## 5 Discussion  
In this study, the mechanical behavior of the age-dependent aorta wall was investigated through an experimental and computational approach. To do this, the uniaxial tensile test for aorta wall tissue of porcine considering collagen fiber orientation was carried out, and the numerical experiment using HGO model-implemented ABAQUS was conducted to verify the experiments. After the validation of the simulation technique, the material characteristics of abdominal aorta wall of healthy humans were simulated, and the material parameters of HGO model under uniaxial tension were determined using polynomial regression equations in order to predict the uniaxial tensile behavior of the abdominal aorta wall of healthy humans. According to the literature, aging has a significant effect on the structure and function of the arterial wall [31; 32]. They studied the evolution of structural components for the human thoracic aorta from the strain energy function considering the composition of the soft tissue, and the axial stretch and residual strain in the biomechanical analysis were considered in order to examine the variation of the collagen fiber. In addition, the other research team studied the variation of the behavior of aortic wall according to age from the constitutive modeling approach. As a result, the age-related variation in pressure-load bearing from elastin to collagen was examined due to the decrease in elastin stiffness and the increase in collagen recruitment. In this study, the increase of stretch and the decrease of stress and Young's modulus in the aortic tissue were investigated with the progress of age from the literature [17; 31; 32]. Moreover, the numerical simulations considering the variation rate of the material constants based on the age were conducted. In this simulation, the range of the material constants, which could simulate the behavior of the aortic tissue, were determined from the trend lines of the maximum and minimum values in the experiment results. The trend line of the minimum value could be applied to the study for aortic disease considering the stress and stretch in the severe condition. In the experiments, the tensile test specimens were placed in saline at 38-39 \({}^{\circ}\)C, and the uniaxial tensile tests were performed within 12 h of harvest since it was important to maintain the environmental conditions in the experiment, such as temperature and elasticity. In other words, the environmental conditions in the tensile test should be maintained as the conditions in the porcine body since the material characteristics for soft tissue depend on the environmental conditions. In addition, the uniaxial tensile test specimens were generally produced as the dog-bone type; however, it is quite difficult to produce the specimens as the dog-bone type. Using the method obtained from the literature, the tensile test specimens were prepared by cutting the tissue into the dog-bone type without curvature [29]. This method had the disadvantage of inducing stress concentration in the center of the specimen, but it had the advantage of avoiding the stress concentration in the jig and reducing the failure rate of the experiment. The preconditioning test was adopted to guarantee the straight configuration of the test specimen before the main test. In addition, the viscoelastic effect was not considered during the main test. In other words, a preconditioning test, involving 10 loading and unloading cycles of the specimen strip to 5% of its length at 4% of specimen length/min (1 mm/min), was conducted. After preconditioning, the strip was uniaxially extended at 4% specimen length/min (1 mm/min) until failure. In some articles, the displacement rate of 20% of specimen length/min was adopted during the uniaxial tensile test, for example [PERSON] et al. [11], [PERSON] et al. [17], and [PERSON] et al. [30], to avoid the viscoelastic effect of tissue.