| With the increasing demand for lightweight materials and components in automobile manufacturing,aerospace and defense industry,etc,materials with light weight,high stiffness and specific strength are becoming the hotspot for scientists all over the world.Among many structural materials,magnesium alloys have been widely used in electronic communications,aerospace and other fields due to their low density,high specific strength and good electromagnetic shielding.Among many structural materials,AZ31 B extruded wrought magnesium alloy is widely used in these fields because of its high specific strength and high ductility.As the structural material,AZ31 B magnesium alloys often bear different kinds of alternating loads in actual service,and fatigue failure has become one of the most important failure modes.Therefore,the investigation on the fatigue behavior of AZ31 B magnesium alloy notched components not only has practical value,but also can provide theoretical support for fatigue resistance design of structural components.Based on the summary of fatigue testing technology and the research of fatigue mechanical properties of AZ31 B magnesium alloy at home and abroad,the fatigue module of self-made instrument was redesigned and processed,and the related sensors were reselected and calibrated.And the finite element analysis software ANSYS Workbench was used to analyze the key components,.which verifies the feasibility of the structural design.After the instrument meets the loading requirement,the composition of extruded AZ31 B magnesium alloy produced commercially was determined by energy spectrum analysis,and the basic properties of AZ31 B magnesium alloy were measured by repeated tensile test.According to the requirement of the test,the geometric shape of the specimens is designed.Then the fatigue modular integrated signal generator,oscilloscope,Nikon microscope and scanning electron microscopy were used to study the fatigue properties of AZ31 B magnesium alloy and data analysis method of Paris formula was used to study the fatigue crack growth.Finally,the fatigue parameters of AZ31 B magnesium alloy under different strain amplitudes are obtained,and the effects of different stress amplitudes and notch shapes on the fatigue crack growth rate of AZ31 B magnesium alloy are analyzed.The main test results are as follows:(1)With the increase of strain range,the energy absorbed by AZ31 B magnesium alloy increases gradually in each cycle.At the later stage of cycle,the energy absorbed by AZ31 B magnesium alloy decreases gradually in each cycle.In addition,AZ31 B magnesium alloy exhibits obvious cyclic hardening phenomenon in the early stage of fatigue loading,followed by cyclic saturation,and finally cyclic softening until fracture.Nanoindentation tests on fractured specimens under different strain ranges show that the closer to the notch tip,the greater the hardness of the fatigue specimens.(2)The experimental study of fatigue crack growth under different stress amplitudes shows that under the same stress intensity factor,the greater the stress amplitude,the greater the fatigue crack growth rate,and the fatigue life of components decreases significantly with the increase of stress amplitude.Paris formula was used to fit the test data.It was found that with the increase of stress amplitude,the parameters C and m increased gradually,and the fitting curve tended to be separated.Fracture analysis by scanning electron microscopy shows that fatigue cracks are initiated from the notch position,and the crack propagation zone accounts for the largest proportion in the whole process of fatigue crack growth.The fracture surface in the crack source region is black,showing typical stress corrosion phenomena.With the increase of the stress amplitude,the rapid crack growth zone increases gradually,and the number and size of dimples decrease with the increase of the stress amplitude.(3)The experimental study on fatigue crack growth with different notch shapes shows that under the same stress amplitude,the total fatigue life of U-notched specimens is obviously higher than that of V-notched specimens.Besides,under the same stress amplitude,the crack growth rate curves are very similar.For further study,Paris formula is used to fit the fatigue growth curves of specimens with different notches.It is found that the parameters C and m of the fitting curves are very similar. |
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