Field Indensity Method For Fatigue Life Prediction Of Structures Under Multiaxial Loadings

Engineering structures are usually subjected to multiaxial fatigue loads.Fatigue accumulative damage theory aims at the accumulation of the fatigue damage under variable amplitude loadings.Fatigue life prediction of notched specimen aims at the influence of stress gradient on the fatigue life.These two subjects are very important to the multiaxial fatigue life analysis of engineering structures.In this paper,based on the randomness,the multiaxial loadings are classified into categories,which are step spectrum,block spectrum and random spectrum.Taking the damage cumulating mode and the directionality of multiaxial fatigue damage into account,the multiaxial fatigue accumulative damage theories are classified into the following four categories:(a)the directional linear models;(b)the directional nonlinear models;(c)the non-directional linear models;(d)the non-directional nonlinear models.By comparing the applications of these models,the applicable scopes and application effects of the above four kinds of multiaxial accumulative damage rules are discussed.In this paper,a series of multiaxial 2-stage step loading spectrum tests are carried out on LY12 CZ aluminium alloy.According to the test results,it can be found that,the variations of the loading amplitude and loading path have significant influences on the accumulation of fatigue damage.The accumulation is found to be affected by the multiaxial stress state on the critical plane.In this paper,a new accumulative damage model is proposed to describe the fatigue damage accumulation under multiaxial 2-stage step loading spectra.Compared with the test results,the calculated accumulative fatigue damage of the new model has satisfying results with the small scatters.In this paper,a series of multiaxial block and random spectrum tests are also carried out on LY12 CZ aluminium alloy.The results show that,although the variations of load amplitude and load path could affect the damage accumulation,this effect decreases with the increase of loading randomness.Consequently,the damage accumulation rule under multiaxial block and random spectra can be described by Miner's Law very well.Therefore,it is reasonable and acceptable to use Miner's Law to calculate the multiaxial fatigue accumulative damage for engineering structures.In this paper,the shear stress gradient effect is studied by using thin-walled tubular specimens.Then,by taking the directionality of the multiaxial fatigue damage and the stress gradient effect into account,the stress field intensity method,which is very commonly used in the study of uniaxial fatigue theory,is improved.In order to verify the newly proved method,a series of multiaxial fatigue tests are carried out on the notched specimens made of LY12 CZ aluminum alloy.The experimental results show that the improved model has good life prediction accuracy.In the end,after combining the newly proposed fatigue damage accumulation model and fatigue life prediction method,the analysis procedure of the multiaxial fatigue life for engineering structures is presented.Based on the procedure,the multiaxial fatigue lives of three structures are estimated.The analysis results show that the fatigue damage accumulation model and fatigue life prediction method proposed in this paper are satisfactory.