Mode I critical stress intensity factor KIc obtained by four-point single-edge-notched bending (4SENB) tests of wood and medium-density fiberboard (MDF) was experimentally and numerically analyzed. To determine the critical load for crack propagation, the relationship between the load and loading-line deflection (LLD) and that between the load and crack opening displacement (COD) were measured, and the results obtained were compared with each other. Additionally, a double cantilever beam (DCB) test was conducted and the results were compared with those of 4SENB tests. Similar to the results obtained in several previous studies, the value of KIc was obtained by introducing an additional crack length into appropriate equations as the crack geometry factor used for analyzing the 4SENB test. Additionally, the load-COD relationship was more sensitive in detecting the crack propagation than the load-LLD relationship, so the measurement of COD is recommended for determining the KIc value in the 4SENB test accurately.
Since the Great East Japan Earthquake, many concrete seawalls for reconstruction and tsunami prevention are being built. Concrete having high resistance of chloride attack has been demanded for such coastal structures. To reduce future maintenance cost, concrete structures must be highly durable to environmental effects in addition to the chloride attack resistance. The authors have developed a pozzolanic admixture with excellent resistance to chloride attack as well as excellent general durability. The admixture is a mineral fine powder with a BET specific surface area of 10 m2/g of higher. The pozzolanic material (general mixture of 20-40 kg/m3) can be used as an alternative binder of Portland cement like fly-ash. The admixture is useable even in precast concrete factories having fewer storage facilities such as extra silos. The study focuses on properties of steam-cured concrete to consider the use of admixture in such precast concrete production. The experimental study examined various properties of the concrete incorporating the admixture, such as the chloride penetration resistance; compressive strength; dry shrinkage; carbonation; sulfate resistance, and freeze-thaw resistance. Pore distribution of the concrete was also investigated to confirm the effect of the powder material. The test result confirms the excellent higher resistance to chloride penetration of the concrete. In addition, the study presents that the durability of the concrete is comparable with or higher than that of conventional concrete without the admixture.
Crack arresters were installed to suppress interfacial crack propagation between the skin and core of CFRP/foam core sandwich panels. The effect of the crack arrester on suppressing the propagation of a fatigue crack was investigated under mode II type loading using end notched flexure specimens. Specimens were made from carbon /epoxy fabric prepreg and a polymethacrylimide foam core. The energy release rate, calculated by FE analysis, showed a considerable reduction at the crack tip as the tip approached the crack arrester. Fatigue crack propagation tests were carried out using sandwich panel specimens containing the crack arrester. The crack propagation rate, da/dN, decreased with decreasing distance from the crack tip to the arrester edge. Thus, the suppression effect of the crack arrester was demonstrated experimentally. The net stress ratio at the crack tip is greater than the load ratio owing to the influence of the thermal residual stress during molding. The net stress ratio dependency of the crack propagation resistance was evaluated for the sandwich panel, and was compared with that for the solid CFRP laminate to investigate the driving force under fatigue loading.
The effects of environmental temperature and tensile speed on the fracture of notched plates of glass-fiber-reinforced syndiotactic polystyrene (GF-SPS) were investigated. Since GF-SPS has high heat resistance, it is expected to be a useful material at high temperature. In this study, GF-SPS plates containing 30 % E-glass fiber by weight were prepared by injection molding. The notch radii were 0.5, 1, and 2 mm, while the notch depth ranged from 2 to 5 mm. The tensile test was carried out at 23, 40, 60, 80, 100, and 120 °C with a constant crosshead speed. The effect of the notch geometry on the tensile strength decreased as the environmental temperature increased. Furthermore, at ≥ 80 °C, the maximum nominal stress of notched specimen was approximately equal to that of the smooth specimen. At ≤ 60 °C, the specimen failed in a brittle manner at the maximum load. Tensile tests were performed in this temperature range at tensile speeds of 102, 1, 8.33 x 10-3, 8.33 x 10-4, and 8.33 x 10-5 mm/s. In case of the same temperature, the maximum elastic stress at fracture was determined from the notch-root radius and the time to fracture but it was independent of the notch depth. It was verified that a fracture criterion based on the severity of the stress fields near the notch roots was applicable in this temperature range. Furthermore, a log-log plot of the maximum elastic stress at fracture versus the time to fracture displayed a straight line and it can be presumed that the slope of the line is the same as that of the plot representing the data obtained at 23 °C.
Mechanical damage on tensile strength of carbon filament is tested by three methods with PAN- and pitch-based fibers. No effect on tensile strength by keeping pre-tension at the level of mean tensile fracture load is made sure. By prior transverse compressive loading on cross-piled up filaments, many filaments are crushed by a small load. Tensile strength of the survival filament has no influence on a PAN-based carbon fiber, but has decreased in case of a pitch-based carbon fiber. Two steps torsional-tensile test is carried out, that is, first step is keeping pre-tension to the mean tensile fracture stress at a rotation, and second step is the survival filament is tested by tension without rotation, or the opposite way of testing, the first step is at simple tension until the mean strength, and the second step is estimation the torsional-tensile strength with a shear stress by a rotation. It is found the tensile strength and the torsional-tensile strength have related each other for a PAN-based fiber until the torsional fracture strain, however the torsional-tensile strength of a pitch-based carbon fiber become no relation at higher torsional strain with the tensile strength. Those differences between PAN- and pitch-based carbon fibers are an influence from difference in degree of strength anisotropy in macroscopic viewpoint, and derived from the structural differences in cross section in microscopic viewpoint.
In this paper， we gave a consideration of Griffith’s theory and fracture of a homogeneous isotropic linear elastic body by thermodynamics． From the results， the condition of fracture occurrence by Griffith’s theory is only applied to reversible fracture and it is not able to be the condition of irreversible fracture． But by the theory which surface formation is the change of constitutive equation of the surface area and the generation of strain energy in the surface area， the fracture above mentioned is irreversible and the condition of fracture occurrence by Griffith’s theory can be applied to irreversible fracture.
This study aimed to clarify the influence of the type and concentration of nitrite-type repair materials on ion penetrability in cement mortar. The permeability test for various repair material was carried out in order to compare the physical permeability of nitrite-type repair materials. In addition, repaired mortar specimens were prepared for ion penetrability test using nitrite-type repair material, and the amount of penetration of nitrite and lithium ions were measured under outdoor and indoor conditions. As a result, the impregnation of a lithium nitrite solution became larger than that of a calcium nitrite solution in the permeability test. Moreover, when the nitrite-type repair material was directly coated on the surface of mortar specimen, it was confirmed that the amount of penetration of nitrite ions of a lithium nitrite solution became larger than that of a calcium nitrite solution. It is thought that a coexisting positive ion affects the penetration property of nitrite ions in hardened cement mortar.
The relationship between the microstructure and fatigue limit of a smooth specimen of tool steel SK85 (0.83% carbon steel) was investigated. Also, the fatigue limit of pure copper was evaluated. Push-pull tests were performed under stress ratio R = -1. The initial length of a crack which initiates during the fatigue process is related to the size of a single crystal in the case of carbon steel. In this study, SK85 was chosen as a testing material to investigate the relationship between the microstructure and fatigue limit for the following reason. The original microstructure of the material used includes spherical microstructure. However, after annealing or normalizing under certain conditions, that microstructure changed to a lamellar microstructure. The fatigue limit of the heat treated SK85 could be evaluated by a relation in which the parameters are the hardness and initial crack length. The crack initiation sizes were related to the microstructure. The method of evaluating fatigue limit was investigated in consideration of the crack initiation size or the sizes of microstructures. Also, it was discussed the fact that the crack growth rate of SK85 was related to the condition of ferrite grain distributions. For the purpose of investigating the method of evaluating the fatigue limit, experiments with smooth and holed specimens of copper were performed. Furthermore, we investigated the relationship between the Hall Petch Relation and the method of evaluating fatigue limit in this study.