Issue 33

X. Zhou et alii, Frattura ed Integrità Strutturale, 33 (2015) 209-214; DOI: 10.3221/IGF-ESIS.33.26 209 Focussed on characterization of crack tip fields Load history effects on fatigue crack propagation: Its effect on the R-curve for threshold X. Zhou Materials Center Leoben Forschungs GmbH, Roseggerstraße 12, 8700 Leoben, Austria Austria Erich Schmid Institute of the Austrian Academy of Sciences, Jahnstraße 12, 8700 Leoben, Austria xiang.zhou@mcl.at A. Hohenwarter, T. Leitner Department of Material Physics of the Montanuniversity Leoben, Jahnstraße 12, 8700 Leoben, Austria anton.hohenwarter@unileoben.ac.at , t.leitner@stud.unileoben.ac.at H.P. Gänser Materials Center Leoben Forschungs GmbH, Roseggerstraße 12, 8700 Leoben, Austria hans-peter.gaenser@mcl.at R. Pippan Erich Schmid Institute of the Austrian Academy of Sciences, Jahnstraße 12, 8700 Leoben, Austria Department of Material Physics of the Montanuniversity Leoben, Jahnstraße 12, 8700 Leoben, Austria reinhard.pippan@oeaw.ac.at A BSTRACT . The study deals with the effect of load history during threshold and fatigue crack propagation measurements in the near threshold regime. The compression pre cracking and the stepwise increasing load amplitude test was applied. The effects of pre-cracking as well as the effect of compression and tension overloads after the pre-cracking on the long crack threshold and on the R-curve of the threshold of stress intensity range are analysed. K EYWORDS . Threshold of stress intensity range; R-curve; Crack closure; Compression pre-cracking. I NTRODUCTION he propagation of a fatigue crack in ductile materials is caused by the cyclic plastic deformation at the crack tip, which is governed by the plastic properties of the material and the crack driving force. The discovery of Elber [1] of the premature contact of the crack flanks during the unloading sequence reduces cyclic the plastic deformation, which can be expressed by a reduction of the crack driving force  K or the cyclic J integral to an effective crack driving force  K eff or  J eff . This phenomenon is usually called crack closure effect. Both, the cyclic plastic properties of a material and the effect of crack closure can be significantly affected by the loading history. There are a vast number of papers dealing with the effect of loading history during variable amplitude loading for fatigue crack propagation in the Paris regime [2]. T