PSI - Issue 60

Prakash Bharadwaj et al. / Procedia Structural Integrity 60 (2024) 655–664 Prakash Bharadwaj / StructuralIntegrity Procedia 00 (2019) 000 – 000

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validity). Hence these experiments successfully captured the FCG in both the small-scale yielding (SSY) region and the large-scale yielding (LSY) region. A total of 16 to 18 mm crack growth was observed during the tests. From the total crack growth, 8.5 mm to 10.7 mm crack growth occurs in the SSY region while 6.5 mm to 7 mm growth occurs in the LSY region. The experimental setup for the FCGR tests and DIC system is shown in Fig. 2(b). The prepared sample is mounted on a servo-hydraulic universal testing machine. A COD gauge is applied on the knife edge of the sample to measure the load line displacement. This cod gauge is connected to the digital controller. The servo-hydraulic universal testing machine is operated with the help of a digital controller PCS8000 as shown in Fig. 2(c).

Fig. 2. (a) Speckle pattern on the prepared sample; (b) Experimental setup; (c) Digital controller PCS8000

To measure the strain field, a digital image correlation (DIC) system having two cameras of focal length 50 mm is used to capture the high-resolution images during the FCGR test. The two lighting sources are mounted above the camera, to provide the proper lighting of the sample. The entire assembly is situated on an adjustable tripod stand. The DIC system is operated by a digital controller and data acquisition is carried out with the help of the software ARAMIS version 6.1.1 (ARAMIS, G., 2009). Before capturing the images, calibration was carried out by using the calibration cube of area 25 mm x 25 mm. During the FCGR test, 60 images per cycle were captured at the interval of every 0.5 mm of crack growth. At the different image-capturing stations, a total of 3 to 5 cycles were taken to capture the image. Crack growth measurement is carried out by both the COD-based unloading compliance method as well as by DIC system. 4. Measuring algorithm of the cyclic plastic zone using the DIC system 4.1 Calculation of strain Mathematics of DIC calculation is the key step to get a load-strain curve. The full field image zone is divided into sub-zone images. The deformed sub-zone and reference sub-zone are used to get the strain field around the crack tip. These sub-zones are shown in Fig. 3. The load-strain curve of every pixel inside the strain field is obtained by adding the loading-unloading information recorded during the image acquisition. The equations involved to get the strain tensors are as follows: ∗ = + ( , ) (1) ∗ = + ( , ) (2)