Research Of Fracture Properties Of Pyrocarbon Material For Artificial Heart Valves

Pyrolytic carbon is a kind of hard and brittle material similar to ceramics,which is commonly used as coating material for artificial mechanical heart valves in the biomedical field due to its excellent chemical stability and good biocompatibility.The artificial mechanical heart valve implanted in the human body works in a complex physiological environment,and the pyrolytic carbon coating on the surface of the valve is subject to the cyclic impact of blood flow for a long time.The coating surface may appear defects such as microcracks due to cavitation erosion and other reasons.Over time,these tiny cracks may expand to form macroscopic cracks and even lead to the rupture of the valve,resulting in reduced reliability or even complete failure of the artificial mechanical heart valve,threatening the life of the valve implanter.Therefore,it is of great significance to research the fracture properties of pyrolytic carbon materials for artificial mechanical heart valves to predict the effective life of heart valves and improve their structural stability.In this paper,experimental and numerical simulation methods were used to research the fracture properties of pyrolytic carbon materials.ABAQUS finite element analysis software combined with FRANC3 D fracture analysis software was used to calculate the initial crack initiation angle of composite cracks on the surface of pyrolytic carbon,and the crack propagation was simulated and analyzed.The compact tensile test of pyrolytic carbon and pyrolytic carbon graphite composite disc specimens was carried out by MTS fatigue testing machine.The fracture morphology of the specimens was observed by microscope and the fracture mechanism was analyzed.The specific research contents are as follows:1.The initial crack initiation angle of semi-elliptical center crack on the surface of artificial heart valve pyrolytic carbon coating was simulated by ABAQUS and FRANC3 D software.The results show that the initial crack initiation angle of semi-elliptical crack is symmetrical with respect to the central node of crack front,and the crack propagation direction is opposite.When the crack depth is constant,the initial crack initiation angle of mode I increases with the increase of the crack inclination angle,and the initial crack initiation angle of mode II decreases first and then increases with the increase of the crack inclination angle;under the condition that the crack inclination angle is constant,the smaller the crack depth,the greater the influence on the initial crack initiation angle of the model surface crack,and the influence of the crack depth on the initial crack initiation angle of mode I is greater than that of mode II.2.The fracture surfaces of pyrolytic carbon and pyrolytic carbon-graphite composite disc samples were prepared by MTS fatigue testing machine,and the fracture morphologies of the samples were observed and analyzed by the optical microscope and the scanning electron microscope.The results show that the fracture of pyrolytic carbon under the optical microscope is smoother than that of graphite,and the fracture of graphite is relatively rough due to the presence of more particles.The fracture of pyrolytic carbon under the scanning electron microscope shows columnar particles and lamellar morphology,and the fracture type is mainly intergranular fracture.There are a large number of white cleavage planes and micropores formed by grain shedding in the fracture of graphite,and the fracture type is mainly transgranular fracture;The crack propagation process of pyrocarbon and other ceramic materials is the result of the interaction of internal damage mechanism and external shielding mechanism.3.The crack propagation process of pyrolytic carbon coating on artificial heart valve was studied by ABAQUS combined with FRANC3 D software.The effects of crack depth and crack inclination angle on crack propagation behavior are discussed.The results show that the maximum operating stress of the valve appears in the ear-shoulder area of the valve,and there is obvious stress concentration in this area.Under the action of tensile load,reducing the crack depth and increasing the crack inclination angle can reduce the fatigue crack growth rate of pyrolytic carbon materials and improve the service life of the artificial heart valve.