Recently, carbon fiber reinforced plastics (CFRPs) have been used as high-pressure container materials for fuel cell vehicles and other applications. When used as structural components, carbon fibers in a CFRP are subjected to multiaxial loading. As is well known, multiaxial strength and fracture behavior are significantly different from those under uniaxial loading. However, research on the multiaxial strength evaluation of carbon fibers is insufficient. To date, there have been a few reports on studies under tension-torsion loading, but these have hardly clarified the fracture criterion. In this study, the tensile-torsional fracture strengths of PAN-based carbon fibers were obtained by proportional loading tests. As a result, it was found that the tensile strength along the fiber axis decreased largely with increasing shear stress at the fiber surface under tension-torsion loading. The changes in the experimental results were well explained by adding a correction term to Tsai-Hill criterion, the theory for anisotropic materials. Furthermore, since PAN-based carbon fibers showed extreme differences in strength between uniaxial tension and pure torsion, it was finally concluded that the carbon fibers follow the maximum principal stress theory when using normalized stresses.

In this study, we attempted in-situ push-out delamination (POD) detection during drilling using an electrical impedance method. We measured changes in electrical impedance and the internal strain around a drilled hole to detect the onset of POD, and estimated the critical thrust force for POD. It was proved that the onset and propagation of large POD can be clearly detected by the electrical impedance method, whereas for small POD, the impedance change is relatively less clear. POD detection sensitivity was improved by using circular electrodes on the exit-side surface instead of ring-shaped electrodes. Furthermore, changes in electrical resistance due to workpiece removal and drill indentation load were also examined, and the impedance change was inferred to be caused by damage, material removal and piezoresistivity effect. Because POD does not always occur at the maximum thrust force, POD onset can be detected more reliably by using not only thrust force but also impedance signals.

Previous studies have reported that the mechanical properties of GFRP, composed of E-glass and unsaturated polyester resin, decrease when subjected to 70 or more thermal cycles ranging from -5 to 40 °C (14 hours per cycle). However, there are reports suggesting that the surface temperature of in-service GFRP bridges can exceed 70 °C during summer. This indicates that current data on temperature ranges during thermal cycling tests might be insufficient. This study, therefore, investigates the effects of temperature cycling at elevated temperatures on the mechanical properties of GFRP by conducting cycles from -5 to 75 °C. Results from the high-temperature range revealed that the strength of unidirectional materials increased in both tensile and bending tests. Conversely, the mechanical properties of bi-directional materials remained relatively unchanged in both tests. Additionally, a slight mass loss in the specimens was noted due to temperature cycling. This suggests that a reduction in water content within the specimens might be a major factor contributing to the observed strength increase.

This study focuses on the pultrusion process and mechanical properties of unidirectional carbon fiber reinforced poly-ether-ether-ketone (CF/PEEK) round rods. A laboratory-scale thermal pultrusion machine with a compressive insert mold was developed to manufacture thermoplastic CFRP round rods. The round rods manufactured using CF/PEEK prepreg tape exhibited high density without visible voids or fiber bundle layers. To investigate the effects of fiber orientation on the mechanical properties of CF/PEEK round rods, the prepreg tapes were pultruded in a twisted state. The mechanical properties of the CF/PEEK round rods were evaluated by a three-point bending test and a lateral compression test, and the damage behavior was investigated by cross-sectional observations after the test. CF/PEEK round rods with twisted fiber orientation exhibited lower elastic modulus and reduced maximum load in the three-point bending tests, but fiber twisting has a significant effect on resistance to lateral compressive loads and stress redistribution. It was confirmed that the complex fiber orientation obtained by twisting the prepreg tape suppresses the straight propagation of transverse cracks in the longitudinal direction of CF/PEEK round rod. The thermal pultrusion process developed in this study was realized as an effective reinforcing method against lateral compressive loads by adding tape twisting.

In-plane shear properties of two-dimensional SiC-continuous-fiber reinforced SiC-matrix composite (2D-SiC/SiC) is studied by the modified Iosipescu shear test at elevated temperatures with water vapor environment. The proposed test geometry combines the advantage of Iosipescu pure shear test with a simple compressive loading which makes the test particularly suitable for high temperature testing. Finite element analysis confirms that proposed geometry introduces minor error in pure shear stress state in the gauge cross-section comparing to alternative geometries. A 2D-SiC/SiC composite was oxidized and measured the shear strength in argon/water vapor environment at temperatures between 1200˚ and 1600˚C. Humidity was varied between 0 and 30 %RH at a total pressure of 0.1 MPa and a constant gas velocity of 300 cm³/min. Influence of water vapor oxidation on some mechanical properties and morphological responses are described in terms of the in-plane shear strength, compliance, changes in sample weight and SEM images.

In order to improve ground to planned shape and property by grouting using colloidal silica type grout, it is necessary to use the grout with determined gelation time according to injection time. Therefore, the grout pH, which correlates with gelation time, must be controlled properly onsite of grout mixing. However, conventional mixing plant is not equipped with a function to adjust pH, so the pH adjustment requires time and labor, especially when sulfuric acid-based reactant is used. To address this issue, we have developed "pH-adjusted grout mixing system" that enables real-time grout pH measurement during on-site mixing and amount control of consistent materials. As a result of on-site grout mixing test using developed mixing system, it was confirmed that grout with target pH could be mixed within the target time by determining the procedure for supplying liquid B, considering the increase in grout pH with a time delay caused in grout mixing, based on the assumption of the mixing procedure in which alkaline liquid B is supplied into acidic liquid A.

Electroless Ni-P plating is suitable for aluminum alloy die castings since the plating can be formed uniformly without being affected by the shape or the microstructure. Moreover, the hardness of the plating increases by heat treatment. In this study, the correlation between the hardness and the heat treatment time was investigated for the electroless Ni-P plating on the ADC12 substrate. The change in scratch characteristics was also examined with/without heat treatment of the plating. The samples were heat treated at 553 K~568 K for 0.9 ks~183.6 ks. Then, the samples were tested by Micro-Vickers hardness, scratch and indentation tests. The hardness of the plating reached its maximum value with the heat treatment time. The temperature dependency of the time to reach the maximum hardness was predicted by using Arrhenius type equation. When the plating reached maximum hardness, the hardness of the substrate was similar to that of as cast sample. However, the hardness of the substrate decreased with passage of the heat treatment time. In the scratch test and the indentation test, the plating was easily cracked with increasing in the hardness by the heat treatment. The critical load of the heat-treated samples also decreased in the scratch test. The results of these scratch test indicate that the hardening heat treatment of the plating does not necessarily have a positive effect on surface damage.

Creep damage preferentially extends at a stress concentration portion in high temperature components. Therefore, it is necessary to predict creep damage accurately at a stress concentration portion under multiaxial stress to maintain reliable operation. In this study, creep tests using a plain and round bar notch specimens with different notch radius (notch tip radius of 0.1mm (R0.1), 0.5mm (R0.5) and 2.0mm (R2.0)) of Modified 9Cr-1Mo forging steel have been conducted to clarify influence of multiaxial stress on creep damage extension process. Stress analyses of the notch specimens were carried out by finite element method. Distribution pattern of the maximum principal stress from notch root surface to specimen center was different depending on the notch root radius. Creep rupture times of round notch bar specimens were longer than that of plain specimen under the same nominal stress. Void number density decreased from notch surface to specimen center in R0.1 and R0.5, while it increased toward specimen center in R2.0. These distribution pattern of void number density corresponded to the distribution of the maximum principal stress. Although change in void number density with time for the plain specimen was successfully predicted by the former proposed simulation, overestimation was made for the notch specimens by the simulation. Therefore, calculation of void nucleation period in the simulation was modified by considering influence of multiaxial stress state on void nucleation. Eventually, it was demonstrated that distribution patterns of the void number density through notch root section in all notch specimens were quantitatively predicted by the modified simulation procedure.

Material strengthening can be achieved by means of effectively intercepting the dislocation motion by some obstacles. The representative one is grain boundary (GB) strengthening, called Hall-Petch relationship (H-P effect). Since such an interaction between dislocation and GB always happens at any plastic deformation state, H-P effect could have plastic strain dependence. Also the dislocation motion is extremely sensitive to temperature, thus temperature dependence should be involved as well. In the present paper, both dependences of H-P effect were investigated using pure aluminum specimens with six different averaged grain sizes at four different temperatures. The former dependence leads to a relationship with work hardening behavior which can be fitted to n-th power law, and the latter successfully induces the activation barrier for the plastic deformation to be evolved across GB. H-P coefficient, which is a coefficient of the term with power -1/2 of averaged grain size on the H-P relationship, was summarized as a function with power m of plastic strain, where m value is newly defined as plastic strain grain boundary strengthening sensitivity exponent. Considering two simple models for description of fundamental plastic deformation across GB in comparison with experimental results, H-P coefficient is found to be proportional to power n of plastic strain (that is, m value is equal to n value of work hardening exponent). This corresponding model suggests that the internal stress due to the piled-up dislocations in front of GB activates the dislocation sources in the adjoining grain. However, this relation doesn't hold at elevated temperature. From Arrhenius plots concerning to the relationship between plastic strain and H-P coefficient, activation energy was obtained as 67 kJ/mol, which is almost half of that of creep behavior (140 kJ/mol).