Measuring Material Moisture in Fiber Reinforced Polymers by Integrated Sensors

The environment can influence and change the properties of structures made of fiber reinforced polymers (FRP). Besides changes in temperature, variation of moisture can be a problem for the mechanical stability of the FRP. In addition, structural health monitoring systems based on Lamb wave propagation need to differentiate between moisture up-take and the local defects. This is because humidity and high temperatures can lead to moisture up-take in the matrix, altering the wave propagation. Thus, it is necessary to determine and incorporate the level of moisture for compensation methods like optimal baseline identification. Here, integrated sensors demonstrate to be useful in the determination of water up-take and in the long term, even aging. This paper presents the successful evaluation of commercially available resistive and capacitive sensors and a new flexible interdigital sensor, integrated in the material to monitor moisture up-take.