| In the development of biomedicine,pyrolytic carbon materials have been gradually used in the preparation of artificial heart valves because of their excellent bioco MPatibility and excellent mechanical properties.In the process of preparation and processing,it is easy to produce small cracks on the surface,and the surface cracks will undergo fatigue propagation in the environment of long-term cyclic stress.The artificial mechanical heart valve will serve for life after being implanted into the human body,and will bear the cyclic i MPact caused by blood flow and the myocardial contractility caused by heart beating for a long time.Pyrolytic carbon materials are brittle materials,which are prone to brittle fracture during fatigue expansion,which in turn threatens the life and health of patients.Therefore,the research on fatigue crack propagation and fatigue life of artificial heart valves is of great significance for maintaining its stable operation in the human body within the design life.In this paper,the fatigue life and crack propagation of pyrolytic carbon materials were studied by means of experimental and numerical analysis.The fatigue crack fracture experiments of pyrolytic carbon and graphite samples with prefabricated cracks were carried out by MTS fatigue test machine.The fracture morphology of pyrolytic carbon and graphite was observed by optical microscope and scanning electron microscope.The maximum initial crack size that can be allowed to exist in the design life of the artificial heart valve frame is studied by using the damage tolerance design method.The crack propagation was studied by ABAQUS and Franc3D co-simulation.The specific research contents are as follows:1.The fatigue fracture of pyrolytic carbon graphite specimens was prepared by MTS fatigue testing machine.The macro and micro fracture morphology was analyzed by optical microscope and SEM.The results show that there is no obvious difference between the fatigue propagation zone and the instantaneous fracture zone of the pyrolytic carbon and graphite specimens,showing the properties of brittle materials.The macroscopic fracture of the pyrolytic carbon is smooth and the graphite fracture is rough and there are more particles.There are a large number of micropores on the microscopic fracture surface of pyrolytic carbon,which are mainly caused by crystal shedding during the fracture process,and the fracture form is mainly intergranular fracture.The microscopic fracture of graphite produces many white flake cleavage planes,mainly the traces left by crystal tearing during the fracture process,and the fracture form is mainly transgranular fracture.2.The analysis method of damage tolerance was used to analyze the conservative life of artificial heart valve frame.CATIA software was used for modeling and ABAQUS software was used for simulation analysis.The stress analysis of the operating stress and residual stress of the artificial heart valve frame was carried out.The maximum operating stress and the maximum residual stress appeared in the eight-shaped positioning pit of the valve frame,which were 72.19 MPa and 8.56 MPa,respectively.The fracture mechanics theory is used to analyze the maximum stress,and the relationship between the maximum initial crack size and the fatigue life N is obtained.When the fatigue life is 100 years(the number of heart beats is 4×10~9),the maximum initial crack size is not more than 77.3μm.3.Using ABAQUS and Franc3D co-simulation to simulate the fatigue propagation process of I-II mixed-mode crack of artificial heart valve pyrolytic carbon material.In the case of ensuring that other factors do not change,the depth and inclination angle of the initial crack are changed respectively.The results show that reducing the crack depth can improve the fatigue life of the flap,and the crack propagation path does not change when the depth changes.Increasing the inclination angle can slightly increase the fatigue life of the flap and change the direction of crack propagation. |
