Shigeru Hamada

Tensile properties of precracked tempered martensitic steel specimens tested at ultralow strain rates in high-pressure hydrogen atmosphere

Tensile tests were performed on precracked Cr–Mo martensitic steel (C: 0.38, Si: 0.22, Mn: 0.84, P: 0.024, S: 0.021, Ni: 0.08, Cr: 1.11, Mo: 0.15, Cu: 0.12, Fe: bal. (wt%)) specimens at various strain rates (ranging from 6.5 × 10−8 s−1 to 1.0 × 10−4 s−1) in high-pressure (95 MPa) hydrogen and helium atmospheres. Irrespective of the strain rate, the tensile strength in the helium atmosphere was 1400 MPa. In the hydrogen atmosphere, the tensile strength decreased to less than 600 MPa at a strain rate of 2.0 × 10−5 s−1. However, the tensile strength increased to 900 MPa when the strain rate was decreased to 6.5 × 10−8 s−1. This recovery of the tensile strength was because of the decrease in the local stress in the vicinity of the precrack because of hydrogen.

Influence of RTA parameters on residual stress and stress gradient of multilayered LPCVD polysilicon film

This paper reports the experimental results of examining the residual stress and stress gradient of LPCVD multilayered polysilicon film according to various rapid thermal annealing (RTA) in a nitrogen atmosphere. In particular the stress gradient of multilayered polysilicon film ranging from -17.1 to +1.5 MPa//spl mu/m as the RTA processing time increased could be reduced to nearly zero by selecting the appropriate RTA time. The mechanism responsible for this dependence was examined by using both experimental data and material analysis using secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM). It is concluded that the shift of the stress gradient is mainly caused by the thin nitrided layers at surface and interface of multilayered polysilicon film.

Fatigue-life and leak-before-break assessments of CR-MO steel pressure vessels with high-pressure gaseous hydrogen

Pressure cycle tests were performed on two types of Cr-Mo steel pressure vessels with inner diameters of 306 mm and 210 mm and notches machined on their inside under hydrogen-gas pressures, varied between 0.6 and 45 MPa at room temperature. One of the Cr-Mo steels had a fine microstructure with tensile strength of 828 MPa, while the other had a coarse microstructure with tensile strength of 947 MPa. Fatigue-crack growth (FCG) and fracture-toughness tests of the Cr-Mo steels were also carried out in gaseous hydrogen. The Cr-Mo steels showed accelerated FCG rates in gaseous hydrogen compared to ambient air with an upper bound corresponding to an approximately 30-times higher FCG rate. Furthermore, in gaseous hydrogen, the fracture toughness of the Cr-Mo steel with coarse microstructure was significantly smaller than that of the steel with fine microstructure. Four pressure vessels were tested; then, all of the pressure vessels failed by leak-before-break (LBB). Based on the fracture-mechanics approach, the LBB failure of one pressure vessel could not be estimated by using the fracture toughness in gaseous hydrogen. The fatigue lives could be estimated by using the upper bound of the accelerated FCG rates in gaseous hydrogen.Copyright

Proposal for an engineering definition of a fatigue crack initiation unit for evaluating the fatigue limit on the basis of crystallographic analysis of pearlitic steel

In this study, in order to define a fatigue crack initiation unit, the relationship between the fatigue crack initiation process and the crystal structure in the pearlitic steel used for railroad rails was examined and fatigue tests, focusing on crack initiation, were performed. The fracture surfaces were analyzed using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The crystal structure of the pearlitic steel is composed of “pearlitic colonies” that have the same lamellar structure direction and “pearlitic blocks” that have the same ferrite crystal direction. The SEM and EBSD results revealed that the crack initiation depends on the pearlitic colonies. Therefore, we defined the characteristic dimension for fatigue crack initiation as the pearlitic colony size. However, for safety purposes, the pearlitic block size should be considered the engineering definition of the fatigue crack initiation unit, since decreasing the pearlitic block size should cause an improvement in the fatigue limit of pearlitic steel.

Evaluation Method for Mean Stress Effect on Fatigue Limit of Non-Combustible Mg Alloy

Structures are commonly loaded with a mean stress. Therefore, it is necessary to investigate the influence of the mean stress on the fatigue strength of the structural materials. Fatigue strength of a material, whose fatigue crack is initiated from an inclusion, strongly depends on the size of the inclusion. The fatigue strength of the material is quite variable. Therefore, if we test the effect of the mean stress on the fatigue strength of this material, the effect would be hidden in the scatter and the effect becomes uncertain. In this study, we propose an evaluation method for the mean stress effect of the inclusion-induced scattered fatigue strength using the non-combustible Mg alloy AMX602B (X=Ca) (Sakamoto et al., 1997; Chang et al., 1998; Akiyama et al., 2000). We discuss the equivalence of an artificial defect and an actual defect (inclusion). Figure 1 shows the S-N diagram for the smooth specimens of the non-combustible Mg alloy AMX602B (X=Ca) by the authors (Kitahara et al., 2005; Kitahara et al., 2006; Fujii et al., 2008; Masaki et al., 2008). The relationship between the load stress and the fatigue life of the smooth specimens significantly varies. Moreover, no non-propagating crack was observed in the unbroken specimens. The condition of the surface of the fatigue fracture origin is shown in Fig. 2. The fracture originated at a non-metallic inclusion. There have been several studies that investigated the effect of the mean stress on the fatigue strength of the conventional Mg alloy (Forrest, 1962; Heywood, 1962; Osgood, 1970; Ogarevic & Stephens, 1990; Akiyama et al., 2000). Forrest (Forrest, 1962) reported that the effect of the mean stress on the fatigue strength of a Mg alloy can be evaluated using the modified Goodman diagram. In contrast, Heywood (Heywood, 1962) and Osgood (Osgood, 1970) reported that the fatigue strength of a Mg alloy under a high mean stress became low and that the fatigue life evaluation using the modified Goodman diagram may not be conservative prediction. However, the reason why the fatigue strength under a high mean stress decreases has not been clarified. In this study, rotating-bending fatigue tests and tension-compression fatigue tests were carried out on specimens with an artificial defect (a small hole or a small crack). Especially, we examined why the fatigue strength under a high mean stress decreases and whether the fatigue strength at N = 107 under a mean stress can be applied to an estimation using the modified Goodman diagram. The fatigue testing of the small holed specimens and the small

Ductile-to-brittle transition in tensile failure due to shear-affected zone with a stress-concentration source: a comparative study on punched-plate tensile-failure characteristics of precipitation-hardened and dual-phase steels

The effect of shear-affected zone (SAZ), with a stress-concentration source induced by the punching process, on tensile properties was investigated. Tests using honed specimens (which have the same shapes and stress-concentration without any SAZ) and smooth specimens were conducted to compare the effect with that of the punched specimens. Dual-phase steel, which has a high work-hardening ability and low yield strength, and precipitation-hardened steel, which has a low work-hardening ability and high yield strength, were used in the tests. Materials with two tensile strength grades were prepared from both types of steel. Only the precipitation-hardened steel with higher strength grade punched specimen showed a brittle fracture with extremely short fracture-elongation, whereas the other specimens showed a ductile fracture. The fracture surface analysis revealed that cracks initiated in the maximum shear stress plane of the SAZ under tensile loading at first. We call the crack “shear crack.” The steel which showed brittle fracture used in this study easily exhibited plastic-strain localization compared with the other steels. If the shear crack is sharp, then the transition from ductile to brittle failure tends to occur. Furthermore, the strength characteristics of the punched specimen depend on the crack length dependency of the strength resistance and the failure phenomenon of the original material.

Tensile- and Fatigue-Properties of Low Alloy Steel JIS-SCM435 and Carbon Steel JIS-SM490B in 115 MPA Hydrogen Gas

Slow strain rate tests using smooth specimens of two types of steels, low alloy steel JIS-SCM435 and carbon steel JIS-SM490B, were carried out in nitrogen gas and hydrogen gas under a pressure of 115 MPa at three different temperatures: 233 K, room temperature and 393 K. In nitrogen gas, these steels exhibited the so-called cup-and-cone fracture at every temperature. On the other hand, in hydrogen gas, in both steels a number of cracks initiated on the specimen surface and coalesced with each other at every temperature, which led to a marked reduction in ductility. Nonetheless, even in hydrogen gas, JIS-SCM435 exhibited a certain reduction of area after the stress-displacement curve reached the tensile strength (TS), whereas JIS-SM490B exhibited little, if any, necking in hydrogen gas. In addition, tension-compression fatigue testing at room temperature revealed that in both steels there was no noticeable difference between the fatigue strengths in air and 115MPa hydrogen gas, especially in a relatively long life regime. Considering that there was little or no hydrogen-induced degradation in either TS or fatigue strength in JIS-SCM435, it is suggested that JIS-SCM435 is eligible for fatigue limit design on the basis of a safety factor (i.e. TS divided by the allowable design stress) for mechanical components used in hydrogen gas up to 115 MPa.Copyright

Effect of Hydrogen on the Tensile Properties of Metals

This chapter describes the effect of hydrogen on tensile properties of metals, showing that the hydrogen-assisted fracture is more enhanced for a material with higher tensile strength. It is also demonstrated that the relative reduction of area of austenitic stainless steels strongly correlates with nickel equivalent.

Effect of Friction Stir Welding Condition on Fatigue Limit of Welded Non-Combustible Mg Alloy

The atigue strength of Friction Stir Welding is affected by the structure and the welding defect size. The welding defect sizes have a scatter. Therfore, the effect of Friction Stir Welding condition on fatigue limit of welded noncombustible Mg alloy was investigated with the specimen which has a controlled defect size. As a result, the good correlation between the fatigue limit and the hardness of the welding area was clarified.

Stress Concentration and Surface Roughness Effect on Strength of Polycrystalline Silicon Structure

In order to clarify the stress concentration and surface roughness effect on strength of the polycrystalline silicon (poly-Si) structure, bending tests of poly-Si microcantilever beam specimen and surface roughness measurement is performed. The bending test results are analyzed by means of maximum stress at the notch root calculated by FEM models, and it is found that this approach cannot describe the test results. Therefore, modified approach is taken into account by use of two parameters that are the maximum stress and area where stress is larger than 50% of the maximum stress, which indicates stress extension around the position of maximum stress representatively. By this two parameters approach, the test results are explained quantitatively and a strength design chart for stress concentration area of the poly-Si structure is obtained. On the other hand, relationship between strength and surface roughness are confirmed and useful information for the process quality control are obtained.Copyright