13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- In-situ Observation of Mixed Mode Fatigue Crack Growth Behavior in Heat Affected Zone of a Welded Joint Ming-Liang Zhu*, De-Qiang Wang, Fu-Zhen Xuan, Shan-Tung Tu Key Laboratory of Pressure Systems and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China * Corresponding author: mlzhu@ecust.edu.cn Abstract In-situ scanning electron microscope (SEM) observations of fatigue crack growth behavior in heat affected zone (HAZ) of a welded joint were performed with CTS (Compact-Tension-Shear) specimens under mixed-mode loading conditions in vacuum. Results showed that mode I fatigue behavior in the HAZ was influenced by local microstructure anisotropy and the ‘zigzag’ mode crack profile tended to deviate into the lower strength weld metal. With increasing the mixity of mode I/II loadings, fatigue crack growth rate was decreased with significant crack branching. Shear and branch cracks were competitive along the crack path, and fatigue cracks were often initiated from grain boundary and the interface of lath martensites. The cracks tended to orient themselves into a pure mode I condition. Three existing models were discussed with respect to their capabilities to predict crack direction under various loading conditions. Keywords in-situ SEM, fatigue crack growth, crack growth path, mixed-mode loading, heat affected zone 1. Introduction Generally, the various loading types in actual practices are often idealized as being mode I, mode II and mode III based on linear elastic fracture mechanics. Majority of crack growth investigations under both static and cyclic loadings were concentrated on single-mode loading mode such as the mode-I load condition. Unfortunately, single-mode loading seldom occurs in practice, and many service failures occur from cracks subjected to mixed mode loadings which involve more than one crack-tip mode. Even in pure mode I cyclic testing, any deviation of crack path or alternation of external loading direction would lead to combinations of mode II deformation in crack growth process. Under mixed mode loading conditions, it was of importance to consider the evolution of crack growth direction. Several criteria for crack growth path prediction such as maximum tangential stress (MTS) criterion by Erdogan and Sih [1], minimum strain energy density (SED) criterion by Sih [2], the criterion of energy release rates by Nuismer [3], and the criterion of Richard [4], have been proposed and well documented in [4-6]. However, all these concepts were based on the isotropic material and limited to the linear elastic fracture mechanics [5]. Another issue in combined loading mode was the crack growth rate. Unlike the conventional stress intensity factor range ΔK in pure mode I fatigue, many parameters have been proposed to correlate mixed mode fatigue crack growth (FCG) rates. For example, Tanaka [7] and Yan [8] suggested the effective stress intensity factor range ΔKeff, Patel and Pandey [9] insisted the strain energy density factor range ΔS, while Socie et al [10] proposed the equivalent strain intensity factor for small cracks. Moreover, the fatigue thresholds [11, 12], crack closure [13, 14], microstructure [15] and loading path [16, 17] influences under mixed mode loading conditions have been investigated. While cracks were observed to propagate in homogeneous materials as widely reported, complications associated with the mixed mode fatigue crack behavior in heterogeneous microstructures, i.e., the gradient microstructures in the heat-affected-zone (HAZ), were far less well understood. In this case, the effect of microstructure anisotropy on crack growth morphology needed to be elucidated.
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