PSI - Issue 60

Shreebanta Kumar Jena et al. / Procedia Structural Integrity 60 (2024) 115–122 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

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DIC technique is of the order of 0.2 mm.

2.3. Base line LCF tests and cyclic plasticity parameter calibration

Preliminary test investigations have been carried out on solid LCF (un-notched) specimens following ASTM E606 at different strain amplitudes ranging from 0.25% to 1.25% under completely reversed strain-controlled condition to generate the base line test data. Additionally, the saturated material behavior was studied w.r.t. Masing idealization for this grade of steel. It was brought out by Arora et al. (2021) that SA333Gr.6 material shows Masing behavior with linear shift along elastic line indicating simulation of post yield behavior using single set of Chaboche material constants. However, a strain amplitude dependent saturated cyclic yield strength is required to be modelled to capture linear shift along elastic line. The calibration of the Chaboche constants have been evaluated by comparing the cyclic saturated hysteresis loop obtained from experiment with its corresponding FE results corresponding to 1% of strain amplitude as reported by Jena et al.(2023). The test fatigue crack initiation life corresponds to ~1mm crack size or 25% drop in axial reaction load (material response for strain-controlled test), whichever is earlier. 3. Test on notched tube specimens Based on calibrated material constants on un-notched LCF specimens, pre-test FE analyses have been performed under pure remote torsional strain conditions on notched specimens, as tested. The FE analyses were useful to obtain remote torsional strain amplitude as input test parameter to arrive at intended peak equivalent strain amplitude at hole tip. Thus, pre-test FE analyses is important to study the separate effects of equivalent strain gradient and peak equivalent strain amplitude on fatigue life. The individual effect of peak equivalent strain and strain gradient on the fatigue life of notched tube specimens have been brought out by Jena et al. (2023) by conducting 14 tests (10 under pure axial and 4 under pure torsion condition (present study)) under completely reversible strain/angle of twist-controlled condition. Test fatigue life criteria for notched tube is same as that of un-notched tube specimen. However, the drop in global load/ torque reaction by 25% takes place after initiated crack at hole tip becomes significantly higher than 1mm. Thus, for notched specimens, 1mm crack on outer surface of tube at hole tip is the criterion for test fatigue life. This has been consistently followed in earlier studies by Jena et al. (2023) and present test investigations. 3.1. Effects of strain gradient and peak equivalent strain amplitude The test fatigue life (following above criterion) of notched specimens has been compared for different hole sizes with a given peak equivalent strain amplitude as hole tip under remote pure axial and pure torsion conditions in Fig.3 and Fig. 4, respectively.

Fig. 3. Effect of different hole sizes (effect of strain gradient) and peak

Fig. 4. Effect of different hole sizes and peak equivalent