Fracture analysis of a castellated shaft

Transcription

1 Engineering Failure Analysis 14 (2007) Fracture analysis of a castellated shaft Y.J. Li *, W.F. Zhang, C.H. Tao Failure Analysis Center of AVIC, Beijing Institute of Aeronautical Materials, P.O. Box 81-4, Beijing , People s Republic of China Received 21 March 2006; accepted 23 March 2006 Available online 6 June 2006 Abstract In this paper, an in-service fracture of a castellated shaft was investigated. Visual inspection, SEM observation on the fracture surface, hardness test and microstructural analysis were performed to determine the cause for the fracture. The results showed that the immediate cause for such a failure was the overload impact and torsional deformation. However, deficient hardness of the hardening layer contributed to this failure as an additive factor. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Fracture; Castellated shaft; Impact; Torsional deformation; Hardness layer 1. Background When a vehicle was diverted from a large pothole to the slushy road, the steering shaft suddenly fractured. Fig. 1 shows the sketch of the steering shaft with three splines (Nos. 1, 2 and 3 ). Spline No. 1 is used for power input, while spline Nos. 2 and 3 are for power output. In service, spline Nos. 2 and 3 transfer the torsion moment to the matched splines in order to drive the vehicle or change the direction. The castellated shaft material is a 20Cr2Ni4A steel. After quenching and high-temperature tempering, Spline Nos. 2 and 3 are laser quenched and the other dimension is carbonized. Technical specifications of the castellated shaft are given below: (1) hardness of the hardening layer is above 48 HRC and depth of the hardening layer is up to mm; (2) hardness of the centre is up to HRC. 2. Observations 2.1. Visual inspection Visual observations showed that the shaft failed by the fracture of spline No. 2. The fracture surface, normal to the longitudinal axis, was the common section of the contact segment and untouched segment. Spline No. 3 was in good condition. Fig. 2 is the macroscopic view of spline No. 1 before and after fracture. Compared with * Corresponding author. Tel.: ; fax: address: liyunju960@163.com (Y.J. Li) /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.engfailanal

2 574 Y.J. Li et al. / Engineering Failure Analysis 14 (2007) Spline No.2 Spline No.1 Spline No.3 Fig. 1. Sketch of the castellated shaft. Fig. 2. Details of spline No. 1 before and after fracture: (a) view of spline No. 1 in good condition and (b) view of spline No. 1 after fracture. the good condition of spline No. 2, there was an obvious plastic deformation because each spline tooth was changed from straight shape to contorted one and accordingly, the width of the tooth became narrower. The distortion angle was about 25. Judged from the contorted direction of the spline teeth, the rotation of the shaft was in a clockwise direction (arrow in Fig. 2(b)). Although there was severe damage by friction on the flat and bright fracture surface, convoluted marking near the central area could be seen (Fig. 3). In the vicinity of the reference-circle on the fracture surface, there was a small step on each tooth and all the steps were distributed on the same side of spline No. 2 (arrows in Fig. 3(a) and (b)). Fig. 3. Macro photograph of the fracture surface: (a) view of the fracture surface and (b) small steps on the reference circle.

3 2.2. SEM observation on the fracture surface The fracture surface was carefully observed by the aid of the scanning electron microscope. The micro-characteristics of all the steps were similar. There were obvious deformation traces around the steps. Fig. 4 shows the low magnification of one step, in which region O is the initiation zone with severe abrasion, and region P is the deformation trace zone. The deformation traces propagated in a length of about 500 lm and the area of region P was about 0.4 mm 2.InFig. 5, region P is further magnified and shows that the deformation traces are locally interjected and the distance between the neighboring traces is at random. Accordingly, the deformation traces around the steps were not fatigue striations. Between the two neighboring traces, there were small ductile dimples with somewhat elongated shape (Fig. 6). From Fig. 2(b) and Fig. 6, it was suggested that spline No. 2 had suffered shearing force opposite to the shaft rotation direction. From Figs. 5 and 6, it was determined that the deformation traces around each step were caused by the overload impact. Besides the steps, the other area was composed of elongated dimples. Closer to the shaft surface, shape of the dimples became more elongated (Fig. 7) Chemical composition Y.J. Li et al. / Engineering Failure Analysis 14 (2007) Table 1 gives the chemical composition of the castellated shaft and the standard composition of 20Cr2Ni4A. As shown in Table 1, the shaft chemical composition is in accordance with the requirement of 20Cr2Ni4A steel. Fig. 4. SEM photograph of the step on the tooth. Fig. 5. Magnification of region P in Fig. 4.

4 576 Y.J. Li et al. / Engineering Failure Analysis 14 (2007) Fig. 6. Elongated dimples between two neighboring deformation traces. Fig. 7. SEM photograph of the dimples: (a) elongated dimples at the position close to the surface and (b) elongated dimples at the position close to the shaft corner. Table 1 Chemical composition of the steering shaft material Element Standard composition [1] Shaft composition C Si Ni Mn Cr P max S max Cu 0.25 max 0.14 Mo 0.10 max 0.10 Fe Balance Balance 2.4. Hardness test Hardness measurements were carried out on some polished transverse sections. At mm depth from the tooth surface of spline Nos. 2 and 3, the hardness values dropped to HRC, which suggested that the actual depth of the hardening layer is less than that of the technological standard. In the centre of spline Nos. 2

5 Y.J. Li et al. / Engineering Failure Analysis 14 (2007) Fig. 8. Optical micrograph of spline No. 2: (a) the tooth surface and (b) the corner. and 3, the hardness values dropped to HRC, which was in accordance with the requirement of HRC Metallography A sample was cut from a position close to the fracture surface and prepared for optical observation. The sample was etched with 4% nital solution (4% HNO 3 in alcohol) and observed by an optical microscope. Fig. 8 shows the microstructure of the cross-section. The microstructure near the tooth surface was mainly composed of plate-like martensite and retained austenite. The microstructure in the corner of the spline shaft was sorbite. There was no obvious abnormality in the microstructure. 3. Discussion 3.1. Failure mode for the castellated shaft Spline No. 2 was used for torsion moment output. The fracture surface was the common section of the contact segment and untouched segment of spline No. 2, and was normal to the longitudinal axis of the shaft. Visual inspection showed that each tooth changed from straight shape to a contorted one with about 25 angle to the longitudinal axis, and that the fracture surface had steps on the same sides of each tooth. SEM observation revealed that the fracture surface was composed of ductile dimples. The facts above indicated that the failure mode of the castellated shaft was a ductile fracture An analysis of the failure causes for the castellated shaft Generally, when a vehicle runs on a bad road, the steering article will suffer the greater torsion torque than when it runs on a smooth road. Especially for the sudden swerve from a large pothole on the slushy road, random impact may cause a severe impact stress on the spline teeth. For this failure, from the locally interjected deformation traces and the random distances between the neighboring traces, it was concluded that when the shaft rotated clockwise, an abrupt change of the road condition and direction caused an anticlockwise large torsion moment and severe impact on the teeth of spline No. 2. For such a castellated shaft, the impact stress resulted in damage at the reference circle positions and made these positions to be the weakest area. Under the cooperation of the impact stress and the overload torsion moment, a crack was prone to form and propagate from the weak area, causing the ultimate ductile failure. That was to say, the load in-service was above the acceptability strength-limit of the shaft and was the immediate causes for the shaft failure.

6 578 Y.J. Li et al. / Engineering Failure Analysis 14 (2007) Although no obvious abnormality had been found in the microstructure and the hardness of the shaft centre was in accordance with the requirement, the hardness of the hardening layer was below the technical value. The deficient hardness was induced by poor laser quenching. The effect of deficient hardness on the shaft fracture could be considered from three aspects as follows: (1) to some extent, deficient hardness facilitates crack initiation at the surface because poor hardness causes the dynamic load-resistance of the shaft to decrease; (2) hardness requirement on the hardening layer is mainly to improve the contact strength and wearing-resistance of the spline teeth, while the excellent impact capacity mostly depends on the hardness of the shaft [2,3]; (3) the hardening layer ( mm) accounts for about two percent of cross area of the castellated shaft, so deficient hardness has little impact on the load capacity of the shaft. Accordingly, the main cause for the shaft fracture was overloading, and the deficient hardness of the hardening layer had contributed as an additive factor to the fracture. 4. Conclusions 1. Failure mode of the steering shaft was a ductile fracture. 2. The shaft fracture was originated and propagated by the overload impact and torsion moment resulting from an abrupt change of the road condition and direction. 3. Deficient hardness of the hardening layer contributed as an additive factor to such a shaft fracture. References [1] GB/T Structural alloy steel. [2] Yang KZ, Cheng GY, editors. Foundation of the mechanical design. Beijing: Higher Education Press; [3] Zhang D, Zhong PD, Tao CH, et al., editors. Failure analysis. Beijing: National Defense Industry Press; 1997.