## 3 Experimental Study  
The efficacy of the mathematical model developed using finite elements to measure the forced vibration response of an intact and delaminated composite structure was studied by comparing the MATLAB(r) simulated transverse deflections in the time domain with that of laboratory experimental results. The intact and delaminated fiber-reinforced laminated structures were fabricated using a vacuum-assisted hand-lay-up process. First, peel fly fabric treated with a releasing agent was laid upon the mold surface to provide a smooth finish on the structure surface. E-glass unidirectional woven fabric of 225 g/m\({}^{2}\) was placed over the peel fly, and then the calculated amount of LY556 epoxy resin with HY951 was evenly applied over the fiber surface. The lay-up was continued to obtain a stacking sequence of [0\({}^{\circ}\)/90\({}^{\circ}\)]t s with sixteen layers. After obtaining the required sequence of lay-up, the peel fly and breather fabric were laid over the lay-up stack. The entire setup was placed inside a vacuum bag, which was sealed with a sealant. Initially, the laminated stack was cured at vacuum pressure and room temperature for one hour. Later, the laminate was placed in an oven where the temperature was raised to 70 \({}^{\circ}\)C at a controlled rate of 1 \({}^{\circ}\)C/min. In the next stage, the laminate was kept at 70 \({}^{\circ}\)C for two hours. Finally, the laminated composite structure was allowed to cure further for one day at room temperature. The prototypes were cut into 400 mm length \(\times\) 200 mm width. The delamination in the composite structure was obtained by inserting 100 \(\times\) 100 mm Teflon film symmetrically in the central plane during the fabrication process. Both the composite structures maintained an identical volume fraction of 0.4. Figure 1a,b shows the fabricated laminated and delaminated composite structures. The thickness of the laminated and delaminated structures was measured to be 4.158 and 4.032 mm, respectively. The mechanical properties of the unidirectional laminate ply are presented in Table 1.  
The schematic diagram of the experimental setup used to obtain the time-domain response of the laminated and the delaminated composite structure is shown in Figure 4. The composite structures were divided into fifty rectangular elements and sixty-six nodes to measure the time-domain response and impact force at the various nodes. The impact  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Fiber-Reinforced Composite Lamina**} \\ \hline \(V_{f}\) & 0.4 \\ \(E_{11}\) (Gpa) & 30.2 \\ \(E_{22}\) (Gpa) & 6.8 \\ \(v_{12}\) & 0.269 \\ \(G_{12}\) (Gpa) & 2.8 \\ \(G_{23}\) (Gpa) & 2.51 \\ \(\rho_{c}\) (kg/m\({}^{3}\)) & 1745 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mechanical properties of the unidirectional fiber-reinforced composite lamina.  
hammer was used to excite all the nodes of the composite plate. A uni-axis accelerometer (50 g/100 mV/g sensitivity) was fixed at the location of 234 \(\times\) 134 mm from the left end bottom corner of the plate to record the acceleration in the time domain due to impact excitation at all the nodes of the plate. The time-domain response (\(h_{ij}\)) acquired using the four-channel data acquisition system represents the response at the eighth node due to the impact excitation at the \(j^{th}\) node of the laminated and delaminated composite plates. The experiments were performed in a temperature-controlled environment using air-conditioning. The temperature was set to 27 \({}^{\circ}\)C during the transverse response extraction of the intact and delaminated composites.