Humidity Within Composite Sandwich Panels: Visualisation of its Progression Through the Core and its Detection by Thickness Resonance

Abstract

The MRH90 Taipan and ARH Tiger helicopters have recently entered service with the Australian Defence Force (ADF). Both make extensive use of advanced composites in their airframes and a large proportion of this is composite sandwich structure. Sandwich structure consists of two thin, stiff skins bonded to a lightweight core and is used wherever light, stiff structures are needed. The effect of moisture on advanced composite materials, in particular composite sandwich structures, is an important issue because many polymers absorb moisture from their operating environment [3]. There are two issues with this: firstly, the moisture affects the basic material properties which may impact the longevity of the structure and secondly, the moisture absorbed by the composite material affects the ability to repair the structure [3]. This research aims to improve the knowledge available for management of composite airframe aircraft in two areas: development of experimental method with improved precision in the conditioning of composite sandwich structure specimens with respect to the internal humidity and further development of the use of the measurement of thickness resonance for the non-destructive evaluation of composite sandwich structure. Methods for the conditioning of composite specimens have focused on weight gain as an adequate indicator of moisture uptake where a plateauing of the weight of the specimen is considered to be the indicator of the point of maximum moisture uptake. This method is not suitable for the conditioning of specimens at relatively low humidity levels where the weight change of the specimen is small. Additionally, the weight gain provides minimal assurance of the homogeneity of the specimen with respect to internal humidity. The contribution of this portion of the research was to provide a visualisation of the progression of humidity through composite sandwich structure and a method of specimen conditioning that allows a precise knowledge of the state of the specimen with regard to its internal humidity. A visual method of tracking the passage of humidity through the core of a composite sandwich specimen was developed by using lindicator specimensr built with transparent acrylic skins in place of the face sheets of the composite specimen and humidity indicators, placed within the cells of the core which change colour from blue (dry) to pink (humid) at a known humidity level. Within the conditioning chamber the indicator specimens were placed on a flatbed scanner to facilitate imaging remotely. As the indicator specimens were made from the same core material and had the same general geometry as the specimens being conditioned for testing, the state of the specimens for testing could be inferred from the indicator specimens at any point in the conditioning process. The homogeneity of specimens was also apparent visually. No fielded or experimental non-destructive evaluation (NDE) method currently exists that can detect and quantify the moisture absorbed in composite sandwich structure exposed to a humid environment. A study of NDE methods that had the potential to detect moisture absorbed in the core of composite sandwich structure was undertaken using morphological analysis and the measurement of through thickness resonance which had been reported as a method for non-destructive testing of bonded joints [2], was identified for further investigation. Measuring thickness resonance involves local ultrasonic excitation of the sandwich core to detect the through thickness resonant frequency. This method had previously been shown to be capable of detecting liquid water trapped in the cells of honeycomb core [20]. The contribution of this portion of the research has been the expansion of the use of the measurement of thickness resonance as an NDE method by showing that the technique has sufficient sensitivity to detect changes in the moisture absorbed in the core of a composite sandwich panel associated with exposure to different levels of environmental humidity. Because the moisture absorbed in the core material both increases the mass and reduces the Youngrs modulus of the core [6] it was expected that the through thickness resonant frequency of the composite sandwich would decrease as the moisture absorbed in the core increased. Tests were carried with specimens made from carbon fibre/epoxy facesheets and Nomexr honeycomb core conditioned to four relative humidity (RH) levels (dry, 25%RH, 65%RH and 100%RH) using the conditioning method already described. It was found that the expected shift in the frequency of the resonant peaks was evident with the average shift of the resonant frequencies between tests of the specimens in a dry state and after being conditioned to equilibrium in a 100%RH environment being an average 1041Hz shift for the second peak. The lapse rate of the resonant frequency of the second peak was determined to be 10.6Hz decrease per 1%RH increase in internal humidity.n