13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Subcycle Fatigue Crack Growth Mechanism Investigation for Aluminum Alloys and Steels Jian Yang1, Wei Zhang1, Yongming Liu1,* 1 Mechanical Engineering, Arizona State University, Tempe 85281, US * Corresponding author: yongming.liu@asu.edu Abstract In this paper, the existence of crack closure and its sufficiency for crack growth prediction is investigated by multi-resolution in-situ optical microscopy experiment and SEM experiment. In in-situ optical microscopy testing, the digital image correlation analysis is used to measure the plastic zone size in front of the crack tip. In in-situ SEM testing, the crack tip opening displacement and the crack growth kinetics are investigated. Besides, crack closure behavior under constant loading with a single overload is studied in in-situ SEM test. This experimental methodology is applied to two different metallic materials (aluminum alloys and steels). Detailed imaging analysis and experimental results are presented and compared. It is found that first, there exists crack closure phenomena for aluminum alloys, but not for steels in our current researches; second, the crack closure will significantly change the crack tip plasticity behavior; third, the crack closure concept is able to account for crack growth kinetics uniquely for constant loading, but it is insufficient for constant loading with a single overload. Finally, the necessarily and the insufficiency of crack closure for crack growth prediction is discussed. Keywords fatigue, crack closure, crack growth, plastic zone, overload 1. Introduction In the past half century, many existing studies have been done on fatigue crack growth mechanism and prediction models. The most famous and successful model should be Paris’ Law, which based on the applied stress intensity range [1]. However, Paris’ model cannot be applied to loadings with different load ratio and modification is required. In 1970, Elber introduced a crack closure mechanism and modified the applied stress intensity factor range in order to characterize the effect of load ratio [2]. Since Elber’s discovery provides a physical meaning to the modification, many researches have been done to get crack-opening load in theoretical method or experimental way. In 1978, Budiansky and Hutchinson provided an analytical estimation of crack opening load with the assumptions of long crack and small scale yielding according to the ideally plastic Dugdale-Barenblatt model [3]. In 1984, Newman analyzed the crack closure problems by developing a strip yield model, which employs a strip yield type plastic zone for leaving residually stretched material in the wake of the crack, causing plasticity-induced closure [4]. However, it is found that the measurement is difficult and the result depends on the measuring location and technique employed such as by traditional gauge and acoustic method [5]. Others methods like electrical potential method [6], ultrasonic method [7][8], and numerical method [9] are also used to measure crack opening load. These methods mentioned are indirectly way to observe crack closure and measure the crack opening loading, and it might be affected by many other factors, for example, it is found that commonly employed notch-mouth clip-gauge method is not sensitive enough to detect the closure of short cracks in regions of notch plasticity [10]. In this paper, in-situ testing and imaging analysis is used to directly observe the crack closure level and its relationship with crack tip behavior. Compare to the other indirect method, it is non-contact and can provide direct evidence about the crack closure at very different resolutions. Next, the proposed methodology is applied to two different metallic material systems and the existence of crack closure is investigated in detail. Following the testing under constant loading, a variable loading situation is tested to investigate crack closure effects on crack growth rate. In mechanics,
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