Research On Fatigue Strength Of Mechanical Components

Research on fatigue strength can be used in extensive applications not only in the aerospace,aviation,shiphuilding and nuclear fields,but also in genral mechanical product designing.In fatigue deign,how to convert the nominal strength of material into components' fatigue strength under working environment is the primary problem for mechanical design.Thus,it is particularly important to research on fatigue strength influencing factors and modify these factors to obtain fatigue strength of componts. Meanwhile,fatigue failure process can be taken as degrading of materials' performance and decreasing of material's strength under cyclic loading which eventually led to failure of static strength.Therefore,it is vital significant to study degradation law of residual strength.In this paper,combining theoretical analysis and experimental research method, fatigue strength of mechanical components is studied systematically.Especially,it focuses on discussing the interaction between stress concentrations;surface machined and size effect on components' fatigue strength,and analyzed the strength degradation model of 35CrMo steel by experiment,which lays the foundation of establishing dynamic reliability model of component under the stochastic load.The researches are shown in detail as follows below.(1) The interaction between machined surface,size effect and stress concentration is analyzed.Aiming to how to avoid not taking account of interaction among influencing factors of fatigue strength in fatigue design,an expression of comprehensive correction factor including all jinfluencing factors of fatigue strength is proposed.(2) The sensitivity of expressions of different comprehensive correction factor is analyzed versus the theoretical stress concentration factor,surface factor and size factor.(3) The fatigue strength,S-N curve and limit stress curve of components are obtained through applying the comprehensive correction factor.The the distribution of components' fatigue limit is derived from comprehensive correction factor' distribution and material fatigue limit' distribution.(4) Fatigue absolute size effect model which is derived form the Weibull weakest link model indicated that,in absolute size effect problems the shape parameter of specimen or components' fatigue life distribution and the minimum life parameter is equivalent,and the scale parameter in line with a certain expression.Fatigue tests verified the absolute size effect model is correct.In the process of the experimental study,the method for fixing test results is presented based on the principle of "equal failure probability".(5) On the basis of the traditional nominal stress approach,applying the comprehensive correction factor' expression put forward in this paper,fatigue life of component is evaluated.The elastic line of strain-life curve is modified by applying the comprehensive correction factor,considing the influence of stress grads,machined surface and size simultaneously,into the local stress-strain approach.Then based on Mason-Coffin forum,the modified fatigue strength index value is derived.The strain-life expression for evaluation high-cycle fatigue life is proposd as well.(6) Experimental study shows that the proposed exponential model can describe the strength degradation law of 35CrMo very well.Then based on the principle of equivalent damage the residual strength model is derived under variable stress amplitude loading.By analyzing the test data of residual strength,it is shown that in the process of strength degradation the residual strength always follows Weibull distributions,the shape parameter of which remains constant,the location parameter and the scale parameter of which decrease with the increasing of cycle index.