Hisashi Hirukawa

Gigacycle fatigue data sheets for advanced engineering materials

Abstract Gigacycle fatigue data sheets have been published since 1997 by the National Institute for Materials Science. They cover several areas such as high-cycle-number fatigue for high-strength steels and titanium alloys, the fatigue of welded joints, and high-temperature fatigue for advanced ferritic heat-resistant steels. Some unique testing machines are used to run the tests up to an extremely high number of cycles such as 1010 cycles. A characteristic of gigacycle fatigue failure is that it is initiated inside smooth specimens; the fatigue strength decreases with increasing cycle number and the fatigue limit disappears, although ordinary fatigue failure initiates from the surface of a smooth specimen and a fatigue limit appears. For welded joints, fatigue failure initiates from the notch root of the weld, because a large amount of stress is concentrated at the weld toe. The fatigue strength of welded joints has been obtained for up to 108 cycles, which is an extremely high number of cycles for large welded joints. The project of producing gigacycle fatigue data sheets is still continuing and will take a few more years to complete. r 2007 Published by Elsevier Ltd.

Nanoscopic strength analysis of tempered-martensitic steels using atomic force microscopy and nanoindentation

The strength mechanisms of seven tempered-martensitic steels were analyzed on a nanoscopic scale using an atomic force microscope (AFM) and ultra-micro-hardness tester. The relationship between tensile strength and microstructural parameters such as carbide spacing and block width are discussed.

Arrest Function in Elevated-Temperature Fatigue Crack Growth of High Alloy with a Small Addition of Zirconium.

Arrest function in fatigue crack growth at 25 and 550°C in air was investigated for a alloy with zirconium in 0.17 Wt%, Fe-27Cr-35Ni-0.17Zr, and a alloy without zirconium, Fe-21Cr-32Ni. The main conclusions obtained are as follows. (1) The fatigue threshold under closure-free condition was about 40% higher for Fe-27Cr-35Ni-0.17Zr alloy than for Fe-21Cr-32Ni alloy at 550°C, while the threshold was independent of the alloys at 25°C. (2) The fatigue limit at 550°C was about 40% higher for Fe-27Cr-35Ni-0.17Zr alloy than for Fe-21Cr-32Ni alloy. (3) The improvement in high-temperature fatigue threshold were explained, considering that the presence of zirconium strengthened the oxide film at the crack tip. Accordingly, we concluded that a small addition of Zr could gives rise to an arrest function of fatigue crack growth at elevated temperatures.

Arrest Function of Elevated-Temperature Fatigue Short Crack Growth from Notch Root for Metallic Materials Containing Dispersed Y2O3 or Pb Particles.

An arrest function of fatigue short crack growth from notch root at 200 and 550°C in air was investigated for Fe-20Cr alloy containing 1.1 weight percent of dispersed Y2O3 particles and for SUS403 (Fe-12Cr) and SUS304 (Fe-18Cr-8Ni) stainless steels containing 0.2 weight percent of dispersed Pb particles. The main conclusions obtained are as follows. The fatigue Iimits at the high temperatures were 40∼60% higher for the alloy containing dispersed Y2O3 particles and the stainless steels containing dispersed Pb perticles than for their base metals. Above the fatigue limit, the fatigue life was longer for the materials containing dispersed Y2O3 or Pb particles than for their base metals, because the growth rate of a fatigue short crack, which was initiated from the notch root, was low. Those improvements in high-temperature fatigue properties were explained, considering that the presence of Y2O3 or Pb particles strengthened the oxide film at the notch root or at the crack tip of a short crack. Accordingly, we concluded that Y2O3 and Pb dispersed particles show an arrest function of fatigue short crack growth at elevated temperatures.

Arrest Function in Elevated-Temperature Fatigue Crack Propagation for Stainless Steels with Dispersed Lead Particles.

The arrest function in fatigue crack propagation at 25∼550°C in air was investigated, using SUS 304 (18 Cr-8 Ni) and SUS 403 (12 Cr) stainless steels containing 0.2 weight percent dispersed lead particles. The Pmax-constant ΔK-decreasing test, where the minimum load, Pmin, increased with increasing crack length while the maximum load, Pmax, remained constant, was employed in order to avoid crack closure. The main conclusions obtained are as follows : (1) Fatigue crack propagation properties including the threshold were independent of the material at room temperature under the closure-free condition. Fatigue threshold values at temperatures higher than the load melting point of 327°C were about 40% higher for lead-particle-dispersed steels than for host steels. (2) The increase in threshold level at elevated temperatures was explained, considering that the oxide layer spalled at the fatigue crack tip was repaired by the molten lead. Accordingly, we concluded that dispersed lead particles could show the arrest function of the fatigue crack propagation at elevated temperatures.

Observation of Elevated-Temperature Fatigue Surfaces for Stainless Steel with Dispersed Lead Particles by an AFM/STM Hybrid System.

Fatigue threshold for SUS304 stainless steel containing 0.2 weight percent dispersed lead particles was about 40% higher for the host steel at test temperatures above 200°C. This was explained by considering that dispersed lead particles could control or arrest the fatigue crack growth. In this study, using an AFM/STM hybrid system which enables one to obtain topographic and current images at the same time, fatigue surfaces for SUS304 steel with lead particles were examined to clarify the controlling mechanisms in the fatigue crack growth. Topographic and current images showed that the oxide layer spalled at the fatigue crack tip was repaired by the molten lead at test temperatures higher than 200°C. Accordingly, we concluded that dispersed lead particles controlled the fatigue crack growth at elevated temperatures.