13th International Conference on Fracture June 16–21, 2013, Beijing, China —1— A Prediction Model for Low Cycle Fatigue Crack Initiation under Axial Loading Yajun Zhang, Lingqing Gao, Chunfen Wang Luoyang Ship Material Research Institute, Luoyang 471023, China * Corresponding author: zhangyj309@163.com Abstract: A prediction model for low cycle fatigue crack initiation was proposed, based on damage mechanics theory, which indicated the relation of equivalent value d of low cycle fatigue crack initiation to stress attenuation percentage △S/S. Then, parameter d will be calculated if △S/S is given, and the corresponding cyclic life is initiation life of fatigue crack. To validate the model, a group of 10CrNiMo structural steel specimens with a round-section were tested through low cycle fatigue test controlled by constant strain under axial loading, and the results manifested that the model is dependable and applicable; and the error of d to really measured value is not more than 5.0%; when the total strain amplitude is 0.6%, d is close to 2.0mm and the initiation life is approximately 900 cycles, which is nearly 80% of the entire life. Keywords: Low cycle fatigue, Crack initiation，Prediction model, Initiation life 1. Introduction In engineering, a crack which can be observed through a microscope with a low magnification (10×～20×), usually was defined as crack initiation and its length is 0.13mm to 0.25mm [1,2]. In design, crack initiation was deemed as a range from 0.25mm to 2.5mm [3]. When a crack increases to a certain length such as 0.1mm, 0.5mm, or 1.0mm, relevant fatigue cycle is called initiation life of fatigue crack. In general, there are three methods to measure crack initiationi, replication technique, potential drop method and observation through a microscope with a long focal length. Basically, the three methods have a high precision, and can meet engineering requirements. But all of them are inconvenient in real application [4]. In the fields of crack initiation and life prediction, many scholars proposed several models based on different parameters, for examples, Manson-Coffin Model [5] beginning with macro-mechanics, Mesomechanics Model [6] to describe microcosmic dispersivity using macroscopic parameters, Probability Model [7] calculating crack initiation life taking microcrack bulk density as index and so on. Many problems appeared when above models applied in reality [8]. In this paper, a new prediction model for low cycle fatigue crack initiation was proposed, based on damage mechanics theory, by which equivalent value of crack initiation could be calculated in case of an assuming condition, and crack initiation life could be measured combining with some simple tests, which would provide some reference for engineering design. 2. A prediction model for low cycle fatigue crack initiation based on damage mechanics
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