1. Introduction
Laminar flow control through active boundary layer suction shows great potential for more energy-efficient aviation by means of drag reduction [
1]. Numerous configurations for such drag reduction of an aircraft wing have been explored. One approach is the use of plasma actuators that generate momentum in the air, postponing the boundary layer separation [
2]. One concept involves the principle of natural laminar flow (NLF), which can be achieved using a smooth surface and a targeted profile design. Another approach is based on laminar flow control (LFC), which uses active boundary layer suction [
3]. The combination of NLF and LFC has been investigated at the Cluster of Excellence SE
2A—Sustainable and Energy-Efficient Aviation—for an electrically powered regional aircraft. The hybrid laminar flow control (HLFC) concept incorporates LFC on the leading edge of the wing and NLF on the rear part of the wing. This novel extended hybrid laminar flow control (xHLFC) concept is based on active suction through a suction panel consisting of a sandwich structure, with a micro-perforated skin and a triply periodic minimal surface (TPMS) structure as a core on the rear part of the wing [
4]. The proposed suction panel could potentially reduce drag by up to 45.5% [
5].
Previous studies have also investigated active suction on the leading edge of the wing. The tailored skin single duct (TSSD) concept [
6] uses a multi-layered, metallic outer skin, which provides a pressure drop. Schrauf et al. [
7] conducted initial flight tests with an HLFC system on the vertical tail plane of an Airbus A320. The system consisted of a simple double-skin structure with the inner and the micro-perforated outer skin connected by stringers. The results of the first tests indicated that the turbulent transition was delayed due to active or passive suction. Lobitz et al. [
4] investigated an integrally, additively manufactured sandwich suction panel consisting of a gyroid core and a micro-perforated outer skin for xHLFC in the rear part of a wing. The study compared different materials and gyroid configurations regarding the loading on the wing and the suction panel. The material Nylon11CF was identified as the optimal material, in terms of light weight considerations. The adhesive and erosive effects of atmospheric pollutants, such as aerosols, organic matter, and rain, were also discussed. Adhesion of organic matter on the wing surface may cause an increase in drag. Frequent washing may be needed, since a smooth surface is essential for HLFC systems. This may reduce the benefit of the fuel savings with HLFC, depending on the mission [
8]. Clogging of the micro-perforations may also counteract the benefits of HLFC, since the major part of debris is located at the leading edge. In the same manner, erosion mostly occurs at the leading edge part of the wing [
9]. Additionally, the accumulation of ice on the leading edge can alter the geometry of a wing and cause drag. Therefore, anti-icing techniques, such as application of heat or anti-freeze agents, need to be considered. This is more of a system design problem than a technical obstacle for HLFC [
10].
Positioning the suction panel at the leading edge necessitates the imposition of additional requirements on its core structure. All parts oriented in flight direction are susceptible to bird strike damage, which makes bird strike a design-driving issue [
11]. Cihan Tezel et al. [
12] conducted a numerical study on different design options of a wing leading edge, concluding that a sandwich leading edge structure with an aluminum skin and a cellular core was the lightest design fulfilling the given requirements for bird strike resistance.
Based on these findings, a new approach for a novel wing leading edge for transport aircraft was investigated. This approach combines the benefits of active suction on the wing, which is characterized by an increase in efficiency through drag reduction, and a lightweight sandwich structure. A schematic representation of the wing leading edge with a sandwich structure and incorporating a TPMS-based core resistant to bird strike is provided in
Figure 1.
Cellular structures can be classified into two categories: stochastic (foams) and periodic (lattices). Lattice structures can be optimized for end-user products or applications in a topological manner and include honeycombs and TPMSs. TPMSs are a subcategory of minimal surfaces that locally minimize the surface area within a given boundary [
13]. TPMSs are defined by implicit equations consisting of a combination of sine and cosine functions, making TPMSs periodically duplicable in all three dimensions. The first TPMSs described in the literature were the diamond and primitive surfaces by Schwarz [
14] in 1890. Multiple other TPMSs were introduced by Schoen [
13] in 1970. One of them was the gyroid surface. The description of the gyroid surface is presented in Equation (
1):
with
An isosurface is generated at all points where the equation is equal to the constant
c [
15]. The constant
c modifies the geometry of the gyroid [
16]. A typical value for this parameter is zero. The total length of the structure in each dimension, designated as
Li,tot, and the unit cell size in each dimension, designated as
ni, are employed to define the periodicity parameter, designated as
. Two distinct methodologies may be employed to generate a structure from the surface: The extrusion of material in a perpendicular direction to the surface generates a solid network structure. Extruding the material in both directions perpendicular to the surface generates a sheet network structure [
17]. Numerical investigations by Li et al. [
18] indicated that a sheet network unit cell tends to be isotropic, while a strut-based unit cell tends to show anisotropic behavior. Furthermore, the sheet-based gyroid was found to be theoretically more suitable for energy absorption than the strut-based gyroid [
18]. The behavior and mechanical properties of gyroid structures are contingent upon the material, unit cell size, manufacturing process, and relative density [
17]. The relative density
is defined as the density of the cellular structure
divided by the density of the solid material
:
In general, lattice structures deform, when under macroscopic loading, through a combination of bending and stretching of the struts or sheets [
17]. For the majority of lattice structures, the force-displacement response commences with an elastic region, which is succeeded by the initial stage of plastic deformation. Stretching-dominated structures display a softening after the elastic region, followed by a force plateau until densification. Bending-dominated structures exhibit a force plateau following the elastic region, without softening [
19]. Oftentimes, it is observed that the plateau force exhibits oscillatory behavior due to the geometry of the lattice [
17]: In sheet network gyroid structures, collapse of the layers can be seen [
20,
21,
22].
The mechanical properties of a cellular structure are dependent on its relative density
and its geometry. This relation can, in principle, be described by a power law of the following scheme [
23]:
The ratio of the physical property of a gyroid structure PG to the physical property of the solid material PS corresponds to a geometric constant C times the relative density to the power parameter n. The parameter n is dependent on the structure’s behavior.
A comprehensive overview of experimental and numerical investigations of sheet-based gyroid structures is presented in [
17]. Experimental studies were conducted to investigate the mechanical behavior of such structures under compressive loading. Maskery et al. [
24] explored the compressive behavior of cubic gyroid specimens made of Al-Si10-Mg with an edge length of 18
, unit cell sizes of 3 mm, 4.5 mm, 6 mm and 9.5 mm, and a relative density of 30% under a quasi-static compressive strain rate of 5 × 10
−4 s
−1. The specimens exhibited three distinct modes of failure, depending on the unit cell size: A successive collapse of cells in planes perpendicular to the loading was observed for cell sizes of 4.5 mm and 6 mm. The second failure mode was indicated by brittle fracturing of the cell walls. A crack propagated with its main component in the plane parallel to the loading direction. This phenomenon was observed for larger unit cell sizes. The third failure mode was characterized by the formation of a diagonal shear band, which occurred exclusively for the smallest unit cell size. Other studies have investigated the mechanical behavior of gyroid structures of different materials and low strain rates, including Al-Si7-Mg0.6 at a strain rate
of 10
−3 s
−1 [
22] and stainless steel at a strain rate
of 10
−3 s
−1 [
20]. Additionally, the mechanical behavior of PA2200 at a strain rate
of 10
−2 s
−1 was investigated in [
21], while [
18] examined the behavior of a polymer at a strain rate
of 4.6 × 10
−3 s
−1.
Liu et al. give an overview of studies on the impact behavior of additively manufactured metals and structures in [
25]. In their survey, less than 40 studies on the dynamic behavior of lattice structures were found. Of the 40 studies, only a few addressed sheet-based gyroid structures. In their study, Li et al. [
26] examined the mechanical behavior of cubic gyroid structures made from 316 L stainless steel with an edge length of 20 mm, five unit cells per dimension, and a relative density of 30% under four different strain rates, ranging from 1 × 10
−3 s
−1 to 8.5 × 10
−2 s
−1. Lu et al. [
27] analyzed the mechanical behavior of different 316L stainless steel cubic gyroid specimens with an edge length of 20 mm and a relative density of around 12% under loading velocities ranging from 5 ms
−1 to 50 ms
−1. Ramos et al. [
28] performed low-velocity impact tests on cubic Al-Si10-Mg gyroid specimens with different loading velocities.
However, as presented, not much research has been carried out on the dynamic behavior of polymeric gyroid structures subjected to impact loading. Therefore, in the present study, experimental tests were conducted on various gyroid configurations of different additively manufactured polymeric materials. For application as a lightweight structure and the suction of the boundary layer flow, the relative density was restricted from 7% to 23%. This study aimed to help fill the gap in research on polymeric gyroid structures under dynamic loading. Research on HLFC deals, to a great extent, with the aerodynamic effects of the applied technique. This study is intended to address aircraft safety in a specific HLFC approach using a sandwich panel structure with a gyroid core.
The outline of this present study is as follows. The investigated materials, the manufacturing process, and the methods employed to generate the gyroid surface are presented. This is followed by the results and a discussion of the compressive tests under quasi-static and dynamic loading, where the failure behavior as well as the plateau force and the weight-specific energy absorption of the specimens are discussed. A brief summary of the study’s findings is presented in the conclusion section.