Assessment of a novel neutron tomography instrument and other nondestructive technologies for the characterization of degradation in honeycomb composites.

摘要

The feasibility of developing a nondestructive evaluation technique (NDE) or combination of techniques capable of characterizing degradation in honeycomb composites was investigated. To enable the determination of the exact location of water ingress inside a honeycomb composite structure, a novel neutron tomography instrument (NTI) was designed and developed at RMC. The system represents the only NTI available in Canada and allows a range of objects to be investigated including honeycomb coupons and complete CF 188 rudders. In order to produce 3D volumetric reconstructions of sufficient quality to assess the location of water, the system was optimized in terms of optics, spatial resolution and signal-to-noise ratio (SNR). An imaging test object was designed to enable the quantitative measurement of the spatial resolution in 2D images and 3D reconstructions, filling a gap in the current neutron imaging standards. Several noise reduction filters were applied to 2D and 3D images produced by the NTI, which improved the spatial resolution and SNR.;Appropriate coupons that were purposely degraded to represent honeycomb composites subjected to water ingress were designed, constructed and tested. To produce coupons with different degrees of degradation in the skin to core bond, varying numbers of freeze-thaw cycles were used. Destructive flat-wise tension tests were then performed to evaluate the coupons and the results showed a strong first-order linear decay relationship between the number of freeze-thaw cycles and the filet bond strength. The method developed to reliably degrade the filet bond, provides a more appropriate degradation mechanism compared to other available methods for producing degraded coupons. The degraded coupons were subsequently inspected using several adapted NDE techniques: neutron tomography, infrared thermography, through-transmission ultrasonics and acoustic bond testing. Neutron tomography was capable of detailing the exact location of water in the composite using 3D volumetric reconstructions and individual axial slices. Both through-transmission ultrasonics and acoustic bond testing were shown to be capable of detecting degradation in the test coupons. Finally, development of NDE techniques towards the reliable quantification of varying degrees of adhesive degradation was recommended.