Developments in structural proof testing methodologies for aircraft composite structures
Creators: name: Herman, Alexander
Publisher: name: RMIT University
Year: 2024

description: A major cost in the fabrication of carbon-epoxy composite structures for aircraft is the non-destructive inspection (NDI) performed at the end of the manufacturing process to reveal the presence of any defects and damage such as porosity, backing paper and bond-line cracks. The inspection can account for up to 50% of the cost and time involved in the fabrication of aircraft composite components. Compounding this problem is that NDI often fails to detect damage in difficult to inspect regions, such as the centre fillet of T-stiffened panels. <br><br>Structural proof testing is a potentially cheaper, faster and more reliable inspection method, which typically involves externally loading the component (static or dynamic loading), and assessing the structural response for the presence of damage and qualification of structural integrity. The current proof testing body of knowledge is limited to detection of active defects that propagate within a structure under load, such as delaminations and cracks. Currently no work has been performed to detect small and passive damages that do not propagate during proof loading, such as voids and porosity. <br><br>In the current work, the feasibility, methodology and technical challenges of surface strain monitoring and mode shape curvature (MSC) analysis as damage detection techniques within a proof testing scenario were investigated numerically and experimentally. A carbon/epoxy T-stiffened panel was used as the case study to assess the two damage monitoring techniques in detecting delamination, voids, and porosity.<br><br>An experimentally validated finite element model was used in a parametric study to assess the ability of surface strain monitoring to detect and locate damage in the skin-stiffener bond-line and central fillet region of a T-stiffened panel. Not only are these regions susceptible to manufacturing damage, but the central fillet region is difficult to inspect using conventional NDI. The proof test conditions necessary to optimise the strain field for damage detection were determined, and the detection and location of delamination and voids throughout the T-joint was successfully achieved. However, surface strain monitoring was limited to the detection of porosity in high, wide-spread concentrations, and could not reliably provide damage location.<br>The difference in mode shape curvature from undamaged-to-damaged structures was investigated experimentally and numerically. A scanning laser vibrometer was used to experimentally measure the mode shape displacements of the T-joints under sinusoidal excitations from 0 – 5 kHz. The results of the MSC differences yielded accurate and consistent results with respect to damage presence, location and severity. Delamination damage was detected and located. Porosity was also detected although the exact location was not as clear due to the high stiffness of the structure affecting the indication of damage location. However, the MSC difference technique successfully detected porosity throughout the T-joint including the central fillet region, which cannot be easily inspected using conventional NDI. <br><br>The results of this investigation have provided the foundations necessary to successfully implement a novel structural proof testing methodology for certification of aircraft composite components, capable of detecting and locating damage in complex structures.