Smart Structures for Health Monitoring

Laminated composite structures have high specific strength and stiffness, and the materials are applied to many aircraft and spacecraft structures. However, the laminates easily have delamination cracks by slight weight drops and fatigue loading. The delamination cracks are unfortunately invisible or very difficult to inspect by eyes. This decreases the reliability of advanced composite laminates.

In our laboratory, smart health monitoring systems are developed for the laminated composites. The first method is an electric potential method. Reinforcement carbon fiber itself is used as sensors to detect the delamination cracks in the system. Carbon fiber is not straight in laminated composites, and has large number of contact points with adjacent fibers. The contact between fibers makes a large network of carbon fibers. Delamination crack growth breaks the network and changes the electric resistance of the laminates.

We adopt a response surface methodology as an artificial intelligent system to judge the delamination crack existence. Designs of experiments are conducted for collection of data of electrical resistance change. By using the response surfaces, high precise predictions of delamination position and size can be obtained without considerations of fracture mechanism at low cost.

The second method is a method using Electro-luminescent devices. Electro-luminescent (EL) devices are usually used for LCD back light for cellular phones. For transparent composites like glass/epoxy or Al2O3/expoy composites, usually we can easily find defects by our eyes. However, in the case that the composite structures are inside of the vessel to prevent thermal conduction for MAGLEV super-conductive coil support structures, health-monitoring system is required.

We adopted a method to detect the damage by the transparent-light-brightness change. Since the EL device is very flexible, we can mount it on the curved surface of the support structures. Optical fiber is also mounted on the other surface. The brightness is measured by using optic sensors. We can judge the damage in service by measurements of the brightness.

A lot of different methods are planed for the health-monitoring of laminated composites. Please visit and join us.