Tatsujiro Miyazaki

Estimation of Particle Size Distribution in Materials in the Case of Spheroidal Particles Using Quantitative Microscopy

In this paper, an estimation method is proposed for the size distribution of particles in materials. The configuration of particles is assumed to be spheroidal and all observed information is gathered from cutting planes. The apparent size distribution measured from 2D is corrected to the 3D true size distribution, then the expected size distribution can be estimated from a small size up to an extreme size. This estimation method is most versatile, and involves Saltycovs method and extreme statistics at the same time. The procedure is applied to an artificial material with a given particle size distribution in computer simulation in order to examine its validity. The estimated results are compared with the given distribution, and the validity of the method is confirmed.

Deceleration and Acceleration of Crack Propagation after an Overload under Negative Baseline Stress Ratio

In a previously reported study, it was found that the fatigue crack growth rate actually accelerated after a tensile overload under a negative value of baseline stress ratio R. This type of crack propagation behavior is related to the change in the compression residual stress to tensile residual stress distributed in the vicinity of the crack tip upon unloading to minimum load after an overload. In the present investigation, focused on the negative stress ratio, it was found that the deceleration and acceleration of crack propagation were associated with crack opening displacement at the overload point and the conditions of applied baseline cyclic stress. The fatigue crack growth rate decelerated after being overloaded at a positive R value. However, that rate accelerated rather than decelerated in some stress conditions at a negative R value. Therefore, the residual fatigue life after overloading became longer or shorter, depending on the overload level and baseline stress conditions.

FATIGUE LIFE AND CRACK GROWTH BEHAVIOR IN ANNEALED AND NORMALIZED 0.83% CARBON STEEL

The variations of fatigue limit and fatigue life of a plain specimen of annealed and normalized 0.83% carbon steel were investigated. This material is used for cutting tools and the original microstructure includes a spherical microstructure. After heat treatment under some conditions, the microstructure changed to a lamellar microstructure. However, the fatigue lives of the plain specimens of this material showed almost the same tendency even after heat treatment under some conditions. In those cases, the initial crack length in the fatigue process is related to the size of the crystal structure and related to the distribution of ferrite. In the present study, the relationship between the distribution of hardness and the fatigue limit was investigated. Also, it was discussed that the fatigue limit of heat-treated 0.83% carbon steel could be evaluated by a relationship in which the parameters are the hardness and initial crack length. Finally, the tendencies of fatigue life of heat-treated 0.83% carbon steel were discussed based on the observations of crack growth behavior.

Improvement of Fatigue Crack Growth Behavior in the Case of the Cracked Specimen with Relatively Narrow Width

The improvement of acceleration behavior of crack growth was investigated with constant stress amplitude under negative stress ratio R=-1. Then a technical method to detect the fatigue crack growth was discussed. For example, when the stress amplitude exceeds a critical value, crack growth rate of overloaded specimen became higher than that of baseline which was obtained by crack growth test without applying overload. In some experimental cases, the acceleration of crack growth was observed and that could be happened on practical cases. Stop-holes were drilled at crack tips or in the vicinity of crack tips to remove the plastic zone and the effect of that on crack growth behavior were investigated. Also, steel pins were inserted into the stop-holes and its effect was discussed. Finite element method (FEM) was used to analyze the stress concentration at the edge of stop-holes. Positions of center of the stop-holes were varied at different distance from the crack tips to investigate the effect of stop-holes on fatigue crack growth. Also, stress intensity of base and stop-holed specimen was calculated. Then, the effect of stop-hole was discussed by both the experimental and analytical results. Specially, it was discussed whether the crack growth behavior was improved or not in the case of relatively smaller width specimen.

Convenient Adhesive Strength Evaluation Method in Terms of the Intensity of Singular Stress Field

A convenient evaluation method is proposed for the debonding adhesive strength in terms of the intensity of singular stress field (ISSF) appearing at the end of interface. The same FEM mesh pattern...

Evaluation of debonding strength of single lap joint by the intensity of singular stress field

In this paper, the similarity of the singular stress field of the single lap joint (SLJ) is discussed to evaluate the debonding fracture by the intensity of the singular stress field (ISSF). The practical method is proposed for analyzing the ISSF for the SLJ. The analysis method focuses on the FEM stress at the interface end by applying the same mesh pattern to the unknown and reference models. It is found that the independent technique useful for the bonded plate and butt joint cannot be applied to the SLJ because the singular stress field of the SLJ consists of two singular stress terms. The FEM stress is divided to two FEM stresses by applying the unknown and reference models to different minimum element sizes. Then, the practicality of the present method is examined by applying to the previous tensile test results of the SLJ composed of the aluminum alloy and the epoxy resin. The ISSFs for the SLJ were calculated by changing the adhesive thickness t 2 and the overlap length l 2. In the case of the SLJ with 225 mm in total length and 7 mm in adherend thickness, it was found that the similar singular stress fields are formed in the range of 0.15 mm ≤ t 2 ≤ 0.9mm and 15 mm ≤ l 2 ≤ 50 mm. It is shown that the critical ISSFs at the fracture are constant in the range.

Effect of Crack Initiation Size on Fatigue Strength in 0.85% Carbon Steel

The fatigue limit of a plain specimen of a tool steel JIS SK85 was investigated. The initial crack length in the fatigue process is related to the size of a single crystal in the case of low carbon steel. However, that is independent of the crystal size in the case of pure aluminum, a lamellar pearlitic steel, etc. In this study, the relationship between the initial crack length and the fatigue limit was investigated. The original microstructure of this material includes a spherical microstructure. However, after heat treatment under some conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of heat-treated SK85 could be evaluated by an equation in which the parameters are the hardness and initial crack length. Finally, this result was discussed based on the observations of crack growth behavior and fracture surface.

IMPROVEMENT OF FATIGUE LIFE OF A HOLED SPECIMEN OF ALUMINUM-ALLOY 2024-T3 BY INDENTATION AND HOLE EXPANSION

A method of improving the fatigue life and crack growth behavior of a center holed specimen was investigated. Local plastic deformation was applied around the hole by indentation to achieve the purpose. A series of fatigue tests was conducted on aluminum-alloy 2024-T3. Push-pull tests were performed under a stress ratio of R= -1 and a frequency of 10Hz. The observations of the crack initiation and growth were performed with a microscope, and hardness around the hole was measured by Vickers hardness testing machine. In the present study, the longest fatigue life was observed in the case of an indentation specimen with the highest load. The indentation was performed on both sides of the hole edges. The crack growth rate was decreased by indentation or expansion of the material around the hole. From the experimental results, it is found that the fatigue life and crack growth behavior of a holed or notched specimen can be improved by a simple technical method that is related to the local plastic working.

Relaxation of Stress Concentration of a Crack by Stop Drilling Holes and Additional Holes and Its Practical Prediction

It is well known that a stress concentration of a crack can be relaxed by drilling a hole at the crack tip. The repair method is called a stop drilling procedure and often used to repair aircrafts, machines and so on. Then, when two additional holes are drilled near the stop drilling hole so as to face each other symmetrically, the stress concentration of the stop drilling hole can be relaxed further, and a fatigue life is extended. However, the size and the location of the additional hole suitable for repairing the fatigue damage are not clarified yet. In this paper, finite element simulations were performed on various plates containing the small center crack at which the stop drilling holes and the additional holes were drilled, and influences of the additional hole on the relaxation effect of the stress concentration of the stop drilling hole were investigated. Then, the size and the location of such additional hole that the stress concentration of the stop drilling hole is minimized were examined.

Effect of Negative Stress Ratio on Fatigue Crack Propagation Behavior after Single Tensile Overload

It is well known that a single tensile overload applied during constant amplitude cycling at a positive stress ratio causes the crack growth retardation. In a case of a negative stress ratio R, however, the fatigue crack growth rate actually accelerated after a tensile overload. This crack propagation behavior is related to the local bulging of specimen thickness, the blunting behavior of crack tips and the residual stress distributions. When the crack tips were blunted during tensile overload and the succeeding compression stress with a high value was applied after the overload, the crack propagation rate got higher level than that before the overload application. In this case, a tensile residual stress was created in front of the crack. To investigate the effect of the negative stress ratio on the fatigue crack propagation, the fatigue tests under R=0, -1 and -1.5 applying the single tensile overload were performed using annealed 0.15% carbon steels. In the present study, a residual life after an overloading was specially investigated, and it was found that the transition phenomenon of the residual life was appeared in the case of the negative stress ratio.