The effect of grain boundary on small fatigue crack growth behavior in α-brass was investigated by means of a small in-plane bending fatigue testing machine installed in the stage of an atomic force microscope (AFM). In-situ observation of transgranular fatigue crack growth behavior by an AFM revealed that the small fatigue crack grew along one slip direction with the highest Schmid factor in contrast with the alternating slipping-off crack growth process in a long fatigue crack. A grain boundary worked as a constraint against slip deformation, resulted in frequent crack branching and kinking behavior near the boundary. It was found that the crack growth behavior near a grain boundary was strongly affected by the difference of grain orientations across the boundary. A large number of dislocations were piled up along a grain boundary due to the constraint of slip deformation, which changed the stress state around a crack tip. The newly developed stress distribution activated the slip plane with lower Schmid factor calculated from the nominal stress, and the crack preferentially grew along that plane.

View full abstract

This paper investigated fatigue strength of flexible bellows made from austenitic stainless steel, SUS316L. First, tensile and fatigue experiments were performed under load control on the base materials for formed and welded stainless bellows, i.e. SUS316L as well as SUS304, with the sheet thickness between 0.15 and 0.2 mm. The materials exhibited extensive strain hardening which influenced on the fatigue strength. It was demonstrated that the fatigue limits of prestrained materials were higher than those of virgin materials. Since the fatigue limits were higher than the 0.2% proof stresses for the base materials, the strain hardening played an important role on the fatigue behavior. Second, model specimens were cut from the formed and welded bellows and Vickers hardness was measured on the specimen section in order to predict the extent of strain hardening. Fatigue experiments were performed under displacement control. Constant amplitudes of cyclic displacement were applied for the specimens and crack initiations were detected by monitoring the maximum and minimum loads. Finite element method (FEM) was employed to correlate the fatigue strength of the model specimens with that of base materials.

View full abstract

A quadratic equation for the Graneisen coefficient γ was derived using a method in which the Walsh-Christian and Mie-Graneisen equations were combined. It was assumed that the constant-volume specific heat is a linear equation for temperature and that γ depends on specific volume. The Hugoniot pressures and the quasi-static pressures at an ambient temperature included in the solution of the quadratic equation γ were obtained from the existing experimental data for Fe. The Hugoniot temperatures were available from the existing theoretical Hugoniots for Fe. The validity of the assumption for the constant-volume specific heat was shown.

View full abstract

The constant-volume specific heats on the Hugoniots C_VH and the temperature-independent Graneisen coefficients γ for close-packed Fe were evaluated using a classical thermodynamic theory constructed in the previous report. The inaccuracy of some existing ab initio temperature Hugoniots for solid Fe was found by showing that the heats C_VHhave too small values. While a C_VH distribution in the solid phase range was demonstrated to agree approximately with a previous ab initio distribution in the previous report, the corresponding γ distribution was significantly higher than the ab initio distribution in the lower pressure region. In the liquid phase range, the γ distribution had a considerably larger slope than the ab initio distribution. The causes of these disagreements are clarified.

View full abstract

In this study, fracture strength of WC-12Co thermal sprayed coating is investigated experimentally and analytically. In the experiments, one pair of butt cylindrical specimen with coating is subjected to combined tension with torsion stresses. Fracture loci were obtained for three kinds of thickness of the coating in σ-τ stress plane. Stress distributions at crack tip singular point on fractured surfaces are analyzed by Finite-Element-Method and is approximated by the expressionσ=KR-1 where R means normalized thickness coordinate. It is found that the normal stress distributions are common to all cases of testing stress conditions and so fracture condition of a brittle coating is represented as K≥Kcr in the normal stress distribution even under mixed deformation mode I with III. Delamination critical shear stress distribution could also be obtained.

View full abstract

The mode I crack growth behavior from delamination was investigated with two kinds of laminates, UD laminates (made of unidirectional laminates) and C laminates (made of unidirectional laminates and cloth laminates), of high strength glass fiber reinforced plastics (GFRP). Crack growth tests were carried out by double cantilever beam specimens under cyclic and monotonic loadings. The fracture toughness at the onset of crack propagation, CIc., was not influenced by the lot of laminates and nearly equal for UD laminates and C laminates. On the other hand, the increase of the fracture toughness with crack extension, G_IR, was very much dependent on the lot, and the value for C laminates was much higher than that of UD laminates. Under cyclic loading with a stress ratio of R=0.1, the fatigue crack propagation rate expressed in terms of a power function of the maximum crack-tip value, GI _tipmax, of the energy release rate was nearly identical in UD and C laminates. The increase of the bridging component of the energy release rate, GI_bridgemax /GI _max, with crack extension was dependent on the lot and the kind of laminates. Fractographic observation of UD laminates showed mode II crack propagation on the surface of fibers, while mode I crack propagation in resin. In C laminates, crack propagation path was on the interface of cloth laminates and fiber bridging caused by fibers of cloth laminates.

View full abstract

Brittle fracture behavior of PMMA was studied using a high-speed extensometer. The single-edge-notched tensile specimens were fractured with a special loading jig so that it could split and fly away after the fracture. The load and displacement diagram, i.e. the external work applied to the specimen was partitioned into three parts : the elastic energy left in the fractured specimen, the nonelastic energy due to viscoplastic deformation and the fracture energy for creating new surfaces. These energies were then determined and correlated with the fracture load and external work, and the results were discussed.

View full abstract

It becomes important problem to obtain the Elastic-Plastic Fracture Toughness J_in of high pressure boiler Tube, in order to avoid brittle fracture at the time of water pressure proof tests. In previous study, J_in tests were able to be carried by using the small size Arc specimens (50 mm in outer diameter). In this study, the specimen thickness was needed to be more than 6 mm, to keep the plane strain fracture toughness (STBA 12). And elastic-plastic fracture toughness Jin of welding HAZ (STBA 24) were tested. According to these tests, J_in test value did not almost differ between STBA 24 material and welding HAZ material.

View full abstract

Recent new finding of unstable displacement burst observed in the relation between the indent load and indent depth of nanoindentation is much possible to be related to the collective dislocation emission. In the present paper, dislocation emission and prismatic dislocation loop formation of a single crystalline aluminum under nano-scaled indentation are simulated by the molecular dynamics. Effects of the stress distribution due to two different types of indenters and of the interatomic description by three different embedded atom type potentials are discussed with much emphasis. In result, the dislocations are emitted from the severely damaged surface atomic layer due to the indenter. Afterwards, the prismatic dislocation loops, which have the same gliding direction but not on the slip planes, are attained by both unstable reaction between the shear loops emitted from the surface and the cross slip mechanism.

View full abstract

In the field of the numerical analysis, response surface method is one of the most practical optimization methods. The optimum design variable is obtained from response surface approximation under constraint condition in the optimization using response surface method. The authors have proposed hierarchized response surface method (H-RSM) using hierarchical clustering analysis in order to make high-precise response surface approximations. Response surface approximation is made in the every hierarchy cluster by the classification of the design pattern. Therefore, hierarchized response surface approximation is high accuracy because of approximate by the local pattern.

View full abstract

In this study, the response surface method is applied to the reliability-based optimum design of a laminated composite plate subject to in-plane loads in order to improve the calculation efficiency. The reliability is approximated by polynomials in terms of the lamination parameters. Where the authors have already clarified that the reliability is distributed smoothly in the lamination parameter space for the symmetric balanced laminate consisting of 0°, ±45° and 90°plies. Using the response surface method, the reliability-based optimization problem is transformed from a duplicated-loop iteration problem into a single-loop iteration problem. In constructing the response surfaces, several experimental points are selected based on the D-optimality criterion using genetic algorithms. The efficiency of the response surface approximation on the reliability-based optimization is demonstrated through the two dual problems ; the reliability-maximized design of the constant-thickness plate and the thickness-minimized design under the reliability constraint.

View full abstract

This paper presents a method to find some local minima by branching at local minimum, based on the Generalized Random Tunneling Algorithm (GRTA). We call this method as Branching GRTA (BGRTA). BGRTA also consists of three phases, that is, the minimization phase, the tunneling and branching phase, and constraint phase. In the minimization phase, local search technique is used. In the tunneling and branching phase, some local minima can be found by branching at local minimum obtained in the minimization phase. The next branching point is chosen from them. In the constraint phase, the feasibility is examined. We apply BGRTA to the topology optimization problem of truss structure and the traffic road design problem. Through numerical examples, we examine the validity of BGRTA.

View full abstract

In this paper a crack identification method for curved beams is presented using both genetic algorithm (GA) and FEM. The crack is simulated by an equivalent rotational spring model. On the basis of this approximate model, the stiffness matrices of a cracked element are derived employing the curved beam theory. In order to calculate the natural frequencies of the curved beam, which are required to identify of the crack location and the depth from frequency measurements, FEM program including the cracked element is developed. The depth and the position of the crack in the curved beam are estimated by using these natural frequencies and GA. The proposed method is verified by numerical simulation of some sample problems.

View full abstract

We developed a basic technology of non-contact, non-destructive inspection method of a concrete structure. It is a current inspection approach that an operator beats surface of a wall with a hammer and hears the sound with his ears, and judges the presence of the defect. The purposes of this study are the improvement of reliability and the curtailment of expenses by the automation of evaluation. The proposed method of this study is that the impact by hammer is replaced with shock wave and the safety check by operator's ears is replaced with the laser measurement. We did a simulation using a finite element method and succeeded detection of a defect of 25 mm diameter and in 200 mm depth. The method of this study is a technology to enable that we inspect concrete structures by non-contact, non-destructive test automatically.

View full abstract

The subloading-friction model is the unconventional elastoplastic friction model which allows the slip displacement due to the contact stress below the sliding surface. It is capable of describing the nonlinear relationship between the normal and the tangential stress on the contact surface and a gradual progress of relative tangential displacement with the increase of the contact stress. In this paper, the compression problem of rectangular block, the indentation problem of circular rigid roller and the cyclic friction problem for the elastic body under the plane strain condition are analyzed by the finite element program incorporating the subloading-friction model. It is revealed that the distribution of contact stress is predicted appropriately and the accumulation of relative displacement due to the cyclic loading of contact tangential traction below the sliding condition can be described. Besides, the computation time is shortened by the present finite element program compared with programs with the conventional friction model.

View full abstract

In this paper, the dynamic electromechanical response of a coated piezoelectric strip with a crack under an anti-plane mechanical and an in-plane electric impact loads is investigated. The crack is oriented normally to the interfaces. Based on the integral transform techniques, the present problem is reduced to the solutions of singular integral equations. Numerical calculations are carried out, and the main results presented are the variations of the dynamic stress and electric displacement intensity factors, the dynamic energy density factors versus time as functions of the geometric parameters and electromechanical loads.

View full abstract

We produced a new piezoelectric fiber with metal core by the hydrothermal method. The insertion of metal core was significant in view that the fragility of ceramics can be overcome and electrodes are not required in the use as sensors and actuators. And new smart board was designed by mounting these piezoelectric fibers on the surface of the CFRP composite. It was shown that these complex fibers function as sensor and actuator in the CFRP board. In order to evaluate the sensor and actuator. However, by the hydrothermal method, piezoelectric layer can be uniformly formed about 20μm thickness on the titanium wire surface, the PZT clad is insufficient in thickness for the actuator use. In this paper, we solve this problem using extrusion method. We developed the metal core further thicker PZT clad fiber by the extrusion method. The sensor signal and vibration displacement are bigger compared to that of the hydrothermal fiber.

View full abstract