Study On Damage Degree Of Surface Crack Fatigue Propagation In Metal Structures

With the increase of service time of mechanical structures,the fracture caused by fatigue damage is gradually increased.Due to the randomness and instantaneity of the fatigue fracture accident,the structures with fatigue damage always threatens its safety running time and the personal safety of the workers.Most of the fatigue cracks are formed on the surface of the metal material,and expand gradually under the action of fatigue load,which seriously affects the bearing capacity and service life of the structures.However,there is still no complete theory to solve the problem of surface crack fatigue propagation.Based on the background,studies of fatigue propagation and damage are carried out to explore the fatigue propagation law of surface crack and provid theoretical basis for damage evaluation.The main research works and contributions are as follows:(1)Due to the difficult to calculate the stress intensity factor under complexity conditions,extend finite element method(EFEM)is proposed to calculate the stress intensity factor under complexity structures and irregularity crack shape.Stress intensity factor of surface crack in plate has been calculated by EFEM and the calculated results are basically identical with the results by Newman-Raju formula,the complexity solution process and special treatment for crack tip are avoide successfully.The influence of structural parameters of T-type for stress intensity factor is analysised and the results can be used to direct the fatigue design of T structures.(2)The shape and size of plastic zone of crack tip are calculated based on the theory of fracture mechanics,the equivalent crack length and stress intensity factor are calculated under the influence of the plastic zone and the influence of crack tip plastic zone on the crack propagation is analysised.The range stress intensity factor is modified based on the Eshelby equivalent squeeze theory,the effect of plastic zone on crack propagation is analyzed by using modified plastic stress intensity factor and it laid the foundation for calculating the shape of surface crack propagation.(3)In order to evaluate the damage degree of structures with surface crack,a calculation method for calculation shape evolution of surface crack growth is proposed based on effective energy release.The relation between shape parameters of surface crack is calculated by the novel method and the evolution progress of surface crack propagation is simulationed by the calculation results.Fatigue crack growth of the surface crack in plate structures of Q345 material has been tested,the experimental results are in good agreement with the calculated results.We also had calculated the evolution of the crack shape under the action of T-type under the tensile load,and the effective release rate of the surface crack was calculated by using the EFEM method,which verified the correctness of the calculation result.Meanwhile,the shape evolution of surface crack in T structures under the pure bending load is been discussed.Finally,surface crack propagation experiment in mental box structure is carried out to prove the validity of novel method.(4)The characteristics of crack propagation acoustic emission(AE)signal is studied based on Griffith energy release theory.The method combined with wavelet transform and ensemble empirical mode decomposition is used to reduce noise of AE signal in complex environment,AE signal of fatigue crack propagation under complex environment is treeded and the effectiveness of the new method was verified.(5)The propagation velocity of AE signal is measured in Q345,the delay for acoustic emission signal is calculated based on the wavelet packet decomposition and cross-correlation analysis,and the positioning of AE source is founded by using three sensor plane positioning principle.Frequency band energy distribution coefficient is proposed as characteristic parameters used to evaluate the damage extent of the surface crack propagation,the relationship between energy distribution coefficient of the signal band and the shape parameters of the surface crack is established.In summary,the evaluation of the fatigue damage degree of metal surface crack propagation in mechanical structures is realized.