Key Engineering Materials Vols. 525-526

Abstract: 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.

Abstract: A Meshless Approximation Based on the Radial Basis Function (RBF) Is Developed for Analysis of Dynamic Crack Problems. A Weak Form for a Set of Governing Equations with a Unit Test Function Is Transformed into Local Integral Equations. A Completed Set of Closed Forms of the Local Boundary Integrals Are Obtained. as the Closed Forms of the Local Boundary Integrals Are Obtained, there Are No any Domain or Boundary Integrals to Be Calculated Numerically in this Approach.

Abstract: Damping characteristics of metals are measured by the use of inversed torsion pendulum method. As the materials, aluminum alloys, magnesium alloys, Inconel etc are used with the temperature and frequency parameters. Mild steel is employed as a reference material for calibrating the apparatus. The measured damping coefficient of magnesium alloys is larger than that of aluminum alloys indicating that the HCP crystal structure has more damping effect than the FCC structure. The damping of these metals increases with temperature. The manner of increment of damping is investigated to analyses the microscopic mechanism of damping.

Abstract: Fracture toughness, stress-strain relation and the damping characteristics of bamboo are investigated. The fracture toughness of bamboo in tearing along the longitudinal direction is measured by the use of newly devised apparatus in which the crack opening displacement is controlled in a constant velocity and a quasi-steady extension of the crack is maintained. The stress-strain relation of bamboo is examined in a reversible elastic range using a conventional tensile test in the longitudinal direction. Repeated tensile loading tests show that the stress-strain relation has a strong non-linear hysteresis and that it converses to a steady loop. The damping of bamboo is measured by the use of inverted torsion pendulum apparatus. The specimen is taken so that damping of twisting longitudinal bar is measured. The damping coefficient of bamboo is much larger than that of metals. The mechanical properties of bamboo are examined in terms of water concentration and fiber density in the bamboo.

Abstract: The fatigue and fracture mechanism of C/C composites material was investigated in notched and smooth specimen. Also, the initiation and growth behaviors of cracks were continuously observed. Specially, the phenomenon of fracture constraint in notched specimen was discussed. This phenomenon was related to the crack growth behavior and shear damage in the matrix. In the present study, fatigue limit was defined by the critical stress level that the specimen was not broken even after applied 1x10⁶ stress cycles. When the fatigue life was longer than 1x10⁶ stress cycles, the initiated crack was stopped to growth after reaching some critical length. Also, the density of crack initiation was low. When the fatigue life became shorter than that, the cracks extended rapidly and coalescences of cracks were observed. The crack growth behavior was strongly related to the fatigue limit and unstable fracture conditions. Also it is found that the compliance or elastic modulus of the specimen was related to residual fatigue life. Now, an observation of crack in C/C composites was performed effectively in the present study.

Abstract: In this work, Lamb wave generation and propagation have been modelled in composite plates. Actuation and acquisition of signals when the PZT transducers are tied to the structure or bonded with an adhesive layer are investigated. The effect of adhesive thickness and actuation frequency of Lamb wave have been examined.

Abstract: Ultra high performance concrete (UHPC), characterized by a high strength and high ductility, is also subjected to large shrinkage due to its low water-to-binder ratio and its large content in high fineness materials. The large amount of autogenous shrinkage of UHPC can induce crack on structural member when it was restrained with reinforcement and form. However, shrinkage of UHPC in plastic state is not generating confining stress, which is the main cause of initial crack. Normally, the setting time in concrete is an index to distinguish shrinkage which occur confining stress or not. An estimation of setting time is conducted in compliance with ASTM C 403 till now however, that test standard reveals error of results due to discordance of test condition as following with concrete type. This study therefore evaluated setting time of UHPC through the modified test method which was proposed by KICT. Test results and analyses proved a discrepancy of setting time between ASTM and proposed method. The proposed method put faith in evaluation of setting time in accordance with UHPC.