Structural Integrity and Design of Railway Components
Research focus
The Peer review has evaluated this group as Excellent
The activity of the research group has begun in 1998 with a series of activities in collaboration with Lucchini Sidermeccanica, Centro Ricerche Fiat and the group of RWD. Since that time the activity has been further developed toward the development of new methods for the analysis of the integrity of the wheelset and the rails. Apart a number of small activities for the integrity assessment of different components, the research can be mainly grouped in these three branches. Axles Considering railway axles, after the previous development of a test bench for fullscale tests of axles, the activity has been firstly devoted to the analysis of the ‘scale effect’ in the fatigue strength of railway axles made of a high strength steel (30NiCrMoV12) and the experimental comparison of crack growth rates in small scale specimens and full-scale axle. The integrity of railway axles has then be addressed by the set-up of a complete methodology for the calculation of inspection intervals of railway axles. The comparison of the growth rates in two steels (A1N and 30NiCrMoV12) has then led to the development of a stochastic model for crack growth based on NASGRO propagation model. The activity on inspection intervals of axles is included in the EU project WIDEM (www.widem.org). In this research area there is also an active cooperation with Prof. U. Zerbst (GKSS, Geeshacht, Germany) about the development of SIF solutions for cracks at the base of the typical notches of axles under rotating bending. Wheels As far as railway wheels are concerned, the research activity has been focused on the analysis of RCF under the action of dynamic service loads obtained by the RWD group. By employing the stress field due to wheel/rail contact resulting from service loads simulations and measurements, the likelihood of fatigue crack nucleation in the sub-surface region under the contact area between wheel and rail has been assessed by integration of fatigue tests and of multiaxial fatigue criteria, developing a design procedure suitable for the sub-surface fatigue assessment of railway wheels. The methodology has been validated by means of multiaxial fatigue tests in which a non-proportional stress state similar to that observed under the contact area in the wheel rim is experimentally reproduced. Moreover, the identification of the parameters contained in an elastic–plastic material model, apt to simulate steady-state ratchetting, critical for railway wheels designed in view of contact loadings, has been studied with reference to cyclic biaxial tests on cylindrical compact specimens. Another part of the research has been oriented to the analysis of railway wheels with sub-surface defects/cracks and assessment of structural integrity and residual life of wheels considering pre-exixsting internal defects or fatigue originated ones. An hybrid numerical approach was developed, being the aim to assess the criticality of 67 these defects as a function of the applied load, their position and dimension. It consists of two parts: the first is aimed to the analytical determination of the displacement field around the crack due to the application of the wheel/rail pressure distribution, the second one consider a FE mesh of the zone surrounding the mesh. The analytical displacements are applied as boundary condition to the FE model, thus enabling the calculation of the stress intensity factors (mode I, II and III) in an entire loading cycle. As regards to the experimental part of the research it was aimed to find experimental confirmation of the numerical results of the stress intensity factor with an original photoelastic approach: araldite models with an internal crack were constructed. The crack was obtained by thermal shock: the araldite model was heated and then cooled with an appropriate thermal law. The results are in good agreement with the numerical ones. Rails The cooperation with RWD group has also allowed the research group to analyze the effect of dynamic loads onto rails. In particular the multiaxial crack propagation and crack path in rail butt welds, subjected to typical service load spectra, has been experimentally and numerically studied. A further research topic on rails is the development of a new three-dimensional model able to improve the ratchetting damage analysis considering the real geometry of contact surface, the multi-hertzian contact, the dynamics of vehicle, the non-linear hardening material and a 3D representation of material damage. This model has been successfully applied to the analysis of damage of tramway rails which are characterized by high of levels surface damage because of dynamic behaviour of tram coaches.
Dipartimento di afferenza
Docenti afferenti
Full Professors
Stefano Beretta
Associate Professors
Mario Guagliano
Michele Sangirardi
Assistant Professors
Andrea Bernasconi
Michele Carboni
Mauro Filippini
Antonietta Lo Conte