Damage Detection in Laminated Composites

There are mainly two types of damages in composites:

• ply failure
• delamination failure.

For ply failure, the stiffness contribution of the corresponding ply to total stiffness of the structure is assumed to be negligible. For delamination, a simple mechanics has been followed. A lamina with delamination has been treated equivalent to two laminae separated by delaminations.

Figure 24,1 A composite laminate with delamination

The constitutive equation for the upper part of the laminated composite (i.e. above delamination) may be written as

(24.4)

where and .   N₁ and M₁ are axial and bending stress resultants of the upper part respectively.
Similarly, for the lower part of the laminated composite (i.e. below delamination)

(24.5)

where and

Thus, at the interface

and

(24.6)

The modified A, B  and D  matrices of delaminated composite may be written as

(24.7)

The modified   and  matrices have been used in the finite element formulation to recalculate the stress and strain profiles and obtain the voltage response from the delaminated plate. Numerical studies are carried out to obtain voltage distribution on the laminate subjected to different known delaminations.