Keywords: grid structure, damage growth, buckling, FEM.

For many years composite grid structures have not been considered as valid alternatives to standard aerospace structure concepts, as a result of the large volume of manufacturing and the lack of numerical models simulating their mechanical behaviour. In the last ten years, remarkable progress has been made in the manufacturing of these structures so that there is renewed interest for their aerospace applications.

Grid structures are characterized by a shell structure (or skin) supported by a lattice pattern (or grid) of rigid and interconnected ribs. Composite materials are particularly well suited for this type of structure because the typical stresses in grid structure ribs are highly directional along the rib length. The high directionality of composite materials allows for the majority of the material's stiffness and strength to be addressed along this directional state of stress, leading, in many cases, to an order of magnitude increase in stiffener strength in comparison with solutions from traditional isotropic materials. Indeed, the grid structure built by composite materials does not produce material mismatch (different layer orientations) associated with laminated structures. The absence of material mismatch implies that the grid structures possess inherent resistance to impact damage, delamination and crack propagation. Automated, low-cost manufacturing is another benefit of grid structure. The ability to fabricate grid structures using an automated, single process is potentially superior to skin-stringer construction where significant hand operations are involved. Nevertheless, the grid structures, as all other structures, are not the best choice for all solutions. One major drawback, which slows down the introduction of the grid structure into industry, is the lack of understanding of their mechanical behaviour, when failure mechanics are involved. In order to improve knowledge concerning the failure of composite grid structures, a parametric FEM model able to simulate the mechanical behaviour of a repetitive volume element (RVE) of grid structures has been developed and presented in this paper. A novel procedure for the damage propagation in terms of matrix cracking and fiber failure, already applied to the study of composite joints [1,2], has been implemented in the research oriented FEM code B2000. This procedure is based on the Hashin's failure criteria to detect the damage on-set and on the material properties degradation rules to correctly follow the damage progression.

Non-linear structural analyses have been carried out in order to simulate the mechanical behaviour of the RVE under investigation. Indeed, two different types of analyses have been performed in order to compare the behaviour of the RVE in terms of deformation and stiffness without the damage progression and with the damage progression. The numerical results obtained from the two types of analyses have been compared and critically assessed.

In the next sections, the theory behind this proposed approach is explained in detail, together with the numerical applications and the most relevant results.

1

A. Riccio, L. Marciano, "Effects of Geometrical and Material Features on Damage Onset and Propagation in Single-lap Bolted Composite Joints under Tensile Load: Part I - Experimental Studies", accepted for publication on Int. Journal of Composite Materials, 2005. doi:10.1177/0021998305052026

2

A. Riccio, "Effects of Geometrical and Material Features on Damage Onset and Propagation in Single-lap Bolted Composite Joints under Tensile Load: Part II - Numerical Studies", accepted for publication on Int. Journal of Composite Materials, 2005. doi:10.1177/0021998305052027

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £135 +P&P)