Research On Crack Growth Mechanism And Fatigue Life Prediction Of Large Modulus Rack Of Tooth Climbing Ship Lift

As the largest rack climbing ship lift in the world,the Three Gorges ship lift has a capacity of 3000 tons and a maximum lifting hight of 113m.The rack modulus is as high as 62.667,which belongs to the super modulus rack.The safety of rack directly determines whether the ship lift can operate safely and stably,because the ship lift is lifted vertically by means of meshing gear and rack.The rack blank is made by casting process.On the one hand,the rack has been exposed to harsh external environment for a long time.On the other hand,due to the influence of defects arised by casting process,surface cracks and slag inclusion appear on the rack after a period of service.In order to accurately predict the fatigue life of large modulus rack with defects,and provide theoretical basis for the later repair of rack,the following work was carried out:(1)The meshing force of the rack was treated equivalently by simulating the real load condition of the rack,the rack tooth surface was divided into 15 loading areas to load the meshing force.The finite element submodel of the large modulus rack was established,and the crack or slag inclusion was inserted into the finite element submodel of the rack by using Franc3D calculate the stress intensity factor(SIF)during the crack growth.The correction coefficient was introduced to characterize the interaction effect between the two collinear cracks,analyzing the influence of crack spacing ratio and offset distance on the correction coefficient.The results indicate that R_S=1.3 is the threshold value of spacing ratio for the interaction of two cracks.When the spacing of double cracks is below this threshold,the interaction between the two cracks begins to appear.The initial offset distance has little effect on the interference between the two cracks.(2)In order to study the coupling effect of slag inclusion and crack and the influence of slag inclusion on crack growth,the rack models of double collinear crack and single slag inclusion were established.The threshold values of spacing ratio and initial spacing of double collinear cracks and single slag inclusion in the growth were discussed,the variation rules of spacing ratio and stress intensity amplification factor of double collinear cracks and single slag inclusion in the growth were calculated,the correction coefficient of load cycle number of double collinear cracks and single slag inclusion in the connection under the influence of surface high frequency induction hardening was analyzed,and the criterion of double collinear cracks and single slag inclusion and the steps of life prediction using the criterion are proposed.The results show that when 0<R_s<1.5,R_k>1.028,the interference effect between double cracks and slag inclusion is obvious.When 1.5<R_s<5.7,1<R_k<1.028,there is interference effect between double cracks and slag inclusion.When R_s>5.7,R_k<1,there is almost no interference effect between double cracks and slag inclusion.When the initial spacing S₀>26C₀,it can be considered that the double collinear cracks and slag inclusions were independent of each other within the design life 50 years of the ship lift.(3)In order to measure the mechanical properties of G35Cr Ni Mo6-6+QT1,an improved small-size sample method was proposed.The fracture toughness of sample of rack material G35Cr Ni Mo6-6+QT1 was measured.At the same time,the plastic elongation strength,tensile strength and hardness of the sample were measured by tensile test and hardness test respectively.The effect of the plastic zone at the crack tip on the stress field was analyzed,the relationship between the condition value of fracture toughness and the thickness of the specimen in the traditional small-size specimen method is corrected,and the final fracture toughness value of the material is obtained according to the condition value of fracture toughness of each size specimen.The results show that the K_IC of G35Cr Ni Mo6-6+QT1 before and after quenching are 140.57MPa?m and124.30MPa?m,respectively;the plastic elongation strength R_p0.2 before quenching is690.46MPa,and after quenching R_p0.2=1286.87MPa;the tensile strength before quenching R_m=898.57MPa,after quenching R_p0.2=1422.45MPa;the hardness values before quenching are HRC24.6?25.4,and the hardness values after quenching are HRC46.2?47.1.(4)Considering the influence of crack closure effect on the crack growth rate,the fatigue crack growth rate expression which can completely describe the initial growth stage,stable growth stage and rapid growth stage was derived.According to the surface defects detection of single rack,the distribution of crack and slag inclusion on the rack was simulated based on Monte Carlo method,and the fatigue life of rack was predicted by polynomial chaos theory and improved nonlinear Wiener degradation modeling theory.The results show that for the the distribution of multi cracks with slag inclusion,the rack life was predicted to be 62 years based on polynomial chaos theory and 61 years by nonlinear Wiener degradation model,and the prediction of large modulus rack by using Miner criterion was evaluated to be 79 years.The results predicted in this work are more accurate than those calculated by Miner criterion.The error of the two prediction methods is 2.29%,which proves the accuracy of the prediction results.Through the research,the influence of quenching and surface defects on the fatigue life of large modulus rack was revealed,and the fatigue life of large modulus rack under the influence of surface defects is predicted,which provides a theoretical reference for the formulation of maintenance cycle of large modulus rack.