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 a noncontact type optical strain measuring technique where the complete displacement and strain field has been evaluated by comparing the deformed image state w.r.t. Unreformed/zero load state. Typical steps followed in present study to measure the localized strain field ahead of notch has been depicted in Fig. 5 (a), (b) and (c). For pure torsion tests on notched tubes, the state of pure shear exists along hoop direction of tube at mid gauge-length location. Along this hoop direction, the shear strain amplitude rises to its maximum value at a distance of nearly 1.7mm from hole tip as shown in Fig. 6 .Variation of shear strain amplitude from hole tip along = 0° line in the circumference of the tube (as analyzed using FE results) is presented in Fig. 6 . This maximum amplitude of the pure shear strain can be captured by putting single element strain gauge at 1.7mm away from hole tip and along 45 ° w.r.t. this hoop direction (Fig. 5(a)), to capture principal strain. This localized principal strain amplitude is same as that of half of the engineering shear strain amplitude and is compared with corresponding DIC results in Fig. 7 (b). Fig. 7 (a) shows half of the applied remote torsional strain amplitude where as Fig. 7 (b) shows that strain gauge measurement is in good agreement with DIC strain measurement.

(a) (b) Fig. 7. Time variations of half of shear strain(a) at remote location using extensometer (controlled variable) and (b) near to hole along = 0 ° line using DIC/ SG (measured variables) 4. Fatigue life assessment To predict the fatigue life under strain gradient and multiaxial conditions, generally two theories are adopted sequentially. To account for strain gradient effect, (i) Theory of Critical Distance (TCD) based models are used, whereas, (ii) Critical Plane (CP) theory is used to account for multiaxial state of cyclic stresses/ strains. In present analyses, a representative material point is chosen at certain distance (or characteristic distance) away from peak equivalent strain location. This material point represents the material volume experiencing fatigue damage owing to gradient. For this grade of C-Mn steel, 70 is taken as characteristic distance as reported by Arora et al (2009). At this characteristic distance, the stress/ strain tensorial time-variations are taken as input information for CP based models. In this study, a recently developed and benchmarked CP model of Arora et al. (2019, 2020) has been used for evaluation of Fatigue Damage Parameter (FDP) at characteristic distance. This FDP is used to predict fatigue life from FDP-versus-test fatigue life curve under uniaxial conditions. The predicted and test fatigue life under remote axial and torsion conditions fall within material intrinsic data scatter band of 2. Fig. 8 shows that predicted and test fatigue lives with test life below 10,000 cycles (typically in LCF regime) are uniformly distributed about the ideal comparison line (Np=Ni line). However, data points with test fatigue life (Ni) higher than 10,000 cycles (nearly in transition regime) are mostly skewed towards lower bound line (Np=Ni/2 line). This observation needs to be substantiated with large number of test data points mostly in transition regime.