non‐linear constitutive behaviour shown thereafter. The application for anisotropic materials is finally demonstrated by modelling a unidirectional lamina based on the measurements of a biaxial load test on a balanced laminate. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

6.
Abstract

By simplifying the impact damages as a single delamination near the surface with an elliptical boundary, the approximate solution of total strain energy release rates can be derived as a function of delamination major axis, minor axis, external compressive strain, Possion's ratio of parent medium, extensional and bending stiffnesses of sublaminate. A linear relation of residual strength versus strain energy release rate can be constructed by correlating the approximate solution with test data of compressive residual strain (strength) after impact (CSAI), indicating that the dimension between the delamination major and minor axis should be dependent. In addition, the delamination aspect ratio is found to be not only a function of the specimen geometry and the extensional stiffness, but also a function of laminate thickness. The approximate solution provides a method for predicting the post impacted strength of composite laminate for only either thick laminate or thin laminate with low impact energy.  相似文献   

7.
A finite element (FE) model using coupling continuum shell elements and cohesive elements is proposed to simulate the compression after impact (CAI) behaviour and predict the CAI strength of stitched composites. Continuum shell elements with Hashin failure criterion exhibit the composite laminate damage behaviour; whilst cohesive elements using traction-separation law characterise the laminate interfaces. Impact-induced delamination is explicitly modelled by reducing material properties of damaged cohesive elements. Computational results have demonstrated the trend of increasing CAI strength with decreasing impact-induced delamination area. Spring elements are introduced into the model to represent through-thickness stitch thread in the composite laminates. Results in this study validate experimental finding that CAI strength is improved when stitching is incorporated into the composite structure. The proposed FE model reveals good CAI strength predictions and indicates good agreement with experimental results, making it a valuable tool for CAI strength prediction of stitched composites.  相似文献   

8.
9.
The static failure behaviour of the fibre-metal laminate GLARE is examined using 3D finite element simulations. The configuration analysed is a centre-cracked tensile specimen composed of two aluminium layers sandwiching a cross-plied, fibre-epoxy layer. The crack and delamination growths are simulated by means of interface elements equipped with a mixed-mode damage model. The mode-mixity is derived from an energy criterion typically used in linear elastic fracture mechanics studies. The damage kinetic law is rate-dependent, in order to simulate rate effects during interfacial delamination and to avoid numerical convergence problems due to crack bifurcations. The numerical implementation of the interface damage model is based on a backward Euler approach. In the boundary value problem studied, the failure responses of GLARE specimens containing elastic aluminium layers and elasto-plastic aluminium layers are compared. The development of plastic deformations in the aluminium layers stabilizes the effective failure response, and increases the residual strength of the laminate. For a ‘quasi-brittle’ GLARE specimen with elastic aluminium layers, the residual strength is governed by the toughness for interfacial delamination, and is in close correspondence with the residual strength obtained from a closed-form expression derived from energy considerations. Conversely, for a ‘ductile’ GLARE specimen with elasto-plastic aluminium layers, the residual strength is also determined by the relation between the fracture strength and the yield strength of the aluminium. The amount of constraint by the horizontal displacements at the vertical specimen edges has a moderate to small influence on the residual strength. Furthermore, the ultimate laminate strength is lower for a larger initial crack length, and shows to be in good correspondence with experimental values.  相似文献   

10.
复合材料层合板稳定性的铺层优化设计   总被引:14,自引:0,他引:14  
修英姝  崔德刚 《工程力学》2005,22(6):212-216
提出采用神经网络和遗传算法来优化设计复合材料层合板,建立了满足铺层结构稳定性的优化铺层体系,优化体系分两步进行优化,第一步,当给出总的铺层数时,由已建立的神经网络模型确定规定角度下的铺层数,确立基本的铺层结构,第二步,采用遗传算法优化这种铺层结构下的铺层顺序,最终在同样重量下获得了最佳的结构铺层。  相似文献   

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