The Multiaxial Fatigue Life Analysys Of Metal Under Non-proportional Random Loading

The fatigue destruction is caused by repeated loads, the time domain methods need large amount of calculation, thus it is necessary to study frequency spectrum method. Engineering practice, the material load environment is complex, such as constant amplitude cyclic loading, variable amplitude proportional loading, nonproportional loading, etc. Correctly handling multiaxial non-proportional random load is the key to the problem of multiaxial fatigue.First of all, analysis of life attenuation about all kinds of metal materials under non proportional loading, the relation of loading path of life and the influence of material hardening effect, the relation of strain and stress amplitude with fatigue life, discuss lifetime decay and ductility.Secondly, through the load matrix derived response and frequency response function matrix of power spectral density, is suitable for multiaxial fatigue problem considering the phase angle of Mises and Lemaitre phase method, unconstrained degree, using phase angle characterization, linear correlation is 0 degrees, 90 degrees according to the proportion, the largest equivalent stress is its corresponding to the power spectral density of the equivalent stress amplitude, thus, we can analyze fatigue problem in frequency domain; The correlation coefficient was established through a large number of experimental verification, the biaxial fatigue characteristic curve, synthetically considering the axial and tangential fatigue characteristic, suitable for orthogonal Gaussian distribution of fatigue life calculation formula, Mises and Lemaitre phase method to calculate the life and the life of the comparison verification, found that Lemaitre method is more close to the experimental value, and within three times of the factor, is a more nice than Mises method with high precision, small error estimation method, especially for non-proportional loading. The calculation results is related to the ratio of tangential and axial stress of the root mean square, the value more close to 1, the more concentrated to the results, the closer to zero, the result is discrete.Finally, study the fatigue design method of fatigue analysis on the basis of the static analysis, with the help of software simulation, established proportional and non-proportional loading environments, get fatigue life and damage value, compare calculation and experimental life, check safety factor; For variable amplitude loading, calculate the analysis of rain flow and damage array, found that most of the circulation in low range and low average range,but high cycles are still the main damage form, small stress can cause considerable influence; Palmgren- Miner criterion is applicable to variable amplitude loading, sawtooth wave and sine wave load history can be dealt with as constant amplitude cycle loads, analyzed the influence of the theory of mean stress in fatigue life, considering load condition and material properties, it is concluded that Soderberg theory is conservative than Goodman and Gerber calculation results, Gerber calculation results is of the most dangerous, we should choose the appropriate average theory.