Abstract
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 ΔK(ΔKo) and the normalized ΔK gradient during ΔK-decreasing were varied on reference to the ASTM E647 standard. Constant ΔP tests, or ΔK-increasing tests, were also conducted to find the practical and appropriate ΔK-decreasing rule. It was found possible to use higher ΔKo and smaller increment of crack length (Δa) in the 4K tests. However, the tests at 300K raised some problems concerning the ASTM standard.Through the ΔK-range tested, two curves of the da/dn vs. ΔK 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 ΔKth, 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. ΔKeff relation was also discussed. These values of ΔKth and ΔKeff, 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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