Modeling of Out-of-Plane Stresses in C/PPS Composite L-Shape Brackets

Abstract

With the increasing use of composites in civil aviation, thermoplastic composites have become an interesting material choice for repeating part numbers such as fuselage brackets. The simple L-shaped bracket geometry can be mass-produced using stamp-forming manufacturing. When subjected to opening loads, out-of-plane stresses, i.e., interlaminar tensile stresses, develop in the 90° corner radius. Those gradually increasing stresses ultimately lead to delamination failure. One approach to evaluate the interlaminar tensile strength is to test a curved beam using a four-point bending apparatus to create an opening load. This test is described in the ASTM D6415 standard. The geometry of the bend (inside/outside radii), the materials and the lay-up sequence have a strong influence on the mechanical performance of the L-shaped bracket. Analytical equations were developed to evaluate the radial interlaminar tensile stresses in the bend. However, these equations can be erroneous for non-unidirectional stacking sequences. Moreover, geometric non-linearity can occur when flexible coupons are subjected to large deformation. In those cases, modeling can be used to evaluate the interlaminar tensile strength between layers at a given applied load. In this study, a finite element model of the ASTM D6415 test was created to calculate the out-of-plane stresses in the radius. The use of parametric modeling enables the simulation of various coupon configurations. The model is compared to the analytical equations and carbon fiber/polyphenylene sulphide composites L-shaped brackets of different stacking sequences, thicknesses and bend radii.