The long term stability of guided wave structural health monitoring systems

Guided waves offer an attractive means of performing structural health monitoring (SHM). It is desirable for such a system to have maximum sensitivity with minimum sensor density. However, in all but the most simple structures the wave interactions become too complex for the time domain signals to be interpreted directly. One approach to overcoming this complexity is to subtract a baseline signal from the measured system thus enabling changes in the system to be identified. Two key issues must be addressed to allow this paradigm to become a reality. Firstly, the system must be sufficiently sensitive to small reflections from defects Secondly, it must be able to distinguish between benign changes and those due to structural defects. In this paper this subtraction approach is used to detect defects in a simple rectangular plate. The system is shown to work well in the short-term and good sensitivity to defects is demonstrated. The performance degrades over the long-term. The principal reason for this degradation is shown to be the temperature of the system.