Some issues and development in the vacuum-assisted resin transfer molding (VARTM) technique and the use of carbon nanotubes (CNT) as additional reinforcement in laminated composite are discussed in this article. There has been considerable interest in the incorporation of CNT as secondary reinforcement in addition to the primary fiber reinforcement in laminated composites. The use of CNT is apparently able to enhance the out-of-plane material properties by improving interlaminar strength and fracture toughness of laminated composites. The addition of CNT may be done through the VARTM technique, a non-autoclave, relatively low-cost method, frequently used to fabricate parts with complex shapes. This survey shows that while the matrix-dominated interlaminar fracture toughness and shear properties may be improved, the improvement in in-plane fiber-dominated properties is very modest.
This paper presents an analysis of the coupling effect on the behavior of piezoelectric wafers used for actuators and sensors. A fully coupled formulation for an eighteen-node assumed strain piezoelectric solid element is used to test the effect of full coupling stiffness on the electric potential distribution through the thickness of the actuator/sensor. Since the assumed strain solid element can alleviate locking, it can be used to analyze the behavior of very thin actuators without locking. In the formulation, the electric potential is regarded as a nodal degree of freedom in addition to three translations at each grid point. The electric potential is then prescribed for nodes at the top and bottom surfaces in the finite element model of a PZT wafer and is left unknown for the other nodes throughout the thickness. Consequently, the induced electric potential and actuation displacement can be computed for an input voltage. The effect of coupling stiffness on the through-the-thickness distribution of the electric potential is examined, considering parameters such as thickness and in-plane dimension. This research shows that full coupling stiffness only affects piezoelectric structures of specific thickness and shape. These new findings can be useful for precision sensor and actuator design.
Finite element analysis for thick composite structures is rather complicated. Two-dimensional modeling, which is relatively easy to make, can cause inaccurate result since the plane stress condition cannot be applied, while three-dimensional modeling is hard to make. In the three- dimensional modeling, it is difficult to model all the layers with different material properties and ply orientation in the structure. In this paper, an equivalent modeling is proposed and numerically tested for analysis of thick composite structures. The method has been verified for the modeling of composite plate and circular composite tube in order to find their bending deflection and natural frequency. MSC/NASTRAN and PATRAN are used for the calculation. It has been confirmed that three-dimensional analysis must be conducted for thick structures and the equivalent modeling is proven to be accurate when layers with same characteristics are properly grouped. The proposed modeling technique has been applied to analyze hingeless composite rotor hub system designed by Korea Aerospace Research Institute (KARI). Detailed three-dimensional modeling for this structure is almost impossible to make due to its complex geometry of thick composite structures. Using the proposed equivalent modeling technique, failure analysis was performed based on stress/strain criterion and the safety of each part was checked. Deflection of the hub system was validated comparing with the result from the simple analytical beam model, and the numerical result will be used for the next design cycle of the composite hub system.
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