PSI - Issue 60

S.K. Chandra et al. / Procedia Structural Integrity 60 (2024) 203–213 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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5

Fig. 2. Microhardness indentations for δ 5 measurements. The specimens (both SENT and DENT) were first polished to metallographic quality, and the microhardness indentation marks were placed. The specimen was (i) loaded at the chosen ramp rate up to a pre-determined actuator displacement; then (ii) the actuator movement was arrested and load was allowed to relax; and finally (iii) the specimen was unloaded to a load value of ~ 1 kN (Fig. 3), this unloading step could be quite helpful for ∆ determination from unloading compliance data, especially when the material is susceptible to large crack tunneling. This load-relax-unload sequence was repeated several times and by this process, almost the entire load-displacement curve was generated. During each load relaxation step, the crack tip region visible at the specimen surface was photographed by using a light microscope integrated with a digital camera. By this technique, each test delivered a series of photographs for subsequent optical measurements of ,  5 , and ∆ .

Fig. 3. Representative P−v s plot showing loading- relaxtation-unloading sequences in CTOA testing.

2.3. CTOA Determination The optical measurement of CTOA from series of photographs was carried out using the two - point method described in ASTM E2472 [18]. Accordingly, on each image, the crack tip was located and then pair of points along