Yoshinori Ono

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.

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.

Notch Effects on High‐Cycle Fatigue Properties of Titanium Alloy at Cryogenic Temperatures

To evaluate material risk caused by human‐error, the notch effects on the fatigue properties of forged Ti‐5A1‐2.5Sn ELI alloy have been investigated at cryogenic temperatures. Smooth and notched specimens with the Kt=1.5, 2 and 3 were prepared. High‐cycle fatigue tests were carried out at 4, 77 and 293 K. The S‐N curves of each specimen shifted to higher stress level with a decrease of the test temperature. At 4 K, the fatigue strength of the Kt=3 specimens were substantially lower than those of the smooth specimens. Although the notch effects on the fatigue properties of the Kt=1.5 and the Kt=2 notched specimens are not clear or significant. Fatigue crack initiation sites of the smooth and the Kt=1.5 notched specimens were in the specimen interior (internal type fracture) and those of the Kt=2 and the Kt=3 notched specimens were at the notch root (surface type fracture). The location of the fatigue crack initiation sites showed transition from the internal type fracture for the Kt=1.5 notched specimens t...

Analyses of heterogeneous deformation and subsurface fatigue crack generation in alpha titanium alloy at low temperature

Subsurface crack initiation in high-cycle fatigue has been detected as {0001} transgranular facet in titanium alloys at low temperature. The discussion on the subsurface crack generation was reviewed. Analyses by neutron diffraction and full constraints model under tension mode as well as crystallographic identification of the facet were focused. The accumulated tensile stress along may be responsible to initial microcracking on {0001} and the crack opening.

High-cycle fatigue behavior of Ti-5Al-2.5Sn ELI alloy forging at low temperatures

High-cycle fatigue properties of Ti-5Al-2.5Sn Extra Low Interstitial (ELI) alloy forging were investigated at low temperatures. The high-cycle fatigue strength at low temperatures of this alloy was relatively low compared with that at ambient temperature. The crystallographic orientation of a facet formed at a fatigue crack initiation site was determined by electron backscatter diffraction (EBSD) method in scanning electron microscope (SEM) to understand the fatigue crack initiation mechanism and discuss on the low fatigue strength at low temperature. Furthermore, in terms of the practical use of this alloy, the effect of the stress ratio (or mean stress) on the high-cycle fatigue properties was evaluated using the modified Goodman diagram.

Data Sheet Program and Mechanical Properties of Ti-5Al-2.5Sn ELI and Alloy 718 at Cryogenic Temperatures

In the development of Japan’s self‐developed H‐IIA launch vehicle, it is important to sufficiently comprehend the properties of materials under conditions in which the materials are used in the system for its design and the improvement of its reliability. Through the process of failure analysis of the LE‐7 engine of H‐II No. 8 in 1999, detailed materials data and photographs of the fracture surface were required as reference data to determine in terms of fracture morphology and to analyze the fracture stress. A series of mechanical properties tests, such as tensile tests, impact tests, fracture toughness tests, and fatigue tests, on Ti‐5Al‐2.5Sn ELI and Alloy 718 at room temperature to 4K were mainly conducted by NIMS and NASDA. The obtained tensile and fracture toughness properties were a little bit smaller than those reported by NASA and NRIM, however, the fatigue properties were relatively lower than the data reported so far. Data resulting from the tests were reviewed in detail and published in the fo...

Effect of microstructure on high-cycle fatigue properties of Alloy718 plates

Effect of microstructure on high-cycle fatigue properties of Alloy718 were investigated at 77 K by using samples with three different microstructures; fine-grained (FG), coarse-grained (CG) and bimodal-grained (BG) ones. The BG sample consisted of FG and CG microstructural regions and grain sizes of those regions were close to those of the FG and the CG samples, respectively. High-cycle fatigue strength of the FG sample was higher than that of the CG sample. High-cycle fatigue strength of the BG sample was clearly lower than that of the FG sample and almost the same as that of the CG one. Flat area (facet) was found at fatigue crack initiation site in all specimens. Facet size was similar to the grain size and found to be almost same in the CG and the BG samples. Observations of the microstructure beneath the fatigue crack initiation site of the BG sample revealed that the facet corresponds to transgranular cracking in the course grain, meaning that fatigue crack initiated at the coarse grain in the BG sample. It is deduced that the high-cycle fatigue strength of Alloy 718 with the BG microstructure is strongly affected by that of the CG region in that material.

Effect of stress ratio on high-cycle fatigue properties of Ti-6Al-4V ELI alloy forging at low temperature

The effect of the stress ratio R (the ratio of minimum stress to maximum stress) on the high-cycle fatigue properties of Ti-6Al-4V extra-low interstitial (ELI) alloy forging was investigated at 293 and 77 K. At 293 K, the fatigue strength at 107 cycles exhibited deviations below the modified Goodman line in the R=0.01 and 0.5 tests. Moreover, at 77 K, larger deviations of the fatigue strength at 107 cycles below the modified Goodman line were confirmed in the same stress ratio conditions. The high-cycle fatigue strength of the present alloy forging exhibit an anomalous mean stress dependency at both temperatures and this dependency becomes remarkable at low temperature.