PSI - Issue 45

Xiaochen Wang et al. / Procedia Structural Integrity 45 (2023) 88–95 Xiaochen Wang/ Structural Integrity Procedia 00 (2023) 000 – 000

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3.1. Wall stress The direct cause of AAA rupture is the aortic wall stress exceeding the wall strength. Peak wall stress (PWS) serves as a direct indicator of the rupture index, allowing for the prediction of AAA failure. The PWS calculated by computational simulations can therefore be used directly as a risk index by comparing it with the AAA material strength threshold obtained from experimental work in Table 1. The results from this study show that isotropic material models present lower overall wall stress compared to anisotropic models. From Fig. 2, it can be seen that such a significant underestimation of PWS leads to an inaccurate prediction if the anisotropy of the AAA tissue is ignored. Neglecting the presence of ILT in the AAA modelling on the other hand, leads to an overestimation of PWS by 18%. The presence of ILT in AAA models can act as a "buffer", reducing the overall wall stress experienced by the aortic wall due to its softer material properties. This finding confirms that of previous research (Haller et al., 2018). This highlights the potential for incorporating ILT volume as a more sophisticated standard for assessing AAA rupture risk in clinical trials, beyond using aortic diameter alone. (a) (b)

Fig. 2. (a) Wall stress contour of model with anisotropic wall material properties and without ILT; (b) PWS of the four models within a cardiac cycle.

3.2. Wall deformation The tissue deformation for the four models is shown in Fig. 3. Figure 3(b) provides a visual representation of the contour mapping for models with and without ILT. Results show that the highest deformation occurs in a similar region, proximal and anterior to the aneurysm, in all models. The presence of ILT reduces the deformation of the aneurysm segment, but incorporating the anisotropic behaviour of the aneurysm wall increases the corresponding displacement by 15%. The actual morphology and material representation are critical in determining wall deformation, as it is largely influenced by the orientation of collagen fibres and their properties, particularly the dissection energy. The longitudinal direction requires a much higher dissection energy compared to the circumferential direction. To fully understand the interaction between blood and the AAA wall, advanced anisotropic constitutive models in AAA simulations are necessary and will significantly improve the reliability of the FEM-based simulation.

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Fig. 3. (a) Wall deformation contour of models with and without ILT; (b) maximum wall deflection of the four models within a cardiac cycle.