Research Based On Test On High-cycle Fatigue Of Steel For Automotive Body

Fatigue fracture is a very common phenomenon among the engineering, which isalmost impossible to avoid completely and is particularly prominent in aviation,automobile and engineering industry. Fatigue fracture is one of the main failuremodes of automotive structure and components. Due to the durability of automobiles,the thickness of auto-sheet steel can not be too thin. In other words, fatigue life willrestrict the further development of lightweight of automobile. Being affected by airflow, traffic and any other external factors, cyclic load can more often than notengender on the body when a car is in motion. As a result, there will be fissures orfatigue fracture on the vehicle body after a long time. Because durability has becomeone of the performance indexes which have to be considered in automotive design,there are so many scholars being engaged in the research on fatigue properties ofbody sheet. However, there so many automobiles in different kinds, the kind of steelused on the vehicle body vary from one car to another. And the worst is thatresearches on the fatigue of stamping steel frequently used on the vehicle body areso little that we are basically blind in this academic zone.This paper gives a brief introduction of the steel frequently used on the vehiclebody and introduces the current situation of research on the fatigue of steel used onbody at the same time. The mechanism of fatigue is elaborated in detail, and ofcourse, this paper focuses on the introduction of several common theory of fatigueand methods of fatigue test. The most important achievement is that I havecompleted the research on high-cycle fatigue of three kinds of steel used on the body.Because the stress level on the parts of vehicle body is not high, fatigue failurebreaks down after several cycles. The process of fatigue fracture is basically in theelastic region, that is to say, stress and strain come into a linear relationship. To sumup, the relationship of stress and fatigue life can describe the fatigue properties ofmaterials properly. The common way to obtain the relationship of stress and fatiguelife of materials is the test method which is based upon stress.In order to obtain the true fatigue properties of materials, this paper penetratesinto studying the fatigue properties of the steel body by using test. In accordancewith the《Method of Axial Fatigue Test of Metals of GBT3075-1982》, a number oftest pieces which all have rectangular cross section have been directly cut from auto-sheet steel, then high-cycle fatigue test have been completed in theelectro-hydraulic servo machine with a model number of8801which ismanufactured by INSTRON and is made for fatigue test. During the test, the modelof load is axial load. Because the thickness of test pieces is so thin that symmetricalcyclic stress is very easy to make the piece bend to instability, the asymmetric sinewave as the cyclic load is loaded until the specimen fracture. The purpose of the testis to get cycles of the specimen under different stress level, that is, the relationshipbetween stress and fatigue life. There are so many kinds of material to test and thenumber of cycles of stress is usually too many in a high-cycle fatigue test,considering that too many samples will largely increase the cost and time of test, sothe scheme of test is chosen as small sample. Mean stress correction is done toeliminate the effect of mean stress on the test results while test data is obtained, then,analyze and calculate the test data. The last work has to do is that calculating thefatigue strength and obtaining the S–N curve by least-square linear fitting.Considering the survival rate and confidence level, it is necessary to obtain the lowerlimit of fatigue strength and the design S–N curves by statistical analysis. It is betterto select the most appropriate design S–N curve.