## 3 Results and Discussion
### Shear Test Results  
The mechanical properties of the specimens were measured using tensile testing of dog-bone specimens and shear tests by punch tool. The shear test is helpful for compliant materials that may stretch excessively and demonstrate misleading results when tested in the tensile mode. The most critical parameters are the peak load, the area under the load-displacement curve, and the punch probe's displacement from initial contact up to the shear fracture of the film. From these data, shear strength and energy to shear were calculated. The nature of the test does not allow calculation of Young's modulus or elongation to shear [42]. Shear strength is calculated based on Equation (28):  
\[\text{Shear strength (N/mm}^{2})=F/A_{c}, \tag{28}\]  
where \(F\) is the load required to shear the specimen and \(A_{c}\) is the area of the sheared edge, which shall be taken as the product of the specimen thickness by the circumference of the punch. The equation for energy to shear is similar to that of a tensile test, except for the calculation of the volume term. Hence, the energy to shear per unit volume is calculated by:  
\[\text{Energy to shear (J/cm}^{3})=A_{S}/V_{C}, \tag{29}\]  
where \(A_{s}\) is the area under the load-displacement curve of the shear tests and \(V_{C}\) is the volume of the die cavity of the film holder. The displacement of shear punch until fracture of each specimen has been normalized by each specimen's initial thickness and considered ductility.  
The shear test data revealed that SCB-PE/5 GNPs w BM specimens have the highest shear strength, as shown in Figure 6a. These results agree with the ones derived from the  
Figure 5: SEM images of the fracture surfaces of (**a**) SCB-PE/2.5 GNPs no BM and (**b**) SCB-PE/2.5 GNPs w BM.  
tensile tests since the elastic modulus of SCB-PE/5 GNPs w BM was the highest among all samples examined. In addition, SCB-PE/5 GNPs w BM specimens had the lowest elongation at break. The neat SCB-PE specimens demonstrated the lowest shear strength. The results show a moderate increase of the shear strength for SCB-PE GNPs for both cases of the nanocomposites with increasing filler content. However, the ball-milled samples present higher shear strength augmentation compared to their solely mixed counterparts. The most probable cause of this behavior is that the shear stress required to fracture is higher for the SCB-PE/GNPs w BM nanocomposites due to the better dispersion and interfacial adherence between the GNPs and the polymer matrix [43]. That is, in the case of the ball-milled nanocomposites, the crack initiation is further delayed since there is a higher surface area for interfacial bonding; during the shear stress application, the internal \"planes\" of a fractured sample are strongly bonded by the filler particles causing resistance to sliding.  
The values of energy to shear fracture are shown in Figure 6b. The neat SCB-PE specimens were the most resistant to shear and required the greatest amount of energy to shear fracture. A similar phenomenon occurred for the calculation of ductility presented in Figure 6c. Neat SCB-PE specimens were considered as the more ductile ones. This can be attributed to a combination of their high shear strength and relatively high elongation to fracture. It should be noted that for both tensile and shear testing, the addition of graphene nanoplatelets improved the strength of the specimens. The nanocomposites' ductility was expected to decrease since, once the debonding of the particles from the matrix begins, the GNPs act as a crack initiation site [44; 45; 46].  
Figure 6: (**a**) Bar chart comparing shear strength measured from mechanical tests, (**b**) bar chart comparing energy to shear measured for all SCB-PE specimens, and (**c**) ductility of the specimens.