Simulating Low-Velocity Impact on a Honeycomb Sandwich Panel
Honeycomb sandwich panels, with their high strength-to-weight ratios, have significant advantages over monocoque construction for certain applications. Twin-skinned plates/shells with a honeycomb core are widely used in the aerospace industry for structures such as aircraft fuselages, engine cowlings and impact protection shields. Manufactured from aluminum, fiberglass or composites, they are also utilized extensively in the marine, railway and building industries, where lightweight yet strong designs are increasingly important. The complexity of the widely varying honeycomb-cell structure, though, presents modeling challenges for engineers who are trying to optimize the topology, materials, and core geometry of these panels.
Successfully simulating honeycomb structures depends in large part on the proper modeling of the panel’s hexagonal-celled core. One approach is to use continuum solid elements to represent the core material and geometry. This method requires that physical or virtual testing (for compression, tension and/or shear) be conducted first to determine the mechanical properties for a specific core type before discretizing the core into elements. A second approach is to explicitly model the cellular structure of the core using conventional shell elements. To automatically create honeycomb geometries quickly and accurately, a dedicated Abaqus/CAE plug-in can be employed.
Using Abaqus/Explicit from Dassault Systèmes SIMULIA, a low-velocity impact with an aluminum-based honeycomb sandwich panel was simulated. Geometry and material properties were taken from previous studies. The aluminum alloys for both the face plates and core were treated as having non-linear, isotropic, elasto-plastic properties. The panel was impacted by a 25-mm-diameter hemispherical object with an energy of 15 J.
The finite element model (FEM) was built in Abaqus/CAE using the honeycomb geometry plug-in. The core geometry was meshed with conventional shell elements that were used directly in the analysis (Figure 1). Tie constraints were chosen to attach the face sheets to the core.
When meshing the model, smaller elements were selected for the region of the face plate where the impactor made contact, so as to more accurately model the deformation. Moving outward from the point of impact, face-plate elements gradually increased in size. A fine mesh with smaller elements was also used for the model’s core to capture the detail of the structure’s crushing response; the impact area was somewhat larger than that chosen for the face plate. The impactor was similarly meshed with smaller elements in the impact region so that pressure stresses could be most accurately calculated.
The following additional steps were taken for model setup. Friction between the bottom face plate and a rigid base was ignored because it did not influence the response. Between the top face sheet and the impactor, a dry sliding friction coefficient was chosen to correspond to a steel-aluminum interface; and using the known impact energy and a range of values allowed by the testing equipment for mass (from a previous study), impact analyses with varying drop heights were performed in order to match the predicted impulse with the measured one. The final values used in the simulation were for an impactor mass of 805 g and an impact velocity of 6 m/sec.
The simulation results showed excellent agreement with experimental results: Calculated peak impact forces differed by only 1.4% with actual measured values.
Comparing simulated and experimental results, the following conclusions can be drawn:
- The calculated load forces demonstrated occasional reductions and increases during the impact process, similar to experimental measurements. This reflects the irregular nature of the crushing process in the honeycomb core.
- The simulation results closely mirrored the experimental force-time history for both the loading (ascending part of the curve) and the unloading (descending part of the curve) of the panel structure.
- The response of the honeycomb core to impact was clearly illustrated and suggests how proper mesh refinement is vital for accurate simulation (Figure 2).
The simulation demonstrated that the Abaqus/CAE plug-in can provide an efficient means for accurately modeling honeycomb sandwich structures. Use of the plug-in for modeling is most suitable for small structural parts but is also effective for large ones, where it can determine the mechanical properties of specific honeycomb geometries via virtual testing, rather than through expensive and time-consuming physical experiments. Mechanical properties obtained from virtual tests can be used to successfully model sandwich cores with continuum solid elements.