Straightening out the Adoption of Variable Stiffness Composite Laminates in Aerospace Industry

Significant improvements in the mechanical performance of a laminated composite structure can be achieved by spatial variation of the stiffness properties within the laminate. Such a variable stiffness (VS) laminate, typically is created by steering the fibers within the laminae, resulting in an in-plane variation of stacking sequence and thus an in-plane variation of mechanical properties of the laminate and subsequent load-redistribution within the laminate. Since the early work on VS laminates [1,2] more than 30 years have passed. In these years, significant improvements in the design, analysis and manufacturing of VS composite laminates have been made. Buckling load improvements in excess of 100% (compared to the best constant stiffness design) are reported for VS laminates [3], and there are numerous examples of case studies on (aerospace) structures [4]. Nevertheless, to the knowledge of the authors, there is still no widespread adoption of VS laminates in the aerospace engineering industry. This raises the question, why VS laminates are not being adopted, and what can be done to change this. Recent work done at the TU Delft has demonstrated that it is possible to achieve variable stiffness laminate designs by means of laminate blending and patching operations, instead of fiber steering. The result is a straight-fiber variable stiffness (SFVS) laminate, see Fig. 1. A computationally efficient design method has been developed for these laminates. Preliminary results show similar improvements in buckling load as have been reported in literature for fiber-steered variable stiffness laminates. The advantage of the proposed design method is that variable stiffness laminates could be achieved by either straight-forward tape-placement machines or pick-and-place operations of pre-cut patches of fabric. In this work the authors have investigated whether this novel approach to variable stiffness laminate design can help the adoption of variable stiffness laminates in aerospace industry.