DETECTION OF WATER ACCUMULATION IN AIRCRAFT HONEYCOMB SANDWICH STRUCTURES USING OPTICAL FIBER BASED DISTRIBUTED TEMPERATURE MEASUREMENT

摘要

A water accumulation in honeycomb sandwich structures is a perceived problem for aircraft operators, since it significantly degrades the structural integrity. The authors developed a fiber optic based technique to detect the water accumulation in large-scaled aircraft honeycomb sandwich structures. An optical fiber network was formed in the adhesive layer and Brillouin based sensing system with high spatial resolution (pre-pump pulse Brillouin optical time domain analysis (PPP-BOTDA)) was utilized to detect non-uniform internal temperature distribution during a cooling process by aircraft takeoff. First, the temperature change during the takeoff was investigated to evaluate feasibility of the proposed technique. Then a verification test was conducted using a Nomex honeycomb sandwich panel. The non-uniform temperature induced by the water accumulation was detected from distributions of a peak frequency and width of the Brillouoin gain spectrum, which is the respondency of the PPP-BOTDA. The width of the spectrum, which represents the temperature non-uniformity within the spatial resolution of the PPP-BOTDA, could indicate the presence of the smaller water accumulation compared to the peak frequency, confirming the importance of evaluating the spectrum width. The developed system is quite useful in continuous monitoring of the large-scaled aircraft sandwich structures.