PSI - Issue 37

Behzad V. Farahani et al. / Procedia Structural Integrity 37 (2022) 873–879 Behzad V. Farahani et al./ Structural Integrity Procedia 00 (2021) 000 – 000

875

3

or out-of-plane, as already referred, depending on the test setup. The phase shifting technique consists in moving one of the laser beams, movements usually in the sub-micrometre range, to induce a phase shift in the beam – if the distance between the light source and the specimen is not changed, the wave will be incident on the surface in question with a different phase. As such, by inducing the shifts in the laser beam, the interferometric fringes will move. With some mathematical manipulation, it is possible to extract information from the change in the fringes to obtain a phase difference map. The number of phase shifts may vary, and the algorithm responsible for the mathematical manipulation changes with the number of obtained fringes. In the present work, measurements were performed considering only the Y-direction, for which a piezoelectric actuator materializes this induced movement with its controller, a Thorlabs MDT694B piezo controller. 3. Experimental procedure and results In this study, a Middle Tension (MT) specimen with a central crack has been manufactured according to the Standard Test Method for Measurement of Fatigue Crack Growth Rates (ASTM International 2015), as shown in Figure 1-a). It was made of an aluminium alloy AA6082-T6, with the following material properties: Young’s modulus: = 70 ( ) ; Poisson’s ratio as = 0.33 ; and yield stress of = 250 ( ). The specimen thickness was identified as = 3 ( ) . The specimen was subjected to fatigue under a maximum load of = 10 ( ) , and a ratio of = 0.1 , as to generate a fatigue crack with a specified length. The crack length was measured through the use of a travelling microscope, mounted on both specimen’s sides and , therefore, the average crack length was measured as = 27.48 ( ) . Then, the cracked specimen was statically loaded under the previously specified maximum load, and the crack tip was observed by the ESPI system. Regarding the ESPI experiment, the developed system possesses five laser beams; one pair vertically aligned, and one pair horizontally aligned, both pairs for in-plane displacement measurement; and a fifth beam, for out-of-plane displacement measurement, which pairs up with any of the other laser beams. Figure 1-b) depicts the ESPI setup standing in front of the testing specimen. Besides, Figure 1-c) shows the laser light projected on the specimen. As previously mentioned, in this study only the vertical deformation was considered, so the only laser beams used were the vertically aligned pair, in order to obtain the vertical displacement, ( , ) . Additionally, a camera is used to capture images of the fringes, a 2-Megapixel Basler acA1600-20um camera, with a resolution of 1626 × 1236 ( 2 ) and a Linus − 25 ( ) lens. The distance between the lens and the specimen is equal to = 300 ( ).

a)

b)

c)

Figure 1. a) Studied MT specimen respecting its geometrical dimensions in mm b) ESPI setup in front of the testing specimen and c) laser projection on the specimen surface. The specimen is put on its reference state, where the camera captures the first set of images generated by the Phase