PSI - Issue 54

Behzad V. Farahani et al. / Procedia Structural Integrity 54 (2024) 638–644 Behzad V. Farahani et al./ Structural Integrity Procedia 00 (2023) 000–000

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specimen. DIC has been successfully implemented to monitor the deformation on engineering structures with different material behaviours, c.f. (Farahani et al., 2019, 2020). Another optical experimental tool, Electronic Speckle Pattern Interferometry (ESPI) has proved to be an appropriate technique to analyse the fracture mechanics problems since it is a high-resolution optical approach used for displacement measurements around the crack tip, from interferometric fringe patterns (Tajik et al., 2011). Likewise, Chen et al. (Chen et al., 2019) presented a methodology based on noise reduction technology on ESPI fringe patterns with variable density by constructing a clustering framework. Mathematical development in addition to experimental validation have been accomplished in their work. Hack et al. (Hack et al., 1995) conducted a research study to monitor the real time crack growth and the plastic zone on the crack tip by a 3D ESPI technique, and compared the results with computational data. Recently, a hybrid methodology comprising ESPI with polarizing phase-shifting techniques was proposed by Gómez-Méndez et al. (Gómez-Méndez et al., 2021) for in-plane displacement measurements. In this regard, Pisarev et al. (Pisarev et al., 2017) established a new combined experimental technique for determination of both singular and non-singular fracture mechanics parameters, through a modified version of the crack compliance technique, and ESPI data. Moreover, Sousa et al. (Sousa et al., 2019) developed a 3D ESPI system to study the thermal response of printed circuit boards, which measured the displacement distribution caused by the thermal effects. Crack closure acts as a shielding mechanism which reduces the effective stress intensity factor (SIF) range at the crack tip, thereby decreasing crack growth rates (Carroll et al., 2009). Therefore, the finding of fatigue crack closure play an important implication for the life prediction and safety estimation on engineering structures (Elber, 1970). Elber (Elber, 1970, 1971) showed that crack opening would be accompanied by a change in specimen compliance due to a configuration change when the crack opens. He discovered that the load level corresponding to this compliance would change by using a displacement gauge behind the crack tip. Therefore, it is feasible to measure the relative displacement of the crack flanks via the predefined gauges. Hence, the opening displacement on a crack is determined as a function of the load over fatigue cycling test (Esteves et al., 2022). Therefore, a linear variation of displacement versus load regardless the effect of small-scale plasticity at the crack tip can be obtained according to the linear elastic fracture mechanics (LEFM). Nevertheless, Elber’s experiments exposed two different regimes: one with the expected linear relationship and a second (at lower loads) where nonlinear behaviour took place (NOWELL et al., 2010). Theoretically, since Elber’s discovery, several other techniques have been developed for measuring crack closure. Some researchers use visual observation techniques to determine closure levels (Sehitoglu, 1893, 1985). Other methods for measuring crack closure such as the electrical potential drop method, ultrasonic/acoustic methods, and the eddy current method have been used with limited success. Experimentally, DIC has been applied to analyse the fatigue crack closure by Sutton et al. (Sutton et al., 1999). Eremin et al. (Eremin et al., 2017) studied near-tip strain measurements under simple variable-amplitude loading by DIC to assess the crack closure phenomenon on the Al-Cu alloy single edge crack tension specimens. However, the Elber’s suggestion for crack closure analysis was successfully employed on a multiscale analysis through DIC (Carroll et al., 2009) and therefore this methodology is taken into account in this study to assess the crack closure phenomenon using DIC data. This work studies the crack closure phenomenon in AA6082-T6 Middle Tension (MT) specimen by optical experimental techniques with different system resolution. Therefore, it brings a complementary multiscale analysis in the crack closure occurrence. Therefore, the specimen is loaded under fatigue condition to generate the fatigue crack with a predefined length. Then, the fatigued MT specimen is monotonically loaded under incremental steps to reach the peak load and then it is unloaded respecting the same load increments. During both loading and unloading, experimental data is collected by two optical contactless tools, ESPI and DIC. As an outcome, the force/displacement variation in predefined gauges, displacement and deformation contours in the cracked area will be monitored. In crack closure assessment, following Elber’s hypothesis (Elber, 1970, 1971), the vertical virtual gauges can be placed across the crack flanks within a distance to the notch or crack tip. These gauges should not physically contact the crack faces. Thus, in the DIC study, two vertical clip gauges (VCG-1 and VCG-2) were defined on the region of interest on the crack flanks. Regarding ESPI analysis, it was not possible to follow the Elber’s methodology due to the presence of a plastic area where fringes de-correlate, therefore, as an alternative approach, the crack opening phenomenon was studied by discontinuities occurred due to the crack existence.