# Tensile Test Analysis of Tire Tread Rubber's Tendency to Resist Cracks
## 4 Experimental Results and Discussion
### Results of Crack Tip Opening Displacement  
Figure 8a-c summarize the hardness, elastic modulus, elongation at 300% of the tensile test, and the related standard deviation for the three types of specimens at the room temperature (25 \(\pm\) 3 \({}^{\circ}\)C) test. Moreover, Figure 8c is the maximum elongation shown on the graph equal to the elongation at the break. According to the results, the hardness of the winter specimen was the same as that of the summer specimen and was less than that of the all-weather specimens. However, at 300% of the tensile test for the winter specimens, the modulus of elasticity was 33 kgf/cm\({}^{2}\) lower than that of the summer specimens and 63 kgf/cm\({}^{2}\) lower than that of the all-weather specimens.  
Further, for the winter specimens, elongation was 180% higher than that of the summer specimens and 250% higher than that of the all-weather specimens.  
Figure 9a,b show the CTOD for each specimen based on the time taken to analyze the images collected by the camera during the tensile test, at speeds of 5 mm/min and 50 mm/min, using the digital image correlation method. Due to the characteristics of viscoelastic and super-elastic materials, the CTOD increased infinitely over time. Cracks, such as those in metals and fine cracks, occurred around the initial crack tip. In other words, fine tearing occurred discontinuously and unevenly.  
Figure 8: Mechanical properties: (**a**) hardness (Shore A); (**b**) elastic modulus at elongation of 300% (kgf/cm\({}^{2}\)); (**c**) elongation (%).  
Figure 9a shows the CTOD of the three types of rubber at the low-speed condition of 5 mm/min. It was confirmed that the CTOD of the three types of rubber exhibited non-linear behavior. This difference can be attributed to the difference in performance, derived from the constituent elements of each material. It can be said that the effect of elastic modulus played a major role rather than the effect of the hardness or elongation at the low speed of 5 mm/min.  
In the case of the all-weather specimens, the growth of the CTOD was about 62% faster until the 35-s section compared to the summer and winter specimens. However, after 35 s, the CTOD growth rate of the summer and winter specimens increased rapidly, reducing the difference to about 10%. In other words, it can be interpreted that the hardness of the all-weather specimens was low, which was disadvantageous for CTOD growth. It also had a larger value than the summer and winter specimens. After increasing by a certain amount or more, it was found that the CTOD growth rate of the summer and winter specimens was faster than that of the all-weather specimens due to the elastic modulus and elongation. In other words, the mechanical properties of the material, such as elastic modulus, elongation, and strength, had an absolute effect at the speed of 5 mm/min.  
Figure 9b shows the CTOD of the three types of rubber at 50 mm/min, the high-speed condition. At the high-speed condition of 50 mm/min, all types of rubbers exhibited non-linear behavior, but the growth trend was similar.  
Regardless of the type of specimen, the CTOD increased rapidly in the section from about 2 s to about 6 s after the start of the experiment, and this increasing trend reduced after 6 s. Once the crack began to open up, it indicated that destruction could occur rapidly due to the influence of speed. In other words, at 50 mm/min, the destructive behavior was expected to appear differently under the influence of the external environment, such as temperature and road conditions, rather than the characteristics of the compound.