Hideshi Sumiyoshi

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.

Variation of fractographic appearance for different microstructures in welded joints having the same fatigue crack propagation properties

Abstract Fatigue fracture surfaces were examined with a scanning electron microscope to investigate the influence of the different microstructure between weld metal and heat affected zone. The specimens were centre-cracked type transverse butt welded joints. The relationship between macroscopic fatigue crack propagation rate and the stress intensity factor range is the same in spite of the difference in microstructure for both materials. It is shown that the fractographic appearance changes with microstructure even in the very low growth rate region near fatigue threshold. This suggests that fractographic appearance is not necessarily a guide to the rate of fatigue crack growth.

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.

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...

High-Cycle Fatigue Properties at Cryogenic Temperatures in INCONEL 718

High‐cycle fatigue properties at 4 K, 20 K, 77 K and 293 K were investigated in forged‐INCONEL 718 nickel‐based superalloy with a mean gamma (γ) grain size of 25 μm. In the present material, plate‐like delta phase precipitated at γ grain boundaries and niobium (Nb)‐enriched MC type carbides precipitated coarsely throughout the specimens. The 0.2% proof stress and the tensile strength of this alloy increased with decreasing temperature, without decreasing elongation or reduction of area. High‐cycle fatigue strengths also increased with decreasing temperature although the fatigue limit at each temperature didn’t appear even around 107 cycles. Fatigue cracks initiated near the specimen surface and formed faceted structures around crack initiation sites. Fatigue cracks predominantly initiated from coarse Nb‐enriched carbides and faceted structures mainly corresponded to these carbides. In lower stress amplitude tests, however, facets were formed through transgranular crack initiation and growth. These kinds o...

Effect of hydrogen in steel on fatigue fracture for carburized steel.

浸炭焼入れ鋼の疲労破壊におよぼす鋼中水素の影響を検討するため, SCM 420鋼に浸炭後水素チャージした後, 回転曲げ疲労試験を行い, 疲労特性, 破壊機構を調べた. 浸炭材は水素チャージすると, 疲労強度は約50%低下し, 破壊様式も浸炭まま材と異なるが, 水素を拡散させて試験片表面から放出させると疲労強度の回復が生じることから, 浸炭焼入れ行程で導入される水素の悪影響はないことがわかった.