The performance of a composite part is primarily determined by the orientation of fibers in the plies. Designers wishing to exploit the full potential of composite materials, while avoiding manufacturing problems and part failures, must define and control fiber orientation. Anticipating true fiber orientation for a single ply is seldom intuitive, and predicting the behavior of an entire laminate made of tens or hundreds of plies is nearly impossible.
Traditionally, verifying the design of a composite part has required repetitive, expensive physical prototyping and testing. The part design could be refined only through lengthy trial and error involving costly redesigns and additional prototyping. Engineers, hampered by time constraints and unable to predict true fiber orientations, often resorted to over-designing composite parts, which added weight and cost to the product.
Now designers are getting faster and more reliable feedback about the producibility and behavior of composite parts by using specialized software that is fully integrated into the engineer's CAD environment. Such software enhances the CAD model by adding features to it that are specific to composites, such as zones, plies, cores, and rosettes. The software provides a configurable library of composite materials specifications, as well as the ability to define the surface, boundary, thickness, and orientation of plies. It also simulates material drapability for different composite manufacturing processes, displaying areas of problematic or unacceptable material deformation and fiber deviation. Designers can export data representing true fiber orientations to other tools such as finite element analysis (FEA) software so the ultimate performance of the part can be more accurately predicted and design revisions can be quickly made.
The examples below show how, by simulating ply draping, engineers can detect and correct problems created by material deformation or fiber deviation. In the first example, deformation caused the part to fail under standard in-service conditions. In the second, fiber deviation produced an unbalanced laminate that caused the part to warp during the curing cycle.
- A gas turbine spinner (see Figure 1). Drape is affected by the formability characteristics of the composite material and by the process of laying up flat patterns on a curved tool. The lay-up starting point and the initial fiber direction significantly influence deformation of the ply. A simulation revealed how draping a ply by starting from the top of the spinner resulted in deformation that caused material scissoring at the edge of the part. The scissoring in turn created unexpected variation in laminate stiffness, which reduced the strength of the part. Moving the lay-up starting point to the rim reduced deformation along the edge, eliminating the in-service part failure.
- A composite sandwich helicopter fairing (see Figure 2). The fairing laminate is made of a foam core sandwiched between two skins. Each skin consists of several ply layers. For optimal part performance, the design should create a quasi-isotropic lay-up of plies in which there is an equal distribution of fiber directions. A simulated lay-up revealed a difference between target and true fiber orientation, resulting in an unusable part. In the initial design, lay-up of a 0/90-degree ply started at the front. The fiber direction was expected to remain the same across the entire fairing, but the shape of the tool shifted fiber orientation 45° at the back of the ply and created a large region of non-quasi-isotropic fiber orientation in the manufactured part. This resulted in a non-symmetric laminate, which in turn caused the part to warp unacceptably during the curing cycle. In this case, the design was revised to achieve a quasi-isotropic lay-up of plies, which eliminated the warping and brought the finished part within specifications.
This feedback loop entails far less physical prototyping and thus reduces design cost and time. It also enables engineers to minimize the weight of composite parts while optimizing part performance much more rapidly. The software tools that help define composite parts also generate downstream manufacturing data from the CAD model, including flat patterns, documentation, nesting, cutting, and laser projection data for lay-up. Employing the CAD model as a single upstream information source for composite design ensures the closest possible match between the specifications and the manufactured part.
This work was done by Dr. Olivier Guillermin of VISTAGY, Inc.