We investigated an optimizing process to improve hardness after the final heat treatment through modification by the Equal Cannel Angular Extrusion (ECAE) processing in a martensite stainless steel (corresponding to SUS420J2). We expected the more refinement in both the chrome carbide particles with relatively large sizes more than 0.5μm and the grain size of the matrix by the sever plastic deformation from the ECAE processing, by the way the less refinement in sizes of neither the particles or the grain size could be obtained than we expected. However, the lattice faults with a high density could be introduced into the matrix by the ECAE processing under the condition without any cracking along the interfaces between the chrome carbide and the matrix. It could accelerate drastically the ability in solid solution of the large chrome carbide particles into the matrix during the final heat treatment by quenching at higher temperatures more than 1323K that led successfully increasing the carbon mass in solid solution into the matrix. In the present study by optimizing the ECAE processing to accelerate the ability in solid solution of the large chrome carbide particles the hardness of the matrix could be improved to Hv = 700 from Hv = 600 achieved by the conventional process. We conformed an affectivity to enhance the hardness of the present martensite stainless steel by the modification from the ECAE processing.
Toward the challenging question, “What dominates an unstable behavior in the stress-strain curve of inhomogeneous system ?”, we have performed again molecular dynamics simulations on pseudo-polycrystalline and amorphous nickels under uniaxial tension. Here the nano-polycrystals are made by dividing a cuboidal periodic cell and rotating the crystal orientations of each cubic grain randomly. Stress drops are observed in the stress-strain curves of pseudo-polycrystals with the grain size of 8.76, 5.28 and 3.52nm, while the amorphous doesn't show distinct peak stress under the strain control of εzz = 1.0 × 10-7 or 1.0 × 10-6 [1/fs]. It is also true for the stress control of σzz = 1.0 × 10-5 [GPa/fs] that the polycrystals show flexion points in the stress-strain curves as the onset of unstable strain burst, while the amorphous comes up to the strain burst without showing determinable flexion point in the continuous nonlinear curve. Then the positiveness of the elastic stiffness coefficients for whole the system (global stability) and those for each atom (local stability) are investigated in detail, but we still couldn't find out common tendency for the prediction of the unstable behavior under uniaxial tension. Nevertheless, we have separately evaluated the stress acting on the unstable and stable atoms according to the local stabilities, finding out the following facts : (1) the unstable atoms in the polycrystals, or mainly grain boundary atoms, largely deform and show higher stress than the stable atoms, (2) the stress increase on the unstable atoms seems to saturate at the first transition point in the stress–strain curves of the polycrystals, (3) the unstable atoms in the amorphous show negative residual stress at the initial equilibrium, and (4) the stress increase on the unstable atoms catches up with that on the stable atoms, at the point where the plateau region begins in the stress–strain curve of the amorphous.
At room temperature, plastic deformation of amorphous metals is localized in narrow bands, so called shear bands (SBs), which is a key issue of their brittle/ductile deformations. The extremely disordered structure and the abrupt fracture obtained in the experiments hinder us from making clear the nature and dynamics of SBs. In this study, atomistic modeling to produce the multiple shear bands (MSBs), not single shear band, was proposed, and the evolution process of MSBs including their coalescence and stationary was investigated using molecular dynamics simulations. We prepared a plate of Cu-Zr binary amorphous alloy model by the melt-quench process, and then performed uniaxial tensile and compressive tests under plane stress condition. During the loading, the deformation was suddenly localized in narrow bands shortly after the onset of yielding. The propagation of SBs was accompanied with drastic stress drop and significant local heating caused by the friction of atoms. Also, critical stresses of SB nucleation considerably differed between under tension and compression. This result indicates that Tresca or von Mises criterion, commonly used as a yield condition in the crystalline metals, is not appropriate to describe the yielding of the amorphous metals. The SB angles to the loading axis are observed to be 45∼57° under tension, while 40∼46° under compression. These angles agree well with the fracture angles observed in the experiments with multi-component metallic glasses. It is concluded that the critical stress state of SB nucleation is dependent on not only the shear stress but also the stress normal to the SB, and it can be described by Mohr-Coulomb criterion.
In this study, we investigate primary buckling modes and stresses of a Kelvin open-cell foam under uniaxial compression in , , and  directions. For this purpose, a homogenization framework (Ohno et al., 2002 ; Okumura et al., 2004) is employed to analyze the macroscopic instability as well as the microscopic bifurcation depending on periodic unit length. In this analysis, ligaments of the Kelvin open-cell foam are assumed to be made of an isotropic elastic material and to have a nonuniform cross-section described by two geometric parameters. It is shown that the buckling behavior is anisotropic in the three directions of uniaxial compression ; the primary buckling modes in the  and  compressions are caused by long wavelength bifurcation accompanying the onset of macroscopic instability, while that in the  compression is done by short wavelength bifurcation. However, their primary buckling stresses are nearly identical, and moreover are in reasonably good agreement with an analytical prediction combined with experiments. It is also shown that the non-uniformity of the cross-sectional area plays a dominant role in the buckling behavior.
Ferritic stainless steels such as Fe-2.25Cr-1Mo-1Zr, Fe-25Cr-1Zr and Fe-13Cr-8Ni-2Mo-1Zr with ultra-fine grain size of 0.2μm were prepared by hot extrusion after mechanical alloying. Axial fatigue life tests for plane round bar specimens of these steels were conducted at 473K. The higher fatigue strengths of these steels at 473K were observed as compared with those of conventional steels such as SAE8740 and SNB16. Fatigue strength expressed as maximum stress at 3 × 106 cycles for Fe-2.25Cr-1Mo-1Zr, Fe-13Cr-8Ni-2Mo-1Zr and Fe-25Cr-1Zr were 1150, 1150 and 1350MPa, respectively. Transranular fracture surfaces were predominantly observed at crack initiation and propagation area. Striation was observed at defined region of crack propagation area. It can be concluded that the higher fatigue strength of ferritic stainless steels with ultra-fine grain size of 0.2μm is strongly governed by delay of crack initiation.
The resistance-curve (R-curve) method was used to assess the effect of the diameter of drilled holes on the fatigue threshold of a carbon steel (JIS S45C) and a spring steel (SUP190). The specimens of the carbon steel with various hole diameters were subjected to tension-compression fatigue tests, and those of the spring steel to rotating bending and torsional fatigue tests. The fatigue crack made near the fatigue thresholds of drill-holed specimens of the carbon steel was modeled by the ring crack around the equator of the semi-cubic hole or by the quarter-circle at the corner of the cylindrical hole. When the diameter of drill holes was d = 0.05 and 0.1mm, the experimental data of the fatigue threshold agreed well with the prediction obtained under the assumption of the propagation threshold of the ring crack. The assumption of the quarter-circle crack gave a better prediction for the diameters of d = 0.3 and 0.5mm. For the case of the spring steel, on the other hand, the threshold value predicted from the crack propagation was far below the experimental data. The critical value of the stress intensity factor for crack nucleation was determined in order to obtain a better agreement with the experimental data. The critical value thus determined was nearly double the threshold stress intensity range for crack propagation. The nucleation condition of fatigue cracks controlled the fatigue threshold of the spring steel.
Rotating bending fatigue tests were carried out for notched specimens of a SUS316NG austenitic stainless steel at 300°C. Nevertheless any non-propagating crack was not recognized at the root of the notch for specimens endured at 108 stress cycles, the fatigue strength at 108 cycles did not decrease continuously with increasing the stress concentration factor. An area of the root of notch hardened during fatigue test at 300°C by dynamic strain aging. The hardening behavior became remarkably with increasing the stress concentration factor. Effects of the stress concentration factor and hardening behavior on the fatigue strength cancel each other, and then dependency of the fatigue strength on the stress concentration factor becomes insensitive.
Fractoluminescence (FL) has been observed by three bending tests of KCl single crystals doped with Ca2+, the size of which is 3 × 1 × 12mm3. FL has occurred only in the vicinity of fracture surface (within 0.3mm) and FL intensity as well as dislocation density has sharply decreased with increasing distance from fracture surface. And the dislocation density has approached to the initial one. Collecting data from many samples, it has been found that the FL intensity is proportional to the three-half power of the dislocation density. The relation between FL intensity and dislocation density has been explained by dislocation intersection.
A Mercury target for spallation neutron source will be installed in MLF (Material and Life-science Facility) in J-PARC (Japan Proton Acceleration Research Complex). The mercury target will be suffered from pitting damage by cavitation erosion. The cavitation will be caused through the pressure wave propagation that induced by thermal expansion of the mercury due to proton beam injection. Surface modification to the vessel wall contact with mercury, such as coatings, surface hardening etc., could exhibit prospective potential to suppress the pitting damage. Hence, cavitation erosion tests on surface modified materials in mercury were performed using electro-Magnetic IMpact Testing Machine (MIMTM) to evaluate the resistance against pitting damage. As a result, it was confirmed that crack and separation occurred on coating layer, because of shear stress due to the mechanical properties change in the interface between the coating and substrate layer.
In the present paper, press molding for thermo-plastic resin toughened by carbon nanofiber were discussed. Polycarbonate (PC) and vapor grown carbon nanofiber (VGCF) are used for the resin matrix and the reinforcement. In order to investigate and simulate the molding process of the PC/VGCF composite by finite element method, the constitutive equation, namely thermo-viscoelastic property of the composites are required. The relaxation modulus (Master curve) and shift factor to characterize it's thermo-viscoelastic behavior were estimated by unidirectional compression creep test in this paper. The relaxation modulus for the composite were approximated by the discretized series form using generalized Maxwell model. The displacement history of the creep test were calculated by finite element method. The numerical and experimental results are compared and discussed in detail to confirm the validity of the estimated relaxation modulus obtained the present procedure.
Bioactive ceramics, one of the most popular biomaterial, is now widely used for bone-repairing. However, its application is limited to less loaded regions because of its low fracture toughness. We focused on Perhydropolysilazane (PHPS) which is an organosilicate polymer consisting of cyclic (SiH2-NH)n units. PHPS converts easily to silicon dioxide by heat-treatment lower than 150°C and is capable of compounding with low-melting polymers. By compounding with such materials SiO2 film bearing several mechanical properties can be cured from PHPS. In this study, SiO2 films were cured under various conditions. In order to investigate the existence of Si-OH, inducing the nucleus formation of apatite, sample surfaces were characterized by Fourier transform infrared reflection (FT-IR). Bioactiveness of each samples was tested by immersing in Simulated body fluid (SBF), solution which ion concentration is nearly equal to that of human blood plasma, for seven days. From the immersion tests, some samples showed bioactivity. Zeta potential of each samples were also measured. The dissolution of SiO2 films under immersion were examined by Atomic absorption spectroscopy (AAS). From these results, SiO2 films of (i) relatively less Si-H, witch repels the Ca ion bonding, (ii) surface Si-OH existing without condensation, (iii) Si-O-2NBO (Non-bridging oxygen) containing siloxane structure witch encourages the dissolution of silicate ion, showed bioactivity.
The authors demonstrate the inscription of several Bragg gratings at the same location of a photosensitive fiber. This superimposed Bragg grating was fabricated by two-beam interference method using UV laser light. Effects due to the inscription of multiple gratings at the same location of the fiber such as changes of reflectivity and shift of wavelength are studied.
This present study is performed in order to understand stabilization effect of volcanic cohesive soil “Akaboku” with quicklime. Volcanic cohesive soil “Akaboku” in north-central Kumamoto prefecture originates from Mt. Aso and Mt. Kujyu. The stabilizer used in this study is quicklime. The added stabilizer is quantified as a percentage to soil dry weight. Cone penetrate tests and unconfined compression tests are performed on quicklime-treated samples. Assessments of the stabilization effect measured in both tests are made through a number of compaction, mixing time, mixing method and curing method. The following conclusions are obtained. (1) The unconfined compression strength of quicklime-treated volcanic cohesive soil increases with reducing remolding influence as avoiding over-compaction of specimens and decreasing mixing time by using machine. (2) In the case of exposing samples before compaction, the unconfined compression strengths and cone indices depend on exposing days without additive amount of quicklime. (3) It is confirmed that the stabilization effect of quicklime-treated soil after fixed days of a combination of exposing and curing is larger than that of curing only in spite of same amount of quicklime additive. (4) It is suggested that cost for soil stabilization can be reduced, if the amount of quicklime additive could be decided following mix proportion test is carried out under the earth working conditions.
In NATM tunnel constructions passing through unconsolidated soil and shallow depths, it is inevitable that ground displacement will occur around tunnels, which may significantly affect nearby structures. Experimental and numerical analysis approaches have been undertaken to understand deformation behaviors during tunneling. This paper consists of two phases. In the first phase, a numerical model that can account for reduction of shear stiffness and frictional strength parameters simultaneously was proposed to investigate the effects of key parameters on the deformation mechanisms on key parameters, e.g. nonlinear parameters, horizontal stress ratio, and shotcrete thickness. The second phase was performed to compare model test results with those of numerical modeling, for validating the proposed strain-softening analysis method.