The effect of static and fatigue loading on the residual stress induced by laser peening has been investigated on the aluminum alloy JIS A2024-T3. X-ray diffraction was applied to laser-peened samples to measure surface residual stress before and after subjecting the sample to static tensile or fatigue loading. Plastic deformation of the internal surfaces before and after loading was evaluated from the diffraction peak width associated with the loading. During static tensile loading, the interior of the sample started to deform plastically due to the superposition of external loading and residual tensile stress that compensated for the surface compression by laser peening; this resulted in the relaxation of the surface residual stress after unloading. In the case of fatigue loading, the residual stress on the surface was relaxed during the first loading cycle due to the same mechanism as that observed in the static tensile loading. The degree of the relaxation could be estimated taking the results of the static loading into account. Significant further relaxation was not observed after the first cycle until just before the final fracture.
The railway carbody structure for commuter and suburban services in Japan is often made of austenitic stainless steel, which is used in the form of the thin metal sheets by cold rolling. It is difficult to observe the strain distribution around the weld zone, which is the critical area of the body structure strength. The objective of this study is to get the strain distribution in stainless steel around the weld zone. To observe the distribution, the strain scanning method by using high-energy synchrotron radiation was applied to the strain measurement of austenitic stainless steel. The transmission method was applied in order to observe internal weld zone. Using this method, we could measure the distribution from surface to inside of the weld zone. The specimen of lapped joint prepared by welding 2mm thick plates by laser weld method was used for measurement. This measurement provides the strain distribution of both residual strain and strain under loading. The result by this measurement shows strain distribution in the weld zone successfully. In addition, the different tendency between the distribution of residual strain and that of strain under loading made be clear. The full width of half maximums, FWHM, of measurement shows a difference between the tendency of measured value of weld zone and that of base material.
The residual stress distribution below the specimen surface of a severe surface deformed medium carbon steel was investigated nondestructively by using high-energy X-rays from a synchrotron radiation source. The carbon steel plate was shot-peened with fine cast iron particles of the size of 50μm. By using the monochromatic X-ray beam with three energy levels of 10, 30 and 72keV, the stress values at the arbitrary depth were measured by the constant penetration depth method. The stress was calculated from the slope of the sin2ψ diagram. Measured stress corresponds to the weighted average associated with the attenuation of the X-rays in the material. The real stress distribution was estimated by using the optimization technique. The stress distribution was assumed by the polynomials with the order from the first to the third in the near surface layer and the second order polynomial in the deeper region. The coefficients of the polynomials were determined by the conjugate gradient iteration. The predicted stress distribution agreed very well with that measured by the conventional surface removal method.
An imaging and a strain mapping in the vicinity of a crack tip in material were investigated using a high energy white X-ray obtained from BL28B2 beam line at SPring-8 in Japan. Low-alloy and high-tensile steel (JIS G3128 SHY685) was used as a specimen prepared in the G-type geometry. A fatigue crack was introduced into the specimen by a cyclic loading. The imaging of the crack in the specimen was carried out by using the X-ray CCD camera that can detect the X-ray transmitted through the specimen. To measure the strain, the synchrotron white X-ray beam, which had a height of 80μm and a width of 300μm, was incident on the specimen with the Bragg angle θ of 5 degrees using the energy dispersive X-ray diffraction technique. The internal strain in the vicinity of the crack tip was mapped out by scanning the irradiated X-ray position around it. As the results, the imaging of the crack, with about 1mm length, in the specimen under the loading of crack opening was practicable by using the synchrotron white X-ray. The map of the internal strain near the crack tip of the steel of 5mm thickness could be obtained using the white X-ray with energy ranging from 50keV to 150keV. The plastic region estimated from the distribution of the FWHM of diffracted X-ray profile almost agreed with the theoretical value calculated by fracture mechanics. It was confirmed that the synchrotron white X-ray is useful for the imaging of the internal crack and the strain mapping near it.
CoNiCrAlY was pressureless plasma-sprayed on a substrate of nickel-base superalloy as a bond coating. Under the substrate rotation, zirconia with 4mol% yttria was coated as the top coating by the electron beam-physical vapor deposition (EB-PVD). The rotation speed of the substrate was 5, 10 and 20rpm. The top coating was made by the columnar structure which consisted of the core part and the peripheral part. The top of the core part had a pyramidal shape made by piling up the zirconia (111) planes. The peripheral part showed feather-like structure, which was made by growth of the zirconia <100> and <111> directions. For the specimen with 20rpm, the in-plane residual stress was small within the range from –20 to 30MPa, the out-of-plane residual stress was a very small compression. By the increase in the rotation speed, the column becomes thick and the inter-columnar space becomes large. The substrate rotation is useful for the reduction of the residual stresses in the EB-PVD thermal barrier coating.
It has been found that the shear strength of the A2024BE-T6 aluminum alloy is much lower than the tensile strength and that this alloy exhibits severe initial anisotropy. The purpose of this paper is to clarify the relation between initial and subsequent anisotropies by comparing with the results of the previous paper. Off-axis torsion tests by means of the combined loading of compression, internal pressure and torsion are carried out on thin-walled cylindrical specimens of the A2024BE-T6 alloy subjected to a torsional prestrain. In this test method, it is possible to detect both Bauschinger effect and planar anisotropy by changing the principal shear stress direction from 0 deg to 90 deg. Furthermore, the anisotropic hardening behavior is analyzed precisely by applying the subsequent yield function to the experimental results, and the degree of anisotropy is expressed as a function of offset strain. It is proved that the maximum anisotropy based on the flow stress corresponds to the Bauschinger effect when small offset strains are used in specifying the equi-strain locus, while it is related to the residual initial anisotropy when large offset strains are used.
The deformation mechanism of the extruded cylinder of AZ31 magunesium alloy depending on a condition of temperature and loading path was studied by a multi-loading test under various temperature conditions and an observation of its microscopic structure after the plastic deformation. From the experimental results, AZ31 magnesium alloy is plastically deformed at room temperature by a twin deformation mechanism. AZ31 magnesium alloy is hard to be plastically deformed at room temperature because the critical shear stress of the twin deformation is much larger than that of a trans-granular slip. Thus, due to the twin deformation mechanism, the yield surface of AZ31 magnesium alloy at room temperature does not coincide with the surface estimated by von Mises criterion, and then the yield stresses of a compressive test and torsion test are lower than the yield stress of a tensile test. From the shape of the yield surface and the existence of the twin in microscopic structure, the deformation mechanism at 423K is similar to the mechanism at room temperature. The superplastic deformation driven by a grain boundary sliding and the grain refinement occurs at 673K. Then, concerning the yield surface, the yield stress of a compressive test is equal to that of a tensile test, and the shear yield stress shows 80% of the tensile yield stress. Such as a shape of the yield surface appears at the temperature of 473K, 498K and 523K and the grain refinement is observed in the specimen deformed at 523K. Therefore, at these temperature conditions, the influence of the grain boundary sliding mechanism appears on the plastic deformation. Because the grain boundary sliding mechanism and the grain refinement leading to the superplastic deformation reveal at a temperature more than 473K, it is supposed that the plastic workability of AZ31 magnesium alloy can be facilitated at a comparatively lower temperature.
In this paper, the crashing behaviors of axial- and ring-stiffened circular tubes subjected to axial compression are studied using the finite element method. The numerical results show that the ring can lead the folding process of the circular tube to the ideal shape of wrinkles, in which case the circular tube represents the axisymmetric deformation mode. On the other hand, the effect of the axial stringer (rib) on the crashing behavior of the circular tube is to increase the compressive load by jointed parts between the cylinder and the ribs. However, the deformation falls into the unstable mode since the wavelength of folds becomes long. Therefore, in order to press down the fluctuation of the compressive load in crashing of tubes and to raise its compressive load, it is an expectable method for the energy absorption material that introduces the coupling effect of the ring and the rib.
An effect of the solidification rate on the hardness was investigated in Fe-Cr-Mo-C stainless steel (SUS420J2) produced by rapidly quenched casting process with single-roll at various rolling speeds from 5 to 40m/s. Furthermore the optimizing process to improve hardness after final heat treatment in industrial scale was carried out for the ingot operated with a rolling speed of 10m/s which could exhibit sound state without any materials defaults. A final goal of the higher hardness more than Hv = 800 after final heat treatment could be obtained along the process condition with the tempering (baking) treatment (air cooling after heating at 973K for 120m/s) and the quenching treatment (air cooling after heating at 1323K for 60m/s). The microstructure observation clarified that the origin of the obtained higher hardness in the optimum processed material compared with the as-cast material resulted from the finer grain size of the old austenite phase at 10μm then 20μm of the as-cast one.
Recently, in place of the welding, the new rivets such as the self-piercing rivet are noticed in the auto industry. In this paper, the new impact riveting method (Single Impact Riveting Method) in which an aluminum rivet is struck into aluminum sheets by the impulsive force is proposed. Procedures of the joining are as follows : (1) the axis of the impact rivet is struck into sheets by the impulsive force ; (2) the rivet axis penetrates the sheets and collides with the die ; (3) by collision, the tip of the axis forms the other head of the rivet and the axis also becomes thicker ; (4) the sheets are joined tightly by the plastic deformation of the rivet axis. The proposed method does not need prepared holes in sheets and improve the productive efficiency further than the conventional method. From experimental results, it was clarified that the joint strength by the single impact riveting method is higher than that by caulking, and the sheet warp is smaller than that by caulking.
Cyclic indentation tests were carried out on TiN or SiC film sputter-coated on titanium or tool steel substrate by using a Rockwell C-scale indenter, and delamination behavior of the films was investigated. The observation using a laser microscope shows that the delamination of TiN film on titanium substrate occurs around a region just outside of indenter-film contact edge after the initiation of circular cracks near the region. Delamination probability is larger when the cyclic load and number of load cycles are larger. The SiC film on titanium substrate does not show delamination within a maximum cyclic load of 10N and 1000cycles perhaps due to a larger interfacial strength. The delamination of SiC film on tool steel substrate occurs by spalling also around the region just outside of indenter-film contact edge. In some cases the delamination of SiC film occurs from bottom of indentation. FEM analysis of indentation into TiN film on titanium substrate shows that a large tensile stress appears on the film surface in the radial direction just outside of indenter-film contact edge. When there is a crack in the edge, both shear and compressive stress appear in the radial direction along interface. These stresses will cause the cracking of film and delamination of film after cracking, respectively.
This paper discusses the fracture criteria for predicting the strength of unidirectional discontinuous glass fiber reinforced composites with side-notches. The point stress criterion (PSC) for notched composites was employed so that the Tsai-Hill criterion for orthotropic materials is satisfied. The fracture criterion proposed on the basis of linear notch mechanics (LNM) proposed by Hyakutake et al. was also discussed in association with the above two criteria. Strength of intact and notched unidirectional composites was experimentally obtained for various kinds of off-axis angle and notch depth. The stress concentration factor in the vicinity of a notch was calculated with the use of finite element analysis. It was found that the notched to intact strength ratio is almost independent of off-axis angle though it slightly depends on notch depth. The characteristic length for the PSC estimated through the above experimental and analytical results increases with increasing off-axis angle and notch depth. The critical maximum stress of the LNM-based criterion was shown to be a function of fiber orientation angle, but almost independent of notch depth. The LNM criterion was proved to be equivalent to the PSC that satisfies the Tsai-Hill criterion.
In this paper, tensile and fatigue strength of high-modulus (HM) type (258GPa modulus) poly-p-phenylene benzobisoxazole (PBO) fiber improved tensile modulus by heat-treatment have been investigated. The tensile tests of a monofilament were carried out at a gauge length of 12.5mm and deformation rate of 0.5mm/min. The fatigue tests of a monofilament were carried out to determine the S-N property at a frequency of 10Hz with three stress ratios of 0.1, 0.5 and 0.7. Standard modulus (As spun : AS) type (187GPa modulus) PBO fiber that authors reported its properties before was employed as standard specimen. It was found that the tensile strength of HM type PBO fiber was well represented by a two-parameter Weibull distribution, and indicated a size effect in diameter direction. The tensile strength of HM type PBO fiber on the basis of the concept of effective volume was higher than that of AS type fiber. The fatigue strength of HM type PBO fiber was higher than that of AS type fiber over all fatigue lives. The relation between the stress amplitude and fatigue life depended on the stress ratio. However, the relation between the maximum stress ratio and fatigue life was independent of the stress ratio. Therefore, it was found that the maximum stress was useful to describe the fatigue lives at the different stress ratios. Additionally, the factor governing fatigue fracture tended to vary from the mean stress to stress amplitude at the low stress ratio because crystalline regions increased by heat-treatment and fiber became brittleness.
The objective of this study is to evaluate the chloride corrosion rate of some stainless rebars in mortar specimen with bending cracks. Especially, the corrosion resistance of 7 Cr bearing rebars having different Cr contents is evaluated. Also, the chloride corrosion rate of Cr and Ni bearing stainless rebars with flaws or welds is evaluated. Therefore, in the specimen exposed to an accelerated chloride environment, the corrosion current of the stainless rebar is measured. In conclusions, the corrosion rate of the SUS304 with flaws or welds is lower than that of the carbon rebar SR235 without flaws or welds. Especially, the Cr bearing rebar whose Cr content is 7% or higher has corrosion resistance that is about 10 times larger than that of the SR235. Also, the Ni or Cr bearing rebar (the Cr content is 11% or higher) has extremely high corrosion resistance to the SR235 in a mortar specimen with bending cracks. Based on the above, it is thought that the rebar containing much Cr will be used under severe chloride environments in the future, by considering LCC (Life Cycle Cost).