Near-threshold fatigue crack growth tests on SUS 316L stainless steel were conducted at liquid helium temperature (4K) and a reference temperature of 300K. The crack length and the crack closure point in CT-specimens in a cryostat were automatically calculated by a 16-bit computer system based on the compliance theory. The initial Δ
K(Δ
Ko) and the normalized Δ
K gradient during Δ
K-decreasing were varied on reference to the ASTM E647 standard. Constant Δ
P tests, or Δ
K-increasing tests, were also conducted to find the practical and appropriate Δ
K-decreasing rule. It was found possible to use higher Δ
Ko and smaller increment of crack length (Δ
a) in the 4K tests. However, the tests at 300K raised some problems concerning the ASTM standard.
Through the Δ
K-range tested, two curves of the
da/
dn vs. Δ
K relation at
R=0.4 and 0.7 lied almost parallel at both temperatures of 4 and 300K, but they showed a complex temperature dependence. With a decrease of temperature from 300 to 4K, the Δ
Kth, corresponding to
da/
dn=10
-7mm/cycle, increased from 3.4MPa√m to 6.5MPa√m at
R=0.4, and did from 2.8 MPa√m to 5.5MPa√m at
R=0.7. By taking the crack closure effect into consideration, the
da/
dn vs. Δ
Keff relation was also discussed. These values of Δ
Kth and Δ
Keff, th for SUS 316L were compared with those of other 300-series ausenitic stainless steels.
SEM fractographs and micrographs of cross-sectional view indicated that the fracture surface appearing at 4K was much smoother than that at 300K.
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