A small crack shows anonalous growth behavior in comparison with a large crack. In both large and small cracks, however, the fundamental factor controlling the crack growth rate is the cyclic displacement near the crack tip. In the present review, two fatigue crack growth laws in small and large cracks, dl/dN∝σⁿ_al and dl/dN∝ΔK^m, which formally contradict each other unless m=n=2, were explained consistently using the assumption dl/dN ∝r_prev. Moreover, the validity and the limitation for application of the small-crack growth law were explained by experimental results and the practical prediction method for fatigue life was suggested.

View full abstract

When applying ceramics to machine components, the probabilistic approach or reliability engineering approach is useful to handle rather large scatter in the strength of ceramics. This has led to the development of reliability evaluation systems in several countries. Up to the present time, the two-parameter Weibull distribution has most often been used for ceramics. However, use of the three-parameter one will also be important in the near future. Thus, this review addresses application of the three-parameter Weibull distribution in relation to reliability evaluation of ceramic components, and some problems to be encountered in its application are discussed. The problems discussed include methods for parameter estimation, the relationship between the shape parameter and degree of scatter, the definition of the effective volume, extension of the three-parameter Weibull distribution to polyaxial stress-states and the stress-state dependence of the shape parameter. In particular, the last two problems are examined in detail. Probabilistic design methodology using the three-parameter Weibull distribution is also discussed.

View full abstract

Solutions to a state of plane stress and to a state of generalized plane stress of transversely isotropic, moderately thick plates in rectangular Cartesian coordinates are obtained. The two solutions are derived from the generalized Elliott solution which includes five independent potential functions. Expressions for components of displacement and stress are explicitly presented in series form, with reference to the plates. As applications of the solutions, in-plane stretching and pure bending of a transversely isotropic, rectangular thick plate are analyzed. E glass-epoxy and graphite-epoxy as transversely isotropic materials, and an exactly isotropic material are treated in numerical calculations. Numerical results for displacements and stresses in the materials are presented.

View full abstract

This paper deals with a two-dimensional stress analysis of a butt adhesive joint with rigid fillers in an adhesive subjected to a tensile load. Assuming that adherends are rigid and replacing an adhesive with a finite strip including rigid fillers, the analysis was done using the two-dimensional theory of elasticity in order to examine the joint strength. The effects of the location and size of rigid fillers on the stress distributions around the fillers and at the interfaces were shown by numerical calculations. For verification, photoelastic experiments were performed. The analytical result was in fairly good agreement with the experimental result. It was seen that based on the amount of shift of the location and the filler diameter, the joint strength was more improved than that of the joints without fillers.

View full abstract

This paper deals with the strength of joints combining adhesives with bolts. The stress distribution in the adhesives and a variation of axial bolt force were analyzed using an axisymmetrical theory of elasticity when an external load was applied to the combination joint, in Which two pipe flanges or two circular flanges were clamped by nuts and bolts with an initial clamping force after being joined by adhesives. In addition, a method to estimate the strength of the combination joint is proposed. Experiments were performed and analytical results are consistent with the experimental results concerning the variation of axial bolt force and the strength of combination joints. It is seen that the strength of the combination joint is greater than that of adhesive joints, and that it increases with a decrease in the bolt pitch diameter. Furthermore, it is seen that the sealing performance of the combination joint is improved when the adherends are pipe flanges.

View full abstract

Theoretical modeling of acoustoelastic effects in plastically deformed solids has been accomplished through the use of an elastoplastic coupling strain rate. This coupling strain rate, which remains effective even under subsequent elastic loading, is considered to be generated at the yield surface vertex and causes elastic modulus degradation due to the growth of plastic anisotropy. The purpose of the present paper is to formulate precisely a generalized acoustoelastic theory for plastically deformed solids with finite deformation, and moreover, to provide a method of nondestructive evaluation of the plastically deformed state, i.e., yield surface, texture change due to the plastic deformation and the occurrence of the instability associated with the microslip band.

View full abstract

The present paper proposes a constitutive equation for rubberlike materials through the strain energy function using a new strain measure called`rotationless strain'. This strain is defined as the accumulated strain defined by integrating strain rate excluding effects of rotation from the stretching tensor, which is derived from the polar decomposition of the deformation tensor. The proposed constitutive equation takes the form of a nonlinear stress-strain relationship, and includes an isotropic linear relationship for metal alloys as a Special case, where the objective stress rate is taken as the Green-Naghdi rate in both of the constitutive equations. The present paper is also concerned with theoretical treatments for incompressibility and the formulations in terms of principal values. Good agreement with the experimental results for typical loading problems, such as simple shear and simple elongation as well as uniform 2-dimensional extension cases, shows the validity of the present constitutive modeling.

View full abstract

It was previously reported by the author that the limit to ductility of a thin sheet is well predicted theoretically by the use of the J2G constitutive equation with the localized necking condition as the critical condition under the assumption of a plane stress state, where the J2G stands for the J₂-Gotoh's corner theory which was previously proposed by the author. In this paper, the critical conditions for the vertical and oblique types of shear band bifurcation in a block subjected to severe compression are deduced by the use of the J2G constitutive equation. It is concluded that the vertical type shear band is followed almost immediately by fracture, whereas the oblique-type shear band does not necessarily induce an immediate fracture and it corresponds rather to the "oblique shear-band like patterns" which appear in the materials under severe compressive plastic deformation such as in rolling. If an additional condition (e.g., the critical value of the circumferential stress to overcome) is satisfied before the oblique type shear band bifurcation, then this bifurcation will induce the immediate fracture, whereas if the additional condition is satisfied after the bifurcation, then fracture will occur at this point. These conclusions are derived from the comparision of the theory with the corresponding experiments reported by other investigators.

View full abstract

This paper deals with numerical simulations for nonuniform yielding processes in mild steel through use of the finite element method. The yield drop is taken into account in the simulation by means of Hahn's constitutive equation. The yielding processes in straight and notched specimens are discussed based on the simulation. The simulated result agrees well with the actual behavior of nonuniform yielding observed in annealed mild steel. The simulation also shows that the nonuniform yielding phenomenon is relevant to yield-drop behavior of the material element.

View full abstract

In order to investigate stress relaxation behavior at the specimen surface during the corrosion fatigue process caused by the initiation and propagation of many cracks, rotating bending fatigue tests were carried out in sodium chloride aqueous solution using specimens of 6 and 12 mm diameters. Stress amplitude decreases of about 25∼50% were observed during the corrosion fatigue process performed at low stress amplitude. This stress relaxation behavior is well evaluated by calculating the compliance increase of the specimen. Computer simulations of the corrosion fatigue process of the unnotched specimens were conducted. The results showed good agreement with the experimental results. In the simulations, probabilistic crack initiation behavior during the corrosion fatigue process, the stress relaxation effect at the cracked parts and crack coalescences among many distributed cracks are taken into consideration.

View full abstract

The cyclic behavior of recovery stress under constant residual strain and recovery strain under constant maximum stress in TiNi shape memory alloy was investigated experimentally. The influence of residual strain, maximum stress and the heating temperature on cyclic characteristics of recovery stress and recovery strain was discussed. The main results are summarized as follows. (1) The stress at high temperature, the stress at low temperature and the recovery stress decrease in the early cycles, but become almost constant after these cycles. (2) The recovery stress under thermal cycling decreases with residual strain. (3) Both the recovery stress and the stress at high temperature increase with the heating temperature, but the stresses' rates of increase decrease. (4) The transformation temperature under constant stress rises with the number of thermal cycles.

View full abstract

Structural ceramics are attracting attention in the development of space planes, aircraft and nuclear fusion reactors because they have excellent wear-resistant and heat-resistant characteristics. However, in some applications it is anticipated that they will be exposed to very-high-temperature environments of the order of thousands of degrees. Therefore, it is very important to investigate their thermal shock characteristics. In this report, the distributions of temperatures and thermal stresses of cylindrically shaped ceramics under irradiation by laser beams are discussed using the finite-element computer code (MARC) with arbitrary quadrilateral axisymmetric ring elements. The relationships between spot diameters of laser beams and maximum values of compressive thermal stresses are derived for various power densities. From these relationships, a critical fracture curve is obtained, and it is compared with the experimental results.

View full abstract

The effects of a gradient distribution of components on the deformation and fracture of functionally gradient materials (FGM) in the corundum/plaster model system were examined as a function of the volume fraction of corundum powder and the multiples of gravity. The radial crushing strength of the FGM was measured under the combined conditions for cast rings with three different volume fractions and three different multiples of gravity. The distribution patterns of stress and strain in FGM rings were analyzed by the conventional curved beam theory considering the change of elastic modulus. The brittle fracture was initiated from the inner surface of the ring on the load action line independent of the distribution of corundum powder. The variation of strength with the gradient distribution of corundum powder and with the centrifugal force was also studied.

View full abstract

Some ceramic-coated metals are nominated as first-wall materials. TiC-coated type-316 stainless steel is expected to be superior to other materials in high-temperature strength and in endurance properties under heavy irradiation. Delamination between the ceramics layer and base metal is considered to be one of the most important problems when such ceramic-coated metals are used in a gradient temperature field such as the first wall. In this report, the high-temperature strength of TiC-coated type-316 stainless steel, which is compared to the first wall of the fusion reactor, is investigated experimentally and computationally. A simple and precise thermal stress testing system is developed. The effects of surface roughness as well as the thermal stress and the residual stress on bond strength are investigated. Experimental and numerical results on residual stress distribution are compared mutually for confirming the reliability of the inelastic analysis of the finite-element method. It is expected that the suitable surface roughness decreases the residual stress in the coated film. The optimum range in the TiC-coating temperature is found by the inelastic FEM analysis for the heating condition used in the experiments.

View full abstract

In the previous study, the authors developed a Hopkinson bar-type, high-speed impact fatigue testing machine, and carried out impact fatigue tests on two types of carbon steels. In this study, after some modifications and improvements to the testing machine, a new method was introduced to generate impact stress patterns with various loading times at maximum stress levels, and impact fatigue tests were carried out on S10C (0.1%C) steel by changing the loading time from 470 to 940μs. Fatigue crack growth tests were also carried out on the same material using another Hopkinson bar-type testing machine, with the loading times of 780 and 1050μs. It was confirmed that the impact S-N property in the high-cycle range was dependent on the loading time and there was a relationship of σ_max(N_fT)^m=D, where σ_max is maximum stress, N_f the number of cycles to failure, T the loading time, and m and D are material constants. It was also confirmed that the fatigue crack growth rate was affected by the loading time, and NT (the product of the number of stress cycles and the loading time)was an important parameter governing the crack growth rate.

View full abstract

In order to clarify the tribological behaviour of SUS304 steel in seawater, the electrochemical approach was taken in this work. Wear tests were carried out at different potentials from -800mV to 600 mV (vs SCE) under the following conditions: a normal load of 10 N, a slip amplitude of 1.5mm, a frequency of 8.3Hz and total sliding distance of 3O0m (10⁵ cycles) . Wear volume increased by three orders of magnitude with an increase in potential from -800mV to 600mV. The relationship between wear volume and corrosion volume was discussed. Corrosion volume was calculated from the anodic current in the polarization curve for a fresh surface, which was obtained by the potential pulse method. In conclusion, the electrochemical factor, dissolution of metal, governed the wear in seawater, while the mechanical factor was only to produce the fresh surface on the wear surface during sliding.

View full abstract

A method is proposed for determining proof strength of ceramic components based on fracture behavior. The strength for short time fracture mainly depends on the sizes of initial defects in the ceramics, but it cannot be uniquely determined by the constant fracture toughness criterion which is well known for applicability to brittle materials. For static fatigue behavior, a static fatigue limit below which cracks are arrested is observed. Both short-time fracture strength and static fatigue limit are shown to have nonlinear relationships to defect size. By considering these dependences, the method gives proof strength which is effective in designing ceramic components.

View full abstract