# Characterization of Shear Strain on PDMS: Numerical and Experimental Approaches
## 3 Materials and Methods  
### Experimental Tests  
It was necessary to manufacture two molds in which the resin of the elastomeric polymer will be cast before polymerization to produce the tensile and shear specimens. The molds to obtain tensile and shear specimens were manufactured in an aluminium alloy (2011-T3) by a CNC milling machine ([PERSON]--DMU 60 T from Germany); each mold had three and two cavities for tensile and shear specimens, respectively. The geometry and dimensions of tensile specimens were chosen to match the American Society for Testing of Materials ASTM D412 TYPE C standard [35].  
The accuracy of tensile testing results is very depending of the dimensional and geometric precision of the specimen [36; 37] which is depending of the stress state of the material namely the level of residual stresses of the material that will be machining. There are some authors who showed that for materials with a high level of residual stresses after a machining process the final dimensions and geometry could change significantly [38]. For this reason, after mold machining was controlled dimensional accuracy, surface finish and defects follow the recommendations of [PERSON] et al. [39]. This evaluation allows us to guarantee that the geometric and dimensional tolerances are within those recommended by the ASTM D412 TYPE C standard.  
The polydimethylsiloxane (PDMS) used was Sylgard 184 (r) by enterprise Dow Corning from Wiesbaden, Germany, To obtain the PDMS resin, it was necessary to mix the curing agent with the prepolymer; for each 10 g of prepolymer, 1 g of the curing agent was added (10: 1).  
The mixture was placed under vacuum conditions for 40 min in a desiccator connected to a vacuum pump to eliminate air bubbles present in the uncured mixture. After removing all air bubbles, the mixture was poured into the tensile and shear molds, and the molds were placed back in the desiccator for approximately 30 min to ensure that all remaining bubbles were removed entirely. Part of this PDMS mixture was taken to measure the density using the pycnometer method, and the value of 1059 (kg/m\({}^{3}\)) was determined which is very similar to other references [40].  
At the end of the process, the tensile and shear molds were taken to a temperature- and time-controlled oven, the mixture was cured at 80 \({}^{\circ}\)C for 45 min. A total of 3 samples were prepared for the tensile tests and three samples for each thickness of the simple shear test.  
To verify the absence of voids (bubbles) with sizes that could influence the experimental tests, a scan was made in different regions of the specimen to identify any bubbles present inside it. For this, an inverted microscope CKX41 by Olympus Iberia from Japan, with an objective lens NCHROPLAN, 32\(\times\) by ZEISS from Oberkochen (Germany) was used. After the images obtained, the image] software was used to check the bubble sizes found in the specimen, Figure 1 shows the microbubbles found. Through this test, it was found that the dimensions of the bubbles found were small (<5 \(\upmu\)m), not significantly influencing the results obtained in the tensile tests.