Short Fatigue Crack Propagation Thresholds of Railway LZ50 Axle Steel

were machined from a production railway axle of Chinese LZ50 axle steel. Chemical composition of the material (wt %) is 0.52 C, 0.74 Mn, 0.26 Si, 0.07 Cr, 0.03 Ni, 0.03 Cu, 0.036 Al, 0.001 S, 0.007 P, and the residual Fe. Monotonic mechanical properties at room temperature are 705 MPa ultimate strength, 385 MPa 0.2% proof strength, 25 % elongation, and 50 % reduction of area. Average conventional fatigue limit corresponding to the life of 107 a.
Fig. 1. Specimen geometry for the present study. Fig. 2. Metallurgical photograph of 5# specimen. Before testing, the test pieces of specimens were polished into mirror finish and then were etched with a solution of 25 ml HNO2 + 75 ml HCl to reveal the material microstructures. Metallurgical photograph for 5# specimen is given in Fig. 2. It reveals that microstructures are consist of web like ferrites and layered pearlites. Using Auto Montage photograph analysis system to treat the photo, it indicates that the ferrites have an equivalent diameter range of 1.1 to 21.6 mum and an average value of 9.1 mum. While the pearlites are distributed with an equivalent diameter range of 1.67 to 24.85 mum and an average value of 17.04 mum. Experiments. Full reversed fatigue tests with the stress ratio of -1 were performed on Rumul 25 t test machine at room temperature under a sine wave loading mode with the frequency of around 100 Hz. Stress ranges were between 170 and 200 MPa which is lower than the material conventional fatigue limit of around 230 MPa with 107 stressing cycles, close to the real service condition of the production axle. The short crack behaviour was observed with a replicating technique. Fatigue tests were interrupted at set time intervals in order to replicate the specimen surfaces. The intervals are relatively short during the test initial piece so as to obtain more detailed micro-cracks information in the MSC regime. Number of replicating actions for each specimen is around 10 to 15. when replicating about 10 MPa tensile stress was maintained to help keep the cracks open and then two 5×20 mm acetate strips were progressively pasted on the opposite parts of the specimens. Crack Originations Checking. After testing, fracture surface of a specimen was checked using SEM to determine the initial locations of the ESFCs result in the specimen failure. Result indicates that the ESFCs were all initiated from the surfaces of specimens. Combining the following surface cracks observations, it cleans that the ESFCs were almost initiated from the material weaker phase, the weakest surface ferrite particles. Cracks Observations. After testing, the dried replicas were examined using an integrated computer system of combining an optical microscope, a digital formed photographs, and the β2 2á Auto-Montage graphs analysis. To avoid differences of the test data resulting from β θ 1 magnifications in observations, detectable size of the micro-cracks on each replica was kept to about 5 to 10 mum. In accordance with the above mentioned “effective short fatigue theory”, a “reversed observed policy” was applied to obtain Fig. 3. Four elements for each micro-crack. information of the micro-cracks:
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Chemistry Research Institute, China Academy of Railway Sciences, Beijing 100081
a
yxzhao@, bhezhong@, cyb@
Keywords: Railway; Axle; Fatigue; Threshold; Short crack.
Abstract. Collective short fatigue behaviour nearby material micro-structural barrier threshold is experimentally observed for the smooth specimens of Chinese railway LZ50 axle steel. Effective micro-cracks which result in the specimens’ failure were initiated from the material ferrite particles on the surfaces of the specimens. Four elements i.e. crack size, orientation angle, and the two crack tip orientation angles should be contained for each micro-crack. The collective micro-crack behaviour should be described by an equivalent dominant effective short fatigue crack (ESFC) concept using an ESFC theory. And an equivalent method is then developed with an equivalent growth driving energy concept. A random modeling of the equivalent dominant ESFC growth rates is further constructed for describing the random behaviour of the collective ESFCs nearby the material micro-structure barrier threshold. And the kinetic threshold is subsequently conducted from the modeling. Introduction After Pearson’s work in 1975 to verify that the crack growth law by standard notched specimens can not properly predict the behaviour of the cracks with physical small size in the range of 0.5-1.0 mm [1], the short fatigue crack problem has been focused on all over the world. Two professional meetings had been held respectively at UK [2] and USA [3] in 1986. Since then, short/small crack behaviour has become an important theme of fatigue study. The theme can mainly be subdivided into two fields. One is for a notched specimen or structure with a character of only physical small size crack. Another is for a smooth or blunt specimen or structure with a character of interactive collective micro-cracks. The former has a fatigue damage behavior mainly depending on stress amplitude level and mode. The equivalent stress intensity factor method should be a representative work for well solving the relative engineering structure analysis [4-7]. The later has damage behaviour not only depending stress amplitude level and mode but also the material micro-structures. The damage process was therefore appropriately subdivided into two sequent evolutionary regimes of micro-structural short crack (MSC) and physical short crack (PSC) [8-10], and the fatigue limits should be physically relative to material micro-structural barriers and loading levels [11]. Considering the interactive collective cracks, equilibrium of crack numerical density and equivalent crack size close to the maximum crack size were respectively introduced [12,13]. Distinguishing the fatigue damage difference of the micro-cracks with different orientations and locations and revealing the kinetic evolutionary cracks in a collective and interactive mode, an effective short fatigue crack (ESFC) theory should be previously proposed [14-19]. Railway axle is an important component affecting vehicle’s dynamic behaviour and the transportation safety. The material fatigue properties play a key role for the safety assessment. Consistency with the fatigue damage physics, short crack behaviour is the main damage process. Process work investigates the fatigue threshold on basis of the previous ESFC theory. Material and Experiments Material and Specimens. Smooth axial hourglass shaped specimens of 10 mm diameter (Fig. 1)