13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- gigacycle fatigue behaviors in Fusion Zone and Heat Affected Zone of Q345 LA steel welded joints He Chao, Liu yongjie, Tian Renhui, Wang qingyuan Dept of Civil Eng & Mechanics, Sichuan University, Chengdu 610065, Sichuan Abstract: The fatigue behavior in Fusion Zone (FZ) and Heat Affected Zone (HAZ) of widely used low alloy steel, Q345, in very high cycle fatigue regime was experimentally studied with ultrasonic fatigue testing system. It was found that the fatigue failure of HAZ specimen still occurred beyond 107 cycles, while a horizontal platform was obtained for FZ specimen in VHCF regime related to internal welding defects in fusing zone. A high-sensitivity infrared imaging system was used to measure the temperature changes during fatigue testing and the relationship of temperature variation and fatigue damage was discussed. The temperature of BM specimen would rise rapidly with the beginning of crack propagation. Fatigue crack propagation lives of HAZ and FZ were estimated by monitoring the natural frequency of specimen, which was varying with the crack size during the fatigue testing. Results showed that more than 99% of the total fatigue life was occupied by the crack initiation process for base metal (BM) and HAZ. However, this number was scattered for FZ specimen at relatively low stress amplitude owing to the crack propagation from welding defects directly, without crack initiate life. Keywords: VHCF, Welded Joints, Fatigue Crack Propagation Life, Heat Affected Zone, Fusion Zone 1. Introduction In the applications of aircrafts, automobiles, off-shore structures and railway equipment, many components would experience nominal vibratory stress conditions over a long period of time, running up to several hundred million cycles in their working service [1,2]. Conventional fatigue design specifications address the stress at 107 cycles as the fatigue strength limit, which was not applicative for mechanical equipment working in super long life. By the statistics, the fatigue failure at welded joints accounted for over 80% of the total and became the most dangerous failure mode for welded structure [3]. The study on the fatigue behavior of welded joints is of significant importance. The research on very high cycle fatigue has been an important issue with the development of ultrasonic fatigue test system. In recent years, many researchers have studied VHCF properties. Bathias et al.[4,5] made great contribution to the development of bending and torsion ultrasonic fatigue test system. Murakami et al. [6] proposed a semi-empirical model for high strength steel to predict the fatigue strength in considering of hardness, inclusion size and stress ratio. Wang et al. [7] analyzed the fatigue failure mechanism and predict the fatigue strength and fatigue life for six ultra-high strength steels. Xue et al. [8] and Huang et al. [9] studied the heat dissipation of specimen in VHCF with the help of infrared video camera. Marines et al. [10,11] proposed a model of fatigue crack propagation for internal “fish-eye” and surface crack initiation, based on the fatigue crack propagation law proposed by Paris [12 ]. In the past twenty years, most of the studies were performed on base materials, while fatigue behaviors of welded joints were rarely studied, especially in very high cycle fatigue domain. He et
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