In this study, the influence of droplet distribution on fatigue strength of SUS 304 with CrN film was examined. Three point bending fatigue test were carried out for two series of CrN films coated onto austenitic stainless steel SUS 304 by arc ion plating (AIP) method. The CrN films were deposited under two different coating conditions for obtaining the different droplet distribution. The fatigue strength was decreased by coating under both coating conditions. The fatigue cracks of the coating specimens initiated at stress range below the fatigue limit of uncoated specimen. The difference between the two samples appeared in the crack initiation and propagation behavior. In the case of a few droplets sample, the fatigue cracks initiated on the film surface at the very early stage but the crack growth rate was slow. In the case of the film with many droplets, the crack initiation life was long, however, the crack growth rate was higher than that of a few droplets sample. In addition, It was easy for the film with many droplets to coalesce the fatigue cracks. The fatigue behavior was caused by the difference of these crack initiation and propagation between two samples.

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Ultrasonic fatigue tests at 20 kHz were conducted under R=-1 and R=0 for a low-temperature-tempered JIS-SCM 440 low-alloy steel, together with 50 Hz tests using a servo-hydraulic fatigue testing machine. The ultrasonic fatigue testing machine, with which tensile mean stress could be applied, were specially prepared in this research. In these fatigue tests, fish-eye fracture occurred both under R=-1 and R=0. Although the fatigue test results were scattered, difference between the 20 kHz and 50 Hz results were little in case of fish-eye fracture. The origins of fish-eye fracture were mostly an Al₂O₃ inclusion, whose sizes ranged from 5 to 35μm. ODAs, i.e. optically dark areas, were observed on the fracture surfaces of fish-eye fracture both under R=-1 and under R=0. The ODA sizes under R=0 were, however, smaller than those under R=-1. The scattering of fatigue test results in the fish-eye fracture region were caused by difference of the inclusion sizes and could be reduced in the modified S-N diagrams in which stress amplitudes were normalized with fatigue limits estimated by Murakami's equation. In the modified S-N diagram, the results at 20 kHz showed good agreements with those at 50 Hz under R=0, as well as R=-1. These results supported the validity of the ultrasonic fatigue testing under tensile mean stress in case of fish-eye fracture.

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For an actual crack growth in structures subjected to the applied stress from the various directions, it is important to study about the fatigue crack propagation behavior under mixed-mode condition. In particular under the condition, crack surfaces tend to contact when the load is applied because of the compressive residual stress distributed near the crack and the zigzag crack surface morphology. In this study, using slant cracks with compressive residual stress induced in mode I fatigue crack propagation, discontinuous displacements (Mode I : crack opening displacement, Mode II : crack sliding displacement) were measured along the slant crack. The stress distributions along the crack were calculated from the discontinuous displacements and the stress intensity factors (K_I)_est and (K_II)_est were estimated from the calculated stress distributions. As a result, the residual stress distribution along the crack was evaluated from the calculated stress distribution. Using the estimated stress intensity factors, (K_I)_est and (K_II)_est, an interaction between the compressive residual stress and the crack surface contact behavior was discussed.

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Frequent start-stop operation of gas turbine sometimes causes coating cracks on the surface of first stage blades. The coating cracks at suction side of blade surface are caused in the form of multiple parallel cracks with narrow intervals. The objective of this paper is to evaluate the propagation behavior of this multiple cracks. Temperature distribution of cracked area during operation was evaluated by observation of microstructure change with long term operation and the resultant thermal stress distribution was calculated. A series of finite element analyses were performed for multiply edge cracked plate with finite width subjected to crack face pressure. The results were used to establish evaluation method of K values of multiple parallel cracks. The established method was used to evaluate crack propagation behavior of the blade surface together with the evaluated temperature distribution and the calculated stress distribution.

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The 3-dimensional finite element analysis was carried out for the compact normal and shear specimens made of the acrylic homogeneous material and the aluminum-acrylic dissimilar material subjected to mixed-mode loading. The 3-D local hybrid method based on an inverse problem was applied to evaluate the 3-D stress field using displacement data surrounded the crack tip of the surface of 3-D full model FEM. The error of stress intensity factor between the full model FEM and the local model hybrid method was discussed. The optimal hybrid domain size was discussed by changing the specimen thickness. Moreover, the optimal depth of the error evaluation was discussed to evaluate an equivalent nodal force by using displacement data obtained by the 2-D intelligent hybrid method.

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In order to investigate the fracture mechanism in fatigue of Ni-base superalloy at elevated temperatures, rotating bending fatigue tests were carried out for Inconel 718 at 500°C and 600°C up to 10⁸ cycles. At both temperatures, fracture occurred from a subsurface of the specimen in long life region, though the origin of fracture was the specimen surface in short life region. Consequently, S-N curves showed a two-step shape in elevated temperature tests. Although a surface crack was observed similar to the result at room temperature even in long life region at elevated temperatures, the surface crack stopped propagating. In this case, interganular cracks were observed at an origin of subsurface fracture. The subsurface damage was initiated at the early stage of fatigue life.

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This paper deals with the bulge process for forming the single-stage bellows of TiNi shape memory alloys, which is proposed as a new type of seismic applications, and especially considering the material's special behavior. Thin walled tubes with 20% cold work, whose composition is Ti51.0 at% Ni, were prepared. First they are appropriately heat-treated and then the rubber bulge process is introduced for the tubes under the condition of austenite phase at room temperature. Displacement control method is adapted to the process. Theoretical prediction of change in outer diameter of the tube on compression is derived, and modified taking into account the progress of the stress-induced martensite transformation on tube's surface by observing the detachment of the oxide layer of the surface. Finally theoretical relationship between compressive displacement and the outer diameter of the tube, which is the most inportant for the design of the bellows shape, is cleared.

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This paper presents the characterization of nonlinear viscoelastic behavior in vinylester resin. The description of nonlinear viscoelastic behavior is based on well-known Schapery's constitutive equation. Cardon proposed the method that can estimate nonlinear parameters in Schapery's constitutive equation, separately by creep-recovery tests in consideration of permanent deformation nonexistent in original Schapery's constitutive equation. However, formulation of permanent deformation has almost never been discussed in these articles, In this study, the irreversible strain is formulated and nonlinear viscoelastic constitutive equation taking into account the permanent deformation is proposed. It is found that the behavior of irreversible strain is nonlinear and it has a threshold stress value between the linear behavior and the nonlinear behavior same as nonlinear parameters in Schapery's constitutive equation. Tensile tests in which crosshead speed is changed as time-dependent factor is performed to verify that the proposed constitutive equation can predict time-dependent behavior. Fine agreement between the experimental results and prediction is observed and the validity of the constitutive equation is confirmed.

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The quadratic equation in which Young's modulus is a function of porosity of sintered iron was applied to the experimental results of sintered Fe-Cu materials. Vickers hardness and Poisson's ratio were also described as the quadratic equation of porosity of sintered Fe-Cu materials. The experimental constants were affected by the type of Fe and Cu powders and sintering temperature. In comparison with Young's modulus against porosity, the relationship between hardness and porosity were affected by those factors. It was clarified that Young's modulus and Poisson's ratio can be described as a linear function of hardness in the case of sintered Fe-Cu materials at the porosity below 20%. Consequently, in the case that the linear relations of Young's modulus and Poisson's ratio to hardness for use powder and sintering condition are known, those can be calculated from the measured value of hardness.

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Smooth specimens, semi-circular notched and V-notched specimens of porous silicon carbide with porosity of 35% were fatigued under cyclic four-point bending at a stress ratio of 0.1. The reduction of the fatigue strength due to notches can be estimated from the reduction of the crack initiation stress in static bending fracture tests. The crack propagation curve in V-notched specimens was divided into stages I, II and III. In stage I, the crack propagation rate decreased even though the applied stress intensity factor got larger with crack extension. The crack propagation rate was nearly constant in stage II, and then increased in stage III. The anomalous behavior in stages I and II was caused by crack-tip shielding due to microcracking and asperity contact, while the crack tip shielding was minimal in stage III. For the case of semi-circular notched specimens, stage I was followed by stage III without stage II. The crack length at the end of stage I took a constant value of about 0.068 mm for semi-circular notched specimens and 0.12 mm for V-notched specimens. The stress intensity factor at the onset of unstable fracture was about 1 MPa·m ^1/2 for both types of notched specimens.

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Specimens of a zirconia-alumina composite were deformed superplastically to pre-determined tensile strains at various temperatures with different initial strain rates. Microstructures evolved during the superplastic deformations and variations in mechanical properties at room temperature were investigated on the specimens. Cavities were formed in the specimens during the deformations. The amount of the cavities in the specimens increased with an increase in the strain. Fracture toughness of the specimens changed complicatedly with the increase in the amount of cavities. It was found that the fracture toughness was fairly improved when the cavity volume fraction becomes around 1%. The fracture toughness was also affected by the cavity size. We concluded that the cavities with diameters of less than about 0.3 μm can improve the fracture toughness, whereas the cavities with sizes larger than about 0.3 μm have a negative effect on the fracture toughness.

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This paper describes the experimental results of the ultrasonic welding of ceramics and metals. In comparison to other welding methods, ultrasonic welding has made welding of various ceramics, such as ZrO₂, SiC, and Si₃N₄, and metals such as aluminum, magnesium, and copper in the atmosphere easier and quicker. In this study, we accomplish the ultrasonic welding of ceramics and magnesium under the following conditions : amplitude, 30 μm ; welding pressure, 10 MPa ; required duration, 1.0 s. The ultrasonic welding of the ceramics with metals became possible when the condition E =KPⁿ < f (P, E) (E : energy density, P : welding pressure) was satisfied and the welding interface temperature was in the range of 300°C-400°C. When the ceramics were preheated, welding became possible even in a short time and under a low pressure. The ultrasonic welding material had good weldability even in a high temperature (200°C) environment. In this environment, it is expected that the oxide and organic films are efficiently removed from the bonded interfaces by the vibration of an ultrasonic wave for ultrasonic welding.

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The influence of flow velocity and cavitation number on erosion rate was studied by using the cavitating liquid jet method. The optimum stand-off distance was established at four cavitation numbers, and the distance coincides with that given in the ASTM G 134 standard. The cavitation erosion tests were carried out at a constant cavitation number at various flow velocities. The maximum mean depth erosion rate (MDERmax) increases with the flow velocity with the power of 3.9 to 4.6 and the power does not change with cavitation number. The MDERmax increases with cavitation number between 0.015 and 0.03. A linear relationship was obtained between MDERmax and ∑F²_i (summation of squares of impact loads which is equivalent to the impact energy) under various cavitation.

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