PSI - Issue 10

I.D. Gavardinas et al. / Procedia Structural Integrity 10 (2018) 18–24 I.D. Gavardinas and A.E. Giannakopoulos / Structural Integrity Procedia 00 (2018) 000 – 000

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suture line bleeding, and possibly reduced rates of infection. Nevertheless, its use is associated with long-term com plications, such as restenosis, pseudoaneurysm formation, infection, fibrosis, calcification, and thrombosis, as mentioned by Li et al. (2011). Under this point of view, currently there are no guidelines concerning the material selection in the quotidian practice of carotid endarterectomy, implying that material selection primarily depends on the material availability. This work is part of an ongoing project by Gavardinas et al. (2018) developing novel meth odologies for the mechanical characterization of other types of patches used in CEA (polyesters etc.), in an effort to provide insight into their dissimilar mechanical behavior. Complications linked to the mechanical properties of bovine patches fall into two major categories: If the patches are elastically stiff, this may lead to restenosis. If the patches are elastically soft, this may lead to aneurysmal dilatation and possible wound hematoma at the suture line reinforcement, as suggested by Kim et al. (2001) and Ho et al. (2012) recently. This paper is organized as follows. First, the necessary analysis is developed. Second, the obtained results are presented. Finally, the importance of the proposed method is discussed, highlighting the synergy required among vascular surgeon and engineers for a successful outcome of CEA.

Fig. 2. Shore Durometer tester with a bovine sample mounted on.

2. Materials and methods

Bovine patches are known to follow non-linear constitutive equations that are often described by a strain energy density given by an exponential form, see for instance Sacks (2000). In case of initially isotropic response, such strain energy is given by:     1 2 1 exp 3 1        W d d I (1) i , with the incompressibility condition: λ 1 λ 2 λ 3 =1. d 1 and d 2 are positive material constants. d 1 has dimensions of stress and d 2 is dimensionless. Zisis et al. (2015) have shown that the indentation response of such hyperelastic models (for the case of moderate indentation depths) follows the results of linear incompressible elasticity where E 0 is the initial elastic modulus (as λ 1 λ 2 λ 3 →1 ) and ν =1/2 (Poisson’s ratio). Reg arding the exponential strain energy given above, E 0 = 6d 1 d 2 . These researchers also showed how to obtain the other material parameter d 2 via indentation, provided that the indented substrate is prestretched by a known amount. In this work, we indented bovine patches without prestretch, obtaining where I 1 = λ 1 2 + λ 2 2 + λ 3 2 is the first strain invariant in terms of the principal stretches λ