Composite honeycomb sandwich structure is a type of advanced structural composite with excellent strength-to-weight and stiffness-to-weight ratios. These properties make them ideal for use in the aerospace industry. However, these composites are subject to moisture ingression, which weakens the structure and can even cause failure. This is why reliable non-destructive testing (NDT) methods are required for honeycomb sandwich structure, so that moisture ingression can be detected and repaired. This project is a fundamental investigation into the use of ultrasonic NDT for determination of moisture ingress in these sandwich structures. It was hoped to establish a quantifiable relationship between moisture ingress and characteristics of the ultrasonic response.
The sandwich structure investigated in this project is comprised of Nomex® honeycomb core and CFRP facesheets. This sandwich structure is representative of panel used in the construction of the ADF Tiger ARH and MRH90 helicopters. Moisture ingression was performed by submerging samples in liquid water. Later this was changed to injecting liquid water directly into the core of the test samples, simulating ingression caused by cracks in the facesheet. Three experimental methods of NDT were formulated and investigated. The first used an impact hammer to create an impulse excitation. The second and third methods used an ultrasonic squarewave pulser/receiver signal generator for excitation of the sample. The P/R signal was transmitted into the samples by the use of a conventional 5MHz transducer for the second experiment and via 1MHz bonded piezoelectric transducer for the third experiment. The through-thickness vibrations for all three methods were measured by a laser Doppler vibrometer. The experiments were performed across a range of moisture levels. Thermographic infrared imaging was performed on some of the samples as a validation of liquid moisture content. The resulting signals were analysed to produce parameters
calculated from both the time and frequency domains. The parameters considered are the total energy of the response in the frequency domain (frequency-energy), the total energy of the response in the time domain (time-energy), the absolute maximum peak value of the response in the time domain and two estimates of the ‘effective damping ratio’. The effective damping ratio estimates are determined by a curve fitting process (linear and exponential curves).
With regards to the methods developed and investigated being feasible for determination of moisture content, the results are inconclusive. A relationship between moisture content and the parameters considered was not established. The impulse experimental method was found to excite the global vibrations, which act almost independently of moisture content, and is a possible cause of the high variability in the results. With regards to the ultrasound experiments, liquid water is a possible cause and explanation of the high variability witnessed in the ultrasound results. It is revealed that conventional pulse-echo ultrasound was able to determine the location moisture content, given the moisture content is higher than 0.3g. It is also established that absorbed moisture (with no liquid water) is approximated to be in the range of 0g to 0.3g. It is recommended that more moisture content levels be investigated with this project’s current ultrasonic methods in the range of 0 to 0.3g as there is a possibility that a relationship between the response in this range and moisture content (absorbed moisture) is achievable.