To identify the creep behavior of Zircaloy-4 during operating time of nuclear power plants, creep tests were performed at 673 K. The alloy showed an abnormal creep behavior, i.e., two-stage steady state creep, at low stresses of less than 66 MPa. The first steady state was observed at strain of 0.05, whereas the second one appeared at that of 0.15 at 55 MPa. Moreover, the first steady state creep rate was faster than the second one. It resulted in the changing of the stress exponent from 4.9 to 14. Microscopy after the creep tests revealed that dislocation structures at each stage were totally different. In the first stage, EBSD results showed that strain distributed uniformly in grain interiors, where dislocations moved individually. In the second stage, the interaction among dislocations became stronger with increasing strain because cell structure was generated. Therefore, the creep was suppressed and the creep rate decreased in the second stage. It concluded that the changing of dislocation structure during creep led to two-stage steady state creep behavior in Zircaloy-4.
MCrAlY alloy thermal spray coatings are applied to protect the substrate from oxidation and corrosion under high temperature environment. Various MCrAlY alloys can be coated by different thermal spray processes; LPS, HVOF, and APS. MCrAlY coating by APS is said to be inferior in oxidation resistance, however few studies have been conducted on oxidation property of APS MCrAlY alloy coatings. In this study, CoNiCrAlY, NiCoCrAlY, and NiCrAlY alloys of two different particle sizes were prepared and their coatings were formed by APS process. These MCrAlY alloy powders and coatings were exposed to high temperature and their surface oxide formations were analyzed to determine the growth rate and the activation energy from the growth of the oxide layer thickness. Furthermore, microstructure observations and chemical composition analysis of coatings were conducted. From these results, influence of MCrAlY powder size difference and oxides contained in the coating on high temperature oxidation property of APS MCrAlY coatings was revealed.
The propagation behavers of naturally initiated small crack in a conventional cast polycrystalline Ni-base superalloy were studied under the thermomechanical fatigue conditions. The small crack propagation tests were carried out under the in-phase (IP) and the out-of-phase (OP) type thermomechanical fatigue (TMF) conditions as well as the isothermal low-cycle fatigue (LCF) condition at the maximum temperature of the TMF conditions. The experimental results revealed that the initiation and propagation morphologies of the naturally initiated small crack were affected by the thermomechanical loading condition. Under the LCF and IP-TMF conditions, the small cracks were initiated and propagated at the grain boundaries perpendicular to the loading axis. On the other hand, under the OP-TMF condition, the small cracks were initiated and propagated with the transgranular modes. When the crack growth rates of naturally initiated small crack were correlated with fatigue J-integral range, which is the continuum mechanics parameters for homogeneous and isotropic materials, the crack growth rates under the TMF conditions were almost similar to those under LCF condition even if the propagation morphologies of small cracks were varied with the thermomechanical fatigue conditions. In addition, it could be considered that the crack growth morphologies, grain structure and grain orientation affect the scatter of the small crack growth rate. The prediction method for TMF and LCF lives was proposed based on the small crack propagation considered with the crack opening-closing behavior. The TMF and LCF lives of Ni-base superalloys could be predicted with higher accuracy by the proposed method.
In the hybrid molding of CFRTP, short or long fiber-reinforced thermoplastics, which compose rib and complicated structures, are injected on top of the continuous fiber-reinforced thermoplastics that are used for the outer shell material. In previous studies, it has been reported that fractures often occur at the interface between short or long fiber-reinforced thermoplastics of the rib structures and the continuous fiber-reinforced thermoplastic laminate of the outer shell structure. By applying cut prepreg having high formability to flow into their interface, improvement of the bond strength of the interface at the rib root portion can be expected. In this study, the press and injection hybrid molding was conducted by supplying cut prepregs to the continuous fiber-reinforced thermoplastic laminate of the outer shell structure, and T-shape specimens extracted from the rib root portion were evaluated by the tensile tests to clarify the effect of the penetration height of cut prepregs to the rib structures on the bond strength of the interface. Higher interfacial strength can be obtained when the reinforced fibers of the penetrated cut prepregs in the rib were oriented to the vertical direction to the interface.
In the press and injection hybrid molding of CFRTP, short or long fiber-reinforced thermoplastics are injected on the continuous fiber-reinforced thermoplastics, which are used for the outer shell material. Since its fracture often occurs at a rib root portion, which is the interface of the injected materials and the continuous fiber-reinforced thermoplastics, the rib root portion has to be strengthened. By using slit material having high formability for the outer shell material, the penetration height of the outer shell material into the rib becomes higher, therefore, improvement of the strength of rib root portion can be expected. In this study, to clarify the effect of penetration height of slit materials on the mechanical properties of the rib root portion of the press and injection hybrid molding, slit materials were used for the outer shell material, and tensile tests of the surface layer and bonding strength at rib root were conducted. Experimental results reveal that the press and injection hybrid molding using slit materials in half of the continuous fiber-reinforced thermoplastics prepreg possesses the good combination of high bonding strength of rib root portion and high tensile strength of the surface layer.
The use of Carbon Fiber Reinforced Thermoplastics (CFRTP) is expected to achieve lightening in mass-produced products. Although composite materials have the advantage that the complex structure can be integrated when being molded, many joints exist for real products. We have been developing a new welding technology in which carbon fiber itself was used for the heating elements, however further improvement of bonding strength is needed. Grafting carbon nanotubes (CNT) on the surface of carbon fibers was reported to improve the fiber matrix interfacial properties. In this study, CNT grafted carbon fibers were used for the heating element of resistant welding, and the effect of carbon nanotube grafting on resistance welded strength of CFRTP was discussed. Welded properties of CFRTP were evaluated by the tensile shear test and the higher welded strength by using carbon nanotube grafted carbon fibers was obtained.
Water-swelling friction reducing materials (WSFRMs) are more frequently used as a “pull-out assisting material” for temporary sheet-piles or piles that are required to be removed and collected after use. Generally, WSFRMs are applied to the steel sheet-piles, piles or H-steel piles before the piles are driven into the ground or placed in the mortar fluid. The WSFRMs absorb moisture in the ground or mortar to swell and form a swelling membrane over the piles. Then, the membrane works also as a lubricating membrane and as a result it can reduce friction. The authors pay attention to these characteristics of WSFRMs and try to develop a special material that can swell only when soaked in an alkaline moisture environment without swelling in acid or a neutral water environment, in addition to the conventional material that swells in any type of moisture environment. In this paper, given that the amphoteric and alkaline WSFRM is coated to temporary materials, to facilitate the pull-out, the authors carry out some experiments to evaluate the fundamental characteristics such as the swelling ratios of the amphoteric and alkaline WSFRMs and the forces to pull out the iron flat bars on which such WSFRMs were coated in advance. As one of the findings, the amphoteric and alkaline WSFRMs coated in advance reduce the friction on the contact surface between the flat bar and the mortar and enables the pull-out with less force.
This paper deals with the problem of dynamic stability of angle-ply laminated cylindrical shells subjected to impact torsional moment. First of all, the motion of cylindrical shells under impact torsional moment is defined as axially symmetric motion. Following this definition, certain perturbations are superimposed on this motion and their effect on the behavior of the shell is investigated. The symmetrical state of the shell is called stable if the perturbations remained bounded. The solutions for pre-buckling motion and the perturbed motion are obtained using the Galerkin’s method. Stable regions are determined by utilizing Mathieu’s equation. The inevitability of dynamically unstable behavior is proved analytically and the effects of various factors, such as torsional stress ratio, lamination angle and dynamic unstable mode are clarified.
Mass moment of inertia is an important criterion in rating a performance of mechanical systems with rotational motion. In fact, moment of inertia of actual products may be different from design values because manufactured products are uneven. In injection molding, it is important to detect voids occurred after injection process to eliminate inferior products. Even in non-defective products, moment of inertia may vary over time due to factors such as spoilage, corrosion, and drying. In order to improve the inertia moment of the product, the weight and the shape must be modified. Therefore, it is important to evaluate mass distribution and internal shape nondestructively from the view point of quality assurance. Mass distribution and internal shape can be investigated by cutting, but such a destructive method is effective only for sampling inspections. The author proposes a new non-destructive method evaluating mass distribution in the entire component of portable solid even though material densities and dimensions are unknown. In this method, mass distribution is estimated by an inverse analysis from mass moments of inertia measured at several rotational axes nondestructively. Voids in a body can be detected by this method because estimated values at voids become zero. Note that density distribution is also evaluated from estimated masses and the shape. In this study, a new concept estimating mass distribution was shown theoretically. As well, numerical simulations for butt-cylindrical rods were carried out to prove the effectiveness of this method. Furthermore, estimation accuracy of this method was improved using the response surface methodology without any additional measurements of mass moment of inertia.