Issue 35

Takamasa Abe et alii, Frattura ed Integrità Strutturale, 35 (2016) 196-205; DOI: 10.3221/IGF-ESIS.35.23 201 Figs. 12 and 13 show different sections of the same test piece. The test was stopped after 8×10 3 cycles (the longest life N f at this load amplitude was 1.6×10 5 cycles ( N / N f =5%)). As in the previous macro observations, the fatigue crack in Fig. 12 initiates at the tip of the unwelded portion and propagates through the welding material. As already mentioned, fatigue crack propagates at some angle from the vertical load axis. However, tiny cracks appearing immediately after the main crack propagate along the vertical load axis. Large welding defects near the unwelded portion, (Fig. 13) exert much less influence on fatigue crack initiation behavior than small defects, because the fatigue crack begins from the root tip of the unwelded portion. Figure 12 : Crack propagation from tip of root region. Figure 13 : Crack propagation from tip of root region with welding defect. Crack propagation behavior In this subsection, we investigate how the fatigue crack propagates into the welding material. Crack propagation behavior from the tip of the unwelded portion is difficult to observe in one-side welded joints such as the present test piece. Thus, we observe fatigue crack by a special three-dimensional observation method, which provides a detailed picture of the fatigue crack propagating into the welding material. The observation method is described below. First we estimate the fracture lifetime N f of the test piece, then run a fatigue test up to x% of the estimated lifetime cycles. Next, we grind one side of the test piece, etch it, and acquire images under a light microscope. The surface is ground to 300 – 500μm in the welding direction, and welding photographs are taken from the side. By repeating this process many times in the welding direction, we compile the images into three-dimensional pictures using three-dimensional construction software. Observations were performed under a low and high test force ( F a=6kN and F a=9kN, respectively), and a three-dimensional picture was compiled in each case. The fracture life N f under each test force was assumed as the longest lifetime obtained in the fatigue test. ( N f =1.1×10 6 and 1.6×10 5 cycles for F a=6 and 9kN, respectively). The three- dimensional pictures compiled from repeated grinding, etching and observation under 6 and 9kN loads are shown in Figs. 14 and 15, respectively. Both figures show the crack propagation at three stages of the fracture lifetime: 5, 25, and 50% N f . The figures are scaled such that the propagating direction is 30 times larger than the width direction (welding direction) of the test piece. We first investigated the crack aspect at 5% of the estimated fracture lifetime (panel (a) in Figs. 14 and 15). At both force amplitudes, there are cracks extending 0.01 - 0.3 mm across the width of the sample (50mm). This indicates that cracks initiate at a very early stage of the estimated rupture life N f . It is suggested that the lifetime of crack propagation chiefly determines the fatigue lifetime, and that fatigue crack propagation is very relevant when evaluating fatigue damage in fracture mechanics experiments. At 25% of the estimated fracture lifetime, fatigue damage has progressed (panel (b) of Figs. 14 and 15), and the early stage cracks have propagated into the welding material. The leading edge of the crack is straight and lacks any loose semi elliptical forms, but exhibits protuberance into the crack propagation direction, developing complex overlapping convexities. At 50% of the fracture lifetime, more fatigue damage is evident (panel (c) of 100μm 100μm