High-performance methodology for residual stress measurement in railway wheels

Considering monoblock railway wheels for freight applications, a compressional state is required in order to guarantee the structural integrity during service. Unfortunately, adverse thermo-mechanical stresses, caused by hard braking, can reverse the beneficial compressive hoop stress to a tensile one, easing the triggering of fatigue phenomena. For this reason, periodical in-service stress measurements are required. The present paper, carried out in the frame of the EMATWHEEL Italian project co-funded by Regione Lombardia and MIUR, aims to define qualitatively and quantitatively, through non-contact ultrasonic EMAT transducers, the residual stress of railway wheels. In particular, it is here derived via the acoustical birefringence effect and elaborated by a more refined approach, with respect to the usual one. Tests are firstly performed onto specimens, cut from railway wheels and subjected to a known biaxial stress state, and then to wheels coming from production and made of ER7 and ER8 steels.