Tetsumi Yuri

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.

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.

Internal crack initiation in high cycle fatigue at cryogenic temperatures

Abstract High cycle fatigue properties at cryogenic temperatures have been investigated for various structural materials like titanium alloys, high manganese steels, and austenitic stainless steels. Some of the test materials show internal crack initiation which is not associated with pores or inclusions. The internal crack initiation occurs in low peak stress tests, although the surface initiation occurs in high peak stress tests. This internal initiation, however, has microstructural origins as follows: a duplex structure phase with low Al content for a Ti-5Al-2.5Sn alloy, a grain for Ti-6Al-4V alloys, Mo-rich zone for a cold-rolled SUS316LN steel, and grain boundary crack for 0.1N-32Mn-7Cr steels. In Ti-6Al-4V alloys, the size of initiation site depends on the peak stress. Transmission electron microscopy demonstrates heterogeneous dislocation structure in fatigued specimens with internal cracks for the Ti-5Al-2.5Sn alloy and the 32Mn-7Cr steel. One possible mechanism of internal initiation is microcracking in weak structures by stress or strain concentration due to localized deformation.

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.

Tensile Properties, Ferrite Contents, and Specimen Heating of Stainless Steels in Cryogenic Gas Tests

We performed tensile tests at cryogenic temperatures below 77 K and in helium gas environment for SUS 304L and SUS 316L in order to obtain basic data of mechanical properties of the materials for liquid hydrogen tank service. We evaluate tensile curves, tensile properties, ferrite contents, mode of deformation and/or fracture, and specimen heating during the testing at 4 to 77 K. For both SUS 304L and 316L, tensile strength shows a small peak around 10 K, and specimen heating decreases above 30 K. The volume fraction of α‐phase increases continuously up to 70 % with plastic strain, at approximately 15 % plastic strain for 304L and up to 35 % for 316L. There was almost no clear influence of testing temperature on strain‐induced martensitic transformation at the cryogenic temperatures.

High-cycle fatigue properties at cryogenic temperatures in forged- and rolled-Ti–5% Al–2.5% Sn ELI alloys

Abstract High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti–5% Al–2.5% Sn ELI alloys, in which mean alpha grain sizes were about 30 mm in the rolled material and 80 mm in the forged material. The ultimate tensile strengths of both materials were almost same and increased with decreasing temperature. The fatigue strength of each material also tended to increase with decreasing temperature. At 293 K, the fatigue strength of each material was almost equivalent. At 4 and 77 K, however, the fatigue strength of the rolled material was higher than that of the forged material. Concerning the rolled material, the fatigue strengths at 106 cycles at 4 and 77 K were about 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the forged material, it should be noted that the fatigue strengths in longer-life region (over 106 cycles) were almost equivalent not depending on test temperatures. Fatigue cracks initiated in the specimen interior independently of test temperatures and materials (we call this type of crack initiation ‘sub-surface crack initiation’) and formed facet-like structures at the sub-surface crack initiation sites at 4 and 77 K. The size of each facet-like structure corresponded closely to the grain size itself. The sizes of crack initiation sites were smaller in the rolled material than in the forged material. Since sub-surface cracks, which form facets or crack initiation sites, are supposed to act as defects, it is concluded that grain refinement leads to reduce the size of crack initiation site and this contributes effectively to improve the fatigue strength in high-cycle region at cryogenic temperatures.