Research On Probabilistic Methods Of Fatigue And Fracture For Mechanical Structural Components

With the complexity of structures and loading history,the life assessments of structures are more and more complicated,including multi-crack failure,the uncertainty of load,etc.In order to ensure the safety of structural components,it is necessary to analyze the failure behavior of fatigue and fracture by using probabilistic methods,and to establish the reliability models of complex structures.Major works of this thesis are listed as bellow:For the fatigue proprietary of structures and materials,fatigue test methods are proposed.First of all,according to the principle of sample aggregation and numerical approximation,the small sampling fatigue life curve with high accuracy is improved by reducing the possible interval.Then,a new flexible test method of probabilistic fatigue limit,which can avoid the restriction of "paired stress levels" and "only one stress step",is proposed.The new test method of probabilistic fatigue limit has the advantages of saving samples and test time for the situation that samples have a large dispersion of material properties(e.g.the number of stress levels is more than 4).For the structural components that do not allow cracks occur,their fatigue life assessment is on the basis of probabilistic fatigue life curves.Based on the probabilistic fatigue life curves,two kinds of reliability models for multi-site damage structure,which contain the situations of deterministic loading and random loading,are established according to the analysis of system reliability method.And these models are compared with the traditional model.In damage tolerance,mechanical products are with initial defects.With cyclic loading,the initial defects can grow up to be cracks,and then cracks grow to fracture.For the complex mechanical structures,the shape,size and quantity of the initial defects are uncertain,and the cracks produced by these defects are widely distributed and difficult to detect.The multiple cracks will significantly reduce the residual strength,and will lead to disastrous consequences.Therefore,based on the analysis of the relationship between the fatigue crack length and initial crack size,two probability models of multi cracks for structural components,which contain the situations of deterministic loading and random loading,are established.For the random loading,the load history is described from two aspects of macro and micro,and the method to determine the equivalent load is presented according to monitor crack growth.Model results are compared with the experimental results.The influences of the number of details and the load history on the results are analyzed as well.For the plate with hole-edge cracks,which is a comnon mechanical structure in engineering,the interactions of collinear cracks were analyzed by using finite element method.Firstly,the stress intensity factors of collinear cracks were calculated under the plates with one hole and with two holes.Following these,a formula of stress intensity factor of collinear cracks in the plate with several circle holes is obtained according to superposition principle.Numerical examples showed that the formula was easily applicated to calculate the stress intensity factors of both symmetric and asymmetric cracks.Finally,for the structural components that allow cracks to occur,the probability damage tolerance of collinear multi-crack structure is analyzed,and applied to the probability damage tolerance assessment of the structure of a plate with several holes.For equivalent initial flaw sizes,new model is improved by using interval linear interpolation method.The new model is independent of load history,and can also be applied to the determination of the equivalent initial flaw size for complex structures.For failure criterion,the competitive failure criterion is proposed based on the Ligament yield criterion,Net ligament loss criterion,and K-apparent criterion.The procedures of probabilistic damage tolerance not only have complete reliability analysis,also can be applicable for the collinear multi-crack structures under both constant and variable amplitude loads.