Issue 48

K. Okuda et alii, Frattura ed Integrità Strutturale, 48 (2019) 125-134; DOI: 10.3221/IGF-ESIS.48.15 127 by hammer peening [7] or shot peening [8] as methods to improve fatigue strength. However, in recent years, new UHTS with different microstructures are developed to improve fatigue strength of UHTS by microstructural control. Thus, further studies are required to evaluate the fatigue properties of UHTS welded joints with different microstructures. In this study, fatigue tests and crack observations have been performed to evaluate the fatigue properties of UHTSs and ultimately improve the fatigue properties of steels. In order to improve the fatigue characteristics of UHTS plates with different tensile strengths and microstructures were evaluated using flat specimens with notches simulating the stress concentration factor at the toe of the overlap fillet joint. E XPERIMENTAL PROCEDURE Materials and specimens urface notch specimens, which simulate lap fillet joint, and smooth specimens were used in this study. Current material (590 MPa class ferrite-bainitic steel), and 3 types of UHTS (980 MPa class bainitic steel, 980 MPa class martensitic steel, and 980 MPa class precipitation hardening steel) are prepared for the fatigue test. Fig. 2 shows microstructures of 4 types of steels. Bainitic steel has low ratio of martensitic phase inside bainitic based steel, while martensitic steel consists of full martensitic phase. Precipitation hardened steel of this study was a precipitate of nano-sized particles in full-ferrite steel. The mechanical properties of the tested steels are listed in Tab. 1. Steels Tensile Strength [MPa] Yield Stress [MPa] Elongation [%] Vickers Hardness [HV 1kg] 590 MPa class steel 610 464 26.0 192 Bainitic steel 988 860 15.0 307 Martensitic steel 1095 1008 11.1 356 Precipitation hardened steel 1059 1014 17.6 333 Table 1 : Mechanical properties of four types of steel. Figure 2 : Microstructures of 4 types of steels (a) 590 MPa class steel, (b) bainitic steel, (c) martensitic steel, (d) and (e) precipitation hardened steel [9]. Fig. 3 shows the sizes of fatigue test specimens. The notch depth is 1 mm, and the bottom of notch was processed with a curvature radius of 0.1 mm or 0.2 mm. The stress concentration factor of lap fillet joint in suspension frame, shown in Fig. S