PSI - Issue 68

Yuki Tampa et al. / Procedia Structural Integrity 68 (2025) 681–686

684

4

Y. Tampa et al. / Structural Integrity Procedia 00 (2025) 000–000

3.2. Stress–strain curves and fracture behaviour of poly-crystalline specimens Figure 5 shows the nominal stress–nominal strain ( ε S ) curves obtained for polycrystalline specimens. Note that the apparent strain ε S was determined using the stroke of the loading stage. Therefore, the slopes of σ – ε S curves are somewhat small at the initial linear regime. The FC-O and FCV-O specimens showed some strain hardening after the onset of yielding at 171 MPa and 215 MPa, respectively. Then, both specimens fractured at the ultimate tensile stresses of 290 MPa for FC-O and 530 MPa for FCV-O. In contrast, the FCV-D specimen showed moderate strain hardening after the onset of yielding at 400 MPa, and the ultimate tensile stress was ~740 MPa under plastic instability conditions, i.e. according to the Considère criterion. The addition of V increased the yield stress and tensile strength. For the FCV specimens, disordering further increased these strength values. Figure 6 shows the deformation and fracture morphologies of millimitre-scale poly-crystalline specimens. In the FC-O specimen, surface undulation appeared for each grain after the onset of yielding and subsequently brittle cracking occurred at the middle of the specimen (Figs. 6a and 6b). Fractographic observation of FC-O reveales mixed intergranular and clevage fracture features (Fig. 6c). The FCV-O specimen exhibited more significant surface relief than the FC-O specimen, and was fractured at the shoulder of

Fig. 5 Nominal stress–nominal strain (ε S ) curves obtained for millimetre-scale polycrystalline specimens. ε S was determined from the stroke in the piezoelectric actuator.

Fig. 6 Optical micrographs captured in the middle of tensile processes (a, d and g), SEM images showing the deformation morphology with the crack path after failure (b, e and h), and the fracture morphology (c, f and i) in the millimetre-scale polycrystalline specimens.