Plates with initial imperfection

The composite layered panels with fibers are the most usually used in shipbuilding industry and in engineering constructions as well. Taking into account that fabrication technologies of composite materials are hand made based, the probabilistic occurrence of defects is quite too high. These panels, unfortunately, have one important characteristic connected to big sensitivity on geometrical imperfections (initial transversal deformation).

In this chapter is analyzing the buckling behavior of the plates placed between two pairs of stiffeners of the ship hull structure. Objective is to present the results obtained after buckling analysis of ship hull plates made of composite materials taking into account the transversal imperfection (spatial cosine form) due to fabrication. Due to the special behavior of the layered composite plates, the nonlinear analysis of the buckling behavior of the plates is to do. In certain cases, to determine the buckling load (ultimate strength), the failure criterion is applied. The buckling load is determined when the first failure occures in an element, based on the Tsai-Wu failure criterion, who provides the mathematical relation for the strength under combined stresses.

1.1 Plates with delaminations

Delamination in composite structures can be a serious threat to the safety of the structure. Delamination leads to loss of stiffness and strength of laminates under some conditions. This is particularly so in the case of compressively loaded structures as the loss of stiffness may lead to buckling, the consequences of which can be catastrophic.

Causes of delamination are many. In shipbuilding and aerospace applications, this includes manufacturing defects, as well as operationally induced defects such as bird strikes hits due

to runway debris and tool drops (aerospace), or cargo operating and slamming loading (shipbuilding).

The type of delamination that is dealt with in this report is the one that is already initiated by one of the above causes.

When a laminate is subjected to in-plane compression, the effects of delamination on the stiffness and strength may be characterised by three sets of analytical results:

a. Buckling load;

b. Post-buckling solutions under increased load;

c. Results concerning the onset of delamination growth and its subsequent development. Many of the analytical treatments deal with a thin near surface delamination. Such approaches are known as "thin-film" analysis in the literature. The thin-film analytical approach may involve significant errors in the post-buckling solutions.