In order to investigate the brittle-striation formation mechanism of a low carbon steel JIS S10C fatigued in a hydrogen gas environment, fractographic observations of the visualized fracture phenomena during some processes of brittle-striation formation were conducted. The following results were obtained. A striation line is formed during the loading part of the cycle as a trace of blunting by slip. A stable ductile crack then starts growing. These processes are similar to those during the normal ductile fracture from a crack; that is, a ductile tearing process in tension. Based on the experimental results, a brittle-striation formation model, in which hydrogen only enhances the microscopic ductile tearing process just ahead of a crack tip, was proposed. The model rationally explains the peculiar load-frequency effect in the quasi-cleavage range on the fatigue crack growth which reveals a lower growth rate in spite of lowering the load-frequency.

This paper summarizes the NIMS fatigue data sheets and makes a new gateway available to access them. The NIMS fatigue data sheets are a huge database of the fatigue properties of structural materials. This project covers fundamental fatigue properties at room temperature and at high temperatures, and the fatigue properties of welded joints. The fundamental fatigue properties recorded include high-cycle, low-cycle and gigacycle fatigue test results for steels, aluminium alloys, titanium alloys and so on. The high-cycle fatigue test results determine the fatigue limits. The low-cycle fatigue test results reveal not only the fatigue lives but also cyclic stress-strain curves. The gigacycle fatigue tests were conducted at 100 Hz for three years up to 1010 cycles, as well as at 20 kHz for a week. The fatigue properties at high temperatures were evaluated chiefly for steels, via low and high-cycle fatigue tests. The low-cycle fatigue tests were conducted by employing various strain rates and waveforms. The fatigue properties of welded joints were evaluated using thick plates of steels and aluminium alloys, conducting high-cycle fatigue and crack propagation tests employing large specimens in as-welded condition. The high-cycle fatigue tests were conducted using various specimen sizes, welding procedures, stress ratios and so on. The crack propagation tests were conducted for the base metal, the weld metal and the heat-affected zone. Many new findings were obtained with these fatigue data as reviewed in this paper.

Observations by Transmission Electron Microscopy and Electron Backscatter Diffraction corresponding to Fractography were conducted in order to investigate the brittle-striation formation mechanism of low carbon steel JIS S10C in a hydrogen gas environment. The main results are as follows: (1) The quasi-cleavage (QC) facets plane with brittle-striations does not coincide with the (100) cleavage plane. (2) Slip deformation distributions reflecting the brittle-striation formation processes are observed by TEM. One of the conceivable brittle-striation formation mechanisms which can explain these results is as follows. A striation line is formed during the loading part of the cycle as a trace of blunting by slips. A stable ductile crack then starts growing. These processes are similar to those in the normal ductile fracture from a crack; that is, a ductile tearing process in tension.

To clarify the effect of microstructural changes on the fatigue property of the weld heat-affected zone (HAZ), low-to high-cycle fatigue tests were conducted on 16 types of simulated HAZ specimens that had been prepared using thermal processes. The results showed the fatigue S-N curves of the HAZ to be widely scattered as a function of strength level. These fatigue data were divided into two groups: coarse grain (CG) and fine grain (FG) HAZ, when strain amplitude was used to represent S-N curves. The fatigue data for the CGHAZ group showed a relatively short fatigue life. Based on surface observations, the initiated fatigue crack size of CGHAZ was larger than that of FGHAZ as a function of microstructural unit size. Hence, fatigue crack growth life, which is almost the same as total fatigue life of CGHAZ, decreased. K E Y W O R D Sfatigue, heat affected zone, low-carbon steel, welding

In Qrder to clarify the ！ oading − frequency effect oll the fatigue crack growth behavior of a low carbon ． steel JIS S ユ OC il 〕 hydrogen gas environment ， fullyreversed bending fatigue tests were carried Qut ． The main obtained results are as follows ． L ' ｝ ading − frequency e 貸ect on FCG reveals complex beh己 vlor ； that is ， not only acce ］ eratiorl but also deceleration e ＞ en 歪 n the same range of low I ｛ ，且 ding − frequency ， Slight acceleration appears in the bw grow 廿l rate range where し he ductile fracture mode ts predo憩 inant． Deceleration appears due to the transitiGn bchav め r from quasi − c ］ eavage fracture mode with a higher growth rate tQ ductlle one with a lower growth ra 亡 e ． This shows that bwering the load − frequency d｛＞es llot necessari 且 y lead dangereus fatigue crack grewth ．