Characterization of fracture toughness exhaustion in pig aorta

Spontaneous rupture of the aorta (SRA) without aneurysm, dissection, inflammation or infection of the aortic wall can be of two types: traumatic and non-traumatic. SRA is most of the time a fatal event. Consequently, it is important to understand the conditions which lead to the aortic rupture, and, in the case of non-traumatic SRA, to predict the temporal likelihood of rupture.

The present work incorporates the temporal aspect by examining the effects of fatigue on aortic wall properties, and adopts an energy approach, based on fracture toughness, to evaluate the aorta's resistance to rupture. Fracture toughness characterization is a destructive testing process and as a consequence cannot be implemented as a clinical tool. However, using concepts in damage mechanics, in theory, it should be possible to indirectly assess fracture toughness from other mechanical properties, such as aortic wall stiffness. Tissue samples from non-aneurysmal porcine aortas were fatigued and were subjected to both biaxial and guillotine tests to assess wall stiffness variations and fracture toughness exhaustion, respectively.

The experiments reveal that aortic wall stiffness variations and fracture toughness exhaustion decreased as a function of loading cycles and can be modeled with exponential functions. After one million loading cycles, the stiffness ratio between the non-fatigued sample and the fatigued sample, dropped to about 0.85, while the fracture toughness ratio counterpart fell to about 0.80.

Consequently, the changes in both stiffness and fracture toughness as a function of applied fatigue cycles can be measured in aortic tissues. Moreover, these results suggest the possibility to use fracture toughness exhaustion curves as a fatigue criterion.