Optimization of Ply Drop-offs in Bonded Patch to Cylindrical Shell Structures

Abstract

Ply drop-off is a technique widely used to achieve gradual thickness change in composite laminate, and it can be utilized to form boundary tapering of a composite patch bonded to a parent structure. This paper firstly presents a new adhesive element formulation with a nonneutral reference plane that takes into account the geometrical features of ply drop-off and can be applied to conduct 2.5-D simplified stress analysis of bonded repairs to curved structures. The new formulation is numerically validated by comparing results calculated using both the present formulation and a full three-dimensional finite element formulation available in commercial software. Sequential Linear Programing (SLP) method is then employed in conjunction with a fully implemented automatic mesh generation algorithm to minimize adhesive stresses, namely, the peak peel, shear or Von Mises stresses, through optimally selecting ply drop-off parameters. The objective of this paper is to ensure effective bonding between a patch and an undamaged parent shell structure via minimizing maximum stress along the boundary of adhesive layer. Several numerical examples are presented to demonstrate that an optimum ply drop-off in a bonded patch can dramatically reduce peak stresses in the adhesive layer, particularly the peak positive peel stress.