Sizing of Hole and Joint in the Composite Structures

Composite structures are widely used for structures requiring high specific characteristics, such as in the aeronautics, space and general transport sectors. The advantage of this type of structure is that it can be made to measure; it is often said that in the case of composite materials, the material does not pre-exist the structure, unlike the case of a metallic material where the material exists and the engineer's job is to design a structure by adapting its geometry in order to optimize the structure (this often involves minimizing mass in order to lighten the structure). Composite materials are generally stratified, i.e. they are made by stacking several layers of composite material. The designer's job will therefore be to design the structure, and in particular to optimize its geometry, but also to determine the layers of composite material to be used, and in particular their nature and direction.

Generally speaking, the problem is as follows: given the forces imposed, design the lightest and least expensive structure possible (this criterion is obviously of prime importance, although it will not be dealt with here, as it depends enormously on the type of industry and the trade agreements of the industry concerned) that can withstand these forces. The optimization problem is therefore generally one requiring a large number of iterations, and at each iteration it will be necessary to check that the structure will withstand the forces imposed. If this is not the case, then you'll have to rethink your design, basically increasing the thicknesses and therefore the number of plies of the composite laminates, or using a stronger material. If this is the case, then you'll either have to stop, or seek to remove material in order to reduce the mass of the structure.

In this article, we'll focus on the dimensioning stage, i.e. verifying that the structure will withstand the forces imposed. To do this, it is necessary to apply failure criteria at every point of the structure, and in particular for each ply of the laminate, in order to verify that failure will not occur. It should also be borne in mind that the fracture criteria used are only simplified models of reality! Experimental tests are always used to check that the real structure can withstand the loads imposed.

In order to establish the failure criteria at every point of the structure, finite element models are most often required to determine how the forces are transmitted through the structure. For laminated composite structures, plate models are generally used to simplify the problem: it would be unthinkable to model a complete aircraft with 3D finite elements representing each ply of the laminate! In this case, a plate finite element is used to model the structure using 2D elements,...