Buckling and postbuckling behavior of antisymmetrically angle-ply laminated composite plates

The buckling and postbuckling behaviors of perfect and imperfect antisymmetricallyangle-ply laminated composite plates under uniaxial compression have been studied byperturbation technique which takes deflection as its perturbation parameter.In this paper,the effects of in-plane boundary conditions,angles,total number oflayers and initial geometric imperfection on the postbuckling behavior of laminated plateshave been discussed.     

Free-edge stresses in antisymmetric angle-ply laminates in extension and torsion

The first-order shear deformation theory and the layerwise theory of laminated plates are employed to analyze the edge-effect problem of an antisymmetric angle-ply laminate subjected to arbitrary combinations of extensional and torsional loads. The first-order theory is used for predicting the unknown constant parameters appearing in the reduced displacement field of elasticity which, on the other hand, signify the global behavior of the laminate. A layerwise theory is then utilized to determine the local interlaminar stresses within the boundary-layer regions of laminates. In order to closely examine the behavioral characteristics of interlaminar stresses, various numerical examples are presented for different antisymmetric angle-ply laminates under an axial force and a torque.     

Effect of transverse cracks on the mechanical properties of angle-ply composites laminates

A modified shear lag analysis, taking into account the notion of stress perturbation function, is employed to evaluate the effect of transverse cracks on the stiffness reduction in [±θ_n/90_m]_S angle-ply laminated composites. Effects of number of 90° layers and number of ±θ layers on the laminate stiffness have also been studied. The present results represent well the dependence of the degradation of mechanical properties on the fibre orientation angle of the outer layers, the number of cracked cross-ply layers and the number of uncracked outer ±θ layers in the laminate.     

Buckling analysis of pristine and defected graphene

The buckling behavior of monolayer graphene (pristine and vacancy-defected) and bilayer graphene (pristine) loaded in the armchair direction was simulated for different boundary conditions using a truss FE model, representing the exact atomic lattice of graphene, and a FE model of an equivalent 2D plate. The critical buckling stress of pristine monolayer graphene was derived as a function of aspect ratio. The results from the two FE models coincide and are in very good agreement with established analytical solutions. With increasing the aspect ratio, the critical buckling stress of monolayer graphene decreases until the value of 2 from which the effect starts to diminish. Using the truss FE model, the effect of randomly dispersed vacancies on the critical buckling stress and buckling mode of monolayer graphene was studied. It was found that the critical buckling stress decreases dramatically with increasing the defect density: for a defect density of 10%, the critical buckling stress decreases by almost 50%. Moreover, the presence of defects was found to affect the highest buckling modes (above 3) even at low densities. Bilayer graphene has a totally different critical buckling stress than monolayer graphene due to the effect of van der Waals forces which depends on the applied boundary conditions.     

碳纳米管/碳纤维增强复合材料层合板低速冲击响应和破坏的数值模拟

碳纳米管/碳纤维增强复合材料（carbon nanotube/carbon fibre reinforced plastic,CNT/CFRP）是一种多尺度复合材料,比传统CFRP有更好的综合性能和更广阔的应用前景。对CNT/CFRP在低速冲击下的响应和破坏进行了数值模拟研究。首先,基于先前的研究通过引入基体增韧因子、残余强度因子并改进损伤耦合方程,建立了新的FRP动态渐进损伤模型;然后,利用新建立的本构模型并结合黏结层损伤模型,对4种碳纳米管含量的增韧碳纤维增强树脂基复合材料层合板在5个能量下的冲击实验进行了数值模拟;最后,将模拟结果与文献中的相关实验结果进行了比较,并讨论了冲击速度的影响。结果表明:新建立的FRP本构模型能够预测CNT/CFRP层合板在低速冲击载荷作用下的响应、破坏过程和分层形貌,模拟得到的载荷-位移曲线和破坏形貌与实验吻合较好;冲击速度会影响CNT/CFRP层合板拉伸和压缩破坏的比例,相同的冲击能量下,更大的冲击速度会造成更多的拉伸破坏。     

Further assessment of a generalized plate model: stress analysis of angle-ply laminates

This study is a continuation of the investigation presented in reference (International Journal of Mechanical Sciences 44 (2002) 287) and deals with an assessment of the stress analysis performance of the generalised shear deformable theory presented in (Acta Mechanica 123 (1997c) 163) when dealing with angle-ply laminated beams. One of the main conclusions is that the existing elasticity solutions for simply supported laminates cannot anymore be considered as safe means in checking and testing the effectiveness of other, conventional shear deformable theories, at least as far as angle-ply laminates are concerned.     

Buckling and vibration of rectangular laminates with cut off regions

The pure global buckling and vibration of four sides simply-supported as well as clamped orthotropic rectangular laminates having multiple rectangular cut off regions that are symmetric with respect to mid-plane have been studied by treating the remaining cut off regions as uniform plates with reduced stiffnesses. The variation of stiffness of the plate is represented by Fourier series. Some numerical results for the pure global buckling bad prediction due to its reduced flexural stiffness for typical cases of rectangular middle-plane symmetric cut off regions are presented.     

A twofold strength and toughness criterion for the onset of free-edge shear delamination in angle-ply laminates

Free edge delamination in composite structures results from very localised stress fields which induce a stress concentration promoting the nucleation of an interfacial crack. To predict such a delamination onset at the free edge of a $(\pm \theta {)}_s$ laminate in traction, use is made of a strength and toughness criterion which combines a stress condition with an energy analysis. A generalised plane strain model allows to determine the stress distribution near the free edge and the energy released by the nucleation of an interfacial crack. The results show that this approach can predict the delamination onset for $((\pm 10{)}_s\text{,}(\pm 20{)}_s)$ laminates provided the interfacial fracture energy and interlaminar shear strength are known. These characteristic values can be identified with the help of traction tests performed on samples with different thicknesses.     

Buckling of reinforced fibers during explosive welding

This paper is devoted to the study of processes of buckling of fibers while fabricating fiber-reinforced composite materials by explosive welding. The process of welding of metals by itself is not studied in this article. Up to the moment of explosion, the multi-layered workpieces consist of alternate layers of plane sheets and parallel bars of rein-forcement. Matrix is fabricated as a result of welding of plane sheets. In the course of collision at explosive welding, the reinforcing bars are subjected to intensive longitudinal impact. The modes of buckling of reinforcement obtained in a theoretical analysis are compared with experimental modes. It has been established that, to describe satisfactorily the experimental results, in a theoretical analysis one must take into account the interaction of longitudinal and transversal oscillations of bars.     

Effect of transverse cracks on the effective thermal expansion coefficient of aged angle-ply composites laminates

A modified shear-lag analysis, taking into account the concept of stress perturbation function, is developed and applied to evaluate the effect of transverse cracks on the effective thermal expansion coefficient of aged angle-ply composites laminates. Effects of number of 90° layers and number of θ° layers in the outer angle-ply laminates on the reduction of the effective axial coefficient of thermal expansion have also been studied. The results of this paper represent well the dependence of the reduction of the effective axial coefficient of thermal expansion on the hygrothermal conditions, the fibre orientation angle of the outer layers, the number of cracked cross-ply layers and the number of un-cracked outer θ° layers in laminate.     

Intermittency and multiscale dynamics in milling of fiber reinforced composites

We have analyzed the variations in cutting force during milling of a fiber-reinforced composite material. In particular, we have investigated the multiscale dynamics of the cutting force measured at different spindle speeds using multifractals and wavelets. The multifractal analysis revealed the changes in complexity with varying spindle speeds. The wavelet analysis identified the coexistence of important periodicities related to the natural frequency of the system and its multiple harmonics. Their nonlinear superposition leads to the specific intermittent behavior. The workpiece used in the experiment was prepared from an epoxy-polymer matrix composite reinforced by carbon fibers.     

Buckling of soft-core sandwich plates with angle-ply face sheets by means of a C0 finite element formulation

In order to avoid using C¹ interpolation functions in finite element implementation of the previous zig–zag theories, artificial constraints, in which the first derivatives of transverse displacement will be replaced by the assumed variables, are usually employed. However, such assumption will violate continuity conditions of transverse shear stresses at interfaces. Differing from previous work, this paper will propose a C⁰-type zig–zag theory for buckling analysis of laminated composite and sandwich plates with general configurations. The first derivatives of transverse displacement have been taken out from a displacement field of the proposed zig–zag theory. Thus, the C⁰ interpolation functions are only required in finite element implementations of the proposed model. Without use of any artificial constraints, an eight-node quadrilateral element based on the proposed model is presented by incorporating the terms associated with the geometric stiffness matrix. In order to verify performance of the proposed model, several buckling problems of sandwich plates with soft core have been analyzed. Numerical results show that the proposed model is able to predict accurately buckling loads of the soft-core sandwich plates with varying fiber orientations of face sheets.     

Bending of beams with bisymmetrical cross sections under non-uniformly distributed load: analytical and numerical-FEM studies

Archive of Applied Mechanics - The paper is devoted to simply supported beams subjected to non-uniformly distributed loads. Shapes of bisymmetrical cross sections of the beams are expressed by...     

Buckling behavior of compression-loaded composite cylindrical shells with reinforced cutouts

Results from a numerical study of the response of thin-walled compression-loaded quasi-isotropic laminated composite cylindrical shells with unreinforced and reinforced square cutouts are presented. The effects of cutout reinforcement orthotropy, size, and thickness on the non-linear response of the shells are described. A high-fidelity non-linear analysis procedure has been used to predict the non-linear response of the shells. The analysis procedure includes a non-linear static analysis that predicts stable response characteristics of the shells and a non-linear transient analysis that predicts unstable dynamic buckling response characteristics. The results illustrate the complex non-linear response of a compression-loaded shell with an unreinforced cutout. In particular, a local buckling response occurs in the shell near the cutout and is caused by a complex non-linear coupling between local shell-wall deformations and in-plane destabilizing compression stresses near the cutout. In general, reinforcement around a cutout in a compression-loaded shell can retard or eliminate the local buckling response near the cutout and increase the buckling load of the shell. However, results are presented that show how certain reinforcement configurations can cause an unexpected increase in the magnitude of local deformations and stresses in the shell and cause a reduction in the buckling load. Specific cases are presented that suggest that the orthotropy, thickness, and size of a cutout reinforcement in a shell can be tailored to achieve improved buckling response characteristics.     

Buckling test and optimum design of fiber reinforced composite plates under compression

In this paper, the conclusion that the experimental results coincide with theoretical analysis has been got through buckling test of 283 composite rectangular plates. It is confirmed that the critical loads of composite plates calculated by buckling theoretical formula of anisotropic plate are reliable. The selections of optimal content of matrix and optimal off-axis which make fiber reinforced composite plates reach biggest critical loads are also discussed in this paper. The result of analysis may be used in the design for the products.     

Experimental validation of multiscale modeling of indentation of suspended circular graphene membranes

In recent studies, the in-plane elastic properties of graphene have been computed via Density Functional Theory (DFT) and expressed in the form of a higher-order continuum elastic constitutive model. The studies predict that graphene exhibits an anisotropic and non-linear elastic response at high strains. However, one study predicts that the rupture mechanism of graphene at its intrinsic strength is due to elastic instability whereas another study predicts the rupture mechanism at its intrinsic strength is due to phonon instability. In the present paper, we use the higher-order continuum elastic constitutive model within the context of the finite element method to simulate a set of experiments of the indentation of circular freestanding monatomic graphene membranes. There is a close correspondence between the measured and predicted measured force vs. displacement responses of indented graphene, providing experimental validation for the constitutive response. Further, there is a close correspondence between the measured and predicted breaking force of graphene via the elastic instability mechanism. Thus, the results suggest that the elastic instability precipitates failure of pristine graphene at its intrinsic strength, and also provides further experimental validation of the constitutive response.     

Phenomenological and numerical modelling of short fibre reinforced cementitious composites

In this paper, a constitutive model for short fibre reinforced cementitious composites will be presented. This model is based on the St. Venant–Kirchhoff model, which is a special case of a hyperelastic material. This model is refined to include the fibre orientation distribution. Numerical FEM simulations with the developed constitutive model and fracture simulations using the discrete element method are presented. The outcomes of these numerical methods demonstrate how important it is to monitor and further to control the fibre orientation distribution during the manufacturing process. As the manufacturing process might involve casting, as, e.g., in the case of steel fibre reinforced concrete, an outlook on simulations of the manufacturing process in order to predict and to control the fibre orientation distribution is given.     

Buckling of nano-fibre reinforced composites: a re-examination of elastic buckling

Elastic buckling of layered/fibre reinforced composites is investigated. Assuming the existence of both shear and transverse modes of failure, the fibre is analysed as a layer embedded in a matrix. Interacting stresses, acting at the interfaces are determined from an exact derived stress field in the matrix. It is shown that buckling can occur only in the shear buckling mode and that the transverse buckling mode is spurious. As opposed to the well known Rosen shear buckling mode solution (predicated on an infinite buckling wavelength), shear buckling is shown to exist under two régimes: buckling of dilute composites with finite wavelengths and buckling of non-dilute composites with infinite wavelengths. Based on the analysis, a model is constructed which defines the fibre concentration at which the transition between the two régimes occurs. The buckling strains are shown to be (approximately) constant for dilute composites and, in the case of very stiff fibres, to have realistic values compatible with elastic behaviour. For the case of non-dilute composites, the strains are found to be in agreement with those given by the Rosen shear buckling solution. Numerical results for the buckling strains and stresses are presented and compared with the Rosen solution. These reveal that the Rosen solution is valid only for the case of non-dilute composites. The investigation demonstrates that elastic buckling may be a dominant failure mechanism of composites consisting of very stiff fibres fabricated in the framework of nano-technology.