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a b s t r a c t This paper presents a new approach based on the application of a multiaxial high cycle fatigue criterion together with the use of El-Haddad correction for investigation of fretting fatigue in railway axles. Stress path along the axle–wheel contact, determined by the FE analysis, was implemented into different mul- tiaxial fatigue criteria in order to predict critical sites of nucleation. The equivalent fatigue limit expressed by the applied criterion is compared with the crack size dependent fatigue limit described by El-Haddad correction in order to define a defect size acceptability criterion. Verification of the pro- posed approach was done by post-test failure investigation of the full-scale axle tests conducted as a part of Euraxles project. Scanning electron microscope (SEM) examination of the failed press-fit sections revealed a critical defect size in the order of 200 lm in depth for non-propagating cracks. The obtained results were found to be consistent with the estimations made by the proposed approach.69574
1. Introduction
Investigation of fretting damage and its prolonged conse- quences in fatigue life assessment is an important issue in railway axle design. Fretting fatigue in the axle–wheel contact can be described as the repetitive micro sliding of the wheel assembly on press-fit seat due to applied bending and vibration. Multiple- site surface damage caused by fretting is considered to be the source of crack nucleation which can become a propagating crack with further application of cyclic loading. A decreased fatigue life up to 60–75% due to fretting damage has been reported in the lit- erature [1,2].
The present study is a part of research activity devoted to vali- date the fatigue limits described by the European Standards. Main objective of the present study is to develop an acceptability criteria that can be applied in assessment of surface defects inspected at axle press-fits by magnetic particle inspections (MPI). Further con- tribution to the European Standard EN-13260, which necessitates verification of no crack formation after 107 cycles at fatigue limit currently, by evaluating the applicability of presents fatigue limits in the presence of non-propagating surface cracks was also aimed [3]. In the study, press fit surfaces of different EA4T axles have been investigated. A nominal bending stress, not exceeding 240 MPa,
was selected in accordance with the EN standards [4–6]. The allow- able stress limit at the press fit was reduced to 132 MPa for an axle with a diameter ratio of D/d = 1.12 (Fig. 1a) [4,5]. Formation and developments of non-propagating cracks at the press-fit under the influence of fretting was investigated.
A previous study on fretting fatigue of railway axles identified four typical regions for initiation sites located at the root of transi- tion fillet (T-transition), contact edge, stick–slip interface of the contact and sub-surface under contact [4,5]. Hirakawa et al. reported formation of an annular band of fretting corrosion with a width of 7–9 mm starting from the edge of press-fit to the inner press fitted surface for railway axles. Within this band several min- ute cracks can be observed. However, the cracks which propagated to final failure always observed to initiate from the sites 2–5 mm away from the edge of the press-fit [7].
Mechanism of fretting fatigue in railway axle is explained by the multiple-site nucleation of non-propagating cracks under the influence of fretting damage described as multiple-site damage (MSD) [8]. Beyond certain limits of loading, framed by application of multiaxial fatigue criteria along the stress path, transformation of non-propagating cracks to a state of propagation might take place. Propagation of long-cracks, driven by the applied bulk stress, results in final fracture of the axle as cyclic loading proceeds [8]. Experimental fretting studies on different materials has shown that majority of fatigue life has been spent in formation of a engi- neering crack in the cases of cylindrical contacts [1,8–13]. In other