The application of low frequency acoustic methods is one that long been implement for non-destructive testing (NDT), probably originating with a simple coin tap technique. These methods have successfully been applied to a variety of materials and are commonly used for assessing the quality of bonded sheet metals. They have also proved useful in the NDE of sandwich composite panels, particularly honeycomb panels. However, these methods, namely: Membrane resonance, Mechanical impedance & Velocimetric method, are only sensitive to relatively large or shallow defects. Since access to both sides of a panel is rare, the need for one-sided in-service inspection is usually a necessity. Whilst the three low frequency inspection methods mentioned above have been found to adequately detect and size near-side defects in sandwich panels, such as arising from impacts and disbonds on the near-side skin, they have proved inadequate at resolving the equivalent far-side defects, producing, at best, a diffuse image of the defect. Also, determining the detect type, depth and 3D location is currently not possible using existing techniques.
This project aims to resolve these challenges by applying a wide frequency sweep, using a pitch-catch probe, to excite various modes of the panel – including the ‘Through Thickness Resonance’ mode which has the ability to detect far-side defects in sandwich composite panels (honeycomb, balsa, and foam core sandwich panels). This method of excitation, combined with comparison with modelled low-frequency responses of a range structures with different defect types, should lead to the ability to classify defects, including determining their depth, location and size. For this project, Abaqus FEA Software will be used to model various structures with different defects, then a database method coupled with multi-dimension optimisation to invert the data will be applied initially, with potential for refinement of the method in a variety of ways.
So far, models created using Abaqus Software have been used to investigate and understand the problem in greater depth. By representing disbonds and delamination as a flat bottom hole (FBH), the concept of a Local defect resonance (LDR) can be explored numerically (see Figure 1). Analytically, when the mass of the defect and the effective stiffness is known, the corresponding LDR frequency can be calculated using:
Where, t is the defect depth, r is the defect radius, E is the young’s modulus, 𝜌 is density and v is Poisson’s ratio. This forms the basis of the investigation carried out in the initial investigation of this project.
Figure 1 – Schematics of a plate containing a FBH defect, and the corresponding mode shape of its 1st mode of resonance
In attempt to gain better understanding of the problem, the LDR amplitude across the FBH defect was plotted and compared to the compliance and the mode shape. The result is presented in the graph below:
Figure 2 – Graphs of normalised Mode shape, Compliance and LDR Amplitude across the radius of the FBH defect
The defect compliance (static model), mode shape (modal analysis) and LDR amplitude (dynamic frequency-domain model) shows similar but different trends. Therefore, an average stiffness model of the LDR amplitude across the defect would be inaccurate, as the results suggests that the LDR amplitude across the defect is also dependent on the effective mass being displaced and the damping in the system. This means the LDR amplitude must be simulated in a dynamic model going forwards, to obtain an accurate solution.