Basic Research On Characterization And Improvement Of Bending Fatigue Behaviors Of Remanufactured Crankshaft By Spraying Method

As an extension of the manufacturing industry,remanufacturing aims to realize the high-quality repair of waste electromechanical products.It is also an important measure to achieve the goal of energy conservation and emission reduction and develop the circular economy in China.As China's economic growth model develops in a sustainable direction,remanufacturing developes rapidly in recent years.The concept of remanufacturing is increasingly popular among people,and the scopes of remanufactured products are increasingly diversified.However,the remanufacturing still faces many scientific and technological problems which need to be solved urgently,the quality of remanufactured products which are born out of waste products is the key to affecting social acceptance,and needs to be paid attention.In this thesis,the engine crackshaft,a typical example of remanufacturing,was used as the research object.The whole process of crankshaft from new product to remanufacturing blank,and to remanufactured part was systematically studied under the laboratory condition based on the two-level experimental design of“material-level”and“part-level”.In the“material-level”experiments,the changes of surface properties of 42CrMoA nitrided steel during the bending fatigue process,the optimization process of spraying on the nitrided steel and the fatigue damage behaviors of remanufactured parts during the bending fatiuge process were studied.The“part-level”experiments were carried out based on the theory and guidance provided by"material-level"experiment,the fatigue damage behavior of crankshaft parts was studied and the fatigue damage model was established.The behaviors of residual stress of 42CrMoA steel during the fatigue tests were studied.Goodman curves with different loading levels were obtained by the staircase method,and the bending fatigue limit of nitrided steel was about 8%higher than the non-nitrided steel's.Based on the statictical life results with different stress amplitudes and mean stresses,the weight of stress amplitude and mean stress on fatigue life was about 4:1.Based on the finite element simulation and experimental method,the residual stress redistribution behavior was determined by the sum of applied loading stress and local residual stress was studied.The preparation process of sprayed coating on the nitrided steel surface was optimized and improved.The lasers with three different powers(0W,10W and 15W)were used to pretreat the surface of steel with four different grinding depths(0?m,50?m,200?m and400?m).The process of preparing the spraying coating was optimized based on the results of filling behaviors of spraying droplets in the texture on the adhesive strength of coating.The coating with complete filling of spraying droplets had the excellent adhesive strength.The optimized process solved the problem of crankshaft fatigue performance decline after remanufacturing.The spraying coating was experienced with nitriding treatment which could improve the wear resistance about 50%and had no obvious influences on the adhesive strength of coating.The evolution of acoustic and thermal signals of sprayed coating during the bending fatigue tests were studied,and the propagation behaviors of fatigue cracks was quantitatively characterized.Based on the results of the Discrete Time Fourier Transform of AE energy signal produced by the substrate fracture,the frequency differences of micro-cracks,micro-holes and dimples in fracture surface are distinguished.Quantitative characterization of fatigue cracks in substrate was realized based on the fact that the gray level in the crack zone was much higher than the non-crack zone.The fatigue life of the sample with thick nitriding layer was significantly higher than that of the sample with thin nitriding layer.The Fourier Transform and wavelet analysis were used to analyze the AE energy signal which could distinguish the frequency differences of the through-wall crack and the interface crack and quantitatively characterized the propagation behavior of the interface crack.The cracks in the thin coating were mainly the surface vertical crack while the cracks in the thick coating were mainly the interface crack.Baesd on the results of finite element simulation and Suo-Hutchinson calculation model,the propagation behaviors of interface crack were determined by the applied stress?,initial interface crack length a₀,and constant m.The evolution of acoustic and thermal signals of remanufactured crankshaft during the fatigue tests were studied,and the propagation behaviors of fatigue cracks was quantitatively characterized.Based on the failure analysis of crankshaft fillet and the near-exponential evolution law of acoustic and thermal signals,a fatigue crack propagation model of crankshaft was established,and the surface length and internal propagation area of fatigue crack at the fillet were quantitatively characterized.According to the law of near-exponential growth of the main control factor in the monitoring signal,a neural network model for fatigue crack propagation of remanufactured crankshaft was trained and optimized.