A methodology to predict the stiffness properties and buckling load of variable stiffness panels

Abstract

Automated fibre placement technology has made possible to place bands of composite fibre tows along curvilinear path to create laminates with spatially varying stiffness properties called variable stiffness panels. These panels have shown improvement in stiffness properties, buckling load and post buckling failure load, over conventional straight fibre laminates. However, automated fibre placement machines can allow a minimum radius of curvature for fibre steering. Due to this, bands of fibre tows with same orientations are placed next to each other to produce a panel. This placement strategy gives rise to mismatch between the two bands and leads to fibre angular distortion. It can also lead to thickness build up but, to maintain the constant thickness of the panel, fibre tows are cut perpendicular to the placement direction which give rise to the defects called tow drop defects. Tow drop defects are the resin rich areas. In recent years, optimization of the variable stiffness panels for different load cases have been studied. These studies do not account for tow drop defects during optimization. To take account of these defects on the properties of the variable stiffness panels certain finite element approaches were proposed. For applying these approaches, location and geometry of the tow drop defects need to be identified in the variable stiffness panels. Based on it, property assignment to the mesh elements were done. However, the location and geometry of tow drops are dependent on fibre angle, width of the fibre tows and number of fibre tows in a band and can vary for different configuration of variable stiffness panel. It involves excessive computational effort to determine the location and geometry of the tow drop defects in the panels and analyze it. This makes it difficult to integrate the finite element approaches to optimization study. Thus, there is a need to develop a methodology which could make this integration possible.