Issue 35

Takamasa Abe et alii, Frattura ed Integrità Strutturale, 35 (2016) 196-205; DOI: 10.3221/IGF-ESIS.35.23 199 process. Focusing on the heat affected zone, we observe that the hardness slowly decreases as the base material approaches the welding material. Distance from the front of the tip of root region (mm) Fillet welded area Hardness (Hv) 0 50 100 150 200 250 -6 -5 -4 -3 -2 -1 0 1 2 3 Base material Figure 7 : Hardness distribution from the edge of fillet weld. Fatigue testing results The fatigue test results are plotted in Fig. 8. The vertical and horizontal axes indicate the amplitude of the force applied to the test piece, and the number of cycles to failure, respectively. Apart from F a=7kN, which exhibits widely spread lifetimes, the dispersion of the S-N curve is small. At F a=5kN, the lifetime extends to 10 7 cycles. Number of cycles to failure N f (cycle) Force amplitude F a (kN) 4 4.5 5 5.5 6 10 4 10 5 10 6 10 7 6.5 7 7.5 8 8.5 9 9.5 10 Figure 8 : F a -N curve. Figure 9 : Location of the root region and fracture aspect. Macroscopic fracture morphology and fractographic study result We observed the macroscopic fracture morphology of all fractured test pieces. A representative sample is presented in Fig. 9. This piece was tested at F a=6kN and observed from the side. The crack was initiated in the unwelded portion and propagated under the welding material at a small angle from the load perpendicular, eventually leading to breakage. The macroscopic fracture morphology and the angle of crack propagation are independent of load under load amplitude. Fig. 10 shows the macroscopic morphology viewed vertically to the load under load amplitudes of F a=9, 8, 7, and 6kN. The number of cycles to failure is stated above each image. Many welding defects of various sizes appear in the test pieces. However, the sizes and numbers of the welding defects are unrelated to the number of cycles to failure. Later, we will also demonstrate that crack initiation and propagation is insensitive to the welding defects around the unwelded portion. The details of fracture were examined under a scanning electron microscope (SEM) (Hitachi, Ltd S-3000N). Fig. 11 shows representative SEM images of fractures under a load amplitude of 6kN. Typical fatigue fractures spread over a wide area (panel (b) of Fig. 11), and the ductile fracture extends to 5mm from the tip of the unwelded portion (panel (d)). According to the macroscopic and microscopic observations, cracks are initiated at the tip of the welding defect, and propagate 5mm into the welding material leading to final fracture.