Repair of Thin Carbon Fibre Composite Laminates

Abstract

The establishment of repair methodologies for composite structures is vital in developing military repair concepts that rapidly return aerospace platforms to service and maintain the sortie fleet during conflict. However, composite and metallic structures have vastly different damage mechanisms, with the knowledge and understanding of composites relatively immature, necessitating testing and analysis to develop new methodologies and enable efficient and optimised structural design. Therefore, this research was undertaken to; • Improve the understanding of and predict the failure mechanisms of undamaged, damaged, mechanically fastened and bonded scarf repaired carbon fibre composite laminates. • Evaluate the capability, and identify the limitations of the current Royal New Zealand Air Force non-destructive inspection techniques. • Compare carbon fibre reinforced polymer computational analysis methods to quantify validity, variance and identify influencing factors to develop a practical set of engineering design tools. Approximately ninety percent of the NH90 helicopter's primary and secondary structure is carbon fibre reinforced polymer construction. However, obtaining NH90 laminate and honeycomb construction materials is problematic due to cost and availability. Therefore, based on the NH90 design substantiation data available, suitable alternative materials were selected from which test panels mimicking typical NH90 laminate structure were manufactured. Edge-wise compression testing was performed using ASTM D7137 as guidance, as the specimen dimensions were larger to accommodate laminate repairs. Acoustic emission monitoring was used during the initial testing block and proved to be an invaluable tool, and was subsequently employed during the remaining two blocks. The acoustic emission monitoring detections indicated matrix cracking/delamination as the primary laminate failure mechanism, and though events were located within the sandwich panel laminates' bonded repair regions, the damage did not appear to be associated with adhesive failure. However, tailored tests are required to characterise the true acoustic emission characteristics for these test scenarios. Flash-thermography and through-transmission ultrasonic non-destructive inspection methods were used to inspect 33 specimens, only detecting one surface crack, while coin-tapping and mechanical impedance identified possible delamination's/debonds in two additional panels. Six panels were sectioned and examined under optical magnification, however, only voids and porosity were found. It was not possible to accurately or reliably estimate the efficacy of or compare the bondline predictions to the test results due to the assumptions made regarding the repair adhesive ultimate and shear strains, and bondline coefficients, all of which directly influence the bondline analysis. The influence of the ballistic impact and a 25 mm open hole on the residual strength was unable to be determined due to the various unrelated failure modes and the locations that they occurred. The test results demonstrate that the design methodology for calculating laminate stiffnesses and the Tsai-Wu plane stress failure criterion for estimating laminate strain are practical prediction tools. The research highlighted the unpredictable nature of carbon fibre laminate failure and raised concerns around the efficacy of the flash-thermography and ultrasonic non-destructive inspection techniques used to detect subsurface damage. The test results demonstrated that the laminate stiffness and strain prediction tools were of practical use, however, further work is required to obtain the material-specific data needed to refine the mechanically fastened repair analysis and determine the adhesive bondline analytical methods' usefulness.