# Characterization of Shear Strain on PDMS: Numerical and Experimental Approaches
## 3 Materials and Methods
### Experimental Tests
#### 3.1.2 Shear Testing  
In preparing the shear test specimen, two steel (DIN-Ck45) bars with \(100\times 20\times 1.5\) mm\({}^{3}\) dimensions were used. The bars' surfaces were sanded and degreased with acetone to improve the adhesion of the PDMS to the steel. After the cleaning, the PDMS bonding was done using a cyanoacrylate structural adhesive Locttice Super Glue 3 (Germany), presenting the following configuration Figure 3.  
The bonded region of the steel plates had a superficial treatment. The process consisted of abrading the steel plate surfaces at the overlap region with fine sandpaper and cleaning with acetone before the application of the cyanoacrylate adhesive. To control and guarantee adhesive thickness, the test specimen (single-lap joint) was manufactured in a mold. The applied cure cycle was 24 h at room temperature. For this test, two PDMS test samples with different thicknesses (2 and 4 mm) were tested. The equipment and procedure were like the tensile test, distinguished in that this test used different test pieces (simple lap joints). During the tensile test of the samples, a commercial Vic-3D by Correlated Solutions \({}^{\textregistered}\) DIC system made it Dutchman Blvd. Irmo, SC, USA was used to measure the strain field in-plane and out-plane of the PDMS surface. The DIC equipment used consisted of two high-resolution digital cameras (Point Gray GZL-CL-41C6M-C 2048 \(\times\) 2048 pixels), using 80 mm LINOS objective lenses, a computer with Vic- 3D and a cold lighting bulb. The experimental set up used in this assay is shown in Figure 4.  
Figure 4: Experimental set-up of 3D digital image correlation. Figure 3: The specimen used in the simple shear test. Figure 2: (**a**) specimen positioned on the tensile machine. (**b**) PDMS stress-strain curve. For a good use of the DIC method, the specimen must be covered with a speckle pattern [41]. A random speckle pattern was artificially created on the PDMS specimen surface. The speckle pattern was produced using an airbrush with an internal mixture of air and paint, connected to a low-pressure compressor. First, the specimen surface was covered with white matte paint (the airbrush nozzle was adjusted to be approximately 0.3 m from the specimen). After drying the first layer, a second fine layer of small black paint dots was applied with the airbrush to produce the high-contrast speckle pattern. The size and density of the speckle were defined to guarantee high fluctuation of the image intensity; thus, higher accuracy measurements could be obtained. System calibration was done first; this procedure is necessary to provide numerical factors that will aid in the analysis of the data and to size the displacement unit. However, the calibration process of the VIC-3D system is direct and almost automatic. For 3D fields, it is necessary to use a standard calibration target; this target is positioned in front of the cameras in different positions, and several images are captured that are used to calculate the intrinsic and extrinsic parameters of the cameras. This procedure removes distortions of the lenses and defines a three-dimensional coordinate system on the surface of the sample. These parameters will subsequently be used to determine the displacement values. The two CCD cameras were positioned at 341 millimetres and a 12.5\({}^{\circ}\) angulation of the specimen, and the correlation equipment was adjusted so that two photos of the sample were captured per second. Once the adjustment was made, the specimen was clamped between the universal machine claws. Two alignment guides were positioned to restrict lateral movement, thus ensuring that the deformation occurred only in the PDMS plane. Then, the tensile test was carried out until specimen rupture occurred. The software that controls the universal test machine controlled the speed of the test (2 mm/min).