PSI - Issue 2_A

Hide-aki Nishikawa et al. / Procedia Structural Integrity 2 (2016) 3002–3009 Author name / Structural Integrity Procedia 00 (2016) 000–000

3009

8

1.E-06 1 -6

1.E-06 10 -6

1.E-06 10 -6

a

Material A (0.07C, 1 ℃ /s)

b

Material B (0.07C, 20 ℃ /s)

c

Material C (0.15C, 30 ℃ /s)

Typical long crack data (SM50B welding, NRIM (1980))

Typical long crack data (SM50B welding, NRIM (1980))

Typical long crack data (SM50B welding, NRIM (1980))

1.E-10 Fatigue crack growth rate d a /d N (m/cycle) 1 -8 1 -9 10 -10 1.E-09 1.E-08 1.E-07 1 -7

1.E-07 10 -7

1.E-09 Fatigue crack growth rate d a /d N (m/cycle) 10 -7 10 -8 10 -9 1.E-08 1.E-07

1.E-10 Fatigue crack growth rate d a /d N (m/cycle) 10 -8 10 -9 10 -10 1.E-09 1.E-08

l

l

Crack coalescence

l

a

a

a= 0.5 l

a= 0.5 l

σ a [MPa] 0.2 ≲ l ≲ 1 mm σ m [MPa]

σ a [MPa] 0.2 ≲ l ≲ 1 mm σ m [MPa]

a

a= 0.5 l

σ a [MPa] 0.2 ≲ l ≲ 1 mm σ m [MPa]

340 320 280

0 0

280 240 210

0 0

0 280 120 280 120

0 240 100 210 130

480 420

0

0 400 300

10 -10

1.E-10

1

10

100

1

10

100

1

10

100

∆ K eff (MPa √ m)

∆ K eff (MPa √ m)

∆ K eff (MPa √ m)

Fig. 9. Small fatigue crack growth rate versus ∆ K eff evaluated with DIC technique.

4. Summary

In this study, to clarify the effect of tensile mean stress and HAZ micro structure on the small fatigue crack growth behavior, uniaxial fatigue testing with small fatigue crack growth observation were carried out for low carbon steel with several simulated HAZ heat treatment under some tensile mean stress conditions. As a results, it is clarified that ε n a type small crack growth law is effective even for HAZ microstructure under zero mean stress. Furthermore it is clarified that small fatigue crack growth rate acceleration under tensile mean stress is able to be evaluated by proposed ε eq n a type modified small crack growth law which includes conventional Smith Watson-Topper (SWT) equivalent strain ε eq . In addition, it is clarified that DIC technique is effective to detect the crack opening point even for early stage small fatigue crack and ε eq is proportional to ε eff which is evaluated by crack opening point measured with DIC. It is considered that since SWT equivalent strain ε eq is implicitly includes small crack growth property and crack closure effect, ε eq is effective to evaluate small crack growth rate as a simple parameter. Acknowledgements This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Structural Materials for Innovation” (Funding agency: JST). References Smith K.N., Watson P., Topper T.H., 1970. A Stress-Strain Function for the Fatigue of Metals, Journal of Materials 5, 767–778. Murakami Y., 2002, Metal fatigue: effects of small defects and nonmetallic inclusions. Elsevier, UK. NRIM Fatigue Data Sheet No. 21, 1980. Data sheet on fatigue crack propagation for butt welded joints of sm50b rolled steel for welded structure. NRIM, Japan. Elber W., 1971. The Significance of Fatigue Crack Closure, Damage Tolerance in Aircraft Structures. ASTM STP 486, 230-242. Nishitani H., 1981. Unifying treatment of fatigue crack growth laws in small, large and non-propagating cracks. Mechanics of Fatigue-AMD 47, 151–166. Rabbolini S., Beretta S., Foletti S., Cristea M.E., 2015. Crack closure effects during low cycle fatigue propagation in line pipe steel: An analysis with digital image correlation. Engineering Fracture Mechanics 148, 441-456.