Rotating bending fatigue tests of normalized 0.37% C steel smooth specimens were carried out in room air. In order to clarify the effect of air conditions on the fatigue damage, both the temperature and humidity of air were changed for each test. Air conditions used in the tests were moist air at 20°C, moist air at 35°C and dry air at 35°C. The effect of atmosphere on fatigue behavior was investigated in detail based on successive observations of the surface. Decrease in temperature increased fatigue life ; the effect of moisture was not clear compared to that of temperature. The statistical investigation of crack initiation and propagation behaviors indicated that the temperature strongly affects the crack initiation process ; conversely, moisture plays an important role in the propagation process of cracks smaller than 0.3mm. Moreover, the distribution characteristics of crack initiation life, crack propagation life, fatigue life and crack growth rate were analyzed by assuming either a Weibull distribution or a log-normal distribution.

View full abstract

An estimation of fatigue damage derived from buckling deformation is one of the important factors in evaluating the safety margin of seismic design criteria for the thin-walled fast breeder reactor vessel, in addition to estimating the critical buckling strength itself. Displacement response waves of postbuckling state obtained from SDOF models were analyzed from the viewpoint of the fatigue failure mechanism. Cyclic postbuckling fatigue tests were performed on 304 S.S.cylinders to obtain basic fracture data and to simulate the seismic response characteristic by the static approach. Based on these works, fatigue damages were found to be correlated to the maximum global displacement response. Finally, the ultimate state definition in the seismic buckling guideline was confirmed to be sufficiently conservative against the fatigue failure limit.

View full abstract

Influence of shot peening (SP) on the fatigue reliability of HIP-treated aluminum alloy casting was investigated with the aim of improving the fatigue strength. The main results were as follows. The fatigue strength of HIP-treated aluminum alloy casting was improved by SP treatment. A plane facet induced by slip was observed at the origin of the fatigue crack in SP-treated casting. Improvement of the fatigue strength resulted from the control of crack initiation due to surface hardening. Fatigue life characteristics were not affected by surface roughness induced by SP treatment. Fatigue strength of aluminum alloy casting was improved by control of material manufacturing process.

View full abstract

A damage model based on micro mechanics was developed in order to assess creep-fatigue interaction in Modified 9Cr-1 Mo steel. The model assumes the following damage processes for each loading. The fatigue life consists of two stages ; damage initiation life and its growth life. On the other hand, creep life is a process of damage initiation only. In the case of creep-fatigue loading, the damage is initiated by creep loading and grows with the fatigue loading. It was shown that the damage model could accurately predict creep-fatigue life of Mod. 9Cr-1 Mo steel under complicated loading histories.

View full abstract

Dynamic thermal stresses and crack growth behavior produced by single and repeated thermal shocks were studied on cermets and cemented carbides. Thermal shock was applied to specimens using a quenching method. Using the precracked specimen, crack growth behavior was investigated under the repeated thermal shock tests. It was clarified that the crack growth resistance of cemented carbides was lower than that of cermets in the low stress intensity factor range. This difference was discussed from the viewpoint of their microstructures.

View full abstract

In this paper, the fractal nature of crack propagation in rocks is demonstrated by means of acoustic emission (AE) method. AE behavior is monitored during fracture toughness tests conducted on compact tension specimens of three types of rocks. It is shown that the microcrack fracture process zone proceeds the macroscopic crack extension, resulting in nonlinear fracture behavior. Iidate granite, Tohoku marble and Akiyoshi limestone were used. In Iidate granite, the fracture process zone grows in narrow region. Tohoku marble shows that the fracture process zone consists of distributed microcracks surrounding the marcroscopic crack. In Akiyoshi limestone which is preheat treated, extensive crack branching is observed. The AE behavior is characterized by a fractal dimension b which is the slope of the log-log frequency and AE energy distribution. The trend in variation of b-value is shown to correspond to the crack propagation bahavior. In addition, it is found that the fluctuation of b-value can also be characterized by fractal. This observation implies that the self-similarity lies in the rock fracture process.

View full abstract

Fatigue strength under plane bending is higher than that under rotating bending at the same stress. This is due to the difference in the crack growth rates. In the present study, rotating bending and plane bending fatigue tests of an annealed 0.42% carbon steel were carried out on smooth specimens and specimens with a small blind hole in order to investigate the difference in the crack growth rates, and a unified evaluation method for crack growth rates in the two types of loading was proposed. The crack growth rate is expressed by the small-crack growth law based on the nominal stress amplitude σ, dl/dN=Cσⁿl, in each type of loading, while the constants C and n differ in these types of loading. However, the crack growth rates in both cases can be expressed by the same small-crack growth law based on the plastic strain amplitude ε_p, dl/dN=C'εⁿ'_pl (C'and n'are constants in both types of loading). Consequently, fatigue lives in both types of loading can be predicted using the former expression, the small-crack growth law based on the plastic strain amplitude, and the cyclic stress-strain curve in each type of loading.

View full abstract

The problem of a crack along the interface of an elliptical elastic inclusion is analyzed using a body force method. In the numerical analysis, a fundamental solution for the force acting at a point in an infinite plate containing the elliptic inclusion is used. Based on numerical results, the effects of geometry and elastic properties of the inclusion on the stress intensity factors are investigated.

View full abstract

In previous papers, the authors discussed the path integral expression of the J-integral range, ΔJ, for short fatigue cracks emanating from notches under uniaxial stresses and showed that ΔJ evaluated based on the crack-opening level was an appropriate parameter for characterizing the near-tip stress and strain fields. In this paper, finite element simulations of fatigue crack growth at notches under biaxial stresses are conducted. It is found that ΔJ values evaluated by the path integral are almost path independent when the minimum-load level or the crack-opening level was selected as a reference level for the evaluation. The examination of the relationship between ΔJ and the crack-tip-opening displacement leads to the conclusion that ΔJ evaluated by the path integral based on the crack-opening level is the most appropriate parameter for representing the mechanical state near the tip of a crack emanating from notches under biaxial stress conditions. Furthermore, it is shown that ΔJ evaluated by Dowling's simple estimation formula based on the crack-opening level agrees well with ΔJ evaluated by the path integral.

View full abstract

In this paper the interaction of multiple edge cracks in a semi-infinite plate is considered. The problem is formulated as a system of hypersingular integral equations using the stress field due to a force doublet as a fundamental solution. In the numerical calculations, unknown functions are approximated by fundamental density functions and Chebyshev polynomials. First, two edge cracks A and B having different crack lengths and inclination angles are analyzed and the effect of crack B upon crack A is investigated. The stress intensity factor (SIF) of crack A is found to be almost constant independent of inclination angle B if the tip of crack B is fixed. Second, periodic edge cracks are systematically analyzed varying the number, distance and angle of cracks. The interaction effect is found to occur mostly due to the distance independent of the angle. Analytical results are also shown when crack parameters are changed slightly from average values in almost equally spaced multiple edge cracks.

View full abstract

A new method for estimating pre-crack length is developed. First, the wavelet transform is applied to a pixel distribution which is obtained from a digital photoimage of a fracture surface. However, the result of the wavelet transform is usually ambiguous. Then the neural network is applied to the result, and the cumulative frequency distribution of the output of the neural network is used to indicate the boundary between crack surface and pre-crack surface. A fracture surface of an Al-Al₂O₃ composite specimen is analyzed to demonstrate the applicability of the proposed method.

View full abstract

Studies using an acoustic emission (AE) method have been undertaken to elucidate the fracture mechanisms of [0]₈ and [0/±45/90]_s AFRP in a static tensile test. In a previous paper, it was confirmed that the fracture phenomena were related to characteristics of the frequency distribution of individual AE waves. Since many AE waves are observed in each deformation process before reaching the final fracture, it is difficult to define a correspondence the representative frequency distribution of AE waves with the representative fracture mechanisms. So, in this paper, a method for new statistical analysis has been proposed. Using this method, it is confirmed that the fracture phenomena can be specified by variation of shape of frequency distribution.

View full abstract

As the authors showed in a previous paper, the criterion for obtaining plane strain at the mid plane of the specimens is established as:β≥0.004<(Κ_c(J)/σ_ys)>²+0.01. However, it is not clear whether this equation can be applied in the region of low transition temperature or in the low shelf region. Therefore, fracture toughness tests were performed in this temperature region to check the validity of the above equation, regardless of temperature, using a mild steel. In addition, the influences of the curved precrack shape on stress state and crack initiation sites near the crack tip were investigated. No difference was recognized between the straight crack and curved precrack shape in the distribution of fracture toughness and crack initiation sites. The results showed that the above equation is valid in the region of low transition temperature but invalid in the low shelf region.

View full abstract

A molecular dynamics simulation is used to investigate the atomic-scale friction generated between two solid surfaces sandwiching a solid thin film. We examine the influence of temperature, load, film thickness and shape of the interatomic potentials on dynamic properties of atoms at the interfaces of the sliding contact surfaces which undergo plastic deformation. The results of the simulations show that the deformation patterns of the two solid surfaces and the force due to frictional sliding depend on the mobility of the thin film and on the temperature. We also find that the film mobility decreases with an increase in the load when the film thickness is small. Moreover, we show that the relationship between the friction coefficient and the temperature in this friction model might be described by the Arrhenius model.

View full abstract

An elastic-plastic viscoplastic constitutive equation for static preloaded materials subjected to dynamic loading is proposed, from which an equation for the speed of an incremental impulsive stress wave is derived. Using the proposed equation, the proposed theory predicts that the slope of the incremental dynamic stress-strain curve dσ/dε coincides with the modulus of longitudinal elasticity Ε at the beginning point of incremental impact, and thereafter, decreases with increasing incremental strain. Using the derived equation, the proposed theory predicts that speed of the precursor of the incremental impulsive stress wave coincides with that of the longitudinal elastic stress wave c₀ at the beginning point of incremental impact, and thereafter, speed of the incremental impulsive stress wave corresponding to any strain except ealstic strain decreases with increasing strain. Using both equations, relation between incremental dynamic stress-strain curves and the speed of an incremental impulsive stress wave is theoretically derived, which agrees with experimental results.

View full abstract

Using an elastic-plastic-viscoplastic theory, numerical analysis is carried out for static preloaded materials subjected to dynamic loading. The theory predicts that the slope of the incremental dynamic stress-strain curve coincides with the modulus of longitudinal elasticity at the beginning point of incremental impact, and thereafter, decreases with increasing incremental strain and that incremental dynamic stress corresponding to strain increases with increasing strain rate. Moreover, the theory predicts that speed of the precursor of the incremental impulsive stress wave coincides with that of the longitudinal elastic stress wave at the beginning point of incremental impact, and thereafter, the speed of the incremental impulsive stress wave corresponding to any strain except elastic strain decreases with increasing strain and increases with increasing strain rate.

View full abstract

Analytical solutions are presented for temperature and thermal stresses in a functionally graded plate subjected to intermittent heating on its surface. The nonhomogeneous thermal and elastic properties of the functionally graded plate are assumed to be symmetric with respect to the midplane. The transient temperature field is obtained from the solution for a transient heat conduction problem in the same functionally graded plate heated continually on its surface that is analyzed by Vodicka's method. Associated thermal stresses are analyzed based on Rogers and Spencer's solutions which are expressed in terms of the solution to the approximate, two-dimensional, thin-plate, governing equations for an equivalent homogeneous plate. Numerical calculations are carried out for the transient temperature and thermal stress distributions in PSZ/SUS 304/PSZ functionally graded plates subjected to intermittent heating on both plate surfaces.

View full abstract

We present an analysis of the relative movement of shrink-fit connections between gears and shafts. First, it is shown experimentally that microslip can occur on mating surfaces even if transmitting torque does not exceed the static frictional torque. Therefore, the relative movement increases with the number of rotations. However, there is a transmitting torque threshold below which no relative movement occurs. The contact behavior and microslip of mating surfaces are also investigated with increasing transmission torque, by three-dimensional finite-element analysis. The experimental threshold values agree well with those predicted from theoretical calculations on the slip model, which are based on the numerical results of shear stress distribution along the interface and on Coulomb's law of friction. These thresholds are functions of the gear/shaft design parameters, such as the ratio of shaft diameter to gear rim thickness, interference, friction coefficient, and gear width. The results can be used to establish qualitative criteria to improve connection design.

View full abstract

An ultrasonic technique was developed to study the characteristic for abrasion pattern of a rubber surface during stick slip motion and to demonstrate its applicability to the precisely contact pressure measurement utilizing Wigner distribution. Experiments were conducted using short cylindrical acril specimens pressed onto the rubber sheet bonded on a rotating disk. It was found that the contact pressure may be nearly zero soon after the locally high contact pressure area moves to the leading edge. In addition, the experimental results showed that the formation mechanism of the abrasion pattern observed on the rubber surface has close relation to locally sudden variation in the contact pressure due to a stick-slip phenomenon.

View full abstract

The elastic-plastic behavior of SNCM 439 steel, glass and silicon was investigated using an ultra-micro hardness apparatus which was newly developed based on an atomic force microscope (AFM). Force-penetration curves showed the yield behavior. Below the yield force where the penetration depth was smaller than 50 nm, no indentation was formed and the experimental curve agreed with the theoretical one obtained from the FEM elastic analysis. The normalized yield stress, τ√(G), was 0.27 for SNCM 439 steel and 0.40 for glass and silicon, where G is shear modulus. It was concluded from these results that the elastic properties such as Young's modulus and yield stress could be measured in the nanoscopic region using the new AFM nanoindenter.

View full abstract

If the quantitative measurement of the out-of-plane component of elastic displacement in a plane stress model is possible, stress analysis in a full field can be achieved by adding photoelastic information from isochromatics. Even if the directions of collimated illumination and observation in holographic interferometry are normal to the diffusible surface of a strained body, one can not obtain isopachics, a fringe pattern showing lines of equi-thickness, because of the perspective effect. However, holographic interferometry based on image-plane holography in which both a telecentric optical system and collimated illumitation are used simultaneously, can yield isopachics and overcome the perspective effect. In very sensitive experiments such as those of holographic interferometry, however, due to lack of rigidity of the total loading system, translational and rotational movements of should be added and comes into fringe pattern inevitably. Consideration of experimentally obtained photographs and incorporated computer simulations enables estimation of the real extent of rigid displacement and evaluates the above deduction.

View full abstract

In this paper, a method of evaluating static strength by applying the newly difined stress intensity factor of an angular corner is considered. Recently, it has been found that the stress field near a corner of jointed dissimilar materials is expressed as a sum of the symmetric state with a singularity of 1/r^1-λ1 and the skew symmetric state with a stress singularity of 1/r^1-λ1 In this respect, the fracture tests are carried out on the plane specimens of acrylic resin having single and double sharp notches. Different artificial notches are introduced into the specimens with varying the opening angle, the notch depth, and the loading conditions. It is found that the fracture toughness K_{1C, λ1} is almost constant independent of the notch depth and the loading condition. In addition, the value of K_{1C, λ1} increases with increasing opening angle because of the decrease of the order of singularity. The fracture criterion for different opening angles of the notch is also discussed on the basis of the stress distribution near the notch. The usefulness of K_{1, λ1} in the evaluation of static strength is confirmed through experimentation.

View full abstract

The strength of tape-wound structures depends on many factors such as winding angle and winding path. Because winding path cannot be obtained explicitly before production, it is very difficult to evaluate the strength. Therefore, a computer system for the design and analysis of wound structures has been developed. This system consists of calculating the winding path and generating a finite-element model to predict the strength. This paper focuses on the algorithm of winding simulation and the modeling method of complicatedly would tape layers. As an example, design and FEM analysis are carried out for CFRP-tape-wound pipe. It is found that the developed system is very useful for design and safety evaluations of winding structures.

View full abstract

This paper deals with numerical solutions of singular integral equations in interaction problems of diamond-shaped inclusions with angular corners under various loading conditions. The problems are formulated as a system of singular integral equations with Cauchy-type or logarithmic-type singularities, where the unknown functions are the densities of body forces distributed in infinite plates having the same elastic constants as those of the matrix and inclusions. In order to analyze the problems accurately, the unknown functions of the body force densities are expressed as a linear combination of two types of fundamental density functions and power series, where the fundamental density functions are chosen to represent the symmetric stress singularity of 1/r^1-λ1 and the skew-symmetric stress singularity of 1/r^1-λ2. Then, newly defined stress intensity factors of angular corners are systematically calculated for various shapes and spacings of two diamond-shaped inclusions in a plate subjected to uniaxial tension, biaxial tension and in-plane shear. The present method is found to yield rapidly converging numerical results for the interaction of diamond-shaped inclusions.

View full abstract

A method is proposed for determining the temperature cycling test conditions for solder joints of electronic devices. First, the purpose of the temperature cycling test is clearly expressed. Next, the concept of the fatigue damage rate based on Miner's law is introduced. The fatigue damage rate is obtained from the finite element analysis of the solder joint and the fatigue strength data of the solder. In this method, the testing conditions are determined so that the damage rate during the temperature cycling test equals that in the Hield after the reliability guaranteed period. The influence of temperature and cycling frequency on the fatigue strength of the solder and the dispersion of the strength of solder joints are considered in determining the testing conditions. Finally, an example of the proposed method is shown.

View full abstract

We present a new discrete beam model of a thin belt plate, such as a sheet, film or tape, for torsional deformation analysis. First the discrete beam model is introduced using the torsion angle and slope angle. This model consists of torsional springs which describe torsional rigidity, and rotational springs which represent bending-torsion rigidity. The formulation for torsional deformation analysis is presented taking into consideration the tension. To evaluate this model, the torsion angles calculated using the model are compared with conventional theoretical results. It is concluded that the results obtained with the discrete model show good agreement with the theoretical results.

View full abstract

This paper describes an application of the hierarchical neural network to defect identification with the ultrasonic method. The present method consists of three subprocesses. First, sample data of defect parameters vs. dynamic responses of djsplacements at several monitoring points on solid surface are calculated using the dynamic finite element method. Second, the back-propagation neural network is trained using the sample data. Finally, the well trained network is utilized for defect identification. This procedure is successfully applied to the identification of sizes and locations of multiple defects hidden in solid, and its performance is discussed in detail.

View full abstract

The two-dimensional mesomechanical simulation method previously proposed by the authors for brittle microcracking solids containing microinclusions is applied to the analysis of R-curve behaviors for a macrocrack under mode-I loading. Influences of grain size, residual stress, total volume fraction of microinclusions and critical microcracking stress at intergranular facets on the fracture toughness of single-as well as two-phase materials are discussed through the calculated results.

View full abstract

In woven fabric composite under tensile loading, microfracture occurs in the transverse fiber bundle at the initial stage of the tensile test. Knee-point on the stress-strain curve appears due to the occurrence of the microfracture. The microfracture is defined as the first fracture phenomenon under tensile test. The stress at the microfracture ; initial microfracture stress is known to be affected by interfacial strength and can be used as the index of fiber/matrix interface. In this study, a new method which can specify material constants of interface was proposed. The initial microfracture stresses obtained both by finite element analysis and by experimental analysis were used, and the combination of elastic modulus and strength was determined by comparing both numerical and experimental results. As a result, on such conditions that the material constants of intersection element were same as those of resin element, when the elastic modulus of interface element in range from 3.2 GPa to 40 GPa was assumed, a unique strength of interface element in range from 20 MPa to 78 MPa could be determined.

View full abstract

There are, at present, too many design parameters for composite structures such as reinforcing fiber architecture, fiber orientation, and volume fraction, among others. Therefore, the material data base system, which can calculate three-dimensional mechanical properties of lamina and laminate using the classical stacking theory, has been developed. The advantage of this system is that it is able to calculate mechanical properties easily and visually by using graphical user interface. As an application of the developed system, a wastewater treatment tank made of GFRP under internal water pressure has been analyzed. The computational results show good agreement with the experimental ones. It is revealed that the proposed system is very useful for three-dimensional structural analysis of composite materials.

View full abstract

Three point impact bending tests on fiber-reinforced ceramics were carried out using a split-Hopkinson pressure bar method. Ramped incident waves were produced by plastic deformation of zinc and were applied to the specimens. Smooth transmitted stress waves without high-frequency fluctuation were obtained to evaluate dynamic stress-strain relations precisely. The bending modes in the impact tests were found to be almost the same as the static ones and dynamic strength can be measured exactly. Numerical analysis using finite-element method showed that analytical results agree with experimental results very well.

View full abstract

In order to evaluate the strength of Si₃N₄/SUS304 joints with Cu interlayers, bending tests were carried out from room temperature to high temperature in air, vacuum, or argon gas. Fractography and fracture mechanics studies were also carried out. The results obtained are as follows, The typical crack initiation site is the interface between Si₃N₄ and a brazing filler. The crack propagates along the interface, and kinks into Si₃N₄. The bending strength is not influenced by the testing atmosphere (air, vacuum, argon). When testing temperature is under 300°C, bending strength is constant, and when testing temperature over is 300°C, bending strength decreases because of the softening of the brazing filler. When α_min is over 30μm, the stress intensity factor considering residual stress, Κ_pr, at which the crack kinks into Si₃N₄ is constant and almost agree with the fracture toughness, Κ_ic, of monolithic Si₃N₄. On the other hand, when α_min is under 30μm, Κ_pr is lower than the fracture toughness of monolithic Si₃N₄.

View full abstract

A new type of load support system for the superconducting magnet of MAGLEV trains has been developed. This connects a coil case kept at liquid helium temperature (4.2K) with an outer case kept at ambient temperature (about 300K). Thus, this support system possesses low thermal conductivity as well as high stiffness and sufficient strength. In this paper we show how this new support system is developed. First, we define a characteristic parameter "specific stiffness" which is the product of support system stiffness multiplied by thermal resistance. Then, we evaluate this specific stiffness for various structural shapes, such as bars, cylinders and cones. On the basis of these evaluations we select a cone-shaped support system made of alumina-FRP. The optimization of cone angle and lamination angle by finite element analysis is discussed. Finally, measured results from several new cone-shaped support systems are presented, including thermal conductivity, stiffness, stress distributions and strength.

View full abstract