PSI - Issue 5

Robert Hannemann et al. / Procedia Structural Integrity 5 (2017) 861–868 Hannemann et al./ Structural Integrity Procedia 00 (2017) 000 – 000

862

2

notch and press-fit stress distributions are still under research using mostly mode I crack growth concepts (Beretta and Carboni (2006), Luke et al. (2010, 2011), Madia, Beretta and Zerbst (2008)). Due to the revolutions of wheelset axles, they are exposed a rotating bending with variable amplitudes during operation. There are numerous experimental investigations on shafts under rotating bending for example by Beretta et al. (2004), Carpinteri, Ronchei and Vantadori (2015), Shin and Chen (2004) or Beretta, Ghidini and Lombardo (2005). However, within this paper the influence of design details of wheelset axles, i.e. transition radii or press-fits, on the crack propagation will be presented. For this purpose a 4-point rotating bending test rig has been developed for s caled solid shafts with design details of a wheelset axle . With this test rig, the specimens of different diameter and length can be subjected both to constant and variable amplitude loading . Moreover, due to special wheel-rail-situations, deceleration and acceleration processes as well as cornering of trains, additional stresses due to torsional loads occur, which result in mixed-mode loading conditions and can affect the crack propagation in wheelset axles. In accordance with the investigations of Fonte et al. (2006) the majority of the disastrous fatigue failures in driving axles are related to the superposition of cyclic bending with torsion. Numerous experimental and numerical investigations exist concerning the superposition of mode I and II or III at thin-walled tube specimens e.g. by Döring et al. (2006), Highsmith (2009), Yang (2014) and Zerres, Brüning and Vormwald (2010). Other investigations analyze crack initiation e.g. by Fonte et al. (2006), Fonte, Reis and Freitas (2015), Freitas et al. (2011) and Yu and Abel (2000a, 2000b). In contrast, within this paper, investigations are done representing the crack propagation conditions of large cracks in solid shafts concerning the superposition of all three crack modes as accurate as possible. The investigated material is the quenched and tempered high-strength steel 34CrNiMo6. The material properties are listed in Table 1. For the determination of the fracture-mechanical parameters, SEN-specimens ( l = 230 mm, w = 50 mm and t = 10 mm) for negative R -ratios of R = [-0.5, -0.7, -1.0, 0] and standardized CT-specimens (w = 72 mm and t = 10 mm) for the positive R -ratio 0.1 are used. The determined crack growth curves depending on the R -ratio are shown in Figure 1. These experimental data are automated analyzed using a MATLAB program, which was developed at the Institute of Structural Mechanics (Lebahn and Sander (2013)). Therewith, the parameters of the FORMAN/METTU equation have been fitted to the crack growth data with different probability of survival ( P S ). The FORMAN/METTU curves determined for a probability of survival of 50% are presented in Figure 1 and the appropriate parameters are given in Table 1. Moreover, the threshold values D K th for the different stress ratios are calculated in dependence of the probability of survival using the approach of Döker (1997). In Table 2 the mean threshold values are shown. Table 1. Material properties and FORMAN/METTU parameter ( P S = 50%) for the high-strength steel 34CrNiMo6 E [GPa] A [%] R m [MPa] C FM n p q D K th,1 [MPam 1/2 ] K IC [MPam 1/2 ] C + th C th α CF σ max / σ F 210 9 1200 9.38 10 -7 1.89 2.39 0.43 1.14 145 3.89 0.05 1.9 0.3 2. Material

Table 2. Mean threshold values for all investigated R -ratios (34CrNiMo6) R -ratio 0.1 0 -0.5 -0.7 -1 D K th [MPam 1/2 ] 7.9 8.3 12.6 13.7 15.3