Stress analysis and vibration of elastic and viscoelastic systems (Composite materials and structures)

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The objectives of this research program are two-fold. One is to provide an understanding of the crack and fracture development in composite structures, such that a criterion for failure initiation can be formulated, and the other is to develop procedures for the efficient manufacturing of thermoplastic composite structures with good quality. For the first objective, over the past few years, the applicant has found a new method that can detect, locate and quantify damages that occur in the composite structures in-situ, meaning that the detection can be done while the structure is in operation. The method is very sensitive such that it can detect matrix cracks that other techniques may not be able to detect. The method is also simple and easy to use. This method will be used to monitor the crack development in the early stages of loading of the composite structure. As such it can follow the crack jumping pattern which is unique in composite structures. The technique depends on the electrical conductivity of the composites by the incorporation of carbon nanotubes within the resin of the matrix. It is essential that an appropriate amount of nanotubes be incorporated by a good mixing method in order to assure uniformity of the electrical conductivity, and sensitivity for the detection. By monitoring the crack jumping pattern in the early stages of loading, the variability in the composite materials can be revealed. For the second objective, while thermoplastic composites offer many attractive characteristics such as no shelf life, good impact resistance, short processing cycle, recyclability etc., due to their high viscosity, processing has been difficult and voids usually exist within the structure of the final part. With the advent of automated fiber placement (AFP) machines, there are more possibilities for the processing of thermoplastic composites. In this project, two aspects will be considered. First, the bonding between substrates made of either thermoset matrix composites or thermoplastic composites, and newly formed layers of thermoplastic composites by AFP will be examined. This type of configuration is common in many aircraft structures, which consist of skins on top of reinforcements such as ribs and spars. The effects of different processing conditions such as temperature, pressure and duration of contact on the quality of the bond will be studied. Optimal conditions will be developed. For the second aspect, in AFP, the deformation of the thermoplastic composite material while they are subjected to melting temperature, and consolidation pressure, will be examined. In addition the relaxation behavior of the material when the pressure is removed will also be examined. The understanding obtained will be used to develop optimal process parameters.