Study On Vibration Fatigue Of The Structure Under Multiaxial Versus Uniaxial Random Loading

Vibration environment test plays an important role on the reliability of products and equipment in the field of aviation,transportation and machinery and so on.At present,most vibration environment tests are conducted by sequentially applying uniaxial excitation to the test specimen along three orthogonal axes,which could lead to short-test or over-test in the application,so its reliability has been widely questioned by the researchers.To overcome these disadvantages aforementioned,the research on vibration fatigue of the structure under multi-axial versus uniaxial loading is investigated in this dissertation.Theory derivation,finite element analysis and experiments on dynamic responses of structures under uniaxial and multiaxial excitations are performed in this work.Results show that under three simultaneous uncorrelated axial excitations,the responses of the structure are the square root of the sum of the squares of each uniaxial excitations results.If the random excitations are correlated,the input correlation have significant effects on the dynamic response,and the effects are regularly.Additionaly,the reasons for the phenomena mentioned above are also analyzed.This study provides references for the vibration failure mechanism analysis of the structure under uniaxial and multiaxial random excitations.The criteria of determining the vibration fatigue of the structure under multi-axial random excitation are presented.The laser vibrometer and dynamic signal analyzer(35670A)are used to monitor the first order natural frequency of the structure.The dynamic strain of the gap position for the structure is monitored by the dynamic strain measurement system in real time.Based on the investigation results,the relationships between the crack growth and the first order natural frequency as well as the strain variation are analyzed,and then two criteria of fatigue failure are determined,which lay the foundation for the multi-axial vibration fatigue experiment.The fatigue behavior of the structure under multiaxial and uniaxial random loading is discussed.The vibration fatigue tests under three cases for typical specimen are conducted which shows that the fatigue life,source of crack and the mode of crack propagation under multi-axial vibration condition are different with uniaxial vibration condition.The fatigue life of test specimen under multi-axial vibration condition is significantly shorter than that of uniaxial vibration condition.The correlation between different axial input loads and its influence on the failure mode are investigated experimentally.Based on the experiment,the relationship between the failure mode and the correlation coefficient as well as the phase difference between the input loads are also obtained.A new frequency domain method for fatigue life prediction of the structure under multi-axial random excitation is presented,according to the multiaxial fatigue damage prediction model and the characteristics of dynamic response of the structure under multiaxial excitation.The multi-axial stress problem is translated to the uniaxial stress problem via equivalent von Mises stress PSD,and the uniaxial frequency domain fatigue analysis is used for the fatigue life prediction.The fatigue life of a typical specimen under the multi-axial random excitation is calculated and compared with the test result.In addition,the fatigue damages of the component caused by the simultaneous multiaxial and uniaxial excitation are analyzed and compared,and the multiaxial effect of the structure under multiaxial excitation is also analyzed.