PSI - Issue 19

Yukio Miyashita et al. / Procedia Structural Integrity 19 (2019) 604–609 Author name / Structural Integrity Procedia 00 (2019) 000–000

606

3

Figure 2(b) shows a specimen used in fatigue crack growth test. A notch with total length of 3.0 mm was machined at the centre of a specimen in fatigue crack growth test. In the present fatigue crack growth test in a weld specimen, a notch was machined at weld part assuming that a fatigue crack initiated from a weld defect. Fatigue pre-crack with length about 0.5 mm was introduced from each notch tip by applying cyclic loading with the same loading condition in fatigue crack growth test. Cyclic loading with sinusoidal wave form and stress ratio of 0.1 at 20-25Hz of frequency were applied in fatigue crack growth test. Stress intensity factor range, Δ K was calculated by equation (1) (Y. Murakami (Ed.), 1987). Where Δσ is applied stress range, W is width of a specimen, 2 a is total crack length, and α =2 a / W . Threshold stress intensity factor range, Δ K th was determined by carrying out Δ K -decreasing test. Threshold stress intensity factor range, Δ K th was defined when a crack was not propagated after applying fatigue loading by 10 6 cycles at certain applied stress. Stress intensity factor, Δ K increasing test was then conducted to complete fatigue crack growth curve. A strain gage was attached nearby the crack tip to observe crack closure behaviour. Crack opening point was determined by using stress-strain curve measured during fatigue crack growth test, and effective stress intensity factor range, Δ K eff was evaluated. Testing environment of fatigue crack growth test was laboratory air. 3. Result and discussion Fatigue strength tests were carried out with TIG weld specimens welded in different institutes, Institute A and B. Appropriate welding condition was found out in each institute and resulted in that welding condition applied was welding current of 110A with welding speed of 200 mm/min in Institute A, and welding current of 110A with welding speed of 190 mm/min in Institute B. Weld materials prepared in Institute A and B are called SA and SB respectively in the followings. Shield gas of Ar was applied with flow rate of 15 l/min during TIG welding. (1)

(b)

(a)

Fig.2 Geometry of specimens for (a) fatigue strength test and (b) fatigue crack growth test (in mm).

140

120

SA SB

100

40 Stress amplitude σ a ,MPa 60 80

20

10 2

10 3

10 4

10 5

10 6

10 7

10 8

Number of cycles to failure N f , cycles

Fig.3 Result of fatigue strength test in weld specimens, SA and SB.