For the purpose of investigating the effect of interfacial bonding between a fiber and a matrix on the tensile strength of FRM, several B₄C coated boron fiber reinforced Al composites were prepared by controlling the hot-pressing temperature. The tensile-test showed that the composites prepared at 550°C exihibited a higher strength than the composites prepared at 500°C and 600°C. The fracture surfaces were changed from a rugged face like a bundle break to a flat face with an increase in the temperature. It was seen from SEM that the interfacial bonding became stronger. So, the transition in the composites' strength was considered as follows ; As the hot-pressing temperature rises, the critical fiber length, a factor determining a fiber's strength in a matrix, becomes gradually shorter, but the stress concentration factor to the adjacent fibers caused by a fiber fracture also becomes larger. Therefore, the higher strength of the composites results in the case that these two factors are properly overlapped. Such a transition of the composites' strength was additionally supported by Batdorf's theory.

View full abstract

The present paper investigates tensile properties of γ-Al₂O₃-fiber-reinforced Al at room temperature and at cryogenic temperatures. Tensile tests of the metal-matrix composite at cryogenic temperatures were carried out by immersing specimens in liquid nitrogen and in liquid helium. During these tests, the deformation behavior of the specimens was monitored by strain gages of foil types. Stress-strain curves of the composite were composed of two nearly straight lines, which reflect earlier onset to plastic deformation in Al matrix, and implied that γ-Al₂O₃ fibers were deformed elastically until the specimens were finally fractured at a strain level of approximately 0.6%. The fracture stress, the elongation, and Young's modulus of the composite followed two-parameter Weibull distributions. The mean of the fracture stress and that of Young's modulus were linearly increased, whereas that of the elongation was linearly decreased as temperature is lowered ; nevertheless, the temperature dependence of the fracture stress and the elongation of the composite was small compared to other candidate cryogenic metallic materials. Scanning electron micrographs revealed brittle fracture surface of the γ-Al₂O₃ fibers in contrast with nearly perfect ductile fracture surface of the Al matrix at both room and liquid helium temperatures.

View full abstract

Commercial base pure iron plate specimens were testes under reversed and pulsating axial stress in a high cycle fatigue range, and the crack nucleation sites and the subsequent growth pass of cracks were analyzed in terms of the crystallographic orientation of grains surrounding the cracks. It was observed that fatigue cracks nucleated at grain boundaries satifying some crystallographic configurations under completely reversed stress, but the crack nucleation sites were along persistent slip bands inside the grains under pulsating stress. the crack growth toward the inside from the surface was observed to take place in the shear mode along dominant slip systems in the same manner as the pencil glides, irrespective of the stress patterns.

View full abstract

the effects of contact pressure between the pad and specimen and the pad contact length on fretting fatigue strength were examined using NiCrMoV generator rotor steel and carbon steel pads. The fatigue limits at a low contact pressure decreased steeply with increasing contact pressure. At pressures higher than 100 MPa, the fatigue limits chenged slightly with increasing pressure. The fretting fatigue strengths depended on the pad contact length. The effects described above were successfully explained by fictitious crack concepts and a stress intensity factor taking account of the applied stress, contact pressure and frictional force.

View full abstract

Measurements of crack closure for small fatigue cracks were mode on fine and coarse grained materials of a low carbon steel, S 10 C, in the region of crack lengths of 2c_m, above which the effect of grain boundaries on crack growth rate disappeared, to crack length of 2c_c, above which crack growth rates for small cracks were consistent with those for large cracks. Crack opening stress increased with increasing crack length and tended to be constant at a crack length near 2c_c. That is, small cracks in the region from 2c_m to 2c_c were in the process of increasing crack closure and then attained the same extent as large cracks. The reduction in crack closure of small cracks would be attributed to extensive plasticity surrounding the crack, and thus plasticity-induced crack closure was reduced because of decreased constraint by the surrounding elastic material on the plastic region.

View full abstract

The fatigue crack propagation behavior was observed under two step variable stresses. In the first step, the stress was below the fatigue limit and the maximum stress was compressive. In the second step, the stress was above the fatigue limit. The testing was carried out on notch sheet specimens of SUS 410 (13% Cr) under repeated bending. The number of fatigue cycles at the first or second step systematically was changed. The results obtained are as follows ; (1) The fatigue life decreased by about 40% when the number of fatigue cycles at the second step (above the fatigue limit) was above 1×10⁵ cycles. (2) The fatigue crack propagation rate decreased when the number of fatigue cycles at the second step was 2×10⁴ cycles.

View full abstract

Rotating bending fatigue tests were carried out on specimens with a small blind hole of an age-hardened Al-alloy in air and in oil. Fatigue crack growth rate of an age-hardened Al-alloy 6061-T6 is higher in oil than in air, contrary to the other many metals. Hence, it is suggested that the fatigue crack growth is accelerated in an age-hardened Al-alloy by the lubricating action.

View full abstract

The fatigue properties of various chains such as high-ten chain, and mild steel chain are confirmed by actual fatigue tests and are analyzed by the finite element method. The main results obtained are as follows : (1) The strain state of chain is an elastic-plastic strain state under a working load, but the relation between load and strain is liner under a proof load. (2) The fatigue cracks initiate on the border of a link bend part and a straight part. It is the same as the largest stress part by the finite element method. (3) The fatigue strength is well estimated by Manson's empirical formula. (4) It is found that the fatigue strength depends on not only the material strength but also the dimensions of the chain.

View full abstract

It has already been reported by the authors that the two indentations facing each other near the tip of a crack are especially effective on retardation of crack propagation. The purpose of this paper is to examine the effectiveness of indentations in plate specimens with various types of notch. The experiments show that the pre-additional indentations near the notch root are especially valid in retardation of a crack. The effect of the indentations can be explained approximately from the existence of residual stresses produced by the indentations.

View full abstract

Pulsating tensile fatigue tests were carried out, using CT specimens cut out in three different directions from a rolled steel having inclusions with a remarkable directionality, in spite of having slightly laminated structures. The crack growth rate in each directional specimen was measured by an optical microscope and the fracture surface was observed by a scanning electron microscope. The main results obtained are as follows. (1) the crack growth resistance is lowest in the case of SL specimens whose cracks propagate along the inclusions with directionality, but anisotropy of the crack growth rate is small in comparison with that of the fatigue limit. (2) The anisotropy of the crack propagation is clearly recognized also by observing the fracture surface in fatigue. (3) The effect of material strength on the crack growth rate is remarkable when the stress intensity factor range ΔK is large, while it is slight when ΔK is small.

View full abstract

Fatigue tests were performed under Mode I and Mode I + Mode II mixed mode loadings using Fe-3%Si coarse grain specimens. The direction of fatigue crack propagation was investigated by changing the crack angle and crystallographic orientation of a notched grain. It was revealed that in the grain whose crystallographic orientation was [001], the fatigue crack propagated in specific directions, showing an aligned chevron pattern. Considering the formation of the aligned chevron pattern, it was found that the crack propagated in the maximum tangential stress direction among these specific directions.

View full abstract

Fracture toughness and fatigue crack growth rate behavior were investigated using six materiels with different toughness which were taken out of a tubular type reactor and heat treated at different tempering temperatures. Repeated internal pressure tests were conducted using actual size cylinders where the fracture toughness of materials ranged from 67 to 144 MPa √(m). Test results suggest that actual reactors satisfy the leak before break (LBB) condition because of their higher fracture toughness.

View full abstract

to study on the leak before break, namely LBB criterion of high pressure reactors for polyethylene production, the validity of the stress intensity factor formulas proposed by Newman-Raju, Iwadate et al. and Nakazawa et al. were discussed. When Newman-Raju's stress intensity factor formula without considering the back surface magnification factor is used, the condition of K₁≥ K_IC(J) gives a good criterion for LBB condition.

View full abstract

This paper is concerned with mixed mode (combined mode I and mode II) fracture criterion in the case where stable crack growth occurs before unstable fracture. Finite element analyses and tensile tests were carried out on thin sheet specimens of 2024-T3 aluminum alloy having a center crack inclined to the tensile loading axis. The specimen width and the angle between the center crack and the tensile loading axis were changed over a wide range. It was found that stable crack initiation follows approximately the criterion of J_I + αJ_II = J_IC, where α is a constant. The direction of stable crack initiation was perpendicular to the tensile loading axis. The net stress at the maximum load was found to be nearly equal to the yield stress of the material tested.

View full abstract

It is important to evaluate the stress intensity factors of surface cracks in fracture mechanics. Numerical evaluations have been done by the three dimensional finite element method witch, however, needs a lot of labor to prepare the data. The boundary element method has the advantage on this point. But, its computational time is long because of the dense coefficient matrix. Therefore, the authors have developed the three dimensional boundary element code BEM 3 D for the super-computer and have shown that BEM 3 D can carry out the elastic analyses of surface cracks in a short computational time.

View full abstract

Using the supercomputer, one can execute the elastic-plastic analyses of three-dimensional surface cracks very quickly and economically. But, a special computer code is necessary to achieve high computing speed in the supercomputer. So, the authors have developed a three dimensional finite element code FEM 3 D for the supercomputer. In FEM 3 D, the solver is the parallel skyline method and the J-integral is calculated by the virtual crack extension method. By FEM 3 D, elastic-plastic analyses and evaluation of the J-integral of surface cracks have been carried out in a short computing time and with good accuracy.

View full abstract

The deformation of a ductile crack and the strain distribution in the vicinity of the crack tip were investigated in order to study the stable growth process of a crack in a center-cracked plate of 70/30 brass. Specimens which have a short fatigue crack in its center were prepared so that the outer boundaries of the specimens did not disturb the substantial characteristics of the strain distribution near the crack tip. The distribution of strain, ε_y, in the tensile direction near the crack tip is approximately expressed by the following equation regardless of the crack length, a. ε_y≌C₁ε_f(a/r)^β (C₁≌0.10∼0.13, β≌0.4∼0.5, 0.01≲r/a≲0.5) where ε_f is the fracture ductility in the tensile test of a round bar and r is the distance from the crack tip. The crack opening displacement, δ(x), near the crack tip has a unique correlation with the distance, x, from the crack tip, that is, δ(x)∝x/a These two equations imply that the following equation holds between the rate of crack growth, da, due to the increase in external loading and the crack length, a, i. e. da/dε₀∝a where ε₀ indicates the external average strain defined by a gauge length of 20 mm.

View full abstract

This paper describes the physical meaning of the representative crack length, 2c_s, and the effectiveness of the strength evaluation by fracture toughness and the representative crack length for specimens with different shapes and sizes. Four point bending tests were carried out for pressureless sintered silicon nitride specimens with different sizes having smooth surfaces or notches. The relation between fracture stresses and the Knoop indented crack lengths showed that the fracture stresses were constant in the region less than 2c_s. Also, the representative crack length 2c_s was constant for specimens with different shapes and sizes. From the fact that the fracture toughness evaluated for the real cracks of specimens was nearly equal to the fracture toughness of the ceramics, 2c_s is the crack length of the semi-circular surface crack, for which the real cracks in the ceramics are substituted.

View full abstract

Recently the authors have derived various new types of path independent integrals in which the theoretical limitations of the so-called J integral are overcome. First, for elastodynamic crack problems, a path independent integral J' which has the physical meaning of energy release rate was derived. Later more general forms of path independent integrals T and T were derived, which are valid for any constitutive relations under quasi-static as well as dynamic conditions. This paper presents the theoretical and computational aspects of these integrals, of relevance in nonlinear dynamic fracture mechanics. Finite element analyses were carried out for an example problem of a center-cracked plate subject to a uniaxial impact loading. The material behavior was modeled by three different constitutive relations such as linear-elastic, elastic-plastic, and elastic-viscoplastic cases. The applicalility of the T integral to nonlinear dynamic fracture mechanics was shown with the numerical results.

View full abstract

An anisotropic model was considered as a rectangular plate made of fiber reinforced plastic with an edge crack parallel to the fiber direction. Its dynamic response was investigated by comparison with the results for an isotropic plate from the point of view of the time response of the dynamic stress intensity factor determined by the finite element method. First, by the qualitative elucidation for the dynamic response of this model subjected to a step function load, the influence of an anisotropic composition on it was made clear. Continually, analyses provided the duration time of the incident stress wave were performed, and according to those results the fracture of the anisotropic model was predicted and it was compared with the experimental results.

View full abstract

High bending stresses occur in the shell-to-annular joint of a cylindrical storage tank due to the hydrostatic pressure and the plastic region grows around the toe of a fillet weld. Because of the cyclic loading by the contained liquid, low cycle fatigue evaluation is required. This paper describes the axisymmetric elastoplastic analysis by the finite element method for this evaluation. In this analysis, the vicinity of the shell-to-annular joint is modeled by solid elements, other members of a tank are modeled by shell elements, the connection element which is formulated by the penalty method is used. The constitutive equation is based on the combined hardening in order to consider both strain hardening and the Bauschinger effect.

View full abstract

The buckling problems of cross-ply laminated cylindrical shells under lateral and hydrostatic pressure loading are solved based on Flugge equations and the simplified method. The effects of the number of layers and the modulus ratio on the buckling pressure are investigated. The effects of coupling between bending and extension for antisymmetric laminates and the cross-ply ratio for symmetric laminates increase with increasing modulus ratio. The accuracy of the simplified method derived, assuming no coupling, is investigated by comparing its results with those of the Flugge equations. In the case of no coupling between bending and extension, the accuracy of the simplified method depends on both the dimension of shells and the modulus ratio. The reduced stiffness is demonstrated to be effective for approximately introducing the effect of coupling in the simplified method.

View full abstract

As a basic study for computer-aided machine design, an interactive shape modification method and a computer program to reduce stress concentration factors for three-dimensional structures composed of curved plates are developed in this report. This method consists of two parts, automated stress analysis and interactive shape modification. Automated stress analysis consists of automated mesh generation and stress analysis. The stress analysis is based on the Assumed Stress Hybrid Method and the Gaussian Elimination Method. Shape modification is classified into three types, inter-plate modification, elliptical modification and thickness modification. Inter-plate modification is the modification of shapes near the intersection lines between two curved plates. Elliptical modification is the modification of shapes of plates such as end closureof a pressure vessel. Thickness modification is modification of the thickness of plates near the intersection lines. The usefulness of the program is illustrated with the help of several examples of practical interest.

View full abstract

Stress wave propagation in an elastic/viscoplastic space containing two spherical cavities is analyzed numerically by the method of direct integration of the governing equations in bipolar coordinates, when axisymmetric dynamic loads are applied to the boundary of one side of the cavities. By obtaining the numerical results, stress distributions on the boundary of the other side of the cavities, displacement of the contour of the cavity, and the plastic wave fronts in the space are presented. The effect of the distance between two cavity are also demonstrated.

View full abstract

A new photoelastic image processing system is produced. This paper describes some procedures for image processing and for separating the principal stresses associated with 2-dimensional stress analysis. The system comprises a high resolution television camera, an interface unit, a microcomputer (μPD8086-2CPU), a color monitor and a printer. Special image processing of thinning and image enhancement are conducted by using the computer. The sum of the principal stresses satisfies Laplace's equation. In solving the equation, the boundary element method is used. The stresses might be separated by combining the sum and the difference of the principal stresses. Stress distributions are calculated for each disk and sheets subjected to compression, with semi-circular notches and with a circular hole, respectively. These were consistent with the theoretical results. Computing times for each example were about 1/2 hour. The system is also applicable to the problems of moire or holography with minor modification of the system.

View full abstract

This paper describes an attempt to develop a structural maintenance support expert system using videotex. Graphic data needed for maintenance support are much too diversified and further, the way to utilize them differs from user to user, so a prototype system permitting each user to utilize graphic data in his own respective way was developed in an attempt to establish the methodology. It was made clear that the NAPLPS videotex system seems to be one of the most favorable candidates for this purpose since it is easily accessed by a network and a large reduction of computer memory is attainable, which will enable the use of a microcomputer. Thus, the integration of videotex and an expert system will provide a maintenance engineer with a most flexible and versatile graphic data base.

View full abstract

Ultrasonic vibration systems consisting of an exponential horn and a terminal rod are analyzed by the finite element method. The influence of the length and the shape of the rod on the vibrational mode and on the resonant frequencies of the systems are made clear, and these results agree qualitatively with the experiments. The results of these studies offer to guide for designing ultrasonic vibration. Further, these studies enable the removal of the limits of application of ultrasonic vibration (i. e., ultrasonic working, ultrasonic fatigue test, ultrasonic surgical knife for operation etc.).

View full abstract

This paper is concerned with the mixed boundary value problems of an infinite plate with a circular hole, into which a smooth elastic circular ring is inserted. Since the infinite plate is subjected to an uniaxial loading (tension or compression) at infinity, apertures are produced in parts along the boundary between the inserted ring and the hole. The contact pressure between the inserted ring and the hole is expressed in a convergent series whose differential form is also convergent, so that the stress and displacement generated along the boundary can be numerically analyzed by the point matching method. Using the numerical results for various parametric combinations the shear moduli of the plate and the ring, the influences of the amount of the misfit and inner radius of inserted ring are shown with respect to this stress. In addition, the problems of a perforated infinite plate under uniaxial loading at infinity inserted by an annular plate subjected to the uniform internal pressure are also investigated.

View full abstract

Coiled springs are often subjected to lateral loads and bending moments in addition to longitudinal loads. Moreover, the nonlinearity increases as the longitudinal load increases. In this paper, two-dimensional deformation of the coiled springs were analyzed under the longitudinal loads, the lateral loads and the bending moments, namely complex loads, by means of the theory of large deformation. Through this analysis, the relation between complex load and displacement was obtained, and it was made clear that the nonlinearity increases as the longitudinal load increases. The behavior of coiled springs which have a slenderness ratio less than the theoretical buckling limit was also obtained, from the relation between the eccentric load and the deformation. Moreover, an experiment was carried out and the results agreed well with the theoretical value.

View full abstract

Interrupted quenching is considered to be effective in avoiding the quenching distortion and in reducing the quenching stress. The numerical calculation for thermoelastoplastic stresses due to interrupted quenching is carried out, using the procedure of Mendelson's method of successive elastic solutions. A₁ and A₃ transformations on heating, and martensitic or pearlitic transformation on cooling, are reasonably taken into account as the typical models of phase transformations during interrupted quenching of carbon steel. Mechanical and thermal properties of steel are considered to be temperature dependent. As a result, the residual stresses due to interrupted quenching are verified to be smaller than those from general quenching.

View full abstract

Optimization of mesh discretization is proposed to improve the accuracy of solution of collapse load analysis by using the Rigid Body-Spring Model (R. B. S. M) under the plane strain condition. Moreover, the fracturing behavior of materials is investigated by employing the fracturing mechanism of a spring connecting the triangular rigid-body elements. It is clarified that the collapse load and the geometry of slip boundary for optimized mesh discretization are close to those of the slip line solution. Further, the wedge-shaped fracture of a cylinder under a lateral load and the central fracture of a strip in the drawing process are well simulated.

View full abstract

This paper deals with the arbitary torsional impact response of a penny-shaped crack. Laplace and Hankel transforms are used to reduce the mixed boundary value peoblems to dual integral equations. The solutions are expressed in terms of a Fredholm integral equation of the second kind having a kernel with fast rate convergence. The numerical inversion of Laplace transform by the use of the FFT algorithm contributes the improvement of the accuracy and the reduction of CPU time.

View full abstract

The distortional wave histories resulting from torsional impact tests on a cylindrical tube of PMMA (polymethyl methacrylate) are resolved into Fourier components in order to determine the complex shear compliance and the viscoelastic model for shear response. In a similar manner, the complex tensile compliance and the associated model are obtained from the analysis of longitudinal impact tests. The results reveal that the 3-element solid model can be applied to the behavior of the material both in shear and in tension over a wide frequency range. By employing the correspondence relation between linear elasticity and linear viscoelasticity, a complex Poisson's ratio and a complex bulk modulus are determined based on the shear and tensile compliances. It is found that the dilatational behavior may be approximated by an elastic prediction and that the relationship between the longitudinal and lateral response is also elastic. As a consequence, it is confirmed that the viscous feature of the material should mainly arise from the shear properties.

View full abstract