Toshio Ogata

Effect of welding structure on high-cycle and low-cycle fatigue properties for MIG welded A5083 aluminum alloys at cryogenic temperatures

Abstract High-cycle and low-cycle fatigue properties of aluminum alloy A5083 base and A5183 weld metals and the effect of welding structure on their fatigue properties have been investigated at cryogenic temperatures in order to evaluate the long-life reliability and safety of the structural materials used in liquid hydrogen supertankers and storage tanks and to develop a welding process for these applications. In the high-cycle fatigue tests, the S–N curves of A5083 base and A5183 weld metals shifted to higher stress levels, i.e., the longer life side at lower test temperatures. The ratios of 10 6 -cycles fatigue strength (FS) to tensile strength (TS) for A5183 weld metals were slightly lower than those of A5083 base metals at each test temperature. Although the ratios of FS to TS for austenitic stainless steels weld metals at 4 K decreased substantially to about 0.4, that of A5183 weld metal was 0.65 even at 4 K and it indicated an excellent high-cycle fatigue property. Fatigue crack initiation sites in A5183 weld metals were occurred from the blowholes if the blowholes were located in the vicinity of the specimen surfaces. However, effects of the blowholes on high-cycle fatigue properties are not clear or significant. In the low-cycle fatigue tests, the fatigue lives of A5183 weld metals were slightly shorter than those of A5083 base metals at cryogenic temperatures. However, the fatigue lives of A5183 weld metals at 4 K were superior to that of conventional A5083 weld metals. The deterioration of low-cycle fatigue properties of A5183 weld metals at cryogenic temperatures were due to the intergranular fracture surface observed in fatigue crack propagation regions.

Effect of welding structure and δ-ferrite on fatigue properties for TIG welded austenitic stainless steels at cryogenic temperatures

Abstract High-cycle and low-cycle fatigue properties of base and weld metals for SUS304L and SUS316L and the effects of welding structure and δ-ferrite on fatigue properties were investigated at cryogenic temperatures in order to evaluate the long-life reliability of the structural materials to be used in liquid hydrogen supertankers and storage tanks and to develop a welding process for these applications. The S–N curves of the base and weld metals shifted towards higher levels, i.e., the longer life side, with decreasing test temperatures. High-cycle fatigue tests demonstrated the ratios of fatigue strength at 10 6 cycles to tensile strength of the weld metals to be 0.35–0.7, falling below those of base metals with decreasing test temperatures. Fatigue crack initiation sites in SUS304L weld metals were mostly at blowholes with diameters of 200–700 μm, and those of SUS316L weld metals were at weld pass interface boundaries. Low-cycle fatigue tests revealed the fatigue lives of the weld metals to be somewhat lower than those of the base metals. Although δ-ferrite reduces the toughness of austenitic stainless steels at cryogenic temperatures, the effects of δ-ferrite on high-cycle and low-cycle fatigue properties are not clear or significant.

Effect of grain size on high-cycle fatigue properties in alpha-type titanium alloy at cryogenic temperatures

Abstract High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti–5%Al–2.5%Sn ELI alloy which was used for liquid hydrogen turbo-pumps of Japanese-built launch vehicles. Mean grain size of specimens was controlled to be about 30 or 80 μm. In the specimens with a grain size of 30 μm, fatigue strengths at 10 6 cycles at 4 and 77 K are 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the specimen with a grain size of 80 μm, fatigue strengths at 10 6 cycles at 4 and 77 K get lower to the same level as that at 293 K. Thus, it is concluded that refinement of α grains is one of important factors to obtain the good high-cycle fatigue properties for Ti–5%Al–2.5%Sn ELI alloy at cryogenic temperature.

A simplified method for charpy impact testing near liquid helium temperature

Abstract A simplified method of examining the impact properties of materials below 5 K is proposed.

Effects of surface roughness and notch on fatigue properties for Ti?5Al?2.5Sn ELI alloy at cryogenic temperatures

Abstract To evaluate material risk caused by human-error, the effects of surface roughness and notch on the fatigue properties of Ti–5Al–2.5Sn ELI alloy have been investigated at cryogenic temperatures. Specimens with surface roughness changed by emery papers (Grade #600, #100) and notched specimens were prepared (Kt = 1.5; 3). The S–N curves shifted to higher stress level with a decrease of the test temperature.Regarding the effect of surface roughness, the fatigue strength of the #100-roughness specimens was a little lower than those of the #600-roughness specimens. Fatigue crack initiation sites of each surface roughness specimen at 4 K were found in the specimen interior (internal type fracture). On the other hand, the fatigue strength of the notched specimens was substantially lower than those of the surface roughness specimens. Although fatigue crack initiation sites of the Kt = 3 notched specimen were at the notch root (surface type fracture), those of the Kt = 1.5 notched specimen were in the specimen interior. The location of the fatigue crack initiation sites changed from the internal type fracture for Kt = 1.5 notched specimens to the surface type fracture for Kt = 3 notched specimens. Therefore, the Kt values of the internal fatigue crack initiation sites correspond between 1.5 and 3. The root area analysis, which is the size of the crack propagation plane as a shape parameter, the fatigue strength depends on the size (internal fatigue crack initiation site size). Fatigue properties of surface roughness and notched specimens at cryogenic temperatures were expected to be more improved when the grain size of the materials was minimized, i.e. fatigue crack initiation sites were minimized.

Discontinuous deformation during load- and displacement-controlled tensile tests and optical observation in liquid helium

Abstract The temperature rise accompanying discontinuous deformation during displacement-controlled and load-controlled tensile tests was measured for high manganese steel at liquid helium temperature. The deformation behaviour and heat transfer at the specimen surface in liquid helium were observed directly with a fibrescope. The relationship between the temperature rise and the boiling helium behaviour was examined. In spite of the large discontinuous deformation during the load-controlled test, the magnitude of the temperature rise during the deformation was not as great as that expected from the displacement-controlled test. It is evident that the change of helium flow and heat transfer at the specimen surface caused the abrupt and large deformation during the load-controlled test.

Temperature rise during the tensile test in superfluid helium

Abstract Measurements were made on the temperature rise of tensile test specimens undergoing plastic deformation and discontinuous flow in superfluid helium. Compared with the results in normal liquid helium, the frequency of load drop increased, and the temperature rise was suppressed.

Local Fracture Toughness Evaluation of 316LN Plate at Cryogenic Temperature

Within the frame work of the VAMAS TWA 17 activity, J evaluation on tensile test (JETT) was performed as an international round robin test. 316LN plate with 30 mm thickness was supplied as a common test material, but it contained irregular microstructure around the center of the plate thickness supposed to be generated during the rolling process. The fracture toughness at 4.2 K according to ASTM standard was invalid because of crack front shape requirement. However, JETT showed the lower toughness at the midsection than that at the near surface part. From this result, it was clarified that the JETT has a potential to evaluate local fracture toughness. Since the material showed serrated deformation during the JETT at cryogenic temperature, the evaluated toughness scattered rather widely, and stiffness of testing machine would affect the results.

VAMAS tests of structural materials at liquid helium temperature

A series of international, interlaboratory comparisons of tensile and fracture-toughness tests for prestandardization and refining of these test procedures at liquid-helium temperature using the same materials has been coordinated under the Versailles Project on Advanced Materials and Standards (VAMAS). High-strength stainless steels, YUS170 and SUS316LN, and a titanium alloy were used for this program. Future plans include testing an aluminum alloy and a composite material. Tensile and fracture-toughness properties were much less scattered under the limited testing conditions that were used in this program.

Effects of Gage Diameter and Strain Rate on Tensile Deformation Behavior of 32Mn-7Cr Steel at 4 K

The cryogenic deformation behavior under uniaxial tensile stress was investigated for 32Mn-7Cr high manganese steel using a diametral extensometer. The tensile deformation and serration behavior were observed for varying gage diameters and strain rates (< 5 × 10-4s-1). A significant effect of gage diameter and strain rate on strengths and total elongation was not seen at 4 K. As the gage diameter became smaller or the strain rate lower, the strain to initiate serration decreased. The geometrical changes in the specimen during serrated yielding are also discussed in relation to the internal specimen heating.