Issue 35

T. Abe et alii, Frattura ed Integrità Strutturale, 35 (2016) 424-433; DOI: 10.3221/IGF-ESIS.35.48 426 strain remained below 50με (the position of the strain gauge is shown in Fig. 3). The fatigue test was performed at room temperature, under the following load conditions: frequency f =20Hz, and load ratio R F (= F min / F max )=0.05. Fracture was defined as the time of complete separation of the welded joint. The fatigue test was conducted through N =10 7 cycles. Fillet welded 10 Strain gauge 4 5 Strain gauge 3 Strain gauge 2 Strain gauge 1 5 Figure 3 : Position of the strain gauge near fillet weld. 1mm (a) ○ △ □ 500μm (b) (c) (d) Figure 4 : Microstructure observation of cross section. Figure 5 : Microstructure observation at the material. Detail (a). (b) 100μm (c) 100μm (d) 100μm Figure 6 : Microstructure observation at the material in detail. R ESULTS Metallographic structure observation and hardness test igs. 4-6 show the microstructure of the one-side filet weld zone. Three structural categories can be recognized; welding material structure, a heat affected zone, and the base material. Observations of each area reveal that the metallographic structure of the welding material had melted and solidified into a dendritic structure, and the base material has a ferrite structure. Closer to the welding metal zone, the crystal grain of the base material enlarges. The hardness results of the one-sided filet welding are plotted in Fig. 7. The hardness was measured along the three horizontal broken lines shown in Fig. 4. The origin of the horizontal axis in Fig. 7 is the vertical line in Fig. 4, which separates the welding material (negative side) from the base material (positive side). According to Fig. 7, the welding material is 1.2 times harder than the base material. The welding material hardened because martensite was generated by the cooling process. Focusing on the heat affected zone, we observe that the hardness slowly decreases as the base material approaches the welding material. F