Within the aerospace and transportation industries, it is always desirable to minimize weight without sacrificing strength. One such method of achieving this in aircraft wings and other aircraft structures is to use honeycomb paneling. Such structures are susceptible to damage from things like hail or tool drop, however, and the damage may not be noticeable from the surface. The face sheet may rebound, leaving crushing to the honeycomb core hidden beneath the surface.
Accurately determining impact damage in aluminum honeycomb aircraft panels is critical to evaluating the remaining lifetime of these structures. The hidden crushed core decreases the residual strength of the panels, so it’s important to detect it. Based on using a dial depth gauge for measuring dents, determining the impact damage in honeycomb aircraft panels can be laborious and subjective. If the impact damage also includes crushing of the internal honeycomb core, a manual tap tester is often used, which is both manual and subjective.
Eddy current testing (ECT) thus represents a promising, semi-automated alternative, which is already a popular non-destructive evaluation (NDE) technique used to inspect cracks in aerospace components. Scanning over dented aluminum honeycomb core covered by a face sheet ultimately revealed that the sizes and locations of crushed core could be detected by scanning with an eddy current array. However, verifying these findings presented some challenges, and engineering simulation helped visualize the eddy current array.
ANSYS Discovery AIM was used to simulate the eddy current testing and frequency response of a coil array hovering above an aluminum honeycomb panel, and ultimately helped verify the experimental electromagnetic findings.
Coil and Honeycomb Panel Geometry
Eddy current array (ECA) systems are complex. In reflection-type probes, the driver coils produce eddy currents, while the pickup coils experience essentially no current flow. The current in the core of a honeycomb panel should then mirror only the driver coils. This setup was investigated in the AIM simulation, which modeled two driver coils and two pickup coils, each made of copper with ferrite cores. The status of a coil as either driver or pickup can change during an evaluation, as the coils are multiplexed; however, modeling the coils as shown provides meaningful insight regarding the behavior of currents generated within a honeycomb panel.
Current Densities in the Honeycomb Core beneath the Face Sheet and Coils
Looking at the honeycomb core, hidden beneath the overlying face sheet, the AIM simulation shows the strongest current densities to be around the perimeter of the two driver coils, as the currents can only flow through the cell walls of the core. This is expected, illustrating how current is diverted away from underneath the coils. It also shows the current direction in the honeycomb core, the current densities generated in the face sheet, and the magnetic field intensity generated by the driver coils within the entire system.
Current Directions in the Honeycomb Core showing Current Flowing
around Perimeter of the Driver Coils
Current Densities in the Face Sheet beneath the Driver Coils
Magnetic Field Intensity, H within the System generated by the Driver Coils
Simulation provides valuable insight on how eddy current testing and measurements in honeycomb panels may vary from typical crack-detecting inspections, where eddy currents flow through essentially continuous medium.
The simulation results showing the behaviors of eddy currents generated in complex geometries are important for designing eddy current probes specifically tailored to complex geometries, as might be the case when designing probes to inspect honeycomb structures, or probes to inspect other complex geometries such as aircraft wings or steam generators. These simulation results are also essential when validating experimental electromagnetic data.
As a result, as companies continue to develop advanced NDE technologies such as eddy current testing for industries such as aerospace and petroleum, they may do so using the power of next-generation pervasive engineering simulation.
To learn more about the electromagnetic, as well as fluid, structural, and thermal simulation capabilities in ANSYS Discovery AIM, we invite you to register for an upcoming webinar showcasing some changes to AIM in ANSYS 19.
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