Fiber Steering for Composite Laminate Design

Fiber steering technology allows for curvilinear (non-straight) fibers within fiber-reinforced plies of a composite laminate. An example steered fiber ply using the parallel method of construction is shown at right. This page outlines the advantages this method offers for laminate design. For an introduction to the basic concepts behind fiber steering construction, see Fiber Steering for Composite Laminates.

Traditional laminates constructed from multiple layers of fiber-reinforced material systems offer versatility in design due to the fact that the orientation angles of each orthotropic layer can be varied to take full advantage of the superior mechanical properties that exist in the direction of the fibers. This is accomplished by designing the stacking sequence with respect to the expected stress state of the laminate. The fiber orientation angles of each ply are the design variables since they form the basis for the stiffness and strength parameters of a fiber-reinforced ply, and they are chosen to improve the response while decreasing the weight of the total laminate. For example, a laminate that undergoes bending in only one plane should be constructed with the majority of plies aligned along the direction of the bend, since the predominant stresses for this loading are in the axial direction. For general loading conditions, stacking sequence optimization can be used to find the most efficient laminate that can withstand the applied loads and minimize the total panel weight. Several numerical methods already exist to find these optimal laminates , but it should be stressed that this type of optimization utilizes the variation of the laminate properties in one direction only, that is, by varying the stiffness and strength properties in the through-the-thickness direction.

Recently it has been shown that even greater use of fiber-reinforced materials can be achieved by varying the stiffness and strength properties within the plane of each ply. This implies that the fiber orientation angle, which was a constant for a given layer of the laminate, is allowed to vary spatially throughout the ply as a function of location. Such a variation produces fiber paths that are not straight, which they are for a constant stiffness case. This ply construction technique is often referred to as curvilinear fiber format, variable stiffness, or fiber/tow steering due to the fact that the manufacture of such a ply is often performed with a tape-laying machine that is capable of steering individual tows of the fiber-reinforced composite. With the possibility of steered fibers, large structures can be designed in which the fiber orientations vary spatially according to the need of the structure. For example, an aircraft fuselage contains some regions along the length that are dominated by bending; therefore plies aligned along the axis will be the most efficient use of the material. In other locations, the bending loads decrease and shear deformation dominates the response, in which case plies aligned at forty-five degrees are the best choice for the ply angles. Fiber steering will allow a design that can slowly change its stacking sequence properties from one region to the next, so that the entire laminate can satisfy the needs for each local stress state.

ADOPTECH personnel have investigated the concept of fiber steered laminates for aircraft structures since 1990 (see the References given below), and have shown that their implementation can greatly enhance the load-carrying performance and reduce the weight of flat panels and cylindrical shells that are used as structural components. Presently, research is being conducted at ADOPTECH to develop tools that can assess the performance of fiber steered laminates for flat rectangular panels under in-plane loading and to integrate these computation modules within an automated design environment. Use of this construction technique should further the advent of composite materials within the aeronautic/aerospace industry.

Fiber Steering Example

A Graphical User Interface has been developed by ADOPTECH to visualize the concept of steered fibers for flat panels. See Fiber Steered Ply Example using Parallel Method.

Design Examples

Presently ADOPTECH is developing analysis tools for steered fiber laminates that can be incorporated into an automated design environment. This will enable the designer to find the optimal fiber paths for a given geometry and load state. Earlier research for flat panels has shown that increased buckling resistance can be achieved by varying the stiffness in the direction perpendicular to the loading, which re-distributes the stresses within the plate. The optimal design directs the majority of the loading toward the reinforced edges, while the center region stacking sequence is designed to best withstand buckling. Similar results can be achieved for circular cylinders, though the lack of true edges in the circumferential direction tends to diminish the effectiveness for the axially loaded case. Further details of the results from the automated design process for flat panels will be included when available.

Related Links

Fiber Steering for Composite Laminates
Fiber Steering Example using Parallel Method

References

• Gürdal, Z., Olmedo, R., “Composite Laminates with Spatially Varying Fiber Orientations: Variable Stiffness Panel Concept,” Proceedings of the 33^rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials (SDM) Conference, Dallas, TX, April 13-15, 1992, pp. 798-808.
• Gürdal, Z., Olmedo, R., “In-plane Response of Laminates with Spatially Varying Fiber Orientations: Variable Stiffness Concept,” AIAA Journal, Vol. 31, No. 4, April, 1993, pp. 751-758.
• Tatting, B. F., Gürdal, Z., “Analysis of Variable Stiffness Composite Cylinders,” Proceedings of the Third Pan American Conference of Applied Mechanics (PACAM III), São Paulo, Brazil, January 4-8, 1993, pp. 608-611.
• Waldhart, C. J., Gürdal, Z., Ribbens, C., “Analysis of Tow-placed, Parallel Fiber, Variable Stiffness Laminates,” Proceedings of the 37^th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials (SDM) Conference, Salt Lake City, UT, April 15-17, 1996, pp. 2210-2220.
• Tatting, B. F., Gürdal, Z., “Design and Manufacture of Tow-placed Variable Stiffness Composite Laminates with Manufacturing Considerations,” Proceedings of the 13^th U.S. National Congress of Applied Mechanics (USNCAM), Gainesville, FL, June 21-26.
• Tatting, B. F., “Analysis and Design of Variable Stiffness Composite Cylinders,” Ph.D. Dissertation, Virginia Polytechnic Institute & State University, October, 1998.