Key Engineering Materials Vols. 321-323

Abstract: Analysis of structures including periodically arrayed in-homogeneities, whether they are microscopic or macroscopic, can be simplified by using homogenized properties. If the unit structure is repeated in a longitudinal direction, the whole structure may be regarded as a beam with effective flexibility. This study proposes a kinematical periodicity constraint to be imposed on the FE model of the unit structure for beams, which improves the accuracy of the effective flexibility along with reduction of the model size. The method is applied to a beam-in-space containing repetitive un-symmetric holes, and it is demonstrated that the deformation behavior of the homogenized beam agrees well with that of the real structure.

Abstract: Dominant frequency characteristics of acoustic emission (AE) from single-edge-notched (SEN) glass fiber aluminum laminates (GFAL) under tensile loads were analyzed in relation to fracture mechanisms. The first and second peak frequencies expressed the characteristic changes of fracture processes in SEN-GFAL specimens such as macro-crack propagation and/or delamination between aluminum and fiber layers. On the basis of the above frequency analysis and of the fracture observation with ultrasonic through-scan and various microscopes, characteristic models for crack propagation of SEN-GFAL was proposed according to various orientations of fiber layer.

Abstract: AE technique was applied to the structural strength tests of the ablative composite liner bonded on steel nozzle to figure out the unexpected problem that happened during the flight test. Two different kinds of specimen, which were specimens with some initial crack and without initial crack each, were used for the test. AE test methodology and parameters were investigated to assess the damage of them. NDT using X-ray technique was performed to improve the test reliability before and after each test. The study revealed that AE technique is a good method to evaluate damage on this kind of specimen with better accuracy.

Abstract: The microstructure and the mechanical property of liquid phase sintered (LPS) SiC materials with oxide secondary phases have been investigated. The strength variation of LPS-SiC materials exposed at the elevated temperatures has been also examined. LPS-SiC materials were sintered at the different temperatures using two types of Al2O3/Y2O3 compositional ratio. The characterization of LPS-SiC materials was investigated by means of SEM with EDS, three point bending test and indentation test. The LPS-SiC material with a density of about 3.2 Mg/m3 represented a flexural strength of about 800 MPa and a fracture toughness of about 9.0 MPa⋅√m.

Abstract: Surface-conductive microspheres consisting of poly(methyl methacrylate) (PMMA) (6.5 μm) core and carbon nanotubes (CNTs)-adsorbed shell were prepared using a simple process involving the blending of two colloidal solution; an aqueous CNT dispersion with surfactants and an aqueous PMMA microsphere colloid. These were adopted as the suspended particles for electrorheological (ER) fluids, in which the electrical conductivity originated primarily from the surface-coated conducting CNT layers. The CNT-adsorbed polymeric microspheres were monodisperse and spherical in shape. The CNT-PMMA composite suspensions in silicone oil showed the typical ER characteristics of forming a chain-like structure under an applied electric field. The CNT-PMMA composite microspheres exhibited a conductivity ranging from 5.2×10-4 to 6.3×10-5 S/cm, which is an acceptable conductivity range for ER fluids. This phenomenon can be explained by the interfacial polarizability of CNTs adsorbed on the surface of the polymeric microspheres.

Abstract: Biodegradable composites consisting of poly(butylene succinate) (PBS) and carbon nanotube (CNT)-coated silk (Bombxy mori) fibers were prepared by melt compression molding. The results show that even with addition of a small amount of reinforcements (about 3 wt%), the tensile strength and modulus of the composites improved dramatically by about 195% and 121%, respectively, compared with PBS. The improvement is attributed to stronger interfacial shear strength between the PBS matrix and the CNT-coated surface of silk fiber, which was obtained by the microbond droplet test between PBS matrix and fibers. Furthermore, scanning electron microscopy images indicated that the interfacial adhesion between PBS matrix and CNT-coated fiber improved in the composites.

Abstract: A modal strain based damage index is proposed to investigate the damage effects of discrete delaminations in a laminated composite structure. The Fermi-Dirac distribution function is incorporated with an improved layerwise laminate theory to model smooth transition of the displacement and the strain fields at the delaminated interfaces. Modal analysis is conducted to investigate dynamic effects of delamination in a laminated structure and to obtain modal strains. The damage index is calculated based on fundamental modal strains of laminated structures. The damage effects of laminated structures are investigated using arbitrary size, number, location and boundary conditions of discrete delaminations.

Abstract: The mechanical property of sintered MoSi2 materials has been investigated, based on the detailed examination of their microstructures. The nondestructive technique by an ultrasonic wave was also used for evaluating the damage behaviors of MoSi2 materials suffered from the cyclic thermal shock. MoSi2 materials were sintered at the temperature of 1723 K. The flexural strength of MoSi2 materials gradually decreased with increasing the thermal shock cycle, accompanying the extensive creation of surface cracks. The increase of thermal shock cycles resulted in a great decrease of ultrasonic wave velocity and a linear increase of attenuation coefficient for MoSi2 materials.

Abstract: Multi-walled carbon nanotubes (MWNTs) were incorporated in electrospun Nylon 610 fibers. The MWNTs used were synthesized by a thermal CVD method. To eliminate metallic catalyst residues in as synthesized MWNTs, they were treated in HNO3 and HCl. The potential adhesion between the MWNTs and Nylon 610 was expected by the interaction between the amide group of Nylon 610 and the oxygen-containing species on the MWNTs such as carboxylic acid group introduced during the purification step. Contrary to the previous works on electrospinning using a simple blend of carbon nanotubes with polymeric materials, we incorporated MWNTs as nanoscale fillers by in-situ interfacial polymerization. We also investigated morphology of the electrospun Nylon 610 fibers with MWNTs.

Abstract: Because of orientation and separation, injection molded products are heterogeneous and anisotropic. These heterogeneousness and anisotropy have a vast influence on mechanical properties of molding material and product characteristics. It is well known that fiber orientation state in fiber-reinforced polymeric composite has a profound effect on dynamic qualities like intensity, rigidity, and etc. To measure the fiber orientation of injection molded product's weld part, we first X-rayed moldings and recognized this photo by using an image scanner. Then, image processing method, which uses intensity difference in measuring fiber orientation state, is applied. Through these procedures, we can analyze the influence of molding's fiber orientation state according to mold gate changes. Fiber orientation is related mainly with the mold gate positions than with fiber content or mold temperature. When the distance from the gate increases, by matrix and reinforcement's flow speed differences, fiber orientation occurred. As diversion flow occurred at the end of fluid flow, fiber oriented at a right angle to the flow, and this is the same effect of weld line formation.