Efstathios E. Theotokoglou

Experimental and Numerical Study of Composite T-Joints

The present paper deals with experimental and nonlinear finite element analysis of sandwich T-joints which are formed by two panels connected by lap joints. Such joints are for instance commonly used in high-speed marine vehicles, and the joints considered herein are typical for such structures. This study is concerned with T-joints subjected to tension (pull-out) force and the behaviour up to ultimate failure. The behaviour of such sandwich joints is very complex, since it involves several types of materials such as GRP, glue filler as well as foam material, and a geometry with potential high stress concentrations. Also, nonlinear material and geometrical effects may influence the ultimate behaviour. In this study experimental specimens have been produced and tested to failure. A nonlinear finite element method is adopted throughout the T-joint, which is taking into account the possible high-stress concentration positions, the nonlinear properties of both the core and the glue materials and the nonlinear displacement deformation of the T-joint. In this way we succeeded in making a significant contribution to furthering the understanding of the response and failure of sandwich T-joints. The effect of some design parameters of such joints affecting joint strength and flexibility is also investigated.

Strength of Composite T-Joints under Pull-Out Loads

This paper examines the non-linear deflection response of highly stressed sandwich T-joints using large deflection and plasticity analyses. The finite element method considering a plane model of the joint, accounts for plastic deformation of the core and glue materials and non-linear geometric effects. The finite element studies are used to identify internal stress states in the various regions of the joint and for different attachment configurations, leading up to and at failure. The results of this analysis have been combined with the measured properties of the materials forming the joint in order to predict quantitatively the failure strength.

Study of the Numerical Fracture Mechanics Analysis of Composite T-Joints

The present paper is concerned with numerical studies of sandwich T- joints which are formed by two panels connected by lap joints. Sandwich GRP skin/PVC core panels are attractive structural components in high speed marine vehicles, especially due to their efficiency to carrying uniform lateral loading. The behaviour of such joints is very complex since it involves several types of materials such as GRP, foam material as well as glue filler, and a geometry with potential high stress concentrations. In this study, a brief review of the problem is first presented and areas of principal interest which most in- fluence joint behaviour are identified. Based on fracture mechanical considerations, a nu- merical crack design procedure has been developed. Finally, numerical results are pre- sented for the J-integral values in the case of T-joints subjected to lateral loads.

Analytical determination of the ultimate strength of sandwich beams

An analytical determination of the ultimate strength of a typical GRP/PVC sandwich beam has been performed. These beams represent common building practise in marine applications. Equations describing the behaviour of a sandwich panel under beam loading and various failure modes have been developed. The method has been applied to predict the ultimate load for a simple supported sandwich beam. The critical loads have been compared with those from the experimental investigation of a typical bulkhead-to-hull GRP/PVC sandwich T-joint under pull out forces.

An alternating coupling of finite elements and singular integral equations for the solution of branched cracks in finite sheets

Abstract The problem of a branched crack in a finite sheet is considered in this paper. The solution is given by Schwarzs alternating method, using two sequences of solutions. The first sequence corresponds to the finite, but uncracked, body, and the finite element method was used, whereas for the other sequence of solutions concerning the infinite cracked sheet, the singular integral equation method. In this way, the well-known capabilities of singular integral equations to describe accurately singular fields with the flexibility and the stability of finite element method are efficiently combined to solve rapidly, and with a reduced computer cost, complicated problems of cracked plates encountered in the praxis. Numerical applications of the method proved the rapid convergence and the stability of the procedure, as well as its accuracy and versatility.

A combined integral-equation and finite-element method for the evaluation of stress intensity factors

Abstract A combination of the classical finite-element and the singular-integral-equation method with the help of Schwarzs Alternating Method (S.A.M.) is presented. In this way the flexibility of finite-element method is complemented by the undisputed power of the singular integral equation method in describing singular fields. The result is the creation of a powerful hybrid method convenient for the efficient solution of problems of stress fields containing any type of singularity. The examples considered in the paper indicate the excellent approximation obtained by the method even when using only a few classical elements.

Study of Composite Beams in Asymmetric Four-Point Bending

This article deals with the numerical study of composite beams in asymmetric four-point bending and the comparison of the induced results with experimental and numerical results. The measurement of the interlaminar shear strength of composite beams, an important design variable in many applications, may be suc-cessfully performed by the recently established asymmetric off-axis bending test. In order to obtain experimental results, specimens had been produced and tested to failure. A linear FEA is adopted throughout the composite beams to obtain the stress distributions in the beams and especially at the loading points and the supports where the stresses vary abruptly and present the most important discrepancies when compared with the classical theory. This is a main purpose of this article since the application of the FEA can elucidate the situation in these areas to a great extent. From the FEA, possible crack initiation positions are determined and compared with the corresponding results obtained from the experimental investigation. Also, a comparison between the experimental results of the deflections and the numerical values was carried out in order to estimate the declination.

Numerical Study of Fractured Sandwich Composites under Flexural Loading

Fatigue crack growth of foam core sandwich beams loaded in flexure has been investigated numerically. Extensive fatigue data from foam core sandwich composites under flexural loading were analyzed. A static finite element analysis is adopted for the uncracked sandwich beam in order to predict crack initiation positions. A face/core debond parallel to the beam axis is considered in accordance with experimental data. A static two-dimensional finite element analysis of the cracked sandwich beam is accounted for the evaluation of stress intensity factors at the crack tips investigating the crack growth behavior. In this way it is succeeded in making a significant contribution to understand the initiation and propagation of a fatigue crack in the foam core of sandwich beams loaded by flexural loading.

An integral-equation solution for cracked half-planes bonded together and containing debondings along their interface

The general solution of an arbitrary system of microdefects (i.e. cracks and/or holes) in an isotropic elastic half-plane bonded partially, along an infinite number of straight line segments to another half-plane consisting of a different isotropic elastic material, is formulated in this paper using the complex variable technique. The solution in terms of complex potentials is given by integrals over the cracks and/or holes with integrands expressed in terms of Greens functions and an unknown complex density function. Finally, the problem is reduced to the solution of a singular integral equation for the complex density function only along the microdefects. The appropriate Greens functions are derived from the solution of the problem of a concentrated force or a dislocation existing in either of the two half planes. Numerical results are presented for the stress intensity factors in three different cases.

A higher order FEM for time-domain hydroelastic analysis of large floating bodies in an inhomogeneous shallow water environment

The study of wave action on large, elastic floating bodies has received considerable attention, finding applications in both geophysics and marine engineering problems. In this context, a higher order finite-element method (FEM) for the numerical simulation of the transient response of thin, floating bodies in shallow water wave conditions is presented. The hydroelastic initial-boundary value problem, in an inhomogeneous environment, characterized by bathymetry and plate thickness variation, is analysed for two configurations: (i) a freely floating strip modelling an ice floe or a very large floating structure and (ii) a semi-fixed floating beam representing an ice shelf or shore fast ice, both under long-wave forcing. The variational formulation of these problems is derived, along with the energy conservation principle and the weak solution stability estimates. A special higher order FEM is developed and applied to the calculation of the numerical solution. Results are presented and compared against established methodologies, thus validating the present method and illustrating its numerical efficiency. Furthermore, theoretical results concerning the energy conservation principle are verified, providing a valuable insight into the physical phenomenon investigated.