# Characterization of Shear Strain on PDMS: Numerical and Experimental Approaches
## 4 Results and Discussion  
### Experimental  
Figure 7 shows the force-displacement curves obtained from the shear tests. The red line represents the results of the specimen with 4 mm PDMS thickness, and the blue one corresponds to the 2 mm thickness. The rupture of the specimen with 4 mm PDMS thickness occurred at higher values of force and displacement, 154 N and 2.8 mm, respectively. The specimen with 2 mm PDMS thickness, broke up for low values of force and displacement, 80 N and 0.8 mm, respectively. Despite the linear curve for both specimens, in the specimen with 2 mm, PDMS thickness an initial disruption happened before the complete rupture of the specimen.  
Figures 8 and 9 show the details of the PDMS region of the specimens for different test phases. Figures 8a and 9a are the images of the specimens before the deformation (reference image), Figures 8b and 9b show the beginning of rupture and, finally, Figures 8c and 9c present the specimen after rupture.  
Figure 8: Image of 2 mm polydimethylsiloxane (PDMS) thickness: (**a**) before deformation; (**b**) beginning of rupture; (**c**) after rupture.  
Figure 7: Force-displacement curve for the shear test.  
For both specimens, the rupture happened in the interface between the metallic plate and the PDMS surface. That is, the rupture occurs in the adhesive region, which is the weakest element of the specimens. It was verified that all ruptures occurred in the interface between the PDMS and the cyanoacrylate. However, for cyanoacrylate shear tests the average of rupture force obtained was 1715 N which indicates that the cyanoacrylate strength is higher when the interface is the steel than when the interface is the PDMS. Another detail was that the rupture always happened in the PDMS surface that was in contact with air, not the surface in contact with the mold, probably because the surface roughness was lower. The authors believe that if they used a primer on the PDMS surface plate, the mechanical strength could improve. However, the goal of this work only analysed the displacement and shear strain before the rupture.  
Figure 10a,b presents the shear strain field for the specimens with a 2 mm thickness PDMS plate and with a 4 mm thickness PDMS plate, respectively. These two images were captured in two different moments of the shear test or, in other words, for two different values of displacement. Figure 10a was obtained when the shear test reached 0.5 mm displacement, and Figure 10b shows the shear test when the displacement value was 0.2 mm. From Figure 10, the maximum shear strain happened in the central region of PDMS for both thicknesses of PDMS plate. However, the maximum shear strain values are different. That is, on the specimen with the smallest thickness, the value was \(-0.1025\) [mm/mm], while the values decreased to \(-0.0213\) [mm/mm] for the specimen with a 4 mm PDMS thickness.  
Figure 10: (**a**) Shear strain field for a 2 mm thickness PDMS plate; (**b**) Shear strain field for a 4 mm thickness PDMS plate.  
Figure 9: Image of 4 mm PDMS thickness: (**a**) before deformation; (**b**) beginning of rupture; (**c**) after rupture.