Vol.09--No.2019
The nAbAt a ICC soApboX
Tuesday,Sep 24,2019 
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Unveiling the secrets of American stealth technology 2 | By Anonymous | July 16, 2019 - 12:23 | Posted in /B/ | 2 Comments

Greetings! We are Anonymous.

In our previous post about American stelth technology, we unlocked many secrets, but this time our focus will be on materials, primarily 3D printed absorber honeycomb which is the most safeguarded classified f-35 program secret. Democratization of this technology will have a positive effect on stability in an increasingly unstable world.

 

Three-dimensional metamaterial absorber based on 3D printed
honeycomb

 

Lightweight structures with multi-functions such as electromagnetic wave absorption and excellent mechanical properties. A three-dimensional metamaterial absorber consisting
of honeycomb and resistive films  fabricated through 3D printing and silk-screen printing technology. The present three-dimensional metamaterial absorber can realize an absorptivity of more than 90% in a wide band of 3.53–24.00 GHz, and improve absorbing efficiency for transverse magnetic (TM) waves of oblique incidence angle from 0° to 70°.

The compression test results reveal that compressive strength of the 3D printed honeycomb can reach 10.7 MPa with density of only 254.91 kg/m3, and the energy absorption per volume Wv and per unit mass Wm are 4.37 × 103 KJ/m3 and 17.14 KJ/Kg, respectively. The peak compressive strength and energy absorption per mass are at least 2.2 and 3 times comparing to metallic lattice cores with the same density.

Outstanding electromagnetic wave absorption and mechanical performance make the present three-dimensional metamaterial absorber more competitive in engineering applications.

 

Metamaterial absorber (MMA) is a composite metamaterial, which usually consists of periodic artificial structures
and dielectric substrate. Through transforming the electromagnetic wave energy into other forms, MMA
can realize electromagnetic absorption. As an important branch of metamaterials, MMA has attracted great
attention in the past decade, and its applications have covered many areas, such as stealth technologies, communication antennas, radars and so on2–5. In 2008, Landy et al. designed a perfect MMA through metal-dielectric composite structure, which consists of electric resonators and magnetic resonators. Their proposal realized nearly 100% absorptivity at 11.5 GHz6. After that, metal-dielectric composite structures are widely used in the design of MMAs in GHz and THz range7–10. However, this kind of MMA can only realize efficient absorption in a narrow
band. In order to broaden the absorption band, multilayer structures are used11–14. In this way, the thickness will
be so large that limits the applications of MMA. Another effective method to achieve broadband absorbing is
using the frequency selective surface (FSS) absorber consisting of lossy resistive patches15–18. These absorbers
usually have various patterns of resistive patches placing on the dielectric substrate. By reasonable design of the
planar patterns, broadband absorbing performance can be easily obtained with an ultra-thin thickness. The realization of resistive FSS absorbers made a breakthrough in the research of radar absorbing.
Previous studies mostly focused on planar structure absorbers. Nowadays, researchers have extended the
planar absorbers to three-dimensional structures. In 2016, Shen et al. found that the folded resistive patches
standing up on a metallic backboard can exhibit not only a wide-band absorbing but also wide-angle absorbing
characteristic. Their proposal gives a new idea for the design of MMA, especially for the large incidence angle
wave absorbing. As aerospace materials, in addition to outstanding electromagnetic properties, they also need
to be strong enough to resist aerodynamic force and thin enough to install on the surface of the aircraft.
However, such three-dimensional absorber may be difficult to resist deformation because of the weak mechanical

properties of the thin resistive patches. Sandwich structures with honeycomb cores have excellent mechanical
performances and are widely applied as engineering structures. Recent research shows that honeycombs and
its composite structures have significant benefits on energy absorption, vibration control, thermal buckling
resistance, acoustic absorption and even electromagnetic absorption properties. Based on these benefits,
honeycomb structures can be used as multifunctional design, such as the combination of electromagnetic wave
absorption, energy absorption and load carrying properties. Compared to traditional metamaterial absorbers of
poor mechanical performances, the honeycomb based design may have more advantages.
In this paper, a three-dimensional MMA was proposed, which consists of hexagonal honeycombs with resistive
patches attached on its walls and they are placed on a metal backboard. Figure shows the front view and top view of its unit cell in plane x-z and x-y. The height, the maximum inner radius and the thickness of honeycomb are h, a and t, respectively. The material of honeycombs is polylactic acid
(PLA), which is a biodegradable and bioactive thermoplastic aliphatic polyester. The dielectric constant ε and
dielectric loss δ of PLA are ε = 3 and δ = 0.01at room temperature30. A copper panel is used as the backboard
with thickness d = 1 mm. And the thickness and resistance of resistive patches are defined as Δ (Δ = 0.01 mm)
and Rs, respectively. Genetic algorithm in software, CST Microwave Studio 2015, is applied into the structure
optimization. Structure parameters h, a, t and resistance value Rs are set as variables. Reflectivity S11 is set as
target value. Then the optimized structure parameters can be obtained, which are h = 15.51 mm, a = 8 mm,
t = 1.9 mm, Rs = 219.28 Ω/sq. Figure 1(c) shows honeycomb specimen for compressive test with a dimension of
78.78 mm × 81.78 mm. The three-dimensional MMA for electromagnetic wave absorption measurement with a
global size of 315.13 mm × 327.48 mm is shown in Fig.

 

Results
Wave absorbing properties. Microwave-absorbing characteristics of the proposed MMA are shown in the
following figure. The red curve in  shows the simulated reflectivity of vertical incident waves in 1–24 GHz,
indicating that this three-dimensional MMA can realize an absorptivity of more than 90% in 3.53–24.00 GHz.
In order to verify this simulated result, experiment was performed in microwave anechoic chamber, as shown in
Fig. 2(a). Then experimental reflectivity can be obtained, as the blue curve shown in . For a metamaterial
absorber, the absorptivity can be expressed as following:

In equation (1), A represents absorptivity, represent reflectivity and transmissivity, respectively. In this work, because of the existence of metal backboard, equals to 0. According to equation (1), when S11 (in
dB) is less than −10, the absorptivity will be more than 90%. Therefore, if the curves are under −10 dB, the errors
of absorptivity between simulation and experimental results are less than 10%. In Fig. 2(b), three frequencies of
peaks with bigger discrepancies between simulation and experimental results are selected to study the errors.

 

In equation (2), parameter σs represents conductivity of resistive patches, which is determined by the resistance
value Rs. The resistive patches are processed using carbon paste through silk-screen printing technology and the
resistance value mainly depends on the concentration of carbon paste. However, it is difficult to control the
parameter Rs in a specific value in actual operation process. The relation between reflectivity and resistance value
was studied and the results are shown in Fig. 3(b). It can be seen that as the value of Rs in 200–400 Ω/sq, the bandwidth of more than 90% absorptivity remains almost unchanged in 3.53–24.00 GHz. Hence, the resistive patches with resistance value in 200–400 Ω/sq can be seen as qualified samples. The preparation of resistive patches and the control of its resistance value are key steps in this research.

Mechanical properties. The out of plane compressive load was applied on PLA honeycomb to study
mechanical properties of the proposed three-dimensional MMA. At least 70% deformation
in strain was achieved in order to record the complete deformation process. The typical measured uniaxial compressive stress versus strain response of 3D printed PLA honeycomb was shown in Fig. 4(b). The deformed images of the specimen at selected points were also shown in Fig. 4(c), and Fig. 4(c)-D shows its top view after compression.
The compressive stress-strain curve is very similar to that of the aluminum foam-filled corrugated sandwich
panel reported by Yan et al.33. After the initial linear elastic response with elasticity modulus of 168.55 MPa at low
strains and nonlinear increase, the compressive stress reached its peak of 10.7 MPa at strain of 0.2, see Fig. 4(b).
As the strain increased furthermore, the stress only slightly declined due to the soft of the 3D printed honeycomb
core web, the initial formed bulges can be seen clearly in Fig. 4(c)-A. However, compared with metallic lattice
cores which have a dramatic stress drop immediately after its peak, such as corrugated cores33, the present honeycomb has much more stabilized cores. When the strain reached about 0.42, the previous bulges propagated and contacted with the newly formed ones which require much higher compressive load, and causing the increase
of stress with further increase in strain till densification. The bulges propagation and generation are showing in
Fig. 4(c), A–C. Top view of specimen after compression indicates that every cell wall of the honeycomb have bulge
generation and propagation, Fig. 4(c)-D.

The energy absorption capacity may be characterized by the area under the uniaxial compressive stress versus
strain curve as shown in Fig. 4(b). The energy absorbed per unit volume, Wv, was defined as:

In addition, as mass was critical for energy absorbers for weight sensitive applications, the specific absorbed
energy (SAE) (or absorbed energy per unit mass) was another important parameter. The absorbed energy per unit
mass, Wm, may be defined as:

where ρc = 254.91 kg/m3 was the average density of the 3D printed PLA honeycomb. When ε = 0.5 was adopted,
the Wv and Wm were calculated as 4.37 × 103 KJ/m3 and 17.14 KJ/Kg, respectively.
Comparison with competing core designs. Yan et al. reported a novel metallic sandwich panel with
aluminum foam-filled corrugated cores, and it’s remarkable advantages in compressive strength and energy
absorption as well as bending resistance properties have been demonstrated33,34. The non-dimensional peak
compressive strength and specific absorbed energy (SAE) of aluminum foam-filled corrugated core was found
to be more competitive compared with the typical metallic lattice cores, such as empty corrugated, diamond,

square-honeycomb and pyramidal truss cores33. The experimental data of the present 3D printed honeycomb
were added and compared in Fig. 5(a,b). Both peak strength and SAE performances of the present honeycomb
have significant advantages compared with aluminum foam-filled corrugated core and other competing lattice
cores. Figure 5 reveals that the peak strength of 3D printed honeycomb is 2.2 times of metallic square honeycombs
while the SAE value is 3 times of that of pyramidal cores with the similar relative density, as the arrow
lines shown in Fig. 5, however, compared with other metallic lattice cores, it is even much more competitive.
Furthermore, compared with previous three-dimensional absorbers19–21, the 3D printed honeycomb MMA have
more advantages due to its excellent mechanical performances.

Conclusion
A three-dimensional MMA consisting of PLA based 3D printed honeycomb, resistive patches and metallic backboard
is reported in this paper. The experiment and simulation results indicate that the proposed MMA canrealize an absorptivity of more than 90% in 3.53–24.00 GHz for vertical incident waves, and can keep this excellent
absorbing effect for oblique incident TM waves from 0°–70°. Out of plane compressive load was applied on
the PLA honeycomb core to study its mechanical performances. Experiment results show that the peak strength
and specific energy absorption of the MMA is much more competitive which are no less than 2.2 and 3 times
respectively compared with the competing metallic lattice core designs under equivalent densities. The outstanding
electromagnetic wave absorption (wide band and large angle) and excellent mechanical performances (high
specific strength and energy absorption) of the multifunctional designed MMA make it have great potential
application as novel lightweight structural materials and electromagnetic wave absorbers.

 

 

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2 Responses to Unveiling the secrets of American stealth technology 2

  1. Where you found this, this seems legitimate…

  2. This is really impressive, thank you!

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