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Mechanical stressstrain hysteresis, temperature and electrical resistance measurements
were performed for the microstructurerelated characterization of the fatigue behavior and for the
fatigue life calculation of metals. The proceeding fatigue damage was evaluated using the change of
the loadfree electrical resistance, which is strongly influenced by the defect density of the
individual material state. A new test procedure was applied for the fatigue assessment under random
loading on the basis of cyclic deformation curves, similar to single step loading. A physically based
fatigue life calculation “PHYBAL” was developed, which requires only three fatigue tests for the
rapid and nevertheless precise calculation of SN (Woehler) and fatigue life curves.
The detailed knowledge of the fatigue behavior and the comprehensive understanding of basic
fatigue mechanisms of metals are fundamental for the systematic development of reliable fatigue
life calculations to ensure safe and economic operation conditions. In general, the material response
to cyclic mechanical loading is described by the plastic strain amplitude ε
a,p
, determined in stress
strain (σε) hysteresis measurements [1]. Additional highresolution temperature T [2,3] and
electrical resistance R [4,5] measurements were performed for the detailed microstructurerelated
characterization of the fatigue behavior of metals under single step and random loading. The
mentioned physical quantities are directly influenced by deformationinduced changes of the
microstructure in the bulk material and describe the actual fatigue state. Related to a reference value
of the virgin material state, the electrical resistance R
loadfree
enables the accurate detection of a
proceeding fatigue damage during loadfree inspections. Apart from geometry, the electrical
resistance depends on the resistivity, which is strongly influenced by the load and cycledependent
defect density, i.e. dislocation density and arrangement, microcracks, pores, macrocracks, of each
individual material state. Especially for material and loading conditions, which lead to small cyclic
plastic deformation, temperature and resistance measurements yield additional information about
the actual fatigue state. The applied measurement methods are equivalently qualified for a fatigue
assessment under single step and random loading as well as for a phy sically ba sed fatigue l ife
calculation “PHYBAL” on the basis of Morrow and Basquin equations [6].
Exemplarily, railway wheels widely used in highspeed passenger and in freight traffic are regarded.
The industrial heat treatment of the wheels manufactured from the unalloyed medium carbon steel
SAE 1050 (R7) consists of austenising, spraying a cooling liquid on the tread, the socalled ‘rim
chilling’, and annealing. Specimens were machined in rolling direction from the rim near the tread
(A1), 35 mm below the tread at the socalled ‘limiting diameter’ (A2) and in the flange (A3) [7].
Additionally, specimens of the quenched and tempered (550°C for 120 min) steel SAE 4140 were
Materials Science Forum Vols. 567-568 (2007) pp. 51-56
online at http://www.scientific.net
© (2007) Trans Tech Publications, Switzerland
Online available since 2007/12/06
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the
written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net . (ID: 131.246.156.23-06/12/07,10:12:12)