Aluminum alloys, A5083 and A7075, have the highest strength among the non-heat treated and the heat-treated aluminum alloys. They are widely utilized in the transportation systems because of their specific strength. Fracture mechanics design needs the information on the fatigue properties of the materials. The experiments were carried out to study the effect of the environment on the fatigue crack propagation of both alloys. The tests were conducted in the atmosphere of controlled humidity. The crack propagation behavior was analyzed using the Paris equation, da/dN = C(ΔK)^m . The constant, C strongly depends upon the environment, while the exponent, m is associated with the fracture mechanism. The ductile fatigue crack propagation shows m 〜 2 for both alloys at any environment. The constant, C is expected to be higher under wet conditions. The aggressive environment together with the frequency can increase the fatigue crack growth rate. The most aggressive environment was moisture which was composed of hydrogen and oxygen atoms. Hydrogen in the water can be decomposed into atomic state on the new aluminum surfaces which are introduced by the fatigue fracture and result in the acceleration of the fatigue crack propagation.

View full abstract

Toughness evaluation of narrow high-energy-beam welds by the conventional Charpy V-notch (CVN) impact test is not all successful because the crack often deviates from the fusion zone into the heat affected- or unaffected base metal-zones. The small-punch (SP) test was applied to the electron-beam welds of a steel with a 490 MPa yield strength to evaluate the toughness by measuring the ductile-brittle transition temperature (DBTT). The steel was electron-beam-welded with various heat-inputs and the welds were examined by Charpy V-notch and small-punch tests at 40-298 K. The SP-DBTTs obtained from the SP energy show a linear correlation with the CVN-DBTTs. The load-deflection curves during the small-punch tests and the fractography of small-punch tested specimens supported the ductile-brittle transitions. The results suggest that the small-punch test can be used to predict the DBTTs of the electron-beam welds, which cannot be accomplished by conventional Charpy V-notch impact tests.

View full abstract

Sensitization by chromium depletion due to chromium carbide precipitation at grain boundaries in austenitic stainless steels can not be prevented perfectly only by existing conventional techniques, such as reduction of carbon content, stabilization-treatment, local solution-heat-treatment, etc. In this investigation, a thermomechanical treatment was tried to improve the resistance to the sensitization by grain boundary engineering (GBE). A type 304 austenitic stainless steel was cold-rolled and solution-heat-treated, and then sensitization-heat-treated. The grain boundary character distribution was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of coincidence-site-lattice (CSL) boundaries indicated a maximum at the small reduction. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material. A high density of annealing twins were observed in the thermomechanical-treated specimen. The results suggest that the thermomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface.

View full abstract

As a part of a research programme investigating the geothermal reservoir potential of deep-seated high-temperature rock masses, experiments have been undertaken to determine the crack behaviour induced by hot-rock/water interaction. Cylindrical specimens of Iidate Granite were heated, whilst subject to static water conditions, under some confining pressure. The effect of temperatures of up to 600℃ was investigated, whilst the pore fluid and confining pressures were maintained at 50 and 100 MPa respectively. The time-dependence of the water/rock interaction, under these conditions, was also determined, with experiments ranging in duration from 0.5 to 96 hours. Experiments to date, have investigated the quartz constituent of the Iidate Granite samples and results show that crack healing in quartz grains is significant at temperatures of 330℃ and above. This temperature effect is also enhanced with increasing time. No crack healing in quartz is observed at temperatures of 250℃. Additional experiments, using the indentation fracture method, show that these healed cracks recover strength to that of the matrix quartz grains. Although these results might appear to indicate that significant permeability enhancement should not be expected in a supercritical geothermal reservoir, micro-structural data show that significant intragranular micro-cracking also occurs at the temperatures greater than 400℃ due to the water/rock interaction. This micro-cracking phenomenon results in an overall increase in the permeability of the granite. This research presents important results which require due consideration in the potential development of a supercritical deep-seated geothermal reservoir.

View full abstract

Cavitating jet can be used to strengthen metallic materials by introducing compressive residual stress into the surface in the same way as shot peening. This surface modification method is called "Cavitation shotless peening", since shot used in shot peening is not required and the impacts of cavitation babble collapses is utilized to peen the surface. In order to demonstrate the effectiveness of cavitation shotless peening, almen strip was exposed to the cavitating jet at several jet conditions. The almen strip is generally used for evaluation of the peening intensity of shot peening by measurement of whose arc height. The almen strip exposed to the cavitating jet bent and the arc height occurred depending upon the jet parameters such as cavitation number, standoff distance, and nozzle diameter. Two types of almen strip, A and N, having different thickness, were used in this study. In the case of cavitation shotless peening, the relation between arc height of thin almen strip and that of thick one was different from that of shot peening. Furthermore, the surface roughness of almen strip exposed to the cavitating jet was significantly smoother than that of shot peening. It is concluded that cavitation shotless peening is effective in the surface modification and that the peening intensity of cavitation shotless peening changing with cavitating jet conditions can be evaluated by using almen strip.

View full abstract

The true stress-strain relation calculated from the relations of load against tensile and compressive surface strains under the Bernoulli's assumption of deformation of beam show nonlinearity and is different between tension and compression. The nonlinearity of the stress and strain relation comes from domain switching induced by external loading. The degree of nonlinearity increased with the easiness of domain switching in bent specimens. The fracture strength in terms of the nominal and true stresses follows the Weibull distribution. The shape parameter takes a large value around, 18 to 22, for PT and PS specimens, while it is the smallest, 7 to 8, for PL specimens. NP specimens take a value in between. The mean strength is the largest for NP specimen and is the lowest for PL specimen. Domain switching during loading will relive local stress concentrations of defects within the material, thus increase the strength and the shape parameter. The change in intensity ratio of the 002 to 200 diffractions can be used to assess the degree of domain switching caused by external straining.

View full abstract

In the present paper, a method for determining the fracture characteristic of the ceramic coat layer/bond coat interface was developed. The uniaxial tension and four point bending tests were conducted on plasma-sprayed rectangular specimens of two-layer system and functionally gradient material (FGM) layer. The top coat layer was yttria-partially-stabilized zirconia (YSZ) and the bond coat layer was MCrAlY, both of which were deposited by plasma-spraying technique. FGM consisted of 5 layers: 100% YSZ, 80%YSZ+20%NiCrAlY, 60%YSZ+40%NiCrAlY, 40%YSZ+60%NiCrAlY, 20%YSZ+80%NiCrAlY. The detailed fracture process in the thermal barrier coating system was examined by using scanning electron microscopy (SEM). It was shown that vertical multiple cracking preceded in the top coat followed by the propagation of interface crack between top coat and bond coat. A shear lag model was used to evaluate the interface strength. The experimental results show that FGM specimens have higher interface strength than non-FGM specimens.

View full abstract

As the initiation of fatigue cracks on smooth surfaces is predominantly a surface phenomenon, extensive efforts have been focused on characterizing a surface damage such as extrusion and intrusion during fatigue. Therefore careful surface analysis techniques such as measurement of the work function (have after denoted as WF) using a Kelvin Probe would have a potential to predict an initiation of fatigue cracks. The WF is very sensitive to changes on the surface and hence it is believed that the formation of extrusions and intrusions during fatigue would affect the work function of the sample and this can be easily detected by the Kelvin probe technique (vibrating capacitor method). Generally, the bare surface of a metal has an active work function following the general e.m.f series. In this present investigation, the Kelvin probe is employed to study the change in work function of a specimen surface after predetermined increments in the number of fatigue cycles. During the initial fatigue cycles, the work function becomes more active than that prior to fatigue loading. This can be attributed to the formation of new surfaces due to the generation of intrusions and extrusions during the fatigue test. When exposed to ambient conditions at room temperature, the work function increases towards a more noble direction with time as the new surface oxidised. Through the continuous measurement of the WF during fatigue, it was observed that the formation ratio of the new surface area rapidly decreased after N/N_f=0.02. On the other hand, the logarithmic rate law constant of oxidation of the new surface gradually decreased with the degree of damage. Furthermore, during the final stages of fatigue, a greater number of intrusions without oxides develop compared to that of extrusions. Oxides formed preferentially on extrusions as observed by the elemental mapping obtained by Time-Of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) analysis. More active development of intrusions can result in the initiation of a crack.

View full abstract

Hydroxyapatite (HAp) ceramics were prepared by hydrothermal hot pressing (HHP) processes through the reaction between octacalcium phosphate (OCP) and ammonia water, at 300℃. When the amount of ammonia was over the chemical stechiometric ratio, pure HAp ceramics was obtained. The high concentration of ammonia water was advantageous for the solidification of the HAp ceramics. The appropriate amount of water improved the toughness, but greatly reduced the tensile strength. With addition of 10% high concentration ammonia water, HAp ceramic was achieved at 300℃ by HHP method. The density, fracture toughness, and tensile strength of the ceramics reached 2.82g/cm^3, 1.07MPam^<1/2>, and 18.2MPa, respectively. The present study demonstrated that the HHP method was a useful method for preparing HAp ceramics at low temperature.

View full abstract

A database-mining project has been introduced to provide an improved capability of envisioning by clarifying the combined effects on material degradation. To make this goal feasible an extensive study has been conducted to determine the data structure, database architecture and search scheme. The design and performance of the first-generation prototype system that can evaluate interactions between material characteristics and environmental features is described.

View full abstract

WC-Co thermal spray coating is widely applied for high wear resistance coatings. But it is also known that the coating has poor corrosion resistance in aqueous solution. In this study, two kinds of powder, WC-12Co and WC-10Co-4Cr, were used to form the coating by high velocity oxygen-fuel flame spraying on mild steel. Corrosion behavior in Na_2SO_4 solution was investigated under anodic polarization, impedance response and distribution of potential on the coating surface. The current density of WC-12Co thermal spray coating in passive regions changed with the pH of the aqueous solution, becoming high in neutral or alkaline solutions, and low in acid. Therefore, WC-12Co thermal spray coating shows poor corrosion resistance in neutral and alkaline solutions. However, since the current density on WC-10Co-4Cr thermal spray coating, in which Cr is added, to the WC-Co system, is lower than that in WC-12Co thermal spray coating for each pH, the former has higher corrosion resistance. On the other hand, it was found that changes in impedance diagram at constant potential corresponded to the changes in the polarization curve. Therefore, it was suggested that the reaction on the corrosion interface could be estimated from the changes in the diagram. The corrosion reaction rate for WC-10Co-4Cr thermal spray coating is better than that of WC-12Co thermal spray coating. Thus, it is thought that control of the dissolution of the coating and substrate components is owing to the early formation and recovery of the passive state of the coatings.

View full abstract

It is known that the impact toughness of polycrystalline Ni-base superalloy, commonly used in gas turbine blades decreases with increasing exposure time at an elevated temperature. For the present study, the change in impact toughness of the Ni-base superalloy Udimet 520, which had been used in service as a first-stage blade in a 1150℃ class gas turbine for 31626 hours, was investigated by small punch (SP) tests. Experimental results revealed that in the pitch line region where the metal temperature was highest there was a decrease of the SP fracture energy E_<SP>, although the E_<SP> measured on the root shank was higher than that on the unexposed blade. That is, in contrast with the root shank, the pitch line region of the present U-520 blade became embrittled due to prolonged in service exposure. It was also found that the increase in the amount of grain boundary M_<23>C_6 carbides, which provide sites for cracks to initiate and/or propagate, was a predominant factor in the thermal aging embrittlement of U-520. Additionally, the influence of rejuvenation treatment, which is usually conducted to restore creep strength, on the toughness has also been examined by SP tests

View full abstract

The environmentally assisted cracking (EAC) behavior of four austenitic steels in high temperature water environments was investigated in order to understand the role of grain boundary microchemistry in irradiation assisted stress corrosion cracking (IASCC) in boiling water reactors (BWR) and pressurized water reactors (PWR) environments. The chemical composition of the materials used was 12Cr-28Ni doped with Si, P or S impurities, to simulate the grain boundaries of highly irradiated 304 stainless steel. The steel was tested using fracture mechanics on compact tension specimens with trapezoidal wave loading. Results showed that, in simulated BWR water at 288℃, the addition of S and sensitization by a thermal treatment raised the EAC susceptibility, whereas the addition of Si or P had no apparent effect. For PWR primary water at 325℃, dissolved H_2 in the water had a significant effect on cracking. Lower H_2 content resulted in a lower EAC crack growth rate. Thermal sensitization treatment reduced the EAC susceptibility in hydrogenated PWR primary water but had no significant effect in the water without H_2.

View full abstract

A systematic survey of reports on abnormal events experienced in Japanese nuclear power plants has been carried out to identify mechanisms that are responsible to anomalies and failures. The idenfitied mechanisms are then decomposed step-by-step to most basic elementary events called primitives. The decomposition procedure has been supported by our unique model of describing onset and development of a failure event as a consequence of interactions among primitives. The potential advantage of the proposed method has been demonstrated though in-depth analysis of a set of anomaly events experienced in several nuclear plants.

View full abstract

The braking performance of brake pad for railway vehicle was investigated in this study. The grooves number on the friction surface was considered in order to evaluate to braking performance. The purpose of this study is to optimize a brake pad design of friction surface on the basis of analyses of a combination of mechanical and thermal load due to braking conditions. The finite element method analysis was used to optimize the groove number of brake pad. The results of the thermal stress analysis on brake pad showed that the thermal stress for three groove pad was reduced to about 8% as compared to that for two groove pad. A full-scale brake dynamo test was carried out to characterize the braking performance such as friction coefficient, wear rate and braking temperature. The result of full-scale brake dynamo test showed that the grooves of brake pad were related to the variation of friction temperature and wear amount of brake pad under braking applications. In conclusion, the braking performance brake pad with three grooves on the friction surface was better than that of two groove pad.

View full abstract

Some plants whose stems lack the strengthening tissues to maintain them in an upright position support themselves by means of thin appendages called tendrils that grasp onto objects they encounter. The tendrils of some species coil after tactile simulation and form spirals. The spirals have reverse spiraling points where the spiral direction is reversed. This differs from normal spiral springs used in the field of industry. This study deals with the observation of the formation of tendril spiraling using a video camera, and the evaluation of the mechanical properties of coiled tendrils of various species using tension tests and finite-element analysis. The results revealed that reverse spiraling points play an important role in forming tendril spirals, and the spring constants, which were obtained as the values of tensile loads divided by the elongations of a tendril, varied with the increase of the elongation of spirals.

View full abstract