Sotiris Koussios

Friction Experiments for Filament Winding Applications

The design procedure of nongeodesic filament wound products requires well-determined values for the available friction situated between the applied roving and the supporting surface. In this paper, we propose a mandrel shape with a specially designed meridian profile that enables a linearly proportional relation between the feed eye carriage translation and the measured values for the coefficients of friction. As a result of this property, the optically or chronometrically obtained measurements can directly be translated into coefficients of friction. Additional features of this approach are the high accuracy, repeatability, low experimental costs, and simple machine control strategies. With the proposed mandrel, we performed several experiments corresponding to the variation of typical filament winding-related process parameters: fiber speed, roving tension, roving dimensions, wet versus dry winding, and surface quality of the mandrel. The results indicate that the surface quality of the mandrel and the type of winding process (wet vs. dry fibers) have a considerable influence on the obtained data. The influence of the fiber speed, roving tension, and fiber material on the other hand, is negligible.

Integral Design for Filament-Wound Composite Pressure Vessels

The most important issues for the integral design of a filament-wound pressure vessel reflect on the determination of the dome shape and applied winding patterns. The goal of this paper is to determine the meridian profiles of continuum-based domes for pressure vessels, and to demonstrate that the utilization of non-geodesic trajectories forms a favorable alternative to the dome design. An integral methodology for the design of such dome structures is outlined, with emphasis on the application of the non-geodesic winding law and the classical lamination theory. Based on the condition of equal shell strains, the governing equation for the shape of the dome meridian and the differential equation describing non-geodesic trajectories on the dome surface are derived. The meridian profiles of non-geodesics-based carbon-epoxy domes are obtained for various slippage coefficients; the structural efficiency of geodesics and non-geodesics-based domes for various polar radii are then calculated and compared to each other. The results concluded that filament-wound domes of pressure vessels designed using the non-geodesics provide better performance than geodesics-based ones.

Development of Filament Wound Composite Isotensoidal Pressure Vessels

Filament wound isotensoidal structures are recently gaining more attention for designing composite pressure vessels. In this paper we present the governing equation for creating geodesic-isotensoids based on the netting theory and geodesic winding law. The feasible intervals of the isotensoid-based design are also determined. The isotensoid-based dome profiles are determined by solving the governing equation with geometrical and initial winding conditions. When the applied axial load reaches a certain magnitude, the isotensoidal toroids can be obtained by forcing the isotensoid-based dome profile to become closed. The comparisons of the cross-sectional shapes between the isotensoidal dome and the hemispherical dome, and between the isotensoidal toroid and the circular toroid, are performed to demonstrate the preferable performance of the isotensoids. It is concluded that the isotensoid-based design leads to uniform fiber tension throughout the whole shell and the resulting structure can thus be considered as optimal for a pressure vessel. In addition, the isotensoid-based profiles show lower aspect ratios than the conventional vessel profiles under the given volume and internal pressure. Therefore the structural performance and the conformability to limited-height storage space of pressure vessels can be improved using the isotensoid-based design.

Analytic Methods for Stress Analysis of Two-Dimensional Flat Anisotropic Plates With Notches: An Overview

The anisotropy of composite plates often poses difficulties for stress field analysis in the presence of notches. The most common methods for these analyses are: (i) analytical means (AM), (ii) finite element analysis (FEA), and (iii) semi-analytical means (SAM). In industry, FEA has been especially popular for the determination of stresses in small to medium size parts but can require a considerable amount of computing power and time. For faster analyses, one can use AM. The available solutions for a given problem, however, can be quite limited. Additionally, AM implemented in commercial computer software are scarce and difficult to find. Due to this, these methods are not widespread and SAM were proposed. SAM combine the (easy) implementation of complex problems from FEA and the computational efficiency from AM to reduce the difficulty on mathematical operation and increase computational speed with respect to FEA. AM, however, are still the fastest and most accurate way to determine the stress field in a given problem. Complex problems, however, e.g., finite width plates with multiple loaded/unloaded notches, require a significant amount of mathematical involvement which quickly discourages, even seasoned, scientists, and engineers. To encourage the use of AM, this paper gives a brief review of the mathematical basis of AM followed by a historic perspective on the expansions originating from this mathematical basis. Specifically the case of a two-dimensional anisotropic plate with unloaded cut-outs subjected to in-plane static load is presented.

Simulation of thermal cycle aging process on fiber-reinforced polymers by extended finite element method

The simulation of long life behavior and environmental aging effects on composite materials are subjects of investigation for future aerospace applications (i.e. supersonic commercial aircrafts). Temperature variation in addition to matrix oxidation involves material degradation and loss of mechanical properties. Crack initiation and growth is the main damage mechanism. In this paper, an extended finite element analysis is proposed to simulate damage on carbon fiber reinforced polymer as a consequence of thermal fatigue between −50℃ and 150℃ under atmospheres with different oxygen content. The interphase effect on the degradation process is analyzed at a microscale level. Finally, results are correlated with the experimental data in terms of material stiffness and, hence, the most suitable model parameters are selected.

Lekhnitskii’s Formalism for Stress Concentrations Around Irregularities in Anisotropic Plates: Solutions for Arbitrary Boundary Conditions

Considering analytical methods in anisotropic elasticity, the complex potentials method (as extensively formulated by Lekhnitskii) may be regarded as a powerful tool. Among the various solutions generated by this approach, the analysis of thin anisotropic plates containing a geometrically simple irregularity is the most classical one as it reflects on an extensive collection of structures: from pinloaded holes to cutouts in aircraft fuselages. In this chapter we outline the complete solution for this particular geometry where the boundary conditions on the edge of the irregularity (forces or displacements) are formulated in Fourier series. The analytical solutions provided here can directly be evaluated as a function of the external boundary loads and the coefficients in the Fourier series, which represent the boundary conditions at the edge of the irregularity. Therefore, the analytical solutions provided here are able to cover a large variety of structural problems. Although Lekhnitskii’s formalism may be regarded as a well-established solution procedure, the availability of engineering-oriented, directly implementable solutions is rather limited. In this chapter we attempt to fill this gap.