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• W1835508921 abstract "A premature failed turbine shaft in an emergency power unit was investigated. Damage and fracture features of the shaft and relative self-lock screw cap and turbine disk were observed with low-powered microscope and Scanning Electronic Microscope. The shaft, including two symmetric fatigue areas and a central instantaneous fracture area, showed typical features of reverse bending fatigue failure. In correspond positions fretting wears were found on both the shaft and the screw cap. This suggests that fretting wear occur before the fracturing of the shaft. Operation principle of the emergency power unit was studied and metallurgical qualities of the shaft and the screw cap were inspected. Grain growth and drop, which implies overlarge electric current in spark-erosion machining, were found near the thread surface in the screw cap. It is reasonable that the grain growth and drop led to crack initiation in screw thread and then fretting in the shaft. Material change and cool forming instead of hot-forming and other preventative measures were provided. KEYWORD Turbine shaft, fatigue fracture, failure analysis, reverse bending INTRODUTION Turbine shafts are the key components in power units. Their fracture failures often imply disastrous accidents. According to their attributes and stress conditions, shafts fails mainly in (bending, tensile-tensile or torsion) fatigue fracturing, tough fracturing, and brittle fracturing [1]. Therefore, based on the analysis of failure modes of the shafts, we can judge the service conditions whether are normal or abnormal, find further failure causes, and propose effective preventive measures. An emergency power unit was discovered flamed on the exhaust pipe in operation testing. After the emergency stop and disintegration, it was found that the turbine shaft was fractured, the turbine disk thrown [2]. The damage features of the turbine shaft and the shelf-lock screw cap were observed and analyzed, then the failure modes were determined. For finding the failure causes, metallurgical qualities of the shaft and the screw cap were inspected and the mechanical and technological factors resulted in this failure mode were investigated. At last, the preventive measures were provided. DAMAGE CHARACTERISTICS OF TURBINE SHAFT AND SHELF-LOCK SCREW CAP Turbine Shaft The shaft fractured in the transitional corner of the screw thread near the behind end of the turbine disc, as shown in Figure 1. The are two symmetric fatigue regions and one central tough fracture region on the fracture surfaces, as can be seen from Figure 2. From the roughness, sizes and colors of the two fatigue regions, it can be referred that A region is the main fatigue region and B region is the second fatigue region. The two fatigue regions were originated from the screw thread and appeared multi-origins, where no metallurgical defects were found. The instantaneous fractured region, which has a large area (approximately 50% of the fractured area), shows typical equi-axial dimples. The above fracture features indicate that the shaft was failed by reversal bending fatigue load. Different damages of the screw thread in the disc hole were found: A side has the most serious damage, showing even wearing plane and high temperature oxidized blue-yellow color (Figure 3). On the surface of the shaft and near the fracture surface 26mm (point C in Figure 1), a wear pit with diameter of approximately 3.0mm was discovered. The bottom of the pit shows typical fretting wear features, which were resulted from the point contact between the disc hole and the shaft. This pit slightly opposites to the main fatigue region. The above features demonstrate that the contact points (A and C) between the disk hole and the shaft surface had a little range of relative movement. Figure 1: Outward and fracture site of the turbine shaft. C is a fretting wear pit. Figure 2: Fracture surface of the turbine shaft. A and B are the chief and secondary fatigue regions. Shelf-lock Screw Cap The wear marks on the side surface of the shelf-lock screw cap which contact with the disk appeared uneven distribution. More serious wear marks can be seen on the site corresponding to fatigue regions (A and B); very slight wear on the site corresponding to the instantaneous fractured region, where silver-plating layer was loomed as shown in Figure 4. This indicates that there were fretting on the contact surfaces of the disk and the screw cap with a boundary of D-D line. Figure 3: Serious wears of the thread near A side. Figure 4: Wear of the screw cap side. A and B sides heavy. The first thread in the screw cap near the disk side fractured a half at the B side. The origin, propagation and instantaneous areas are obvious. The propagation region has small size whereas the tough region has large size about 70% of the fractured area. Figure 5 gives a zoomed appearance of the origin and the growth region, where arrow shows the crack propagation direction (i.e. axial direction). Fracturing initiated at the transitional corner of the thread bottom. Although the fracture surface underwent axial secondary damage, local fatigue features can be seen in the growth region and shear dimples in the tough region. Therefore, the thread failure is low-cycle shear fatigue fracture. METALLRUGICAL ANALYSIS" @default.
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• W1835508921 date "2013-04-04" @default.
• W1835508921 modified "2023-09-27" @default.
• W1835508921 title "Reverse bending fatigue failure of an emergency power turbine shaft" @default.
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