An embedded radio frequency sensor for application in an electrically non-conductive composite structure

Embedding a radio frequency sensor in the form of a balanced transmission line into an electrically non-conductive composite structure is investigated as a means of strain sensing and continuous lifetime testing. Initial experiments established that the reflection coefficient phase shift correlates to length changes in the transmission line using a printed circuit board having parallel copper lines. A transmission line consisting of flexible metal-coated fabric strips was then embedded in a composite sample. This sample was subjected to tensile strain and phase shift data and strain gauge data were collected. Correlations were found between load, phase shift and strain. Lastly, the printed circuit transmission line was used in an oscillator circuit for tuning. This experiment demonstrated that the capacitive effect of an open circuited transmission line could be used to vary the frequency of an oscillator. As the length of the transmission line was shortened, the frequency changed.