Research On Fatigue Performance And Damage Tolerance Of Aluminum Alloy For High Speed Train

6N01 and 7N01 aluminum alloys commonly used in high-speed train body materials.Three kinds of materials 6N01 profile,7N01 profile and 7N01 plate were studied for fatigue performance and surface damage tolerance.The conditional fatigue strength of the three materials was tested by the lifting method;the effects of different surface states?original surface,sandblasted surface,electropolished surface?on the fatigue strength were investigated.The crack propagation threshold values of the three materials were tested.The fatigue test results of the surface notch specimens were used to construct a Kitagawa-Takahashi line graph to quantitatively analyze the surface damage tolerance of the material.The results of the single-point method and the lifting method for the conditional fatigue test showed that under the conditions of confidence?=95%and survival rate p=50%,90%,99%,7N01 profiles have the highest fatigue strength in all three aluminum alloys which are145.36MPa?131.57MPa?122.00MPa,6N01 profiles fatigue strength which are 138.21MPa,125.50MPa,116.67MPa are at medium level,7N01 plate's fatigue strength which are128.93MPa,118.25MPa,110.83MPa with the lowest strength.Under different surface processes,the surface implant compressive stress?sandblasting process?improves the fatigue strength of the profile significantly,mainly because the stress layer and dislocations on the surface after sandblasting improve the resistance of the material to crack initiation.However,due to the difference in surface texture and internal structure of the 7N01 plate,the hardness of the surface layer and the core portion are still big different after sandblasting,so the fatigue strength is not improved.Increasing the surface quality?electrolytic polishing process?is not obvious for the fatigue performance of the three aluminum alloys,mainly because the original surface has a high degree of smoothness,so the improvement of the fatigue strength is not obvious.Measured the threshold crack propagation?K_th of 6N01 profile,7N01 profile and 7N01plate.Completed the fatigue strength test of aluminum alloys sample which introducing different size notch.These two experimental results plot Kitagawa-Takahashi line graph.The harmless defect sizes of the three alloys of 6N01 profile?7N01 profile and 7N01 plate were determined by the Kitagawa-Takahashi line diagram to be 23.92?m,10.81?m and24.32?m,respectively,and the significant theoretical transition dimensions were 79.09?m,72.68?m and 74.56?m.The fatigue behavior of the notch of the three aluminum alloys within a certain range after the transitional dimension does not meet the fatigue behavior of the smooth specimen,nor the long crack behavior of the notched specimen,but the fatigue strength and elasticity below the smooth sample.Failure under fatigue strength predicted by fracture mechanics.It is shown that there is a"short notch effect"in the fatigue behavior of the notched specimens of the three aluminum alloys.For safety,the effective fatigue notch expansion threshold?K_eff.th of the long cracks of the three alloys was re-established.