3.4.2. Polymer

The R-value of the polymer material is measured in this section. A 2.5 mm thick dogbone specimen was used for uniaxial tensile experiments. Since the polymer material is a typical brittle material, the material does not neck during the tensile test. Any rectangular area of 8 × 8 pixels on the material surface can be selected for R-value calculation. Figure 18 shows the R-value curves of the two measurement methods for this material. It can be found that the measurement results of the two methods are linearly increasing, but the

growth rate is quite different. The R-value of the direct measurement is 0.3101, while the indirect measurement result is 0.2499. This states that the polymer material does not entirely follow the constant volume assumption in the tensile test. One possible explanation is the constant volume assumption is based on the crystal structure of metal materials, while polymers are composed of polymer compounds with complex structures and no longer follow the deformation conditions of metal materials.

**Figure 18.** R-value curves of polymer.
