13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Based on cycle strain damage prediction of the crack growth behavior of elliptic surface crack LONG CHEN1,*, LIXUN CAI1, KAIKAI SHI1 1 School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China * Corresponding author: lix_cai@263.net Abstract According to the low cycle fatigue strain damage, a unit average damage parameter over the cyclic plastic zone along the crack extending direction is defined, and a new fatigue crack growth rate prediction model (LCF-FCGM) is established. Then combined with Newman-Raju formula, simulation of the fatigue crack growth problem of a half elliptic surface crack in tension plate under cyclic loading is fulfilled. The fatigue crack growth rate curve of Al 7075-T6 alloy predicted by LCF-FCGM agrees well with that obtained from the corresponding test. Assuming that the shape of the surface crack front keeps elliptic and is controlled by two critical points: the deepest point and the surface point, a numerical approach based on material’s low cycle fatigue properties is developed to analyze the configuration evolution of surface cracks during fatigue crack growth. The new approach is well applied to Al 7075-T6 alloy surface cracked plate, and six different initiate crack shapes are discussed and all reflect the same phenomenon of round first to oval again, which are consistent with the test results. Keywords fatigue crack propagation; low-cycle fatigue damage; Newman-Raju formula; elliptic surface crack; Al 7075-T6 alloy 1. Introduction In engineering the crack is always in three-dimensional stress status, especially for the surface crack in pressure vessel pipe. So study on the surface crack growth problem has great importance. Because of the uncertainty of the crack front shape, it brings much difficulty on solving these nonpenetrating crack problems. Nowadays, there are three methods have been proposed: conservative estimate method, shape assuming method and crack shape tracking method. ASME X1(1997) assumed the crack shape parameter α=a/c is constant. And BSI PD6493（1980）assumed that the crack width c keeps still until the shape changes to round and the round shape kept till the end. Known as the representative of crack shape assuming method, Newman-Raju formula[1] (Newman and Raju, 1981) has been wildly used in solving the surface crack problem, in which the crack shape parameter α is assumed to be changeable. By using the crack shape tracking method, the simulation of nonpenetrating crack is fulfilled with 3D finite element method[2] (Smith and Copper, 1989), the 3D crack propagation problem is studied by boundary element method[3] (Deng Jiangang et al., 2003 ). Based on the continuum damage mechanics method the fatigue crack growth behavior of surface crack is discussed[4] (Feng Xiqiao and He Shuyan, 1997), and by using S-version FEM the process of surface crack growth under mixed mode cyclic loading condition is simulated[5] (Masanori et al., 2011). What’s more, many researchers have proposed different approaches in mathematic way to predict the growth behavior of surface crack. Take energy approach for example, Song, Sheu and Shieh[6] (2002) have well applied it to predict the surface crack growth in 2024-T4 aluminium alloy. And, an approach with data obtained from the testing of through-thickness cracks was established to predict surface crack growth, in which the effects of crack closure[7] was considered (Liu Yanping et al., 2010). Therefore, the study on surface crack growth behavior is still hot to solve the fatigue fracture problem in engineering like the pressure vessels and pipes. However, the past researches on the crack growth behavior of surface crack are all based on the Paris formula[8] (Paris and Erdogan, 1963). As we know, the fatigue crack growth rate behavior and the low cycle fatigue behavior are just different way to describe material’s fatigue properties, so there should be a connection to each other[9] (Cui Weicheng, 2002). And many theoretical models have been proposed based on material’s fatigue properties[10-12] (Castro J.T.P.,
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