## 2 Experiment Test  
In this study, the fabrication of composite material is consisted of plane E-glass fiber plies in a thermosetting polyester resin. The composite laminate plate is manufactured by hand in the lay-up technique. The fabrication of composite laminates is conducted from six layers of E-glass fabric sheets. The fabric sheets are placed in a (24 cm \(\times\) 24 cm)aluminum mold. Two X-ray photo sheets are used to avoid abrasion in a specimen surface. The fiber volume fraction is determined experimentally for the composites plate from the following relationship [13]:  
\[\upsilon_{\mathrm{f}}=\frac{1}{1+\frac{1-\varphi}{\varphi}*\frac{\rho_{\mathrm{f} }}{\rho_{\mathrm{m}}}} \tag{1}\]  
where:  
\(\varphi\): is the fiber weight fraction. And  
\[\varphi=\frac{\mathrm{m_{f}}}{\mathrm{m_{C}}}\]  
When the composite with the dimensions (24 \(\times\) 24) gets dried and gets out from the mold, we can weigh the composite plate to determine (m\({}_{\mathrm{C}}\)). Before fabrication we can weigh the E-glass fiber to find the value of (m\({}_{\mathrm{f}}\)). Divide (m\({}_{\mathrm{f}}\)) on (m\({}_{\mathrm{C}}\)) to calculate (\(\psi\)). By substituting all these values in Equation (1) we can find the value of (\(\upsilon_{\mathrm{f}}\)) as shown in Figure 1.  
The composite laminated plate with length (200 mm), width (100 mm) and thickness (4 mm) is selected in the experiment test to calculate critical buckling load. One and two stiffeners of width (6 mm) and depth (4 mm) are selected along the length of the main composite plate. The lamination angle for all stiffeners is equal to (0) degrees, while the lamination angle of composite plate is assumed to be cross ply. The dimensions of the specimen that were used in the buckling test has been taken from ASTM E1876 [14]. A tensile test machine was used in a buckling test with an axial compression load of (200 KN) in the vertical direction, as illustrated in Figure 2.  
Figure 1: Manufacturing of composite laminated plate. Figure 2: Buckling test. The boundary conditions of the specimen is simply supported by two ends and free at the other two ends (S-F-S-F). The simply supported boundary condition is simulated along the top and bottom edges of the compression test device. The specimen is compressed slowly with a constant cross-head speed of (5 mm/min) until buckling. Figures 3-5 show the buckling test for the un-stiffened plate, stiffened plate with one stiffener and stiffened plate with two stiffeners. For the buckling test loading, the test specimen is placed between two extremely stiff machine heads. The lower head is fixed during the test, whereas the upper one is moved downwards by a servo hydraulic cylinder. The strain gauge is placed in the middle point of the composite laminated plate through the z-axis at the connection point between the composite plate and the stiffener. The strain meter is used to catch the voltage of the bending deflection through the plate thickness due to the in-plane compression mechanical load and the shear force. When the upper head of the servo hydraulic cylinder moves downward, the composite laminated plate will buckle under the effect of the compression load. The data of the bending deflection through the plate thickness against time is stocked in an Excel spreadsheet, and these data were processed using MatLab software. The deflection through the plate thickness is calculated using the following equation:  
\[\delta_{\mathrm{zz}}=\varepsilon_{\mathrm{zz}}*\mathrm{h}\]  
where:  
\(\varepsilon_{\mathrm{zz}}\): is the experiment strain through the composite plate thickness. \(\delta_{\mathrm{zz}}\): is the experiment bending deflection through the composite plate thickness. h: is the composite plate thickness. The Southwell plot is a histogram between the bending deflection (\(\delta_{\mathrm{zz}}\)) and the in-plane compression mechanical load [15].