| The test method adopting two-stage、three-stage sequential loading for traditional random fatigue test was difficult to reflect the effect of random load on fatigue life, so the test results had low precision. High-frequency fatigue testing machine was used and loading method of random load block was adopted in this paper to carry out the random loading fatigue test with two-stage, three-stage load block, four-stage load block and eight-stage as well as the secondary order loading (low-high,high-low) fatigue test,all of the tests using infundibular fatigue specimen whose material is45steel,fatigue damage analysis and life estimation were carried out by Miner theory, modified Miner theory and Corten-Dolan model, besides, the effect of high cycle fatigue damage accumulation characteristic and random load on fatigue life under random loading condition was studied. The effect of different load sequence on fatigue damage was analysed from microscopic mechanism through SEM experiments of fatigue fracture surface.Main research achievements and results were as follows:(1) The study of high cycle fatigue test was carried out under the condition that load block was random by means of the high-frequency fatigue testing machine, which considered randomness and nonrepeatability during the load history, better simulated the practical load history.(2) The experimental results showed that using two-stage, three-stage, four-stage and eight-stage random loading ways, the linear cumulative damage value has the follow distribution range:∑ni/Ni=0.25-1.46,∑ni/Ni=0.18～1.08,∑ni/Ni=0.13-0.95,∑ni/Ni=0.23～1.71, according to the linear cumulative damage theory, analyzing damage deviation is larger. There is no linear relationship between load the block size and fatigue life, change the size of load block does not affect the fatigue life estimation.(3) Using the secondary low-high and high-low loading ways, linear fatigue cumulative damage value is∑ni/Ni=0.12-0.68,∑ni/Ni=0.22～1.24, The error values between average test life of Secondary low-high and high-low sequence load and the life of the two-stages random load test is14.91%and75.10%respectively. The influence of loading sequence on fatigue life is significant.(4) Based on the cumulative damage theory, three kinds of life estimation model calculation results show that:Miner theory and correction Miner theory to random load mode and order load mode life estimation accuracy is not high, error range between about30%～60%. Corten-Dolan theory can well describe the random load influence on the material fatigue cumulative damage,life estimation is more accurate, error basic within30%. Compare the d values of Corten-Dolan theory adopt the secondary random load test with the d values of secondary order load test, the life estimation precision is higher, the two error range, respectively7%～14%and14%～13%. There is no difference on life estimation accuracy between using multistage random load test of d value and secondary randomized trials of d values.(5) The SEM experiments test results of45steel specimens fatigue fracture scanning show that:Relative to the constant amplitude loading mode, sequence loading mode and random loading mode, macro and micro morphology of fatigue fracture is messy, rough, secondary cracks appear earlier.This is a kind of typical fatigue fracture morphology under random loading way. |
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