Showing papers in "Journal of The Society of Materials Science, Japan in 1999"

Effects of fiber length and orientation distributions on the mechanical properties of short-fiber-reinforced polymers

Abstract: Extrusion compounding and injection molding processes are frequently employed to fabricate short-fiber-reinforced polymers. During extrusion compounding and injection molding processing, considerable shear-induced fiber breakage takes place and results in a fiber length distribution (FLD) in final short-fiber-reinforced polymer (SFRP) composites. Also, during compounding and molding processing, progressive and continuous changes in fiber orientation occur and lead to a fiber orientation distribution (FOD) in final composites. Both FLD and FOD are governed by a number of design and fabrication factors including original fiber length, fiber content, mold geometry and processing conditions. The mechanical properties such as strength, stiffness and fracture toughness or specific work of fracture (WOF) of SFRP composites have been shown to depend critically on FLD and FOD. The present paper reviews previous research work on the effects of design/fabrication factors on FLD and FOD and the effects of FLD and FOD on the strength, stiffness and toughness or WOF of SFRP composites. Conclusions which can be drawn from the literature are presented with discussions of areas in which further research is required.

Fatigue. Analysis of the Mechanism of Superlong Fatigue Failure by Optical Microscope and SEM/AFM Observations.

Abstract: Since in superlong fatigue failure with Nf≥108, the average fatigue crack growth rate is much less than lattice spacing (-0.1A or 0.01nm or 10-11--12m/cycle), we cannot assume that crack growth occurs cycle by cycle in the early stage of fatigue process.In this paper, possible mechanisms for extremely high cycle fatigue are discussed. A special attention was paid to a newly found particular fatigue fracture morphology in the vicinity of fracture origin (nonmetallic inclusions) of a heat treated alloy steel, SCM435. The particular morphology looks a dark area inside fish-eye mark by optical microscopic observation. Specimens with short fatigue life of N=-105 do not have such dark area in fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from usual fatigue fracture surface in martensite lath structure. The predictions of fatigue limit by the √area parameter model are -10% unconservative for fatigue limit defined for N=107. Thus, the fatigue failure for N≥108 is presumed to be caused by a mechanism which induces breaking or releasing of fatigue crack closure phenomenon in small cracks. The breaking or releasing crack closure mechanism is presumed to be caused by environmental effects such as hydrogen embrittlement coupled with extremely high cycle fatigue. Some indirect evidences to support this hypothesis are shown.

Crack Healing Behavior and High Temperature Strength of Mullite/SiC Composite Ceramics.

Abstract: A mullite/SiC composite ceramic was sintered. Four point bend tests were conducted according to JIS standard. Semi-elliptical surface crack of 100 or 200μm in diameter was made on the specimen. For four kinds of specimens (asreceived, heal treated, pre-cracked and heal treated cracked), crack healing behavior and high temperature strength were tested systematically. The main conclusions were obtained as follows: (a) Mullite/SiC composite ceramic has ability to heal crack. (b) Best healing condition was found to be 1300°C, in air for 1h. (c) Maximum crack size to be healed is semi-elliptical crack of 100μm in diameter. (d) Crack healed part has enough strength up to 1100°C and most specimen failed outside the pre-cracked zone and healed crack is not sensitive to static fatigue.

Basic Studies on Simplified Curing Technique, and Prevention of Initial Cracking and Leakage of Water through Cracks of Concrete by Applying Superabsorbent Polymers as New Concrete Admixture

Abstract: Recently, many kinds of superabsorbent polymers were developed. In this research, three special applications of the polymers, that is 1) application for simplifying curing of concrete at early age, 2) application for preventing cracking of concrete caused by initial drying shrinkage, and 3) application for preventing leakage of water through cracks, were proposed. The superabsorbent polymer used for this study can hardly absorb alkaline water in fresh/hardened concrete, but can absorb much neutral/acid water and makes gel. Thus when the neutral water is poured on concrete only once after setting, face of the concrete can be coated with the gels, and the concrete can be kept without drying, so that the 1) effect can be achieved. The water confined in the superabsorbent polymer and gels may delay the immediate drying of concrete, and the 2) effect may be obtained. The some of polymers around cracks can catch much neutral or acid water passing through the cracks and the gels fill up the cracks, so that the 3) effect can be achieved.

Fatigue. Subsurface Fatigue Crack Initiation Behavior and S-N Curve Characteristics in High Carbon-Chromium Bearing Steel.

Abstract: In order to investigate high cycle long life fatigue characteristics, cantilever-type rotary bending fatigue tests were conducted in an open environment at room temperature using hourglass shaped specimens of high carbonchromium bearing steel. The S-N curve obtained was clearly classified fracture modes into two groups of different crack origin. One was governed by a crystal slip on the specimen surface which occurred in the region of short fatigue life and high stress amplitude level. The other was governed by nonmetallic inclusion at subsurface which occurred in the region of long fatigue life and low stress amplitude. The inclusion governed fish-eye fracture mode was distributed in a wide range of stress amplitude not only below the fatigue limit defined as the threshold of fracture due to the slip governed mode but also above the fatigue limit. This remarkable shape of the S-N curve was different from a step-wise one reported in previous literature and was characterized as duplex S-N curves which composed of two different S-N curves corresponding to the respective fracture modes. An examination of the initial stress intensity factor range of fracture origin, ΔKini, showed that the ΔKini of the slip governed fracture mode was larger than the threshold stress intensity factor range of the surface crack growth, ΔKth. On the other hand, cracks originate and propagate from inclusion at subsurface with smaller ΔKini than the slip-originating crack and the ΔKth.

Effect of Deformation on the Precipitates in Al-Mg2Si Alloys Containing Silicon in Excess.

Abstract: Precipitates formed by aging in deformed Al-Mg2Si alloys were classified using high resolution transmission electron microscopy (HRTEM) in order to understand the effect of excess silicon contents and deformation on precipitation. Two types of precipitates appeared on the dislocation in the deformed Al-Mg2Si alloy including 0.4mass% silicon in excess (excess Si alloy) at early stage during aging at 473K. First one is the string-like precipitate that consisted of a lot of small precipitates linked as a string-beads along the dislocation line. Second one is the elongated type precipitate that is the end-on of the needle- or rod-shaped precipitate having the elongated cross section. There were a lot of the TYPE-C precipitates on the dislocation after a peak aging. There were the random type precipitates and β" phase in the matrix which were independent on the existence of the dislocation. At the early stage during aging, a lot of the random type precipitates appeared in the matrix. Next, the β" phase appeared and the coarse TYPE-A precipitates finally appeared at the over aged condition. It has been clarified in our recent work that the β" phase, the TYPE-A and the TYPE-C precipitates are typical precipitates in the excess Si alloy. It was considered that the string-like precipitate grows to the elongated-type precipitate and then those transforms to the TYPE-C precipitate with progress of aging, because there is similarity of the arrangement of bright dots and orientation relationship in their HRTEM images between the string-like and elongated-type precipitates, and the TYPE-C precipitate. The β' phase increased and the TYPE-C precipitate decreased on the dislocation in the over aged alloy with decreasing of excess silicon contents. There were no TYPE-C precipitates and the β' phase predominantly appeared on the dislocation in the over aged alloy without excess silicon contents. It was found that the TYPE-C precipitate is a typical precipitate which appears inhomogeneously in the deformed excess Si alloy.

Fatigue Strength Properties of Crack Healed Mullite/SiC Composite Ceramics.

Abstract: Mullite/SiC composite ceramics was sintered. Three point bend specimen was made according to JIS standard. Semi-elliptical surface crack of 100μm in diameter was made on the specimen. By using four kind of specimens (as-received, heal treated as-received, pre-cracked and pre-crack healed), cyclic fatigue and static fatigue strength of the specimen were tested systematically at room temperature. The main conclusions obtained are following: (a) Mullite/SiC composite ceramics has ability to heal crack. (b) Crack healed specimen (heal treated as-received and pre-crack healed) showed higher cyclic and static fatigue strength than as-received specimen, and this fact was caused by crack healing. (c) Crack healed part has enough fatigue strength and most fracture occurred outside the pre-cracked zone in pre-crack healed specimen.

Topochemistry of SiO2 wood-inorganic composites for enhancing water-repellency

Abstract: Methyltrimethoxysilane (MTMOS) was used to prepare the SiO2 wood-inorganic composites. To enhance the properties of these composites, 2-heptadecafluorooctylethyltrimethoxysilane (HFOETMOS) as a property enhancer was added to this reaction system for SiO2 wood-inorganic composites. To elucidate the mechanism for enhancing the water-repellent property in the HFOETMOS-SiO2 wood-inorganic composites, the composites with various weight percent gains (WPGs) were prepared, and the prepared composites with the lower WPG revealed the better property in water-repellency, whereas those with the higher WPG could not attain the high water-repellent property. SEM-EDXA analysis on these composites revealed that the HFOETMOS-derived residues were concentrated on the boundary between the cell wall and cell lumen in both composites with the lower and higher WPGs, while SiO2 gels were almost uniformly distributed within the cell walls in the composites with lower WPG. SiO2 gels in the composites with higher WPG were distributed in a similar manner but slightly higher in its concentration at the boundary between the cell wall and cell lumen. From these results, it was assumed that for the composites with lower WPG, the cell wall would be covered with the long hydrophobic alkyl residue of the HFOETMOS, whereas for the composites with higher WPG, SiO2 gels formed in the cell lumen would have prevented HFOETMOS-derived residues from being exposed uniformly over the surface of cell wall. This may be why the different water-repellent properties were shown between the composites with higher and lower WPGs. These results suggest that the topochemical effects of the SiO2 gels and HFOETMOS-derived residues exist for the enhancement of water-repellent property.

Corrosion Resistance Evaluation of Brittle Materials in Boiling Sulfuric Acid.

Abstract: The corrosion behaviors of ceramics (Si-SiC, SiC, Si3N4, Al2O3, ZrO2) were examined in boiliing 95 wt% sulfuric acid by measuring weight change, concentration of dissolved ions in the sulfuric acid, 4-point bending strength and apparent hardness calculated from Load-Depth curve obtained by Micro- Indentation technique. The corroded surface and the fracture surface of the specimens were analyzed by SEM and EPMA. The corrosion behaviors of these ceramics were divided into 3 types; (1) Si-SiC and SiC showed excellent corrosion resistance because of protective oxide films which developed during immersion, (2) the bending strength, apparent hardness of Si3N4 and Al2O3 decreased with increasing immersion time because of corroded layers produced during immersion, and (3) the bending strength of ZrO2 decreased with remarkable weight loss. The thickness (T) of oxide films and corroded layer produced on Si-SiC, SiC, Si3N4 and Al2O3 was evaluated from the depth (d) which was determined by the slope change on Depth-Load/Depth curve. The relationship between T and d was described as T≈10d. The bending strengths of Si3N4 and Al2O3 were evaluated from thickness (T) of corroded layer and KIc obtained before immersion. As a result, the bending strengths of Si3N4 and Al2O3 could be determined with both the KIc and the equivalent crack length (ae) to 20d.

Stress-strain response of rock-forming minerals by molecular dynamics simulation

Abstract: We show a series of molecular dynamics (MD) simulation to determine material properties (i.e., the elastic moduli and the strength) of quartz, muscovite and albite under uniaxial compression and shearing. Note that these are major rock-forming minerals of granite, and are of anisotropic properties. Interatomic potentials are essentially important for the MD calculation, and we used a generalized potential function [1]. MD basic cells imposed are composed of 900 atoms for quartz, 936 atoms for albite and 1, 512 atoms for muscovite, respectively. Calculated results are agreeable compared with experimental data.

Rate-dependent mode ii interlaminar fracture behavior of carbon-fiber/epoxy composite laminates

Abstract: Effects of loading rate on mode II interlaminar fracture behavior of unidirectional CF/conventional-epoxy (T300/2500, Toray) and CF/toughened-epoxy (IM600/133, Toho Rayon) composite laminates were investigated over a wide range of loading rate from quasi-static to impact at room temperature (displacement rate, δ=10-7-101m/s). A newly developed experimental method using the SHPB (Split Hopkinson Pressure Bar) technique and the ENF (End Notched Flexure) specimen was employed for measuring the accurate fracture toughness at very high loading rates. The mode II fracture toughness at the onset of crack growth showed positive rate dependence (fracture toughness increased with increasing loading rate) at lower loading rates, while it showed negative rate dependence (fracture toughness decreased with increasing loading rate) at higher loading rates; there existed a local maximum value of fracture toughness at intermediate loading rates. The impact fracture toughness was about 13 and 29% lower than the local maximum value for the conventional epoxy composite and toughened epoxy composite, respectively; the toughened epoxy composite was more sensitive to the loading rate than the conventional epoxy composite. Microscopic observation showed that the debonding of fiber/matrix interface was dominant at lower loading rates and that the cohesive fracture of matrix resin was dominant at higher loading rates. The transition point of microscopic fracture morphology approximately coincided with the local maximum point of macroscopic fracture toughness. In addition, the load-displacement relation was non-linear just before the onset of crack growth at lower loading rates but almost linear up to the maximum point at higher loading rates.

Mechanical Properties of Cu-Mo Composites.

Abstract: For applicability in the field of semiconductor, it is important that the thermal expansion coefficient of heat sink materials approximates to those of semiconductor chip and peripheral materials. In addition, high thermal conductivity also is required. The authors have studied a composite of molybdenum (Mo) and copper (Cu) prepared by two kinds of method, the mixing method (M/CM) and the cladding method (L/CM). The mechanical and thermal properties of these Cu-Mo composites can be controlled by varying the composition ratio.In this paper, mechanical properties of the Cu-Mo composites were investigated. The thermal expansion coefficient α of M/CM increased with increase in copper content. The mean measured values of α in the temperature of about 30°C nearly corresponded with those calculated from Turner's equation and Kerner's equation. The observed anisotropy in α values was maximum for 50 mass% Cu content. The measured values of tensile strength, Young's modulus, Poisson's ratio and Vickers hardness showed a behavior similar the linear law of a mixture rule, but their values were a little smaller than the values calculated by the rule. The elongation and Erichsen value were discontinuous with the rule.The α of L/CM was increased with an increase in Cu content. With the rise in temperature, Mo of the middle layer strongly controlled the thermal expansion. The mean measured values of α in the temperature of about 30°C were smaller than the values calculated by the mixture rule of material dynamics. Comparing L/CM with M/CM, the α values of L/CM were smaller than those of M/CM over the entire range of Cu content.

X-ray measurement of stresses in deposited thin film with fiber texture

Abstract: Thin film coating by physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques are advancing intensively in the technology of mechanical as well as electrical fields. Deposition by these techniques essentially develops residual stresses in the film for intrinsic and extrinsic reasons. The microstructure of the film normally exhibits a strong preferred orientation on a crystallographic scale. Residual stresses and the microstructure of the growing film influence the properties of the film as well as the film/substrate system. The present paper demonstrates the recent developments in X-ray stress evaluation of the film having a very sharp preferred orientation. A new method of X-ray stress measurement for such films having a strong texture is presented in place of the conventional sin2Ψ method. C-axis oriented aluminum nitride (AlN) films, [111] fiber textured aluminum and titanium nitride (TiN) films and [110] fiber textured TiN films are investigated to study the residual stress and the effect of heat treatment on the state of residual stress. The effects of patterning the film on residual stress are also investigated.

Estimation of tensile strength distribution for carbon fiber with diameter variation along fiber

Abstract: Tensile strength distribution of carbon fibers was investigated for different gauge lengths ranging from 2 to 100mm. In tensile tests, the fiber diameter was determined as the three-point minimum diameter d3 at neighborhoods of both ends and the center of gauge section by using an optical microscope. Weibull analysis of the data and the effective volume model for tensile strength of carbon fiber showed that the shape parameter a was not constant for different gauge lengths and the effective volume model could not be applied effectively to the results. It was found that these were caused by the fact that the diameter was never constant along a carbon fiber. Consequently, in this study, it was aimed to derive the distribution of the tensile strength σ0 for the true minimum diameter d0 along the gauge length. In order to do this, two-dimensional distributions of d3 and d0 were derived from the data of diameter measured at every 0.1mm along a single fiber with the gauge length of 100mm. Combining these distributions of d3 and d0 with the distribution of the tensile strength σ3 for the diameter d3, the distribution of the strength σ0 was derived analytically, and calculated by using numerical values of included parameters. The distributions of σ0 obtained for respective gauge lengths mutually agreed very well and could be represented by a single normal distribution, and this showed the validity of the present analysis.

Multi-modal vibration control of smart composite plates

Abstract: In this paper, multi-modal vibration control of smart composite plates has been studied. Piezoelectric films and ceramics have been used as sensors and actuators in surface bonded forms. For modeling and analysis of smart composite plates, an efficient finite element method based on the layerwise plate theory has been used. The effect of thickness changes due to bonded piezoelectric materials and variable in-plane displacement fields on the dynamic, actuating and sensing characteristics have been considered in the finite element formulation. This study includes an optimization method based on genetic algorithms to select appropriate locations of piezoelectric sensors and actuators of a smart composite plate for maximization of control performance. The cost function used in the optimization is based on degrees of controllability, observability, and spillover prevention. Experimental multi-modal vibration control has been also performed. A smart composite plate is prepared according to the optimization results for the experimental works. Coupled positive position feedback algorithms have been implemented in digital signal processing (DSP) system. The control results show that vibration level of the controlled modes has been significantly reduced with negligible effect on the residual modes.

Computer simulation method for crack growth using interface element employing lennard-jones type potential function

Abstract: The phenomena of crack propagation and interface debonding can be regarded as formation of new surface. Thus, it is quite natural to model these problems by introducing the mechanism of surface formation. The authors proposed a method in which the formation of new surface is represented by interface element based on the interface potential energy. The general idea of the interface element and its application to peeling test of bonded plates, push-out test of fiber in matrix, dynamic crack propagation and ductile tearing of steel plate are presented.

Influence of Thermal Aging and Notch on Low-Cycle Fatigue Strength of Forged Mod. 9Cr-1Mo Steel at Elevated Temperature.

Abstract: In this study, the influence of thermal aging and notch on low-cycle fatigue strength of forged mod. 9Cr-1Mo steel was investigated at elevated temperature. Total strain-controlled low-cycle fatigue tests using the smooth specimen and load-controlled low-cycle fatigue tests using the notched specimen were carried out at 873K in air. The triangular, tension-hold and compression-hold trapezoidal waveforms were applied to the specimens in a series of the tests. From the experimental results, there is no effect of the thermal aging on strain-controlled low-cycle fatigue life using the smooth specimen with each strain waveform. However, low-cycle fatigue life of the notched specimen decreased by thermal aging under load-controlled condition with triangular and tension-hold trapezoidal waveform.

Fatigue. The Effect of Atmosphere on Fatigue Properties of High Strength Steel.

Abstract: In order to examine the mechanism of so-called fish eye-pattern fatigue failure, usually being caused by initiation and growth of internal small crack, uniaxial push-pull fatigue tests were carried out on a high strength low alloy steel in vacuum, for modifying the environment during internal crack initiation and growth on specimen surface. The test results showed that the fatigue life was elongated and the crack growth rate was delayed in vacuum. Furthermore, the results of fractographic observation of crack initiation at inclusions revealed that the surface type crack initiation in vacuum resembles the internal fish eye-type crack initiation in atmosphere. Finally the fish eye-type fatigue life was well evaluated in terms of small crack growth law in vacuum.

Strengthening and monitoring concrete structures using glass fiber composites and fbg sensor

Abstract: The research works related to the use of fibre reinforced plastics (FRP) in civil concrete engineering application have been found increasingly only in recent years. Most of the studies were concentrated on the use of bonding FRP onto the plain rectangular beams and columnar concrete specimens under three point bending and uni-axial compression tests, respectively. The results gave a compromising solution for both strengthening and retrofitting of plain concrete samples. Unfortunately, the defection of the reinforced sample cannot be measured visibly due to the coverage of the bonding patch. To increase the efficient and precision in monitoring of the strengthened structure, Fibre-optic Bragg Grating (FBG) sensor is being introduced in embedding it into the structure to measure the internal strain under applied load. This paper presents the experimental results of cylindrical concrete specimens with and without strengthening by wrapping glass fibre composites under uni-axial compression loading. FBG sensors were pre-embedded into concrete specimens and at the interface between the reinforced laminate and concrete surface. The test results show a considerable increase in the ultimate compressive strength and confine the lateral expansion of the strengthened specimens. The strains extracted from the embedded FBG sensor compare well with that measured by the surface bonded electrical strain gauges.

Solid mechanics under magnetic field

Abstract: This paper reviews the basis of mechanics for deformable electromagnetic materials, their typical behaviors under the magnetic field and applications utilizing coupled phenomena of magnetism and mechanics. Electromagnetic field equations and the conservation laws for moving deformable materials are laid down, on the basis of continuum theory of electromagnetic materials. If such a material is under the magnetic field, it is subjected to several kinds of magnetic forces. Further if the material is electrically conductive and the magnetic field is changing in time, there may also exist the eddy current, the induced magnetic field, the Lorentz force, etc. In the case of ferromagnetic materials, can not be neglected the magnetic nonlinearity, the hysteresis and the magnetomechanical effects such as magnetostriction and stress-dependence of magnetization. In many cases, the magnetic field is static, periodically changing in time, or applied to the demagnetized material. In these cases, the equilibrium theory of deformable magnetic materials can be applied to each magnetization process in which the magnetic field is changing in one-direction including the case of rotating magnetic fields. Mechanical properties under such magnetic fields and the magnetization properties under the stress are discussed. As related topics to the above phenomena, introduced are magnetoelastic problems, nondestructive evaluations of material properties and deformation, and applications of magnetic forces.

Analysis of Cracking and Delamination Processes of WC-Co Coating under Bending Load.

Abstract: n order to analyze the cracking and delamination processes of hard coating on ductile substrate under bending load, four-point bending tests were carried out for tool steel specimens with WC-Co cermet coating. The coating was divided by cracks repeatedly with increase in the bending load, and finally the delamination of coating occurred. The interval of cracks at the same nominal stress and those at the delamination ware large when the coating thickness was large. The stress distribution in the coating was analyzed by two-dimensional elastic-plastic finite element method. In addition, the stress distribution in the coating was calculated also by an elastic-plastic stress analysis, which introduced the tensile stress in the coating σx1(x) as follows:σx1(x)=β{1-cosh(αpx)/cosh(αpL/2)}where, L is the crack interval, x is distance from the center of divided coating, and αp and β are constants which contain thickness, material's constant of coating and substrate, and nominal bending stress. The cracking process calculated both by the finite element method and the analytical method agree relatively well with the experimental results.

Correlation between Microstructure and Bending Strength Scatter of Water Corrosion Resistant Al2O3/ZrO2 Composites.

Abstract: The influences of ZrO2 particle addition upon improvement in mechanical properties of Al2O3 ceramics and their degradation due to a deionized water environment under high temperature and high pressure were investigated in terms of the microstructural changes and residual strength characteristics. Sintered Al2O3 bodies containing, 3.3, 10.2, 21.7vol.% of 3mol% Y2O3-doped tetragonal ZrO2 were made using a pressureless sintering process at 1550, 1600, 1650°C, respectively. These ceramic composites were corroded in deionized water under its equilibrium vapor pressure of 1.59MPa at 200°C. The effects of microstructure upon mechanical properties of Al2O3/ZrO2 composites were studied for the specimens treated with, and not treated with water under high temperature and pressure, by residual bending strength measurement, FE-SEM with EDX spectrometry observation and XRD and FE-TEM structural analysis. The mechanical properties were found to be superior when specimens were 21.7vol.% of ZrO2 contents and sintered at 1550°C. It was shown that the strength degradation due to the corrosion by deionized water molecule was suppressed in the case of the specimens with higher contents of ZrO2 particles sintered at lower temperatures. The correlation between bending strength scatter and microstructural defects was discussed.

Application of fast multipole boundary element method to corrosion problems

Abstract: Application of the fast multipole boundary element method (FMBEM) to corrosion problems is studied. It is found that this method can be successfully applied to corrosion analysis by employing the previous research results on the potential problems together with the following procedures (1) use of M 0 0 of the root cell (which is obtained in the process of multipole expansion) for the infinite region problems, and (2) modification of the residual calculation formula in the Bi-CGSTAB iterative method to cope with the nonlinearity caused by polarization curves. A pipe element which is suitable to the FMBEM is also developed to increase the efficiency of calculation for pipelines. A few examples are shown to demonstrate the applicability of the FMBEM to corrosion problems.

Strength of Materials at Elevated Temperatures. Heterogeneity of Creep Properties of Welds in 304 Stainless Steel Plate.

Abstract: An welded joint of 304 stainless steel was fabricated by SAW with use of 308 filler metal to investigate the influence of welding pass sequence and sampling location of the specimen in the welded joint on the creep properties of weld metal zone and heat affected zone in the joint. Creep tests were conducted at 823K and 235MPa. In the HAZ of the welded joint, there existed a hardened region of which specimen showed a little smaller creep rate and longer life than base metal. The 308 weld metal had larger creep rate than the 304 base metal and the life of the former was only 10% of that of the latter (under-matching), however, the difference of the lives becomes smaller for long times. 0.2% proof stress and creep properties of the weld metal zone depended on the sampling location in the welded joint due to the effect of the weld pass sequence and the restraint used during welding. The most important factor that affects the creep properties of weld metal zone would be the difference of micro structure among the beads which was generated from the pass sequence on multi-layer welding. From the results mentioned above, it is necessary, on evaluating the strength of the joint and predicting its life by simulation, to take the heterogeneity of the mechanical properties within the welded joint into account.

Basic Properties of Duplex Stainless Prestressing Steel and Flexural Behaviors of Prestressed Concrete Beams Using the Tendon.

Abstract: The duplex stainless prestressing steel has been developed for durable prestressed concrete members. This steel is cold drawn and has microstructure with two phases composed of ferrite (α) phase and austenite (γ) phase.The duplex stainless steel is excellent in durability. But there are few studies about cold drawn duplex stainless steel compared with those for annealed duplex stainless steel. So we have made research on both the basic properties of the duplex stainless prestressing steel and flexural behaviors of prestressed concrete beams using this tendon.This paper presents the results of tensile test, fatigue test, stress relaxation test, corrosion test and pull-out test, and further flexural test of the prestressed concrete beams using the tendon. The outlines of these results are as follows.The properties of fatigue and stress relaxation of the duplex stainless prestressing steel were roughly equal to those ordinary high carbon prestressing steel, and had satisfactory resistance to stress corrosion, chloride corrosion and satisfactory bond strength to concrete. The prestressed concrete beams using this tendon showed similar flexural behaviors to the concrete beams using the ordinary high carbon prestressing steel tendon.The duplex stainless prestressing steel is small in the elongation at ultimate strength and excellent in durability. Therefore, the tendon is suitable for the external cable system in which the cable tension is not expected to increase so large as in the internal cable system and durability is required. However, the behaviors of the cable at the saddle must also be investigated.A defect of the duplex stainless prestressing steel compared with high carbon prestressing steel is that the tensile strength is low.