Structural composite panels are generally assembled using two stiff faces bonded to a lightweight structural core. Specific configurations are engineered for strength, stiffness, or energy absorption characteristics for many applications used in buildings, transportation, aircraft, marine, etc. [1
]. There are many core structural options (e.g., honeycomb, foam, corrugated materials, trusses, grids, lattices, etc.). Each type has its own unique characteristics that can be used to provide optimum performance for different applications. Recently, an efficient interlocking grid system has been developed. It has one set of linear ribs with single slots from one edge for part of the rib’s width, for the other set of linear ribs, they have single or double slots from one or both edges to create an interlocking grid system (Figure 1
]. With this system, it is possible to modify the construction to obtain various grid patterns such as square, rectangular, triangular, hexagonal, or mixed structures [6
]. As the number of linear rib directions is increased, core shear stress resistance is improved for multidirectional loadings. Researchers have shown that panels with structural core ribs aligned 60 degrees apart had better mechanical performance than a honeycomb structure [7
]. However, there are few studies regarding impact damage investigations, so additional analyses are needed to continue the development of this type of structure using isogrid core sandwich structures.
Low-velocity impact can cause different types of damage and significantly affect the carrying capacity of structures. For example, in the aircraft industry, impact strength/resistance of exposed components must be sufficiently robust to prevent catastrophic failure of the structure [8
]. Most of the previous low-velocity impact research has focused on foam or honeycomb structural panels. Schubel et al. performed low-velocity impact tests on sandwich structures with carbon fabric/epoxy laminates faces and polyurethane foam core material [9
]. They analyzed the impact dynamic behavior using analytical and finite element model analyses to determine their effectiveness in predicting the impact behavior of the sandwich panel. Wang investigated the impact behavior and energy absorption of honeycomb sandwich panels [10
]. The honeycomb cell-wall thickness and length, core thickness, and materials were all input factors and analyzed for their influence on panel performance. The results showed the thickness and length of honeycomb cell-wall had a significant effect on its impact behavior. However, different types of structures exhibited different impact behaviors, thus the results were limited to the specific structural panel being studied. Recently, the isogrid-stiffened/syntactic-foam core sandwich structure has been investigated by Li et al. [11
]. The results showed that the design and configuration significantly influenced the impact behavior, and the isogrid triangular cells within the foam showed better impact capacity than just the laminated composite structure without foam. Additionally, the raw material and structural configuration of structural composite panels had significant effects on the impact behavior and damage [9
]. However, there is no literature where the isogrid structural composites were made from bio-based material with an interlocking isogrid structure.
At the USDA, Forest Products Laboratory (FPL), new research is being conducted to develop bio-composite materials to enhance performance for various engineering applications including tactical shelters, packaging, building materials, and air pallets [6
]. High stiffness characteristics were the primary consideration for performance. A phenolic laminated paper made from wood fibers was used for the tri-axial interlocked core and faces of a sandwich panel. In the initial study, both static and dynamic mechanical behavior for these tri-axial wood-fiber-based composite panels was investigated using flatwise and edgewise compression tests, four-point bending static, and fatigue tests [6
]. The results showed that these structural panels have the potential to provide good mechanical performance for various engineering applications. However, impact resistance, which has not been investigated, is considered a critical design parameter for some applications.
This paper investigates the impact behavior of wood-fiber-based tri-axial core composite sandwich panels with different core/face configurations using quasi-static compression tests and low-velocity impact tests. The energy absorptions and contact loads of panels were analyzed through different materials, design configurations, and test parameters.
Experimental investigation on bio-composite structural panels has been presented and analyzed using low-velocity impact and quasi-static compression tests. In the low-velocity impact test, the impact behavior improved when higher stiffness or strength components of panel such as carbon fiber fabric composite layer and higher triangular core density were used to support the face and core. In addition, foam filled core provided improved energy absorption and better energy transfer throughout the structure. The foam core helped to resist impact buckling in the core ribs. The impact head size influenced impact energy density. If the energy density was high enough to cause penetration through the top face, then the residual impact speed and energy density will influence the following impact on the bottom face. A smaller grid element core would provide better energy absorption due to the higher core density.
The impact failures of composite panels could be classified into three stages according to their damage levels. The first stage is top face damage only, followed by the second stage of core buckling failure and the third stage of whole panel penetration failure. Generally, face damage only is not considered a critical failure, whereas damage that impacts the core and bottom face significantly degrades mechanical performance of any panel, thus it is not accepted for most applications. These tests show there are conditions where damage, while undesirable, could occur that only affects the top face. The tri-axial core bio-composite structure with additional modification could be shown to locally fail without significant impact on the other components. Depending on the design limits and impact needs this composite panel could be designed to meet some of these challenges. These tests also show that depending on where the impact occurs may influence the test data especially when the rib construction and impact head are similar dimensions.
Moreover, the results from compression test showed that both quasi-static compression and low-velocity impact tests had similar mechanical responses in the early phase of each test. The compression results could be used as reference to evaluate the low-velocity impact performance. For future applications, the bio-composite structural panels could be engineered to achieve various impact performance characteristics for different applications while reducing the weight and cost by optimizing the panel configurations. For further study, a micro-damage mechanism and finite element model could be used to help analyze impact performance.