Fiber Steering for Composite Laminates

Fiber steering is a method of construction for fiber-reinforced composites that allows the unidirectional fibers to be aligned along curvilinear paths. Fiber steering is performed by an advanced tow placement machine, such as Cincinnati Machine's VIPER™, which is able to steer the fibers along the desired paths through computer controlled trajectories. The shapes of the fiber paths are determined from software programs developed by ADOPTECH personnel. Research has shown that optimal fiber paths exist which can improve the performance of laminates under basic loadings (see Fiber Steering for Composite Laminate Design). This page is intended to introduce the basic ideas behind fiber steering and to define the typical terminology used for composite laminates.

Laminated Composite Structures

Traditional composite laminates are constructed of many layers of fiber-reinforced materials, which are composite materials that contain a strong and stiff fiber embedded in a softer matrix material. Typical fiber materials include glass, boron, and graphite, while the surrounding matrix material is usually some form of epoxy. For traditional straight fiber plies, each fiber is aligned at a specified constant angle with respect to the rectangular axes. This “fiber orientation angle” determines the stiffness properties of the orthotropic ply with respect to the rectangular axes. For plies with constant fiber orientation angles, the paths of each fiber are straight parallel lines aligned at the specified fiber orientation angle. Fiber steered plies allow this fiber orientation angle to change spatially, which produces curvilinear (non-straight) fiber paths.

Fiber Steered Paths – Linear Definition

The initial definition of the fiber orientation angle variation is assumed to be a linear saw-tooth function, with the end point angles being input by the user. The parameter f is used to define the direction of the variation with respect to the xy axes.

Though the fiber angle variation is linear, the actual path of the fiber will be curvilinear and dependent on the four parameters f, T0, T1, and Loc. For example, the figure below displays the fiber path for f = 0°, T0 = 15°, T1 = 45°, Loc = 12.

Note that if T0 = T1, then the fiber orientation angle q is constant and the fiber path is linear. The four fiber path parameters define a reference path that runs through the center of the plate, and the paths of the remaining area of the plate are computed using either the shifted or parallel method.

Shifted Ply Construction Method

The shifted ply construction method constructs additional ply paths that are exactly like the reference path except for a shift in the direction perpendicular to the variation direction. The size of the shift depends on the width of the tow placement head and the definition of the reference path, and is chosen so that there are no gaps between successive passes of the tow placement head. However, due to the curvilinear path variation a constant shift will produce some overlap between passes, and this is taken care of either through ply drops of allowing overlap. Shown on the left is a laminate constructed using the overlap option for the shifted method.

Parallel Ply Construction Method

Parallel ply construction implies that each pass is exactly parallel to the neighboring path. This results in no overlap or tow drop problems, but the fiber orientation angle must be computed numerically based on the reference path parameters. An example ply using the parallel method is shown on the right. A more detailed example program using the parallel method has also been developed by ADOPTECH: Fiber Steering Example using Parallel Method.

Manufacturing Considerations

The only limit on the construction of these curvilinear fiber paths is the turn radius for the tow placement machine and accompanying fiber tows. For too small of a turn radius, the material will buckle and not remain flat during the construction process. Therefore, besides calculation of the fiber orientation angles at each location, a radius of curvature calculation is also required. Comparison of the smallest radius of convergence within the ply to the manufacturing constraint results in an important design consideration involving the manufacturability of the part.

Related Links

Fiber Steering for Composite Laminate Design

Fiber Steering Example using Parallel Method