Statistical fatigue tests were conducted on low alloy steels, SCM 435 and SNCM 439, in room air and in 3%NaCl solution. Fatigue life distributions were examined at three stress levels, and were analyzed by the three-parameter Weibull distribution. In both environments, fatigue life distributions of both steels followed the three-parameter Weibull distributions. The scatter of fatigue life in 3% NaCl solution was smaller than that in room air. From the observation of the fracture surfaces in 3% NaCl solution, it was found that fracture of SCM 435 at all stress levels resulted from the growth of a single crack, while that of SNCM 439 occured by the growth of multiple cracks emanating from corrosion pits. The distributions of the size and aspect ratio of corrosion pits followed the log-normal distributions. The size increased and the aspect ratio decreased, with decreasing stress level. The stress intensity factors at which cracks emanated from corrosion pits increased with increasing stress level.

View full abstract

The influences of the martensite morphology on the characteristics of small fatigue crack growth were investigated under rotating bending using two kinds of dual-phase steels, Steel A which has a microstructure containing isolated martensite phases in a ferrite phase and Steel B which has a continuous martensite phase filled by ferrite phases. In the two steels, most of the fatigue life in plain specimens is occupied by the growth of small cracks less than 1 mm, and in the region corresponding to about 50 % of the total life, small crack growth is affected strongly by microstructures such as the ferrite grain boundary and martensite phase. Even in the region where the growth behaviors are affected by microstructures and the fluctuation of growth rate is remarkable, the average crack growth rate can be considered to be proportional to the crack length. That is, the resistance to the crack growth is approximately evaluated through the static strength. On the other hand, the fatigue limits in the steels are determined by each limit for crack growth which is affected by the microstructure. Consequently, there is a large difference between fatigue ratios (σ_W/σ_B) of the two steels (0.39 in Steel A, 0.49 in Steel B).

View full abstract

It has been reported by the authors that two "Brinell" indentations facing each other near the bottom of a notch are effective in improving fatigue strengh. However, it is not easy to form these indentations precisely. Therefore, this method does not seem to be convenient for practical applications. In this study, the effect of an indentation pressed near the bottom of a notch on the fatigue strength of plate specimens with a central notch having various notch root radii is investigated. The results show that an extreme increase in fatigue strength is caused by the application of an indentation. The effect of the indentation can be estimated approximately from the value of residual stresses produced by an indentation.

View full abstract

A low-cycle fatigue test was carried out under push-pull loading on an annealed low-carbon steel. Measurement of residual fracture ductility ε_FR was also performed after a given number of strain cycles with a special interest in the relationship between the exhaustion of ε_FR and the development of fatigue damage. Results show that the change in value of ε_FR with strain cycling in a low-cycle fatigue regime including an extremely low-cycle regime is controlled by the following basic mechanisms : (i) work hardening of the material, (ii) development of surface cracks and (iii) internal crack originating from the fracture of pearlites. The effects of these mechanisms on the exhaustion of fracture ductility (ε_FR) depends on the level of plastic strain range and on the number of strain cycles involved. The fatigue life in the extremely low cycle regime is primary controlled by two basic mechanisms such as work hardening of the material and increase of internal micro-voids.

View full abstract

The clad plates composed of low carbon and high carbon steels were prepared in order to investigate effects of laminated inhomogeneity and residual stress distribution balanced along the thickness on fatigue crack propagation behavior. The fatigue crack propagation rates of the tempered clad plate without the residual stress were situated between those of both base metals, which showed a behavior based on the law of mixture. The fatigue crack propagation rates of the quenched clad plates with the residual stress were lower than those of both base metals under the same fatigue condition. This was because the crack opening displacement, even in unhardened parts with tensile residual stress, was suppressed by the crack front curved because of the residual stress field balanced along the thickness, in which the stress intensity factor due to the residual stress was estimated to be almost zero.

View full abstract

In this study, the location of the edge crack in the semi-infinite plate and the loading stress at the infinity are detected by the data of strains measured around the region of the crack. In searching for the location of the edge crack, the method of gradient search is employed to minimize the square sum of the residuals between the measured strain distributions and the computed ones for the assumed crack locations. The strain field induced by the assumed edge crack is exactly analyzed by using the hypersingular integral equation method (HIEM). All integrals used in the present analysis are expressed in closed forms and the results can be given with short CPU time. The starting point for the gradient search can be estimated efficiently on the basis of the concept of the body force method where the crack can be represented by a pair of point forces. The results show the location of the crack are determined efficiently with good accuracies.

View full abstract

Time-dependent thermal stress field was analyzed when a crack-face was heated locally in an infinite plate. From the beginning of heating, the crack closes partly on each side of the heat source and bears the compressive stress. The closed region size, growing with time, depends on the heating location and heat dissipation from the surfaces. At the nearer crack tip to the heat source, tip-closing occurs at first and then at the other tip. As long as some part of the crack remains open, the stress field has singularity at the open crack tip. The stress intensity factor there varies with time and attains to its maximum after a certain duration, showing the similarity to the previous study where a point ahead of the crack tip is heated. By the optical interference method, crack opening displacement was measured experimentally using center-cracked glass plates heated by CO₂ laser beam. Measured displacements, varing with time, were well approximated by the analysis.

View full abstract

This paper presents a new method for stress intensity factor analysis of 2-dimensional crack problems including a mixed-mode interface crack between dissimilar materials. The M₁-integral method, an extended version of the J-integral method, was applied for interface crack problems with the finite-element method by Yau and Wang. In the present paper, the M₁-Integral method is combined with the boundary element method. At first, we improved the accuracy of the results of interior points by using adaptive automatic integration for a singular boundary integral. The value of the J-integral or M₁-integral was also obtained by this automatic integration scheme. We applied the present method to a center-cracked plate under tension. Furthermore, we analyzed a plate with an edge interface crack and a plate with a center slant interface crack between dissimilar materials subjected to tension. It is found from these analyses that the present method gives very accurate results, which are insensitive to the contour integral paths except for the region close to the crack tip.

View full abstract

A dynamic small punch (DSP) tester has been developed. DSP specimens are dynamically loaded by hitting an input bar through a striker bar which is accelerated by an air gun. The maximum speed of the striker bar is 13 m/s. Load and deflection are measured with two foil strain gages attached to the input bar and the deflection sensor, respectively. The fracture energy of the DSP test is determined from the total area under the load versus deflection curve up to the maximum load point. The ductile-brittle transition behavior of turbine rotor steels has been investigated by means of the DSP tester. The ductile-brittle transition temperature (DBTT) is determined on he basis of the variation of the DSP energy with testing temperatures. SEM observations of fractured surfaced have shown that the fracture mode transition and the DSP energy transition are consistent. The effect of strain rate on the DBTT is also examined. Based on these results, the relationship between the DBTT obtained from the DSP tests and that from the Charpy V-notch (CVN) impact tests is discussed as a function of strain rate.

View full abstract

The fracture toughness test for specimens of various thicknesses is performed in the ductile-brittle transition temperature region. The statistical property of cleavage fracture toughness, and effect of thickness on the cleavage crack initiation point are investigated systematically. The following are the main results obtained in this study. (1) Cleavage cracks initiated at a certain distance (S') from the surface of specimens. (2) It is possible to determine the plane-strain region from the analysis of the cleavage crack initiation point. (3) The plane-strain condition at the center of the specimen is given by the following equation: B≥0.004(K_c(J)/σ_ys)²+0.01. (4) The thickness effect of cleavage fracture toughness occurs with a statistical distribution of the weakest point ahead of the crack front.

View full abstract

The fracture strength of the adhesive butt joint of a round steel bar bonded by cyano-acrylate for tensile stress waves and alternating waves was measured experimentally. The tensile stress wave initiates in the round bar with the adhesive butt joint as the anvil attached at the end of the bar is struck by the dropping a circular cylinder. The alternating stress waves, which change from compressive to tensile, repeatedly propagate in the longer bar, as two bars of unequal length coaxially collide into each other. The location of the joint is designed to agree with the position at which the intermittent or serial alternating stress waves are observed. The fracture strength of the adhesive decreases as the acting period of waves increases. The fracture strength for intermittent waves is larger than for serial waves. The dynamic fracture strength was compared with the result for static fracture strength.

View full abstract

The mechanical behavior of laminated graphite/epoxy tubes under multiaxial monotonic loadings was elucidated with special emphasis on the effects of fiber orientation, loading condition and laminate structure. Tubular specimens of symmetrically laminated graphite/epoxy of 5 different fiber orientations θ (the angle of fiber measured from the specimen axis) were tested under monotonic combined tension/compression and torsion. Based on the preliminary results of the proceeding paper, the angle θ was specified to be 0°, ±22.5°, ±45°, ±67.5°and ±78.75°. The results on the stress-strain relationships and fracture stress loci were presented. It was shown that stiffness and strength decreased with an increase in the deviation of fiber orientation from the direction of principal stress, and the Tsai-Wu criterion applied very well. The theory which accounts for the fracture loci of arbitrary fiber orientations will be a future objective of research.

View full abstract

The generalized stress intensity factor has been defined at the bottom of the V-notch to represent the stress field, but it is difficult to determine the critical value of the generalized stress intensity factor K_C. Therefore, in the present investigation, we propose the method to determine this critacal value by using the molecular dynamic method. The problem of the band plate with a V-notch which is subjected to the shear deformation is investigated by the molecular dynamic method. The stress field, when the atm at the bottom of the V-notch begins to dislocate, is obtained. Using this stress field, K_C can be given, and the relationship between K_C and the opening angle of the V-notch is shown.

View full abstract

A series of tests for creep and biaxial ratchetting during cyclic loading were carried out to investigate the interaction between cyclic plasticity and creep or ratchetting of SUS 304 stainless steel. Specimens were subject to intermittent creep or biaxial ratchetting at several stress levels during cyclic tension-compression loading with the constant strain amplitude under the constant stress rate. The test results showed that the creep curve during cyclic loading could be explained by the so-called Bailey-Norton law, with stress levels measured from the current center of the yield surface which was determined by the hybrid constitutive equation for cyclic plasticity proposed by authors. Axial ratchet strain induced by cyclic shear stressing was expressed by the shear stress amplitude, the number of cycles and the axial stress level measured from the current center of the yield surface.

View full abstract

In a previous study, the authors proposed an anisotropic yield function for a porous solid. The FE calculations were performed in this investigation for a unit cell of a porous solid and a comparison was made between yield loci derived from the proposed yield function and those determined from the FE model. For two types of FE models, a flat plate with a periodic array of cylindrical holes under biaxial loading and a porous solid with a periodic array of spherical voids under axial load and superimposed hydrostatic pressures, the agreement between the yield function and numerical results were good, especially for the flat plate. Good correspondence can be obtained by introducing a scalar multiplier in the pressure-dependent term in the yield function.

View full abstract

In the present paper, a tensile elastic-plastic-viscoplastic constitutive equation is proposed, where the concepts of understress and overstress are introduced. Using the proposed equation, strain-rate-dependence of stress, high strain-rate-dependence of stress, strain-dependence of wave propagation speed and strain-rate-dependence of wave propagation speed are shown. It is shown that the proposed elastic-plastic-viscoplastic constitutive equation contains an elastic-plastic constitutive equation and an elastic-viscoplastic constitutive equation. Also, a discussion is made on viscoplastic strain and a new type of elastic-plastic constitutive equation is given.

View full abstract

The present paper concerns some unification of hypoelasticity using Jaumann, Green and Hilll's stress rates with different spins. The corotational stress-strain relation on the appropriate intermediate configuration rotated back from the current configuration is proved to be converted to conventional hypoelasticity on the current configuration. Also, the angular velocity of the eigenvectors of stress is shown to be obtained by subtracting the spins of the intermediate configuration from the spin due to deformations. The noncoaxiality at large strain is easily taken into account in standard hypoelasticity in a unified manner by introducing a new intermediate configuration which progresses with deformations.

View full abstract

An estimation method of the dynamic constitutive equations using the observed strain data by means of the method of nonlinear least squares is described. By using this method some experiments were performed such that the constitutive equation for aluminum involving two unknown parameters was estimated using the strain wave and the residual strain profile induced in a finite-length bar subjected to longitudinal impact. The stress-strain rate curves obtained by using the estimated constitutive equations were compared with the ordinary experimental values. As a result, it was concluded that the present method is practically useful. It was also found that the estimation was enabled by either the experimental values of the strain wave or the experimental values of residual strain.

View full abstract

ENF specimen of CFRP laminates is analyzed by the Boundary Element Method. For the anisotropic analysis, the fundamental solution by Cruse is used. The contact problem is analyzed using the Penalty method proposed by Yagawa et al. The contact force distribution, area of the contact zone and the crack opening profiles are obtained by analysing several specimens with different clack lengths. It is shown that the contact zone size keeps nearly constant value for different crack lengths. The effect of the friction coefficient is also evaluated and discussed.

View full abstract

The authers have developed a method for satisfying resultant boundary conditions on each divided boundary by discrete point forces. This method stands high accuracy and short calculating time at the same time. However, to obtain highly accurate solutions, it is necessary to put the point forces at the optimum locations. In this paper, the optimum location of point forces were obtained through several problems in 2D or 3D. The optimum location, that is, the optimum ratio of devided boundary length to distance between point force and boundary is 0.5∼0.7 in the case of notch problems and is 1.5 in the case of crack problems.

View full abstract

An efficient approximation method to determine an optimum shape of minimum weight subjected to stress constraints is suggested. Design variable reduction techniques of isoparametric interpolation and trigonometric series interpolation for the boundary shape are adopted for reducing the degree of freedom of the design problems. The objective function of weight is approximated to an expansion of a second-order Taylor series and the stress constraints to expansions of first-order Taylor series, based on the boundary element sensitivity analysis at a design point. Then the approximated subproblem is solved by a linear complementary pivot method. In the design variable reduction of isoparametric interpolation, an adaptive mesh refinement technique is also adopted to maintain the accuracy of the structural analysis. The efficiency of the approximation method suggested here is confirmed by application to minimum weight design problems to determine hole shapes in a plate under biaxial loadings.

View full abstract

This paper is concerned with a new boundary element approach based on the time-stepping scheme for two-dimensional thermoelasticity in unsteady heat conduction states. The uncoupling is assumed for the temperature and displacement fields. The time derivative in the differential equations is approximated by the time-stepping scheme. The reduced differential equations are transformed into a set of boundary integral equations by using the exact fundamental solutions which can be derived by the Hormander method. The standard boundary element method is applied to obtain numerical solutions at each time step. Computation is started from the initial state, and advanced by a time-marching procedure until the final state is realized. The mathematical formulation is presented in detail. The proposed solution procedure is applied to some example problems, whereby its potential usefulness is demonstrated.

View full abstract

A numerical method is given for the solution of counterformal rolling contact problems. The theoretical basis of rolling contact problems is described in a manner where the physical meanings of the problems can be understood directly. The difference of rolling velocities and the gap of the leading edges of two contacting wheels are calculated as functions of driving forces. Only the contact area is divided into triangular elements. Linear contact pressures are applied on a triangular element. The contribution from a triangular element is calculated analytically.

View full abstract

A numerical method is given for the solution of counterformal contact problems with friction. Only the contact area is divided into triangular elements. Linear contact pressure was applied on a triangular element. The displacements and stresses in the contact body were calculated accurately. The contribution from a triangular element was calculated analytically. An example is given for an inclined roller slipping on a semi-infinite body.

View full abstract

An analytical solution is presented for an asymmetric contact problem of a transversely isotropic layer subjected to moment by a circular rigid punch, with the layer resting on a rigid foundation. In the analysis, dual integral equations are reduced to an infinite system of simultaneous equations by using the technique of expressing the normal stress under the punch as an an appropriate series. Numerical results for the contact stress and the surface displacement in three examples of transversely isotropic materials as well as in an isotropic one are presented. The effects of anisotropy and layer thickness on the stress field are discussed.

View full abstract

In the present study, three kinds of composite wires of continuous SiC (Nicalon)-fiber-reinforced Al alloys, pure-Al, Al-5.7 %Ni and Al-4.0 %Cu, were employed for tensile tests. The strength of the matrix was changed by heat treatment to check the relationship between the tensile strength and the matrix strength. The strength of composite wires was calculated by the conventional rule of mixture. The results showed that the effect of matrix strength on the composite strength could not be predicted as it was. The rule of mixture was modified to relate reasonably well with matrix strength by determining the critical fiber length. The calculated strength of composite wire based on the devised rule of mixture was in good agreement with experimental results. It has been demonstrated that the matrix strength makes the fiber increase in strength by decreasing the critical fiber length.

View full abstract

Finite element analyses were conducted for Al/epoxy butt joints under remote tension, and the singular stresses near the bond edge were determined by the displacement method. The edge singular thermal stresses of the joint caused by uniform temperature change were obtained based on the superposition principle from the solution of singular stress due to remote tension. When the joint was under fatigue loading below the glass transition temperature of epoxy resin, the singular term corresponding to the stress range or the mean stress became frequency dependent or time dependent. Variation of strains near the bond edge of the joint under fatigue loading was well predicted by the analysis with material properties approximated as linear elastic.

View full abstract

Using the eigenfunction-expansion method, crack tip stress singularities for various open cracks and two kinds of different material combinations are calculated for the plane and the antiplane problems for all the boundary conditions at the crack interfaces such as free, fixed and smooth contact surfaces. Many numerical results represented by figures are given in this paper.

View full abstract

It is theoretically shown that the reflection coefficient of an elastic wave in layers of plates depends on the frequency of the incident wave, the width and the number of layers. Then the spectral change of ultrasonic waves reflected in layers composed of aluminum and steel is measured and compared with the theoretical results. It is found that both results agree well if the bond between the materials is taken into account as one of the elastic layers.

View full abstract

The impact bending characteristics of polycarbonate are investigated by means of an instrumented Charpy impact tester. The maximum load per unit area and the specific absorbed energy decrease with increase in the relative notch length. When the relationship between the absorbed energy and BWΦ is plotted as a straight line in an approximate method, the dynamic energy release rate, 𝒢 _ID, can be determined by the slope of the straight line, and the 𝒢 _ID values increase with increase in temperature. The relationship between the dynamic intensity factor, K_ID, which is determined by the dynamic energy release rate, and the temperature shows a tendency similar to the relationship between the maximum load per unit area and the temperature, and has the minimum value at a temperature of 0°C. It is observed that the area ratio of the huckle zone to the fracture surface is larger when the relative notch length is smaller, and ratio of the mist zone to the fracture surface is larger when the relative notch length is larger. It is also found that the crack growth rate is lower on the mist zone and higher on the huckle zone.

View full abstract

Particle impact wad repeated on SiC ceramics and the results were compared with the case of single particle impact. A SiC or steel particle was accelerated to impact a SiC specimen repeatedly by an air gun with a time interval of 2 seconds. Ring cracks were overlapped with each other on the surface of the specimen by the repetition of particle impact. Chippings were initiated at the over-lapped points of ring cracks after the accumulated damage reached a critical value. The relation between chipping initiation life and impact velocity expressed by an inverse power law. The residual strength of the specimen was evaluated by a four-point bending test after the particle impact experiment, and it was degraded with the increase of impact velocity and of repeated numbers. However, the degradation was saturated in a comparatively small repetition. No more degradation in residual strength could be seen, even though the small parts were broken off due to chipping. The erosion rate under repetition of particle impact was expressed by a power function of impact velocity with the exponent of 3.2.

View full abstract

Polycrystalline thin sheet metals under plane strain tension is modeled by employing Asaro's constitutive equation for plane strain deformation of single crystal, and finite-element simulation is performed to investigate the growth of surface roughening and the localization of the deformation. In addition, the effect of the magnitude of prestrain before an annealing, the relative size of grains with respect to the thickness of sheet metals and the orientation of the slip plane on the improvement of formability due to intermediate annealing are clarified.

View full abstract

A robotic manipulator is simplified by numerous beams with pinjoints between them and an anchor pinjoint at the base. The beams under impact loading are allowed to rotate about the axis of the pinjoint. The dynamic response of a vertical chain for a transverse load suddenly applied along every beam is theoretically analysed. The angular and linear acceleration of the mass center of beams, reaction force at the pinjoints, shearing force and bending moment along the beams is presented as a function of impact load. As the vertical chain consisting of one plaster beam and one steel beam, or two with a pinjoint between them, is subjected to the collision of the flier steel rod accelerated by the air gun, the location of the fracture of the plaster beam is measured experimentally. The validation of the theory is confirmed by comparison of the location of the critical bending moment with the position of the fracture of the beam.

View full abstract

A qualitative model on the remodeling of soft connective tissues is proposed, from which follows that the dynamic component of tensile strain will induce the strengthening of tissues and the static component of them will induce lengthening of tissues. Based on this hypotheses, we examine the design rule for elastic prostheses, the use of them insure the proliferation and the maturity of reconstructed tissues surrounding the prostheses. Then we construct artificial ligaments for rabbit's anterior cruciate, and carried out animal experiments. Unfortunately, considerable inflammation responses are induced, the wear debris are supposed to be responsible for.

View full abstract