26 L. Nguyen et al. 5% compressive strain. If we examine the final stress (the stress value at the end of the test) in relation to the changes in compressive strain, shown in figure 5, we do not see any definitive correlation. We expect there to be a high variance within the different biological samples, so the question arises as to whether this peculiarity in the data is a result of the large spread in experimental data or if there is something within the microstructure of the porcine aorta that leads to such a phenomenon. (a) Final normal stress vs applied compressive strain (b) Final shear stress vs applied compressive strain Fig. 5 The (a) normal and (b) shear stress at time t =1000s as a function of the applied compressive strain. The varying shear strains are plotted on the same figures. Conclusions We noticed the relaxation behavior is different in the normal and shear directions. The amplitude of compressive strain appears to have a larger impact on the quantitative experienced stress compared to the amplitude of the applied shear strain. There appears to be some nonlinear correlation between the normal and shear stress responses of the porcine descending aorta. Further investigation is required to better characterize this correlation. Future work Future work for this project will include a creep experiment to examine if the creep behavior can be predicted using this relaxation data. This creep experiment will be conducted using similar testing conditions as those found in this paper. We expect the porcine aorta to behave as a nonlinear material, so the creep behavior should not be able to be directly predicted from the relaxation data. Creep experimental data will be needed to confirm such hypotheses. An analysis on the kinematics and force balances will also be done for the problem given by this experimental set up. With the creep and relaxation data and the mechanics analysis, further improvements on constitutive modeling can be made. The purpose of this study is to provide a robust set of experimental data on the relaxation response of the porcine aorta. As such, testing time was limited to 1000 seconds per sample. In the future, we seek to test samples until the value of the stress plateau in both the normal and shear directions. This will help to determine with certainty if the porcine aorta acts as a viscoelastic solid or viscoelastic fluid. We also seek to perform histology analysis on samples used in these relaxation experiments. This will further examine how the relaxation response of the porcine aorta relates to the permanent damage to the tissue. References 1. Afshin Anssari-Benam, Hazel RC Screen, and Andrea Bucchi. Insights into the micromechanics of stress-relaxation and creep behaviours in the aortic valve. Journal of the mechanical behavior of biomedical materials, 93:230–245, 2019. 2. RJ Bagshaw and Franc¸oise ML Attinger. Longitudinal stress relaxation in the canine aorta. Experientia, 30(9):1046–1047, 1974.
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