Interfacial Fracture Toughness Measurement of Welded Babbitt alloy SnSb11Cu6/ 20Steel

The interface fracture toughness of SnSb11Cu6/20steel was measured by calculating the critical energy release rate and stress phase angle of the interface crack. A three-point bending test was used to introduce cracks into the bonding interface, and the cohesion model of the bonding interface was established through experimental data. Through finite element analysis of load-deflection curves with and without interface crack propagation, the crack initiation point is found. Then the energy calculation model of crack propagation is established, and the critical energy release rate is obtained using the virtual crack growth criterion. The calculation results of the stress phase angle show that the crack propagation is greatly affected by the normal stress after the babbitt alloy layer fractures. If the strength of the substrate material is weaker, the crack will continue to expand in the tangent perpendicular to the crack tip.

A Fluid-Structure Interaction Model of Atherosclerosis at Abdominal Aorta

Atherosclerosis is a form of cardiovascular disease that is a major contributing factor to death and disability worldwide. This study uses computational fluid dynamics (CFD) models as a cost effective and non-invasive method to determine the location and condition of atherosclerosis segments on the arterial wall. It also investigates changes in the abdominal aorta geometry including the inner and outer diameters, the length of the disease segments and the thickness of the arterial wall on the development of disease. Three groups of unhealthy conditions are assumed with each group having eight cases, which are compared to the control case of healthy condition. An invasive catheter pulsatile blood flow is imposed at the ascending aorta and pressure waveforms data is imposed at the four outlets of the aorta and also used to validate the present models. The results show that the stress phase angle at the brachial artery could be correlated to the early stages of atherosclerosis development at the abdominal aorta. This can be detected by measured values of the systolic wall shear stress and elastic strain intensity which increases due to the forward pulse wave resulting from atherosclerosis, while the diastolic values of stresses decreases due to the delay of the backward waves which reach the brachial artery. The three scenarios of atherosclerosis show that the forward and backward waves, which can be attributed to changes in the diameter, length and thickness of the abdominal aorta, can be non-invasively used to diagnose cardiovascular diseases.

Stress Phase Angle for Non-Invasive Diagnosis of Cardiovascular Diseases

In recent cardiovascular studies, the stress phase angle (SPA) has been used as a parameter to identify location for various diseases such as atherosclerosis and aneurysm. This angle represents the phase interaction between the wall shear stress (WSS) and the elastic strain intensity (ESI). It reflects the seriousness of WSS which is a direct measure of these diseases. In this work, a 3D model pulsatile flow in an artery is developed and used to calculate the SPA. Three different models are simulated and the changes in the SPA are observed.

Stress phase angle depicts differences in coronary artery hemodynamics due to changes in flow and geometry after percutaneous coronary intervention

The effects of changes in flow velocity waveform and arterial geometry before and after percutaneous coronary intervention (PCI) in the right coronary artery (RCA) were investigated using computational fluid dynamics. An RCA from a patient with a stenosis was reconstructed based on multislice computerized tomography images. A nonstenosed model, simulating the same RCA after PCI, was also constructed. The blood flows in the RCA models were simulated using pulsatile flow waveforms acquired with an intravascular ultrasound-Doppler probe in the RCA of a patient undergoing PCI. It was found that differences in the waveforms before and after PCI did not affect the time-averaged wall shear stress and oscillatory shear index, but the phase angle between pressure and wall shear stress on the endothelium, stress phase angle (SPA), differed markedly. The median SPA was −63.9° (range, −204° to −10.0°) for the pre-PCI state, whereas it was 10.4° (range, −71.1° to 25.4°) in the post-PCI state, i.e., more asynchronous in the pre-PCI state. SPA has been reported to influence the secretion of vasoactive molecules (e.g., nitric oxide, PGI2, and endothelin-1), and asynchronous SPA (≈−180°) is proposed to be proatherogenic. Our results suggest that differences in the pulsatile flow waveform may have an important influence on atherogenesis, although associated with only minor changes in the time-averaged wall shear stress and oscillatory shear index. SPA may be a useful indicator in predicting sites prone to atherosclerosis.

BIOMECHANICAL ANALYSIS OF WALL REMODELING IN ELASTIC ARTERIES WITH APPLICATION OF FLUID–SOLID INTERACTION METHODS

The effects of age-related hypertrophic remodeling of the thoracic aortic wall on mechanical stresses are quantified using the fluid–solid interaction method. Boundary conditions include physiological flow and pressure waves. Fluid and solid governing equations are solved using the loose coupling method. The results show alteration of hemodynamic and wall mechanical parameters by the remodeling process, including reduction in maximum circumferential stress and lower shear stress fluctuation with smaller portion of negative value and smaller maximum value. Such characteristics are indicators of the reduction of risk of endothelial injury. Remodeling causes elevation of the stress phase angle, an indicator of interaction between shear and circumferential stresses that causes triggering of endothelial cell proliferation, which is necessary for coverage of extra surface required by remodeling. The improvement by remodeling is limited by age-related structural changes such as elastin dysfunction and disorganization of structural components.