Key Engineering Materials Vols. 334-335

Abstract: Conceptual structural design of the main wing for the 20 seats WIG(Wing in Ground Effect)flight vehicle, which will be a high speed maritime transportation system for the next generation in Rep. of Korea, was performed[1,2]. The high stiffness and strength Carbon-Epoxy material was used for the major structure and the skin-spar with a foam sandwich structural type was adopted for improvement of lightness and structural stability. As a design procedure for this study, firstly the design load was estimated through the critical flight load case study, and then flanges of the front and the rear spar from major bending loads and the skin structure and the webs of the spars from shear loads were preliminarily sized using the netting rule and the rule of mixture[4,5]. In order to investigate the structural safety and stability, stress analysis was performed by commercial Finite Element code such as NASTRAN/PATRAN. From the stress analysis results, it was confirmed that the upper skin structure between the front spar and rear spar was weak for the buckling. Therefore in order to solve this problem, a middle spar and the foam sandwich structure at the upper skin and the web were added. After design modification, even thought the designed wing weight was a little bit heavier than the target wing weight, the structural safety and stability of the final design feature was confirmed. In addition to this, the insert bolt type structure with six high strength bolts to fix the wing structure at the fuselage was adopted for easy assembly and removal. As well as consideration of the fatigue limit load for more than 20 years fatigue life.

Abstract: Existing test methods for thick-section specimens (≥ 4[mm]) have not provided precise compressive properties to date for the analysis and design of thick structure. In the current study, the cause of the premature failure for thick specimens (T800/924C and IM7/8552) was identified experimentally and numerically. The premature failure was successfully avoided during compressive testing and the failure mode was quite similar regardless of increasing specimen thickness and specimen volume. Furthermore it was identified that the failure location within gauge section does not have an influence on the compressive strength from the experimental result performed with the unwaisted specimens and the waisted specimens. Finally, clear 1-D thickness and 3-D scaling effects on the compressive strength of unidirectional composite laminates existed. The main factors influencing the effects were due to manufacturing defects such as void content and fibre waviness.

Abstract: Adhesive bonding between different materials has been widely used for a large variety of applications, such as in the aircraft, automotive, and many other civil engineering structures. Adhesive-bonded joints as load bearing components have the potential to save significant weight and cost over conventional riveted or bolted joints. For the last ten years a major problem in adhesive technology has been the difficulty in predicting the accurate load bearing capacity of a joint. This difficulty comes from the fact that the stress distribution in the adhesive joint is very complex and singular stress field exists at the bi-material corner. And for bonded joints, the failure usually occurs at the adhesive/adherend interface. Therefore another difficulty comes from the complex interfacial failure analysis due to the formation of chemical bonds, whose strengths are difficult to measure. Many studies have been conducted to investigate the effects of bond thickness, material properties of adhesives and adherends, and geometric shape of bi-material corner tip to the fracture behavior of bonded joints. In this paper, we analyze the stress fields at the interface corner of composite/steel(anisotropic/isotropic) double lap joint to predict failure by using stress intensity based fracture criterion. And analytical results are compared with experimental results of co-cured lap joints under tensile load condition. Micro-structural features, hardness characteristics, and fracture toughness determinations of the interfaces are also conducted.

Abstract: The improvement of crack resistance is essential to the application of composites for cryogenic use such as structures storing liquid oxygen or liquid hydrogen. In this study, an effort to improve the crack resistance of a carbon/epoxy composite was made by adding MWNTs (Multi-Walled Carbon Nanotubes) to the resin formulation. Ahead of the investigation of MWNT effect, an epoxy matrix system was developed by mixing two kinds of epoxy resins and adding additives for high toughness at cryogenic temperature. The MWNT-added carbon/epoxy unidirectional prepregs were fabricated by way of a filament winding method with different concentrations of MWNTs (0.2wt% and 0.7wt%). The mechanical tests were performed inside an environmental chamber at room temperature and -150°C. The developed material system has little influence on interlaminar shear strength but resulted in higher fracture toughness at -150°C than those of baseline material. Microcrack densities after thermo-mechanical cycles were measured through an optical microscope.

Abstract: Environment-friendly green composites were developed by combining a biodegradable resin with curaua fibers. The composites were fabricated by three different methods, i.e. direct, pre-forming and prepreg sheets methods. Mechanical properties of composites fabricated by all three methods were evaluated through tensile test. Moreover, green composites reinforced by fibers treated with high concentration alkali solution were fabricated by pre-forming and prepreg sheets methods. The results showed that fracture strain of alkali-treated fiber composites increased twice to three times larger than that of untreated-fiber composites. Finally, effect of cyclic load application on tensile strength of curaua single fibers was investigated.

Abstract: Molecular simulation of single chain in the vicinity of nanoparticle in comparison with pure system is presented. According to the Rouse theory, chains were considered as a sequence of beads connected together by harmonic springs. The motion of atoms was supported by thermal energy and retarded by the resistance of surrounding. New atom position, in given time, was determined by the Smoluchowski equation, that consists of two terms: first one includes the influence of the inter-atomic collisions, the sterical obstacles and the strong intermolecular interactions in friction coefficient, second one express the energy field aggregated from potentials of all atoms. Sinusoidal shear stress was applied to the chain. The output of the model was energy as a function of time. The energy course was also sinusoidal but shifted according to the deformation. The amplitudes and phase shifts were analyzed for the chains under different conditions .The chains were subjected to the model first as the standalone objects. Then, barrier was defined and chains placed in the vicinity of it. The barrier acted as a volume excluded hindrance. This type of chain molecular dynamics could be used as a stand-alone model or it could be suitable component for complex models, for example network model of polymer nanocomposite.

Abstract: Recently there has been an increased demand in the use of FRP materials as load-bearing structural elements for the civil engineering applications, since FRP composite materials offer superior properties to conventional structural materials used in civil engineering structures. Among various production techniques, the pultrusion process has enabled the production of structural profiles with large cross sections at relatively low cost. Pultruded structural members are generally manufactured in the form of thin-walled member composed of plate elements and material properties can often be assumed as orthotropic. Due to the relatively low geometrical stiffness, often combined with low elastic modulus, the design of pultruded profiles is governed by the deflection or buckling rather than the material strength. In this paper, the elastic local buckling coefficient of the pultruded structural member having L-, T- and box-shape under the uniform in-plane compression was investigated and approximate equation to find the local buckling strength was proposed by using the classical orthotropic plate theory and energy method. The suggested equation can be used in the development of the design criteria and local buckling analysis of pultruded structural members.

Abstract: In recent years, the FRP-concrete composite bridge deck system has been introduced because of its light-weight and durability. The FRP-concrete composite bridge deck is composed of FRP module and concrete, and they are connected with shear connectors. In order to insure the composite action between FRP module and concrete, appropriate types of shear connector need to be installed. In this study, new type of FRP shear connector was suggested and the experimental investigations are conducted based on the studies of Perfobond. In the experimental study, the push-out test was conducted and the load carrying mechanism was analyzed including the friction effect of sand coating. Considering the load carrying mechanism of perforated shear connector under shear force, the empirical equation for the prediction of shear strength of perforated FRP shear connector was suggested.

Abstract: Environment-friendly “green” composites were fabricated from a starch-based, dispersion-type biodegradable resin and cellulose nanofibers. The mixture of the dispersion-type biodegradable resin and cellulose nanofibers were blended well by using a home-use mixer and a stirrer, and then dried in air or in a vacuum. Composites were prepared by conventional hot pressing at a constant temperature of 140°C and at pressures from 10 to 50 MPa. Their flexural strength as well as flexural modulus increased with increasing the molding pressure, and were also affected by preparation methods and conditions. Their mechanical properties such as strength and modulus had a good correlation with their density. Especially it can be seen that there is significant effectiveness in a stirrer mixing process, which results in the improved uniform dispersion of nanofibers.