13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Measurement of Effective Stress Intensity Factor Range of Mode II Fatigue Crack Growth using Hysteresis Loop Shigeru Hamada1,*, Minjian Liu 2 1 Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, 744 Moto-oka, Nishi-Ku, Fukuoka 819-0395, Japan 2 Department of Mechanical Engineering, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-Ku, Fukuoka 819-0395, Japan * Corresponding author: hamada@mech.kyushu-u.ac.jp Abstract A method was proposed for measuring the effective stress intensity factor ranges of Mode II fatigue crack growth by using the hysteresis loop for a specimen's surface strain. Many cases of rolling contact fatigue failure, such as those that occur in railway rails, bearings and gears are due to repeated high shear loads. In order to prevent such fatigue failures, the resistance of a material to repeated high shear loads must be determined. The fatigue crack growth characteristics are dependent on the Mode II stress intensity factor range. However, conventionally measured Mode II fatigue crack growth characteristics vary according to the measurement methods. Therefore, the authors improved the experimental measurement method proposed by Murakami, and proposed a way to measure the Mode II effective stress intensity factor range. Improvements to the jigs and specimen were made based on the ideal mechanical model of the experimental method. Furthermore, to measure the Mode II fatigue crack growth behavior, strain gauges were applied to the specimen and the hysteresis loop of the strain was measured with high accuracy by using a newly developed subtraction circuit. Keywords Rolling contact fatigue, Friction, Mode II fatigue crack, Effective stress intensity factor, Fracture mechanics 1. Introduction Mechanical failures such as spalling and pitting can occur in rails, bearings, and other components when they are subjected to heavy repeated rolling contact loading. In order to prevent these types of failures, it is necessary to determine the resistance of certain materials to them. A fatigue crack under repeated rolling contact loading, which is what leads to the failure, propagates in Mode II. Therefore, the resistance to fatigue crack propagation which is caused by stress concentration sources such as flaws or inclusions can be evaluated by using the Mode II fatigue threshold stress intensity factor range, ΔKIIth. Methods for measuring Mode I fatigue crack propagation have already been established and standardized [1]. However, for Mode II fatigue crack propagation, systematic research is limited because this type of fatigue crack propagation is difficult to produce in a laboratory and there are no standard tests. Early systematic research was conducted by Otsuka et al. [2]. However, the method they developed could only be applied to soft metals such as aluminum alloys, even though hard metals are used for the components for which Mode II fatigue crack become a problem. After this study, Murakami et al. [3, 4] developed an experimental method that could also be applied to hard metals such as bearing steel. Later, Otsuka et al. [5] improved their method so that it could also be applied to hard metals. However, different values were obtained for the threshold stress intensity factor range ΔKIIth when these two methods [4, 5] were used for the same material. It seems that interference by the crack faces affected the result. The study conducted by Matsunaga et al. [6] on the shear mode threshold proved that friction on the crack face increases the value of ΔKIIth. Therefore, it is thought to be necessary to take the friction on the crack faces into account when determining the Mode II effective stress intensity factor range, ΔKIIeff. In the previous work by the authors, a new method was proposed [7, 8] to measure the friction at the crack faces and ΔKIIeff. Moreover, a more appropriate assumption for the friction distribution on
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