Detecting Moisture in Honeycomb Sandwich Structures using Through-Thickness Resonance measurements

Sandwich structures are frequently used in transport and civil infrastructure applications where high bending stiffness to weight ratio and excellent strength performance are required. One possible damage mode for sandwich components is moisture ingression creating pockets with liquid water or humid cores in the case of paper based honeycomb materials. Both variants may cause the sandwich structure to deteriorate and lose its stiffness and load carrying capacity. Different methods of non-destructive testing have been tried to detect and quantify moisture ingress. However, the development of a robust and simple measurement technique with the required sensitivity has not yet been achieved. This paper describes a non-destructive testing method using through-thickness resonant modes to detect moisture ingress into honeycomb sandwich structures by detecting changes in through-thickness resonant frequencies as a function of humidity levels. Results from analytical modelling, numerical simulation and experimental measurements show that there exists a downward relationship between resonant frequency and volume of moisture ingress. The major conclusion of the work is that the through-thickness resonant technique has the potential to be developed into a robust and simple measurement technique to detect and quantify moisture levels in sandwich structures.