A Theoretical Model to Study the Influence of Tow-drop Areas on the Stiffness and Strength of Variable-stiffness Laminates

Variable-stiffness laminates that have fiber orientation variation across its planform can be manufactured using advanced fiber placement technology. For such laminates, successive passes of the fiber placement head often overlap resulting in thickness build-up. If a constant thickness is desired, tows will be cut at the course boundary, which can result in small triangular resin-rich areas without any fibers. In this article a theoretical, numerical investigation of the influence of these tow-drop areas on the strength and stiffness of variable-stiffness laminates is performed. The effects of tow width, laminate thickness and staggering in combination with tow-drop areas are studied by making use of finite element simulations. A method for the localization of tow-drop areas is presented, and the expressions for implementing the tow-drop areas in a finite element model are given. Subsequently, progressive failure analyses using the LaRC failure criteria are performed. Failure occurs at tow-drop locations in both the surface plies and underlying plies. Wider tows result in lower strength. No correlation seems to exist between thickness and laminate strength, while staggering has a positive influence on strength.