Each separate fuselage section or wing panel is itself a complex design. These parts involve laminate specifications for many zones of different thicknesses, and ply stacks or complex zone transition drop-offs governed by various design, structural, and manufacturing rules. For example, ply boundaries that include corner treatments known as “bird beaks” must be incorporated in the engineering model for an accurate and manufacturing ready tape laying design. Numerous details like these must be accounted for as early as possible in the design process.
One approach to designing composite aerostructures is to subdivide a large, complex part into smaller design tasks — defined either by uniform grids or by zones that present unique requirements and characteristics. Engineers then treat each grid unit or zone as an individual design task, specifying the materials and manufacturing processes peculiar to that zone. Because this methodology is efficient and promotes concurrent collaboration, it can save significant amounts of design time.
Composite parts actually are laminates made by manual or automated deposition of tens or hundreds of layers (plies or tapes) of various shapes and fiber orientations. In particular, engineers need to know the precise topology of the top and bottom surfaces, which define the final laminate. An accurate definition of these surfaces is vital for many reasons: part representation for digital mock up, part volume/ weight calculation, interference checking between mating parts, and generation of accurate tooling or manufacturing surfaces. The ability to appropriately and quickly calculate these surface specifications has been a computational challenge for design software in the past, due to the very small thickness of the plies. But the increased power and speed of computer hardware and software has given engineers the means to perform these calculations.
The manufacturing of composite structures requires careful evaluation. Most composite manufacturing methods can lead to fiber deformation, which impacts production quality and part performance. For example, hand layup of prepregs or dry materials inside a curved mold is prone to deformation such as ply wrinkling, warping, and bridging (see Figure 2), which may affect the strength, stiffness, and fatigue resistance of the final product. Fiber placement or tape laying on large fuselage panels or wing skins can produce uncontrolled deviation of fibers. Forming processes used for spars and stringers may generate stresses and strains inside and between ply layers. Unless the engineer faithfully simulates fiber deformation for all processes, the parts may not meet specifications. That failure may not be evident until physical testing, which will waste development cycles and increase costs.
Composite engineering for aerospace products has become very challenging as composite aircraft parts have increased in size and complexity, calling for new design and manufacturing methods.Specialized CAD-integrated composite engineering software is replacing manual design approaches for capturing and managing large amounts of complex and critical detailed data. Such software helps engineers automate powerful design methodologies and enables accurate virtual simulation of the as-manufactured composite structures. Specialized software will open the door to more reliable and efficient engineering of large composite aerostructures, with reduced costs, shortened development cycles, and controlled manufacturing.
This article was written by Olivier Guillermin, Director of Product and Market Strategy, for VISTAGY, Inc. For further information visit http://info.ims.ca/5787-122.