Online Structural Health Monitoring of Composite Structures using Smart Sensors

Now a days advanced composites having superior physical and mechanical properties are \ud being developed, new means and methods for making them intelligent are eagerly being \ud sought. Modern visions of next generation aerospace vehicles comprise the use of \ud distributed sensors and actuators, which are incorporated into the composite laminate \ud structures on nano level. The eventual goal is to reduce cost, weight, and design \ud complexity while high performance polymers are being considered for use in this \ud endeavor due to their light weight, high thermal and chemical stability and mechanical \ud durability. \ud This research is based on a novel thermoplastic based conductive polymeric blend of \ud Polystyrene/Carbon black (PS/CB) coating for strain sensing in online structural health \ud monitoring system. It has been demonstrated that interfacial bonding between the carbon \ud nanoparticles is greatly improved by the Polystyrene polymer. The strain sensing behavior \ud of piezoresistive PS/CB blend coated on aerospace grade glass fabric is also investigated. \ud The PS/CB was first blended to produce electrically conductive piezoresistive coating. \ud The coating was then applied on glass fabric using doctor blade technique. \ud This coated fabric was analyzed for two types of configurations i.e. PS/CB coated glass \ud fabric and PS/CB coated glass fabric impregnated with epoxy polymer. The two types of \ud specimens were tested at different strain rates and ply orientations. The PS/CB coated \ud fabric showed improved response for relatively slower and intermediate strain rates \ud between 10-4 s-1 and 10-3 s-1. The PS/CB coated fabric infused with epoxy configuration \ud showed promising results for strain rate of about 10-3 s-1. However, the PS/CB coated \ud fabric impregnated with epoxy was ineffective at higher strain rates. The morphology of \ud both types of coatings was obtained using SEM to properly characterize the PS/CB blend \ud as a potential strain sensor. \ud This PS/CB strain sensor is a piezoresistive type electrical strain sensor and it can be used \ud with wheatstone circuitry like a conventional strain gauge. Piezoresistive effect of the \ud sensors is investigated. When strain is produced, they undergo variation in their electrical \ud properties i.e. electrical resistance. Graphical relationship is developed between voltage \ud variations and strain. Gauge factor of the sensors is also calculated. \ud This thesis also examines the strain measuring ability of smart sensing layer for notched \ud composite specimens. The real life stresses in smart skins of future aerospace structures \ud have been investigated under different notch spacing. To find out the condition of strain at the sensing layer, Finite element based numerical simulations of the stress/ strain \ud concentration have been carried out in the presence of different defect configurations by \ud using ABAQUS. This has been experiential that smart sensing layer detects accurately the \ud existence of large deformations and damage due to defects in the structure with evidently \ud distinct peaks at the points of structural damage. \ud Dynamic response of smart PS/CB coating describes the possible use of this smart paint \ud sensor for monitoring structural vibrations, mechanical excitation and cyclic loads. The \ud PS/CB coating sensor showed remarkable repeatability when multicyclic loadings were \ud applied. The intelligent coating effectively responded to the applied strain under dynamic \ud conditions. The cyclic response of the smart coating showed that the sensors are \ud competent of following loading and unloading cycles of specimens without any delay. \ud This experiment determined the efficiency and the structural integrity of intelligent coating used for composite specimens under dynamic loadings. The capacitance effect of coated strain sensor was considered and found to have a minimal \ud effect on the dynamic response. This sensor is able to detect small crack propagation under \ud dynamic loading. The coating remained intact within a safe strain limit and showed no \ud physical damage after 20 cycles. The change in resistivity has been correlated with the \ud change in strain