10-04-2012, 03:54 PM
Finite Element Modelling of Delamination Buckling of Composite Panel Using ANSYS
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Introduction
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 catastropic.
Causes of delamination are many. In aerospace applications, this includes manufacturing defects, as
well as operationally induced defects such as bird strikes, hits due to runway debris and tool drops..
The type of delamination that is dealt with in this report is the one that is already initiated by one of the
above causes.
Description of the problem considered for study
A square panel with central circular delamination is considered. The radius of delamination ® is 40
mm (Fig.1). Various panel sizes in the range 100 x 100 mm to 800 x 800 mm are considered.
Exploiting the symmetry of the panel, a quarter of the panel is modelled for finite element analysis.
Two locations of delamination through the thickness are considered
Finite element model
The finite element delamination analysis is carried out using ANSYS finite element software (ver.5.4).
As discussed in section 2, there are several ways in which the panel can be modelled for the
delamination analysis. For the present study, a 3-D model with 8-node SHELL 99 composite element
of ANSYS is used. The panel is divided into two sub-laminates (Fig.4) by a hypothetical plane
containing the delamination. For this reason, the present finite element model would be referred to as
two sub-laminate model.
Validation of finite element model
Test problem no. 1: Buckling load computation
The aim of this test problem is as follows:
a) To check if the mesh refinement is sufficient for accurate buckling load computation.
b) To check if the coupled nodes facility of ANSYS is useful for the present problem.
Delamination buckling analysis of composite panel
In the present study, we are concerned with only the first buckling load and the corresponding buckling
mode. Using ANSYS, buckling analysis can be carried out by two approaches: (a) Eigenvalue (or
linear) buckling analysis (b) Nonlinear (buckling) analysis.
Concluding remarks
Buckling analysis of square composite panels with embedded circular delaminations has been carried
out using ANSYS 5.4. The following are some important observations:
1. The two-sublaminate model developed in this work provides a convenient method to model
delaminated composite panels. The accuracy of the model in predicting the buckling loads and
strain energy release rates has been verified using a typical test problem. The agreement of results
with theory is quite good.
2. Whenever the delamination is located at the mid-plane of laminate, the panel exhibits only global
buckling, i.e. there is no local buckling of delaminated region. On the other hand, whenever the
delamination is close to the surface (typically t2/t1=0.2), the buckling mode is predominantly local,
i.e. only the delaminated region buckles for small r/a values (typically, 0.3). For higher r/a values,
both global and local buckling co-exists.