Statistical analysis
The data analysis was performed using SPSS 22.0 software (IBM SPSS Inc., Chicago, IL, USA). Levene’s test was performed to assess the homogeneity of variance, and Kolmogorov–Smirnov test was used to calculate the normality of distribution. Since the variance was uneven or the data did not conform to a normal distribution, the nonparametric test Kruskal-Wallis analysis of variance followed by Mann-Whitney U test was used to identify the statistically significant difference between the average DH (or E) values of each subgroup which was the same material, as well as the average DH (or E) values of each material at the significance level of α = 0.05.