With the deepening of the academic research on the fatigue or corrosion fatigue of biological metal materials in the biological environment, the complexity of the fracture process is also increasing, and there are still many unsolved research fields. The current fatigue test methods in the biological environment are all conducted independently in the laboratory.
This topic discusses the comprehensive performance including fatigue corrosion, wear resistance, and the influence of cells on biological titanium alloys. Specifically, the influencing factors of sample shape, repetition rate, load-bearing method, simulated body fluid, pH and dissolved oxygen concentration, etc., and also verified whether the test method is appropriate.
The sample is a Ti-6Al-4VELI alloy (ASTM F136) made of Datong special steel. Fatigue sample shape: Quasi-hourglass round bar with a standard spacing of 12 mm and a diameter of 6 mm. Fatigue test conditions: the frequency under uniaxial tensile load is 10 Hz and it is conducted under a sine wave. The test temperature was conducted in the atmosphere of the laboratory where the indoor temperature was adjusted to 22°C with an air conditioner. The number of repetitive fatigue loads is 107.
The fatigue test parameters of biological titanium alloys are: the surface properties of the sample, the stress ratio, the presence or absence of notches, the repetition rate, the load loading method, the type of simulated body fluid and the choice of pH and dissolved oxygen concentration, all of which are influencing factors. Only the effect of the surface properties of the specimen on the fatigue properties is reported here. Regarding the surface properties, two types of materials, the machined sample surface and the mirror polishing, were discussed.
the surface roughness measurement of the sample in the longitudinal direction using a contact surface roughness meter: the surface roughness of the mirror polished sample is reduced to less than half compared with the machined material. The tensile strength test results (stress-deformation curve) of samples with such surface roughness: regardless of the surface roughness, the results of the maximum tensile strength, elongation, etc. are the same, which means that the surface properties are different. The tensile properties have no effect.
On the one hand, the result of the fatigue test is that the number of repetitions of the mirror polished sample at each stress level is increased compared to the machined profile. That is, the effect of surface properties on fatigue life. As the number of repetitions increases, the starting point of the crack changes from the surface to the inside. The starting position of the internal crack is compared with the mirror polished material, and the machined shaped material appears outside (close to the surface). This is because a material with a large surface roughness has a high stress state near the surface and is relatively sensitive to cracks. That is, the degree of unevenness accompanying the surface roughness promotes the generation of cracks, and the crack texture is large. Therefore, as the number of samples increases, the relationship between the crack starting point and the stress distribution is analyzed in detail, so that one of the issues is understood: the effect of surface properties on fatigue characteristics.
This research focuses on various fatigue-related conditions of biological titanium alloys, and discusses the comprehensive fatigue test method and its characteristics. In particular, the surface properties of the sample are reported, that is, the effect of surface roughness on fatigue characteristics is clarified.