Key Engineering Materials Vols. 297-300

Abstract: For prevention of failure and extension of maintenance interval, many Asian power companies have continuously tried to apply Condition Based Maintenance system to their plants. However, only few companies succeed to change their maintenance system from fixed time based to equipment condition based, although there are many brand new ITs, such as high speed computing system, huge storage device, and advanced development software tools. This paper describes the reasons why they failed to change and proposes the solutions to construct maintenance management system based on equipment condition. The solutions were also applied practically to power plant in Korea.

Abstract: In this paper, CaB6 sintered body was fabricated by hot-pressed sintering with/without nickel as a sintering aid. The microstructure and fracture morphology were observed by means of SEM. CaB6 polycrystalline hot-pressed at 2123K showed insufficient densification. Fracture surface revealed that the existence of pores and the poor grain boundaries made the occurrence of intergranular fracture. When 28wt% nickel was added, nearly full density was obtained, although the sintering temperature is 200K lower. Hardness, Bending strength and fracture toughness of polycrystalline CaB6 were measured. By adding the Ni in CaB6 matrix, the flexural strength and the fracture toughness were enhanced, and the ratio of transgranular to intergranular fracture type was increased notably. The fracture surface showed a transgranular fracture. The crack bridging, micro-cracking and crack deflecting were deemed as the contribution to the improved fracture toughness.

Abstract: The mechanical properties of palladium (Pd) wire absorbed hydrogen were evaluated by the quasi-static tension test and indentation test. The electrolytic method was used for hydrogen absorption. Pd wire with a diameter of 1mm was used. The electrolyte was a sulfuric acid solution and the current density used in the electrolytic method was 200mAcm-2. The hydrogen absorption ratio defined by the molecular ratio (H/Pd) of hydrogen and palladium was controlled by the absorption time. The gauge length for the tension test was 20mm. The ultimate tensile strength increased with the increase of the absorption ratio. On the other hand, the increase of the ratio decreased the strain hardening parameter and fracture strain. A model considering the specimen absorbed hydrogen and a composite material constructed in a concentric configuration was suggested to estimate the hydrogen absorption area and mechanical properties. The indentation test was conducted to clarify the evolution of the embrittlement due to the hydrogen absorption microscopically and determine the absorption area precisely. Vickers hardness clearly increased with the increase of the hydrogen absorption ratio. The hardness distribution was measured to detect the boundary of the absorption and non-absorption area using a Berkovich indenter that is smaller than a Vickers indenter. The hardness boundary of the absorption and non-absorption of the specimen with the hydrogen absorption ratio of 22 percent was observed experimentally at the position around 100-150μm from the outside of the specimen. The position of the boundary estimated using the model was 85μm from the outside of the specimen. When the stress-strain curves of the specimen with the unknown hydrogen absorption ratio were measured, the hydrogen absorption ratio could be estimated using the proposed model.

Abstract: High temperature gas cooled reactor (HTGR) with higher outlet coolant temperature nearly 1000°C is expected for direct utilization of process heat to hydrogen production. The thermal analysis of reactor internals with 3 dimensional, flow paths coupled model was conducted to demonstrate how strictly PSR block gaps must be closed to limit core bypass flow rate ratio lest fuel temperature should exceed admissible level, and the highly heat resistant core restraint mechanism must be developed in consequence. Potential applicability of the core restraint mechanism made of C/C composite, the attractive candidate material, was demonstrated by point design with adequate thickness and FEM stress analysis for material with orthotropic anisotropy .

Abstract: Composite materials have drawn a lot of attention in recent years due to their being in expensive, lightweight and biodegradable. The polyetherimide has been successfully used as an effective toughening modifier for polyfuctional epoxy resins for aerospace use. The material used in this study was solvent-impregnated carbon-fiber fabric reinforced polyetherimide. In this study, effects of water absorption cycles on fracture toughness in carbon fiber reinforced composites (CFRP) have been investigated according to various absorption cycles. As a result, strength of CFRP composite was decreased according to increasing water absorption cycles.

Abstract: A new method is proposed in this paper for robust deign of configuration of multiple layers. The present design tool may enable the designers to estimate the flexural strength for the target combination of lamina. In line with these objectives, an experimental analysis was performed for 3 kinds of combinations of the number of orientation angles and 21 kinds of configurations of the symmetric 26 layers. The experimental results are compared with the theoretical considerations. From the viewpoint of robust design of configurations, the relation between flexural fracture strength and equivalent engineering modulus was also investigated. Finally, the procedure of practical robust design of composite laminate is also proposed.

Abstract: Shape optimization was performed to obtain the precise shape of cutouts including the internal shape of cutouts in laminated composite plates by three dimensional modeling using solid element. The volume control of the growth-strain method was implemented and the distributed parameter was chosen as Tsai-Hill failure index for shape optimization. In order to verify the validity of the obtained optimal shapes, the changes of the maximum Tsai-Hill failure index were examined for each load condition and cutouts. The following conclusions were obtained in this study; 1) It was found that growth-strain method was applied efficiently to shape optimization of three dimensional cutouts in anisotropic laminate composite, 2) The optimal three dimensional shapes of the various load conditions and cutouts were obtained, 3) The maximum Tsai-Hill failure index was reduced up to 68% when shape optimization was performed under the initial volume by volume control of growth-strain method.

Abstract: To prove the suitability the honeycomb composites structure with VARTM, the mechanical properties of the skin materials and honeycomb composites structure were evaluated with the static strength tests. The mechanical properties of honeycomb composites structure made by VARTM were satisfied with the real using conditions instead of the composites structure made by autoclave process. Accordingly, the honeycomb sandwich composites made by VARTM is available for manufacturing various composites parts. VARTM was very effective method to manufacture the honeycomb sandwich composites. It was possible that the manufacturing process was changed from autoclave process to VARTM to solve the problems on the autoclave process.

Abstract: When comparing composite sandwich analysis with an exact solution, the results of finite element modeling with an ANSYS shell 91 element agreed well with the exact solution. The practical applications of the shell 91 element are demonstrated with a four-point bend test conducted on sandwich beam specimens. The specimens comprised carbon/epoxy fabric face sheets and a honeycomb core. Two kinds of honeycomb cores were used to fabricate the composite sandwich specimens: an aluminum one and a glass/phenolic one. The predictions with the shell 91 element were also agreed well with the experimental results. A variety of tests was conducted; namely, a long beam flexural test, a short beam shear test, a flatwise tensile test, a flatwise compression test and an edge compression test. The sandwich plate with the aluminum honeycomb core had a specific bending stiffness that was 1.7 to 2.0 times higher than that of the sandwich plate with the glass/ phenolic honeycomb core.

Abstract: The structural shapes or plates with varying thickness are often adopted in aerospace industries such as a skin structure of an airplane and civil engineering fields where the load effects are varied along the longitudinal axis of the member. Specially, the structural steel tapered plate, so called LP (longitudinally profiled) plate, has been used in many countries as a part of main member of bridges, buildings, etc. This paper presents the result of an analytical study pertaining to the local buckling behavior of FRP (Fiber Reinforced Plastic) flexural member. The flexural member is consisted with uniform thickness web and flange tapered in thickness. The boundary conditions of flange plate in its unloaded edges are elastically restrained, but they are assumed to be either fixed-free or simple-free for simplicity. In the analysis, the Galerkin form of Rayleigh-Ritz method is adopted. The buckling equation of isolated unstiffened tapered plate elements having fixed-free or simple-free boundary condition in its unloaded edges is derived. The plate buckling coefficients with respect to the thickness ratios and plate aspect ratios are calculated and presented in a graphical form. Unlike the buckling of plate with uniform thickness, discontinuous changes of buckling mode are not exist. In addition, the critical slenderness ratio of such an unstiffened tapered plate element is suggested to prevent buckling.