Fracture mechanics modeling of aortic dissection

Aortic dissection, a critical cardiovascular condition with life-threatening implications, is distinguished by the development of a tear and its propagation within the aortic wall. A thorough understanding of the initiation and progression of these tears, or cracks, is essential for accurate diagnosis and effective treatment. This paper undertakes a fracture mechanics approach to delve into the mechanics of tear propagation in aortic dissection. Our objective is to elucidate the impact of geometric and material parameters, providing valuable insights into the determinants of this pivotal cardiovascular event. Through our investigation, we have gained an understanding of how various parameters influence the energy release rate for tear propagation in both longitudinal and circumferential directions, aligning our findings with clinical data.

Cracks in tensile-contracting and tensile-dilating poroelastic materials

Fibrous gels such as cartilage, blood clots, and carbon-nanotube-based sponges with absorbed oils suffer a reduction in volume by the expulsion of liquid under uniaxial tension, and this directly affects crack-tip fields and energy release rates. A continuum model is formulated for isotropic fibrous gels that exhibit a range of behaviors from volume increasing to volume decreasing in uniaxial tension by changing the ratio of two material parameters. The motion of liquid in the pores of such gels is modeled using poroelasticity. The direction of liquid fluxes around cracks is shown to depend on whether the gel locally increases or decreases in volume. The energy release rate for cracks is computed using a surface-independent integral and it is shown to have two contributions - one from the stresses in the solid network, and another from the flow of liquid. The contribution to the integral from liquid permeation tends to be negative when the gel exhibits volume decrease, which effectively is a crack shielding mechanism.

The effect of surface treatments on the adhesive bond in all-ceramic dental crowns using four-point bending and dynamic loading tests

The aim of this study was to determine the effect of sandblasting, grinding and plasma treatment on the adhesive bond strength between framework ceramic (Y-TZP) and veneering ceramic (feldspar ceramic). Therefore, four-point bending specimens (n = 180) were cut from densely sintered 3Y-TZP blanks. Subsequently, 80 of these samples received surface treatment by sandblasting and 80 samples by grinding. A reference group (20 samples) was not processed. Half of the specimens that received a surface treatment were additionally exposed to an oxygen plasma treatment. After processing, all specimens were manually veneered with feldspar ceramic and examined with a four-point bending test to evaluate the strain energy release rate G. The surface treatment parameters that achieved the highest and lowest G were transferred to real geometries of a posterior crown (n = 45). The crowns' ceramic framework was sandblasted and veneered by hand. The all-ceramic crowns were tested in a dynamic loading test and Wöhler curves were evaluated. Four-point bending samples blasted at an angle of 90° at 6 bar and a working distance of 1.5 cm without plasma treatment achieved the highest energy release rate. Samples blasted at an angle of 90° at 2 bar and a working distance of 1 cm with plasma treatment achieved the lowest energy release rate. Overall, plasma treatment did not improve bond strength. In the dynamic loading test, the group blasted with 2 bar showed the best results.

Hamstring muscles rupture under traction, peeling and shear lap tests: A biomechanical study in rabbits

Lesions of the Musculotendinous Unit (MTU, i.e. tendon, myotendinous junction, muscle, aponeurosis and myoaponeurotic junction) are a common injury and a leading cause of functional impairment, long-term pain, and/or physical disability worldwide. Though a large effort has been devoted to macroscopic failure evaluation, these injuries suffer from a lack of knowledge of the underlying tissue-scale micro-mechanisms triggering such lesions. More specifically, there is a strong need for experimental data to better understand and quantify damage initiation and propagation on MTUs. The present study presents original experimental data on muscle tissue extracted from the hamstring muscle group of rabbits under relevant mechanical solicitations up to rupture, revealing elementary micro-mechanisms and providing quantified values of elastic properties as well as initiation stress and energy release rate. More specifically, tensile, peeling and shear lap tests were performed to explore cohesion of muscle tissue along the fibre direction or across fibres (mode I) and in shear (mode II), as well as at the muscle/tendon interface. We show that muscle tissue is weaker in shear than tension (p-value < 0.01) and that the Biceps Femoris had the lowest energy release rate as calculated from mode I peeling tests (G = 0.23 ± 0.16 N/mm) compared to the Semi-Membranous (G = 0.53 ± 0.08 N/mm) and the Semi-Tendinous (0.45 ± 0.20 N/mm), and that this energy is the lowest at the musculotendinous junction. Our study suggests a preferred damage initiation mechanism based on fibre decohesion in mode I or II and provides quantitative data to model these phenomena. Results also suggest that the Biceps Femoris and more precisely its musculotendinous junction could be the weakest point of the hamstring group. These findings could be used as a basis to develop mechanical models (e.g. finite element) to better understand and predict the onset of hamstring lesions and help in preventing such events.

Size Effect on Failure of Pre-stretched Free-Standing Nanomembranes

Free-standing nanomembranes are two-dimensional materials with nanometer thickness but can have macroscopic lateral dimensions. We develop a fracture model to evaluate a pre-stretched free standing circular ultrathin nanomembrane and establish a relation between the energy release rate of a circumferential interface crack and the pre-strain in the membrane. Our results demonstrate that detachment cannot occur when the radius of the membrane is smaller than a critical size. This critical radius is inversely proportional to the Young's modulus and square of the pre-strain of the membrane.