Fig. 7.

Relationship between the plastic strain (ε_p) and the applied strain (ε_i) and the linear regression fitting of the linear part of the curves for both middle-aged (ε_p = 0.60 ε_i−0.0052, and R² = 0.98) and elderly groups (ε_p = 0.61 ε_i−0.0046, and R² = 0.95). The yield strain (ε_y) is determined at the intersection of the regression line with the horizontal axis.