Investigation On The Residual Fatigue Life Evaluation Of48MnV Crankshaft Core

Nowadays, with the rapid development of vehicle industry, China hasreplaced the USA to become the biggest vehicle market in the world, whichmakes the number of scraped vehicles rise up and leads a lot of social andenvironmental issues. Remanufacturing, a production process of restoring thetechnical performance and product quality of scraped products, is one of the bestmethods to solve the problems of scraped vehicles. In China’s existing recyclingsystem, although scraped parts were detected, all the non-destructive testingmethods such as ultrasound, magnetic, etc., are aiming at defects reprensentedby fatigue cracks. If the fatigue cracks of the parts have not yet detected butexisting in the late stage of fatigue crack initiation phase, whether the residualfatigue strength of the parts can support remanufacturing and then fullfill thenext service cycle is hard to judged by the current methods. Therefore, tointroduce a new method to evaluate the residual fatigue life of scraped auto partsbecomes more and more important.Metal Memory Method (MMM) has a unique advantage in the evaluationof early fatigue damage represented by stress concentration of ferromagneticparts. Also, as an core part of engine, crankshaft has a high remanufacturingvalue. Therefore, the widely used48MnV crank was chosen to carry out theresidual fatigue life evaluation technology research and the MMM was chosento be the technical means in this paper, which will has great project value andtheoretical significance for China’s remanufacturing industry.Firstly, this paper analysised the magnetic memory parameter tocharacterize the fatigue damage, discovered that the normal component ofleakage magnetic field is hard to characterize the fatigue damage, then with theoperational requirements of MMM to propose that taking the diffrerencebetween the maximum and the minimum tangential component of leakagemagnetic field on the surface of crankshaft as the magnetic memorycharacterization parameter of fatigue damage. Actual testing results show that the characterization parameter has a good reproducibility and is suitable forcharacterization of fatigue damage.The crankshaft system was simulated to caculate the theoretical minimumsafety factor and then the bending fatigue tests were carried out on single crankspecimens which were cut respectively from the testing crankshafts including anew crankshaft, a crankshaft after500hour’s reliability test, a crankshaft after1000hour’s reliability test and a crankshaft after3000hour’s reliability test tosdudy the change of residual safety factor. The results show as follows:(1) themaximum stress of crankshaft when working was208MPa and thecorresponding theoretical minimum safety factor was1.6;(2) The longer theservice time of crankshaft, the smaller the safety factor of used crankshaft, butits value is bigger than the theoretical minimum safety factor, which verifies theremanufacturability of the crankshaft.Research on the magnetic memory characterization model of fatigueprocess of48MnV steel were carried out based on the tension-compressionfatigue tests and the rotating bending fatigue tests. The magnetic memorycharacterization model of fatigue process of48MnV steel was presented asfollows:(1) When the loads are less than the fatigue limit, magnetic memorycharacterization values keep fluctuating at a narrow range;(2) When the loadsare greater than the fatigue limit, with the increment of fatigue cycles, magneticmemory characterization values increase at first, and then, after reach a peakvalue, decrease with the increment of fatigue cycles. The appearance of the peakvalue has a correlation with the initiation of fatigue cracks.This paper developed a bending fatigue testing system of crankshaft basedon LabVIEW virtual Instruments technology, and then research on the magneticmemory characterization model of fatigue process of crankshaft was studied onthe system. Results show that the model of48MnV crankshafts is consistentwith the above mentioned magnetic memory characterization model of fatigueprocess of48MnV steel, and then the safety threshold of magnetic memorycharacterization parameter was analyzed and determined as438A/m bycaculating with the theoretical minimum safety factor. The safety threshold canbe explained like this: when the detected value of magnetic memorycharacterization parameter of48MnV crankshaft is less than the safety threshold,its residual fatigue life exists in safety area and is longer enough to supportremanufacturing. Based on the previous research works, the residual fatigue life evaluationmethod of48MnV crankshaft was set up and the corresponding residual fatiguelife testing device was developed. Meanwhile, three Chinese patents wereauthorized. The evaluation method and device work well in the actual test andare suitble for engineering practice. The establishing process of the residualfatigue life evaluation method of48MnV crankshaft has a general adaption andcan be applied for other retired crankshafts or ferromagnetic parts.