In-Vitro Testing and Numerical Modelling towards Uncovering Aortic Wall Fracture Mechanisms

QC 251229

Abstract

Cardiovascular pathologies such as aortic aneurysm and dissection remain one of the leading causes of mortality worldwide. Current clinical standards for assessing rupture risk in aneurysmal aortas rely primarily on external diameter and its growth rate, despite the inherently multifactorial nature of rupture. Although tissue fracture plays a crucial role in the onset and progression of vascular diseases, understanding in this area remains limited. The hierarchical histological structure of vascular tissue gives rise to complex mechanical behaviour, while existing experimental protocols for soft tissue fracture are often inadequate for a sound characterisation of the fracture response.

A comprehensive understanding of fracture requires the assessment of fracture mechanisms and the quantification of key parameters, including resistance to rupture and the size of the fracture process zone. Concerning biological soft tissue, most mechanistic information stems from studies on skin, which is extremely resistant to fracture. However, the histological structure of vascular tissue differs from that of skin, and impedes the translation of such information. Moreover, the influence of clinical factors on the mechanics of diseased vessel walls cannot be ignored, as focusing solely on normal tissue may yield clinically irrelevant estimates of mechanical properties. Bridging engineering fracture mechanics with medical application thus represents both a critical and challenging task.

A major part of this thesis was dedicated to the design and application of a fracture test experiment, the symmetry-constraint Compact Tension (symconCT) test. The setup enabled a stable propagation of the crack in a pre-notched specimen orthogonal to the loading direction. Investigations could be carried out up to complete rupture of the specimen, and image analysis captured local mechanisms at the fracture tip. Pronounced rounding/flattening of the crack notch, called blunting, characterised the fracture. Besides, the study demonstrated the strong dependence of crack morphology on loading orientation relative to fiber alignment. Despite a slow displacement rate being applied, the experiments revealed significant strain-rate effects ahead of the notch. The protocol allowed testing of both normal porcine tissue and human aneurysmal aorta, with results linking fracture properties to clinical and histological data. Collagen content increased fracture resistance, while energy dissipation decreased with age, underscoring the relevance of patient-specific factors in rupture prediction. To further validate this hypothesis, mechanical, geometrical, and clinical information were integrated through different machine learning models to assess abdominal aortic aneurysms' rupture. The models outperformed the clinical standard, revealing that rupture identification depends on multiple interacting factors rather than any single dominant parameter.

Based on the experimental data, finite element models were developed to simulate the fracture behaviour during the symconCT test. Elastic and fracture properties were identified at a specimen-specific level, exploring two different methods to fracture: the cohesive zone and phase-field approaches. As the fracture resistance (strength) of notched specimens was significantly lower than that of unnotched tensile specimens, this indicates that conventional tests on flawless tissue overestimate fracture properties, especially in diseased tissues, which contain microvoids and microdamage. Future work should aim to simulate entire vessel walls using patient-specific geometries and boundary conditions.

The combined experimental and computational framework in this thesis advanced the understanding of the fracture processes and mechanical behaviour of the aortic vessel wall. It provided essential groundwork for patient-specific rupture risk prediction, supported the translation of biomechanics into clinical decision-making, and paved the way for future studies addressing more realistic and complex physiological scenarios.

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