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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simulations were conducted, and the results of stress distribution, deformation patterns, and blood flow velocity field were analysed and compared. The results motivate a systematic sensitivity analysis to the selected parameters considered in the costed simulations, in order to reduce computational times while achieving a desired level of accuracy and convergence. Inclusion of the presence and mechanical properties of the ILT and the anisotropic nature of the aneurysmal wall increases the ability to understand and predict the progression of an AAA and guide future clinical and computational efforts towards vascular pathology. 2. Materials and methods The rupture is primarily caused by the increase in intraluminal pressure, leading to the generation of shock waves that propagate in various direction, as previously noted, the presence of collagen fibres in the aortic wall impacts the stresses inside, and high axial stresses may result in transverse rupture, resulting in internal bleeding. The strength of the tissue has been the subject of many experimental studies, and can be quantified through the determination of failure stresses and dissection energy through experimental methods (Khaniki et al. 2023). The failure stresses shown in Table 1 were obtained from uniaxial tensile tests and peeling tests performed on sample strips of the AAA and ILT in various directions (Raghavan et al., 1996, Vorp et al., 2001, Di Martino et al., 2006, Raghavan et al., 2006, Sommer et al., 2008, Tavares Monteiro et al., 2014, Tong et al., 2014). The results showed that the aneurysmal wall tissue was stiffer compared to the controls, while the ILT tissue exhibited significantly higher failure stresses. Previous studies have also reported that the failure stress of the AAA wall decreases with an increase volume of ILT.

Table 1. Experimental studies investigating AAA and ILT tissue strength.

Failure stress (kPa)

Dissection energy (mJ/cm2)

Studies

Cases

Orientation

P

AA (Control)

Longitudinal Longitudinal

2014±394 864±102 2160±340 1380±190

- - - - - -

<0.0005 <0.0005

Raghavan et al. 1996 (Raghavan et al., 1996)

AAA

ILT(t<4mm) ILT(t>4mm)

Circumferential Circumferential Circumferential Circumferential

<0.05 <0.05

Vorp et al. 2001 (Vorp et al., 2001)

AAA

820±90 540±60

0.04 0.04

Di Martino et al. 2006 (Di Martino et al., 2006) Raghavan et al. 2006 (Raghavan et al., 2006) Sommer et al. 2008 (Sommer et al., 2008)

Ruptured AAA

AAA

Longitudinal

1266

-

-

Circumferential

- -

5.1±0.6 7.6±2.7

- -

AAA

Longitudinal Longitudinal Longitudinal

AAA(d<55mm) AAA(d>55mm)

770±237 1030±273

- -

0.027 0.027

Monteiro et al. 2014 (Tavares Monteiro et al., 2014)

Circumferential

- -

4.0±1.2 3.8±0.9

-

Tong et al. 2014 (Tong et al., 2014)

ILT

Longitudinal - Abbreviations : AA, Abdominal Aorta; AAA, Abdominal Aortic Aneurysm; ILT, Intraluminal Thrombus; t, thickness; d, diameter. The material properties used in this study were calibrated into two material models using uniaxial test results obtained from the experimental data in (Amabili et al., 2019). The 5-parameter Mooney-Rivlin model is used to describe the aneurysm wall as an isotropic and hyperelastic material, while the anisotropic material model was obtained by using an exponential-function-based strain energy function (SEF) shown in Eqn. (2). The SEF for 5 parameter Mooney-Rivlin model is describe as (Mooney 1940) 2 2 2 10 1 01 2 20 1 11 1 1 02 2 1 ( 3) ( 3) ( 3) ( 3)( 3) ( 3) ( 1) , C I C I C I C I I C I J d          (1) where C i are five independent constants for material deviatoric deformation characterisation, I 1 , I 2 are invariants of Cauchy deformation tensor, J is the deformation gradient determinant, and d is the material incompressibility parameter (Ranga et al., 2004; Carpenter et al., 2021). The anisotropic exponential-based SEF includes an isotropic part and an anisotropic part that considers fibre stretches in two directions, which can be written with