Nicholas G. Tsouvalis

Buckling of unsymmetric laminates under linearly varying, biaxial in-plane loads, combined with shear

Classical lamination theory in conjunction with the Rayleigh-Ritz method is used for the determination of the critical buckling load of simply supported, unsymmetric cross-ply and antisymmetric angle-ply laminates, under linearly varying in-plane biaxial loads, combined with shear. The algorithms that produce each element of both the stiffness and load matrices in the final generalized eigenvalue problem are obtained. No limitation in the number of terms of the displacement field expansion considered exists. The validity of the presented method is evaluated by comparing its results with those of other investigators. Finally, a parametric study for both types of laminates is given, changing parameters such as the form of the applied load, the E1E2 ratio and the angle of fiber orientation.

Numerical modeling of composite laminated cylinders in compression using a novel imperfections modeling method

The present study presents the finite element modeling procedure of two composite laminated cylinders exhibiting initial geometric imperfections. Using as input a set of experimental measurements of the cylinder geometry, the application of the skinning method leads to the analytical representation of the cylinder imperfect internal, external and middle surfaces. A finite element mesh is then easily constructed over these surfaces. The results of the analysis are in very good agreement with the experimental strains and buckling load measurements and are used to estimate the knockdown effect of the imperfections on the cylinder buckling behaviour. They are also compared to results obtained by other simpler finite element models, in an effort to evaluate the accuracy of various modeling simplifications.

Post Buckling Progressive Failure Analysis of Composite Laminated Stiffened Panels

The present work deals with the numerical prediction of the post buckling progressive and final failure response of stiffened composite panels based on structural nonlinear finite element methods. For this purpose, a progressive failure model (PFM) is developed and applied to predict the behaviour of an experimentally tested blade-stiffened panel found in the literature. Failure initiation and propagation is calculated, owing to the accumulation of the intralaminar failure modes induced in fibre reinforced composite materials. Hashin failure criteria have been employed in order to address the fiber and matrix failure modes in compression and tension. On the other hand, the Tsai-Wu failure criterion has been utilized for addressing shear failure. Failure detection is followed with the introduction of corresponding material degradation rules depending on the individual failure mechanisms. Failure initiation and failure propagation as well as the post buckling ultimate attained load have been numerically evaluated. Final failure behaviour of the simulated stiffened panel is due to sudden global failure, as concluded from comparisons between numerical and experimental results being in good agreement.

The effect of geometric imperfections on the buckling behaviour of composite laminated cylinders under external hydrostatic pressure

Using a new FE modelling procedure for the accurate representation of a geometrically imperfect cylinder, the present study investigates the effect of the initial imperfection magnitude on the cylinder buckling load, when it is loaded by external hydrostatic pressure. The buckling behaviour of a carbon/epoxy filament wound cylinder is initially studied and the modelling procedure is validated through a comparison between calculated and experimental results. Various FE models are developed and evaluated, with increasing degree of complexity. The method is then applied to a cylinder with different end supports, to assess how the boundary conditions together with the imperfections affect the buckling behaviour. Finally, the effect of imperfection magnitude is investigated, leading to the calculation of knockdown factors as low as 0.6.

Large deflection dynamic response of composite laminated plates under in-plane loads

An analytical solution is presented for the dynamic buckling behaviour of a laminate subjected to time-dependent uniformly distributed normal and shear in-plane loads based on the Classical Lamination Theory and accounting for moderately large deflections. The assumed load-time variation is either linear or of a pulse type. The Galerkin method is applied and the obtained nonlinear differential equations are solved numerically by the Runge-Kutta method. The parametric study performed revealed three distinct phases in the undamped centre deflection time-history of a laminate. No significant deformations are produced in phase I, while the subsequent phase II reflects the dynamic buckling behaviour, exhibiting a rapid increase of the deformations. These magnitudes follow an oscillating motion in phase III, analogous to the loading condition under consideration. Relatively large initial imperfection values change completely the response of the laminate, by eliminating both the characteristic dynamic buckling phase II and the oscillations of phase III.

Mechanical behaviour of bimodulus laminated plates

Abstract A new analytical method for the prediction of the mechanical behaviour of laminates consisting of layers having different properties in tension and compression is proposed in this paper. As a case study, bending of simply supported, multilayered, unsymmetric, specially orthotropic laminates under various types of lateral loads is considered. The underlying bending theory used is a higher-order shear deformation theory. With the proposed method, laminates having more than two layers, not necessarily in an antisymmetric stacking sequence, can be analyzed. The validity of the new method is confirmed by comparing its results with the ones of other simpler theories. Finally, as an example, deflections, strains and stresses are calculated for a five-layer, unsymmetric, specially-orthotropic laminate.

Fabrication, Testing and Analysis of Steel/Composite DLS Adhesive Joints

This paper describes experimental and numerical techniques to study the structural design of double lap shear joints that are based on thick-adherend steel/steel and steel/composite, with epoxy adhesive. A standard practical fabrication method was used to produce specimens of various dimensions and materials. These specimens consist of 10 mm steel inner adherend and various outer adherend materials including composite and steel of various thicknesses and overlaps. The composite is largely based on carbon fibre-reinforced plastic. The specimens were tested under monotonic tensile loading and the results showed that joint strength depends largely on materials combination and overlap length. The testing also included the use of an advanced imaging system to determine failure initiation and propagation. Two-dimensional finite element analysis (FEA) stress models were applied and showed the importance of modelling the composite layers adjacent to the adhesive bondline in order to account for the critical local stresses. The FEA results also showed that overall shear stress distributions can be used to characterise joint failure. The paper presents the experimental and numerical details with key conclusions.

A 3D ductile constitutive mixed-mode model of cohesive elements for the finite element analysis of adhesive joints

In this paper, a new traction–separation law is developed that represents the constitutive relation of ductile adhesive materials in Modes I, II, and III. The proposed traction–separation laws model the elastic, plastic, and failure material response of a ductile adhesive layer. Initially, the independent-mode proposed laws (loading and fracture in Modes I, II, and III) are mathematically described and then introduced in a developed formulation that simulates the interdependency of the mixed-mode coupled laws. Under mixed-mode conditions, damage initiation is predicted with the quadratic stress criterion and damage propagation with the linear energetic fracture criterion. For verification and validation purposes of the proposed laws and mixed-mode model, steel adherends have been adhesively bonded with a structural ductile adhesive material in order to fabricate a series of single and double strap adhesive joint configurations. The specimens have been tested under uni-axial quasi-static load and the respective force and displacement loading history have been recorded. Corresponding numerical and experimental results have been compared for each joint case, respectively. Additionally, the developed stress fields (peel, in-plane, and out-of-plane shear) are presented as they evolve during the loading of both joint cases.

Determination of the fracture process zone under mode I fracture in glass fiber composites

This study provides a simple yet effective procedure for the characterization of the fracture process zone (FPZ) developing in the interface of unidirectional laminates under Mode I delamination fracture. Double cantilever beam (DCB) coupons have been manufactured and tested. Three data reduction schemes available in the literature have been utilized for the calculation of the energy release rate (ERR) magnitude as a function of crack extension and the corresponding R-curves have been constructed. The R-curves were then reconstructed in terms of the experimentally registered pre-crack tip opening displacement (δ*) and analytical functions have been used to describe their concatenate trend. The J-integral approach was then applied over the analytical functions to derive the corresponding bridging laws that describe the FPZ. The derived bridging laws were appropriately modified according to three different traction–separation models and implemented into user-developed interface finite elements (UEL) for the simulation of the fracture tests in ABAQUS® commercial software. Comparisons between numerical and experimental results have shown that the proposed straightforward procedure leads to an effective traction–separation law that can be used as a material property of the modeled interface.

Buckling strength parametric study of composite laminated plates with delaminations

The purpose of this work is to investigate the effect of delaminations on the buckling behaviour of a marine composite hull. This is done by using the finite-element method to model delaminations and calculate the buckling strength of a typical marine composite panel. The parametric study is based on a marine panel clamped along all its edges and loaded in compression. The delamination was assumed to have an elliptic shape and the parameters investigated in the analysis were its shape, magnitude and location. The total number of cases investigated is 45. The eigenvalue buckling analyses led in many cases in inadmissible buckling mode shapes. A procedure for eliminating these inadmissible mode shapes in the non-linear analyses is described in the paper. The final results indicated that the greatest effect comes from delaminations which are closer to the laminate surface, are closer to the circular shape and have the largest magnitude.