【作者】 王翔；

【导师】 陈铭；

【作者基本信息】 上海交通大学 ， 机械设计及理论， 2011， 博士

【摘要】 当前，随着汽车产销的井喷式发展，我国已取代美国成为了世界第一大汽车市场。汽车保有量的飞速增长，使得报废汽车的数量也急剧增加，给社会和环境带来了一系列问题。而再制造，是把废旧产品恢复到像原产品一样的技术性能和产品质量的生产工艺流程，也是国际公认的解决报废汽车难题的最好方法之一。现有报废汽车回收利用体系下，退役零部件虽进行检测，但其所采用的无损检测方法，如超声波、磁粉等，均是以疲劳裂纹等缺陷为发现目标，当退役零件尚未检测出疲劳裂纹，而又已经处于疲劳裂纹萌生阶段的后期时，其剩余疲劳强度是否足以支持其再制造进而完成下一个服役周期，当前的检测方法无法做出判断，也就不能准确的评价退役零件是否可以再制造。因此，引入一种新的评价方法，在退役汽车零部件进入再制造流程之前对其进行剩余疲劳寿命测评，已变得十分必要和紧迫。磁记忆检测方法在对铁磁零件以应力集中为代表的早期疲劳损伤评价方面有着独特的优势，而曲轴作为汽车发动机的重要部件之一，具有很高的再制造价值，因此，本文以当前广泛应用的48MnV曲轴为代表，将磁记忆检测方法进行移植，开展退役汽车零部件剩余疲劳寿命测评技术研究，准确判断其是否可以再制造，具有极大的工程价值和理论意义。本文首先对表征疲劳损伤的磁记忆参数进行了分析，发现磁记忆漏磁场法向分量很难表征疲劳损伤，并结合磁记忆检测操作要求，创造性的提出了将轴类零件表面漏磁场最大和最小切向分量之差，即幅值，作为疲劳损伤的表征参数。实际检测效果表明，该表征参数重复性好，适合表征疲劳损伤。本文通过对曲轴系进行数值模拟求取了曲轴理论最小安全系数，并对全新曲轴和经500小时、1000小时和3000小时发动机可靠性试验后的曲轴截取曲拐进行了弯曲疲劳试验，研究其剩余安全系数的变化。结果表明：（1）曲轴正常工作时最大应力为208Mpa，由此计算得到曲轴的理论最小安全系数为1.6。（2）随服役时间的增加，曲轴剩余安全系数略有下降，但其值远在理论最小安全系数1.6之上，从而验证了曲轴的可再制造性。本文开展了基于标准拉压疲劳试验和标准旋转弯曲疲劳试验的曲轴材料48MnV钢疲劳过程的磁记忆表征模型研究，结果显示：疲劳损伤与磁记忆表征参数之间存在着非线性关系，并且，疲劳载荷与磁记忆表征参数值的大小之间存在着对应关系：（1）当载荷小于疲劳极限时，磁记忆表征值在疲劳过程中一直保持在较小范围内波动；（2）当载荷大于疲劳极限时，磁记忆表征值随疲劳循环次数的增加先升高，到达某一峰值后，又随疲劳循环次数的增加而下降，直到疲劳断裂。峰值与疲劳裂纹的出现存在着一定的对应关系。本文自行开发了基于LabVIEW虚拟仪器技术的曲轴弯曲疲劳试验系统，并在其上对48MnV曲轴疲劳过程的磁记忆表征模型进行了研究。结果表明：该模型与上述48MnV钢疲劳过程的磁记忆表征模型相一致，再结合曲轴理论最小安全系数，分析确定了退役曲轴剩余疲劳寿命的安全阈值为438A/m，即，当48MnV退役曲轴测得的磁记忆表征值小于该安全阈值时，可以认为其剩余疲劳寿命处于安全区内，足以支持再制造。在前述工作的基础上，本文提出了48MnV曲轴剩余疲劳寿命测评方法并开发了相应的测评装置，均已获得发明专利授权。该测评方法和装置在实际测试中运行良好，适用于工程实践。48MnV曲轴剩余疲劳寿命测评方法的建立过程具有普适性，可以推及其他各类退役曲轴和铁磁零件，评价其是否适合再制造。更多还原

【Abstract】 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.更多还原