A novel parameter to evaluate fatigue crack closure: Crack opening ratio

A new parameter called crack opening ratio (COR) is proposed to evaluate the fatigue crack closure. COR is expressed as a percentage that can be devoted to identifying the degree of crack opening and to determining the value of crack opening load (COL). Interestingly, theoretical analysis indicates that COR is invariant in different measurement positions behind the crack tip. To verify the effectiveness and reliability of COR-based methods, fatigue tests of superalloy GH4169 in complex environment, overload and high temperature (650 degrees C), are conducted with compact tension (CT) specimens. By means of bi-prism-based single-lens 3D digital image correlation (BSL 3D DIC) technique, the full-field fatigue deformation near the crack tip is in situ measured, based on which the COL is characterized throughout the whole process of fatigue tests. Compared to some other classical methods, including stress intensity factor-crack opening load method (SIF - COL), crack opening displacement compliance method (COD - C), and crack-profiles method (C - P), the COR-based methods have shown a consistent result and better anti-noise capability. Concretely, COR-based methods are not sensitive to the measurement positions, thus the results have little dispersion and can be derived from full-field data. As a result, the determined value of COL is relevant to the characteristics of the region (not just the discrete points), which is steady and reliable.