Key Engineering Materials Vols. 353-358

Abstract: The present paper proposes a new diagnostic tool for the structural health monitoring that employs a Kriging Interpolation. Structural health monitoring is a noticeable technology for aged civil structures. Most of the structural health monitoring systems adopts parametric method based on modeling or non-parametric method such as artificial neural networks or response surfaces. The conventional methods require FEM modeling of structure or a regression model. This modeling needs judgment of human, and it requires much costs. The present method does not require the process of modeling, in order to identify the damage level using the discriminant analysis. This suggest us, this technique is applicable to the health monitoring system, which identifies the damage of the structure, easily. In the present paper, we developed the damage diagnostic methods using Kriging method for identifying delamination from data. Kriging method is a interpolation technique which shown in geostatistic. We applied this method to identifications of delamination crack of CFRP structure. Delamination cracks are invisible and cause decrease of compression strength of laminated composites. Therefore, health-monitoring system is required for CFRP laminates. The present study adopts an electric potential method for health monitoring of graphite/epoxy laminated composites. The electric potential method does not cause strength reduction and can be applied existing structures by low cost. As a result, it was shown that this method is effective for identification of damages.

Abstract: The structure with the sensor and the actuator is called Smart Structure. Installation of the sensor and the actuator to the structure enables the automatic damage detection. In this paper, a novel damage detection method called “SI-F method” is applied to the Smart Structure to detect the delamination in the CFRP beam. The artificial delamination in the CFRP is introduced by the out-of-plane bending. CFRP beam will be exited by the PZT actuator, and the strain of the surface will be measured by the strain gauge. As the result, the delamination is successfully detected by varying the condition of the excitation.

Abstract: This paper suggested that the addition of a proper amount of mischmetal efficiently improved the contact face and its mechanism of contact materials, enhancing their electrical conductivity. At the same time, their inoxidizability and non-fusibility were also highly improved, while the temperature elevation of the contactor during operation was dramatically reduced.

Abstract: A simplified approach for calculating the stress field of the fiber-reinforced composites is developed to improve the work of Theocaris et al. (1985) in this paper. This approach considers that the main factor affecting the stress field of composites is the existence of interphase between fiber and matrix, which possesses different Young’s modulus than those of the constituent phase. A parabolic law is adopted for the variation of Young’s modulus of graded interphase, versus the polar radius from fiber to matrix, while the Poisson’s ratio of the interphase is assumed as constant. Space axisymmetric model is applied to the Representative Volume Element (RVE) of the fiber-reinforced composites to evaluate stress field of the composites. Also, the effective longitudinal modulus of the fiber-reinforced composite can be obtained.

Abstract: The two (Al2O3+TiB2+Al3Ti)/Al composites were fabricated from Al-B2O3-TiO2 and Al-B-TiO2 raw powders by reactive hot pressing, respectively. The microstructure of in situ two composites was analyzed by OM, SEM and TEM. The results showed that coarse Al3Ti blocks with several tens of micrometers size were formed during hot pressing. The equiaxed Al2O3 particulates and hexagonal TiB2 particulates with finer sizes were formed in the composites simultaneously. The microstructure formation mechanism of (Al2O3+TiB2+Al3Ti)/Al composites were discussed. The results showed that Al2O3 reinforcements were formed on the surface of TiO2 or B2O3 powder and TiB2 particles were formed on B or B2O3 powders. The formation of coarse Al3Ti block is result from continuous diffusion of Ti in liquid Al during reactive hot pressing. In addition, there are fine Al3Ti precipitates exist in the composite fabricated from Al-B-TiO2 powders. This contributes to the improved mechanical properties in terms of yield and ultimate stresses and Young’s modulus of the composite.

Abstract: This study explored the feasibility and potential benefits provided by the addition of through-the-thickness reinforcement to foam core sandwich structures. Through-the-thickness stitching is proposed to increase the interlaminar strength and damage tolerance of composite sandwich structures. A low-cost, out-of-autoclave processing method was developed to produce composite sandwich panels with carbon fiber face sheets, a closed-cell foam core, and through-the-thickness Kevlar stitching. The sandwich panels were stitched in a dry preform state, vacuum bagged, and infiltrated using Vacuum Assisted Resin Transfer Molding (VARTM) processing. For comparison purposes, unstitched sandwich panels were produced using the same materials and manufacturing methodology. Five types of mechanical tests were performed: flexural testing, flatwise tensile and compression testing, core shear testing, and edgewise compression testing. Drop-weight impact testing followed by specimen sectioning was performed to characterize the damage resistance of stitched sandwich panels. Compression after impact (CAI) testing was performed to evaluate the damage tolerance of the sandwich panels. Results show significant increases in the flexural stiffness and strength, out-of-plane tensile strength, core shear strength, edgewise compression strength, and compression-after-impact strength of stitched sandwich structures.

Abstract: (Al2O3+Ni) and (Al2O3+Ni)/Ni laminated materials were prepared by tape casting and hot pressing sintering. The mechanical properties of (Al2O3+20wt%Ni) laminated composites were higher than those of (Al2O3+50wt%Ni) composites and Al2O3 sample. The strengthening in the (Al2O3+Ni) composites mainly results from microstructure refinement of the alumina grain size, cracks bridging and crack deflection by the Ni particles. Results showed that the strength and fracture toughness of (Al2O3+Ni)/Ni laminated materials were higher than those of Al2O3/Ni laminated materials with the same layer numbers and thickness ratio. The good mechanical properties of (Al2O3+Ni) and (Al2O3+Ni)/Ni laminated materials result from the second phase of Ni particles in Al2O3 layers.

Abstract: The C/C composite was rapidly fabricated by Multi-factor coupling fields CVI in a home-made reactor with propylene as carbon source and carbon felt as porous preform. The effect of the temperature and pressure on microstructure and properties of the C/C composites was investigated. Microstructure and morphology of the C/C composites were characterized with optical microscopy, SEM, XRD and Raman spectroscopy. The results show the C/C composite has rough laminar pyrolytic carbon and higher density at conditions of 650°C and 12kPa in 12h. The observed regenerative cones were formed by amplified layers on defects. A new type of pyrocarbon was observed with graphitization degree 77.9% and crystallite size 30.5 nm and Optical and SEM morphologies reveal its peculiar structure. It is called “dot structure” pyrocarbon and its formation mechanism is simple proposed.

Abstract: In this paper the tensile and fatigue life tests of carbon fiber-reinforced epoxy prepreg (CFRP) were conducted in order to investigate the influence of volume ratio and fiber orientation. CFRP/Al7075 laminate hybrid composite (CARALL) consists of alternating Al7075-T6 sheets and carbon epoxy prepreg (epoxy 121oC #2560). The fiber orientations of CFRP were applied to the extent of 0/90° and ±45°. The CFRP layers are 1ply, 3plies and 5plies in case of 0/90°, and 1ply and 2plies in case of ±45° of carbon fiber direction, respectively. The tensile strength decreased with the volume ratio of CFRP in both the cases of fiber orientation 0/90° and ±45°. The fatigue life is lower in large volume ratio of CFRP than in small volume ratio in both the cases of fiber orientation 0/90° and ±45°.

Abstract: High reinforcement content TiB2/2024Al composites (Vp=55, 65%) were fabricated by squeeze casting technology, and their microstructures as well as mechanical properties under quasi-static and dynamic loading were evaluated. For 55 vol.% TiB2/Al composite, the bending strength and elastic modulus were as high as 623.5MPa and 218.1GPa. When compressed at a strain rate of 1050s-1, both composites exhibited a higher compressive flow stresses and compressive moduli than those under quasi-static loading. The micro-damage of high reinforcement content composites was mostly dominated by the large particle cracking. In addition, some evidence of aluminum alloy melting was observed on the fracture surfaces of dynamic compression, it was ascribed to the adiabatic heat accumulated in a local region transferred by plastic work.