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

643

6

The discontinuities were detected in the displacement fields obtained from ESPI data at various load stages. The results show that the crack is not opened if the specimen is loaded at F = 300 (N) which is 3 % of the peak load, c.f. Figure 4. Then, the crack opens when submitted to a load of F = 750 (N), and discontinuities were identified. Finally, the crack gets widely opened with a stable condition if the specimen is loaded at the pick load, as confirmed by the discontinuities map. It can be remarked that the obtained displacement profiles from DIC and ESPI are in good agreement in terms of shape and magnitude, demonstrating the crack opening evolution upon loading. The strain contours have been obtained and presented in the previously published paper by the Authors, therefore the readers are referred to (Farahani et al., 2022). 4. Concluding final remarks This work proposed a complementary methodology on crack tip monitoring of aluminium alloy AA6082-T6 middle tension (MT) specimen. The studied specimen compromises an initial notch generated by an Electron Discharge Machine, EDM. The MT specimen was fabricated according to the standard test methods for fatigue crack growth ASTM E647. Thus, the following conclusions could be drawn from this work:  The specimen was subjected to fatigue in a servo-hydraulic MTS machine to generate the crack with different lengths;  Then, to reach fully open cracks, the specimen was monotonically loaded under uniaxial tensile conditions up to the peak load as configured in fatigue tests;  Two optical techniques with different system resolution, ESPI and DIC, were employed to acquire and analyse the experimental data;  Due to the nature of the experimental tools, a portable testing machine was designed and fabricated to perform the uniaxial tensile tests and it was successfully installed in the experimental conditions together with the ESPI and DIC systems;  The displacement and deformation fields have been acquired at different load increments, and the crack path was detected through discontinuities that took place in the fields. It was proved that these discontinuities occurred due to the presence of fatigue cracks;  To assess the crack closure phenomenon, the studied specimen has been loaded/unloaded under different loading increments, and the evolution of the crack opening/closure has been documented through displacement and deformation distributions. In the DIC case study, vertical strain gauges (VSG) were defined on the crack flanks and the force/displacement variations were monitored at those locations. The displacement behaviour followed Elber’s hypothesis regarding crack closure evaluation. Furthermore, an acceptable verification has been achieved amongst both optical techniques in accordance with Elber’s theory and fracture mechanics fundamentals. In ESPI analysis, the crack opening has been studied based on the discontinuities phenomenon which occur due to the fatigue crack presence. Therefore, a good agreement was verified between DIC and ESPI results supporting the crack closure principle. Behzad V. Farahani sincerely acknowledges the funding received from Ministério da Educação e Ciência, Fundação para a Ciência e a Tecnologia (Portugal), under grant PTDC/EME-EME/29339/2017. References ASTM International. (2015). ASTM E647 - 15 Standard Test Method for Measurement of Fatigue Crack Growth Rates . ASTM International. Carroll, J., Efstathiou, C., Lambros, J., Sehitoglu, H., Hauber, B., Spottswood, S., & Chona, R. (2009). Investigation of fatigue crack closure using multiscale image correlation experiments. Engineering Fracture Mechanics , 76 (15), 2384–2398. https://doi.org/10.1016/J.ENGFRACMECH.2009.08.002 Chen, M., Tang, C., Xu, M., & Lei, Z. (2019). A clustering framework based on FCM and texture features for Acknowledgements