Tow-steered Laminates Research Articles

Buckling Load Maximization of Curvilinearly Stiffened Tow-Steered Laminates

Under certain loading conditions, curvilinearly stiffened panels are seen to have better structural performance while being lighter in mass when compared to panels with straight stiffeners. A curvilinearly stiffened tow-steered composite laminate could be manufactured by fastening or bonding metallic curvilinear stiffeners to the composite panel. In this paper, such panels are computationally evaluated for their benefit in improving the panel buckling load. Initially, a parallel processing-based optimization framework to design tow-steered composite laminates with metallic curvilinear stiffeners is presented, where the objective is to maximize the buckling load with only a structural mass constraint and no stress constraint. The results show that the curvilinear stiffeners and curvilinear fiber paths in the composite laminated skin can lead to more than a 75% increase in the panel buckling load as compared to the use of equidistant straight stiffeners and straight fibers. Subsequently, a multiobjective optimization is conducted with the objectives to maximize the buckling load while minimizing the mass of the panels. It is seen that the improvement in the buckling load, by using curvilinear stiffeners and curvilinear fibers over equidistant straight stiffeners and straight fibers, depends upon the prescribed structural mass constraint.

Optimization of Tow-Steered Composite Wing Laminates for Aeroelastic Tailoring

Tow-steered composites are optimized for use in tailoring the aeroelastic behavior of a simple two-dimensional composite wing, with particular emphasis on improving both flutter/divergence airspeeds and gust loads. Symmetric layups are considered where the fibers vary in orientation along the wingspan and chord. Tow-steered laminates were found to increase the instability airspeed by up to 7% compared with optimized straight-fiber laminates and by 13% compared with optimized laminates with standard () plies. Tow-steered laminates were also found to reduce the peak wing root gust loads (up to 52%) and the correlated gust loads (up to 24%). The lowest gust loads were reached with higher order nonlinear fiber angle variations when either all plies were optimized or when two-dimensional fiber angle variations were used. Optimization strategies that allowed the fiber angles to vary freely in each ply generally performed better than optimizations based on the rotation of ()-ply stacks along the span of the wing.

Variable-stiffness composite panels: As-manufactured modeling and its influence on the failure behavior

The introduction of tow-steered laminates in structural applications demands reliable design methodologies to predict and simulate their mechanical response. The full details about the mechanisms that lead to damage and failure of these novel panels are not yet known. In this work, a numerical analysis tool has been developed for structural simulations in a three-dimensional domain. This paper presents a comparison between different configurations and modeling approaches of these panels. Finite element analyses are carried out to simulate the first-ply failure of tow-steered panels under tensile load. The simulations show how matrix cracking is affected by discontinuities in the fiber angle between adjacent courses and fiber-free areas resultant from manufacturing effects; whereas fiber tensile failure is directly influenced by the orientation angles of the fiber, whose distribution depends on the chosen course cutting method: on one side, or on both sides.

Improved aeroelastic tailoring using tow-steered composites

The use of tow-steered composites to tailor the aeroelastic behaviour of composite wings is investigated. A Rayleigh–Ritz type approach is used to model the aeroelastic behaviour of a simple rectangular unswept composite wing combined with modified strip theory aerodynamics. Symmetric lay-ups have been considered where the outer fibres are allowed to vary in orientation along the wing span, and the effects on free vibration, flexural axis, flutter and divergence speeds, and gust loads are examined. It was found that it is possible to influence the aeroelastic behaviour in both a beneficial and detrimental manner using tow steering. By increasing the design space, tow-steered laminates allow improved designs compared to traditional unidirectional composite laminates.

Investigation of thermally induced bistable behaviour for tow-steered laminates

Traditionally, bistable laminates have been developed from prepreg plies stacked up together to achieve a layup which is either constant or discretely varying over the planform of the laminate. Laminates with discrete variation in layup are, in particular, of interest as they offer the prospect of easier blending. Moreover, such laminates can be manufactured to have Variable Angle Tows (VATs) in a ply using a tow-steering technique, doing so, ensures fibre-continuity and may impart additional structural strength. This paper presents an approach to develop finite element (FE) models which can accurately predict the cured shape(s) of tow-steered laminates that are designed to be bistable. Manufactured laminates are characterised using microscopy and resin burn-off tests to identify resin-rich layers, ply-thicknesses and fibre volume fractions (Vf) – which are then translated into FE models. Presented data highlights the influence of the manufacturing process in the thermally induced bistable behaviour of tow-steered laminates.