Study On Fatigue Fracture Behaviors Of Screw Shaft In Polypropylene Extruder

Failure analysis, the technological means of safety accident investigation, might identifythe causes and liabilities of the accident, predicts and prevents the accidents. A Screw Shaft ofpolypropylene extruder was put in service, and very soon fatigue fracture occurred. Theaccident occurred without any evidence, causing unexpected shutdown and economic lossesof the factory.Mostly the failure of shaft is due to fatigue. The main objectives of the study are to findout the main cause of the facture, to explain the fatigue behavior, and to predict the fatiguelife. The results of this work may be a useful information to prevent the similar failureaccidents, and to provide the reference of the material selection and structure design of thescrew shaft, and to promote the safe production and economic benefits of the enterprises.The paper presented the analysis and behavior of screw shaft fatigue facture with systemfailure analysis methods. The main contents of this paper were as follows:(1) Based on the theories of fatigue failure, the paper summarized its trend, the factureprocess, the principle of crack growth, and the facture morphology.(2) The origin of fatigue cracks was on the rim of spline tooth, which was the stressconcentration area. The chemical composition of the shaft met with the standard specification,but its mechanical properties were not evenly distributed across the shaft cross setion, such asthe inhomogeneity of hardness. And the impact roughness was below standard in partial area.With poor thermal treatment, there exist many complex tissues in the shaft, like coarsemartensite and ferrite and retained austenite in zonal distribution. Fractography showed thatthere were brittle inclusions and a large number of micropores and their aggregation groups inthe shaft, which were the causes of the early fatigue facture of the srew shaft.(3) Based on the tension-compression fatigue test, fatigue strength was obtained, and aswell as the function formula of the S-N curve, and the failure probability of10%at aconfidence level of95%of S-N curve.(4) Structure design, service stress and wrong operation were not the factors causedfatigue fracture. Actually, origin cracks occurred from the local stress concentration area. Theinequality of mechanical properties and poor thermal treatment were the material causes.Micropore aggregation also conftributed to fatigue fracture of the shaft.