Multifunctional Structural Composite with Integrated Electromagnetic Shielding
By: Harry R. Luzetsky
Military aircraft operate in a widely varied electromagnetic (EM) operational environment, which can critically affect onboard electrical/electronic systems, especially those characterized as level A or mission-critical. This environment includes emissions from sources that radiate radio frequencies (RF) that are coupled onto internal cables or produce aircraft internal field strengths of 5 to 200 V/m or greater. It also includes transients from some forms of EM encounters, such as lightning, EM pulse (EMP), and high-power microwave (HPM). The result can be upsets and/or hard failures of electronic systems, causing temporary or permanent damage to systems and aircraft function.
While circuit architecture can address some of these concerns, the traditional approach is to create a Faraday cage for aircraft electronics enclosures in the form of a heavy steel or aluminum box. As shown in Figure 1, these boxes are often installed in racks on the aircraft platform. Due to stringent weight requirements, the weight associated with a dedicated Faraday cage or shielding is often traded against aircraft performance. In many instances, shielding is thus eliminated to reduce weight and preserve aircraft performance. Without appropriate shielding, however, intensive energy fields produced by various EM sources, such as HPM and EMP, can instantly overload or disrupt electrical circuits at a distance. And modern high-technology microcircuits are especially sensitive to power surges and EM effects associated with external energy sources.
Through incorporation of EM shielding material into a structural composite, a Faraday cage structure that combines effective EM shielding, ballistic tolerance, low weight, and structural durability is now possible. The result is a lightweight alternative to the traditional means of providing aircraft electronics protection from the deleterious effects of EM radiation, thereby enhancing aircraft survivability. The metal composite hybrid material, developed by the SURVICE Engineering Company, provides a lightweight, affordable approach to achieving the desired level of EM protection of existing and emerging requirements, such as HPM and EMP/high-altitude EMP (HEMP).
MATERIAL EM SHIELDING PROPERTY DEFINITION
EM shielding properties were based on military standards and measurements from an aluminum enclosure that was previously constructed and tested by the Air Force Research Laboratory (AFRL). These military standards were used to establish the basic shielding design envelopes and actual test data from the enclosure to provide a measurement unit for comparison purposes.
Reviews of the military standards for EM environments were used to establish a basic design envelope for the composite enclosure in the harshest RF environments [1–3]. Figure 2 illustrates the relationships between field intensity and frequency that were developed from these reviews and required for the material design. These relationships established the EM environment that electronic components and systems could experience in operation and defined the basic EM protection levels required for the material design.
From the basic EM environment requirements, a comparative physical model using a typical electronics enclosure geometry was created to determine the effectiveness of the developed material in achieving the appropriate level of EM protection. Comparing the EM shielding effectiveness of the physical model against an existing baseline supported quantification of the shielding effectiveness of the multifunctional material.
AFRL provided an aluminum electronics enclosure test box (shown in Figure 3) to support data to evaluate the shielding effectiveness of the multifunctional material. The characteristics of this box established the baseline for the shielding effectiveness comparison. The box measured 8 × 13 × 19 inches, had 1/8-inch-thick walls, and weighed 32.5 lbs.
Shielding effectiveness testing for the aluminum baseline box provided by AFRL was conducted at Wyle Laboratories with the test setup shown in Figure 4 to establish minimum shielding requirements. A baseline measurement was made in the anechoic chamber to ensure that the proper field intensity or electric field (E-field) would be available when the box was present.
The field intensity inside the box was recorded and used with the radiated field to calculate the shielding effectiveness:
The calculated shielding effectiveness for the baseline enclosure from 8 MHz to 18 GHz is presented in Figure 5. Per the constraints of the test setup, the highest field that could be radiated in the anechoic chamber was 200 V/m. When the baseline box was radiated with a field of 200 V/m and the received field inside the box was recorded from 10 kHz to 1 MHz, a field could not be detected. Therefore, it is known that the baseline box provided greater than 120-dB shielding in that frequency range. These test results defined the EM parameters required for demonstration of the multifunctional material.
MULTIFUNCTIONAL MATERIAL COMPONENT SELECTION
The selection of appropriate constituent materials was critical to the development of the multifunctional material as these materials established the potential weight, shielding effectiveness capability, and structural design capability. Making this selection required consideration of the operational environment of the material, including durability, environmental, and handling attributes. The basic weight and physical attributes of the material were driven by the baseline composite material while electrical properties were developed through integration of a uniquely shaped, high electrically conductive material in the design.
The selected baseline resin material was a semicrystalline polyetherether-ketone (PEEK). This materiel was established as the baseline due to a combination of its environmental resistance, tolerance of various types of man-made fluids (including machine oils and fuels), superior impact properties, and excellent durability and wear resistance. These characteristics are a result of the resin highly ordered molecular structure associated with its semicrystalline nature.
With consideration of the operational aspects of the electronic enclosure, the key parameters for the fiber selection was driven by mechanical properties, electrical conductivity, and cost. The fibers considered for potential use in the enclosure design were graphite and nickel (Ni)-coated carbon fibers. Properties associated with the potential fibers are provided in Table 1.
Two fiber types were selected for evaluation in development of the multifunctional material: the AS4 graphite fiber and the Ni-coated carbon (NiC) fiber. The selection process was driven by similar mechanical properties, low AS4 fiber cost, and potential enhanced electrical properties associated with the NiC fibers. Selection of these two fibers permitted evaluation of the effectiveness of the Ni coating in enhancement of electrical properties. In addition, the resin compatibility with each of the graphite fibers supported interchangeability to achieve greater mechanical properties.
In addition to the consideration for incorporating layers of NiC fibers to supplement the electrical shielding provided by the selected resin/fiber combination, metal layers were evaluated as an alternative. These layers were composed of an expanded copper mesh (Cumesh) film, which provides an effective shield against EM interference, RF interference, and electrostatic discharge. It is formed from a solid sheet of metal foil, in which the shape and pattern of the open areas are engineered precisely to match shielding requirements. The effect of mesh shape and size is illustrated in Figure 6. In addition, the mesh can be conformed to nearly any shape, readily bonds to the resin matrix, and has been demonstrated to work in in-situ tape fabrication processes.
MULTIFUNCTIONAL MATERIAL FABRICATION
Integration of the NiC fibers and/or expanded Cumesh with the base graphite/ PEEK was accomplished with “on-the-fly processing,” which provides a methodology to integrate Cumesh with minimal impact to the structural properties while enhancing EM shielding of the developed material. With this process, the part is fully consolidated as the raw material is being put into place. There are no intermediate debulking steps or post-processing needed with the in-situ process. This process uses a nitrogen gas, which is heated as is passes through an electrically resistive heating element to elevate the raw material temperature up to its melting point. The material is then passed between a rigid steel roller and the processing tool to consolidate/compact the material, as illustrated in Figure 7.
The first layer of material is placed onto a cold tool. Subsequent layers are placed on top of the previous layers to form the laminate of desired thickness and fiber orientation. Each new layer is melt-bonded to the previous layer. Through additive manufacturing, the laminate or structure is built to the desired specifications, and then the complete component is removed from the tooling.
The critical integration feature for the Cumesh is minimizing resin required to ensure a good bond between the mesh and base graphite/PEEK composite. Because the resin acts as an insulator, the more resin surrounding the Cumesh, the greater the potential is for lower shielding effectiveness than with minimal resin.
MULTIFUNCTIONAL MATERIAL DESIGN
The design of the multifunctional material was based on percolation theory, where the composite is viewed as a series of conductive and nonconductive regions. The fibers and Cumesh represent the conductive layers, while the resin is nonconductive. As material is added, there is a point where the overall laminate will begin to assume the electrical characteristics of the conductive layers. This occurrence is largely dependent on the electrical characteristics of these components and the overall thickness of the laminate.
Several laminate design configurations were developed and tested to determine the EM shielding effectiveness of various laminate designs and EM shielding effectivity of their constituent components. Laminate configurations evaluated included variations of AS4/ NiC and AS4/Cumesh in a PEEK matrix. All panel designs were compared against an aluminum baseline electronics enclosure design. A custom test fixture was designed that permitted shielding effectiveness measurements of the flat panels while in an electronics enclosure box configuration. This fixture and test setup are shown in Figure 8.
As illustrated in Figure 9, while the NiC fiber tape was worked directly into the lamination process as the base fiber resin, integrating the Cumesh into the laminates required more care to ensure that the mesh was securely encapsulated between layers of the base fiber and resin. In this manner, the resin migrates though the mesh, locking the copper substrate in place, which in combination with the adhesive characteristics of the resin securely bonds the mesh in the laminate in a manner that permits it to function as an integral ply of the laminate and not a “foreign layer” subject to delamination.
A comparison of the panel test data, showed similarity in shielding effectiveness between each of the panels tested in both magnitude and trending. In every instance, effectiveness levels associated with the composite panels exceeded the established minimum 60-dB requirement. From the comparison, it was determined that the combination of AS4/PEEK and expanded Cumesh provided the greatest combination of structural properties and EM shielding effectiveness.
MULTIFUNCTIONAL MATERIAL SHIELDING VALIDATION
To validate functionality of the multifunctional material, an electronics box with the same geometric configuration as the baseline aluminum box was constructed using AS4/PEEK and two plies of expanded Cumesh. As shown in Figure 10, the box was designed with end enclosures that could be either bonded and/or mechanically fastened to the body of the box.
EM shielding tests were conducted on the composite enclosure and compared with those for the aluminum baseline enclosure. The test setup and comparative results are provided in Figures 11 and 12, respectively.
Validation test results illustrated a shielding effectiveness that tracked closely with that of the aluminum box while yielding enhancements at both the low- and high-frequency levels. The selected design successfully demonstrated:
- Weight reduction of 70% over traditional metallic enclosures (e.g., aluminum).
- Equivalent structural/operational characteristics to a base composite.
- Excellent durability and tailorable thermal conductivity.
- Shielding effectiveness exceeding all applicable MIL-STD-461, -464, and -2169B requirements [1–3].
- Resistance to chemicals/fluids (air and ground); chemical, biological, radiological, and nuclear (CBRN) effects; and decontamination exposure.
- Producibility and scalability to large and small structures with the capability to tailor the structure to structural and EMI requirements.
- A level of damage tolerance exceeding that associate with epoxy-based composites.
While other techniques exist for introducing EM shielding to composites, levels are typically below 60 dB and do not address the greater levels demonstrated by this multifunctional material. In addition, with high structural properties, the surrounding structure can be designed with integrated shielding, reducing the need for high protection levels in electronic enclosures. Compared to these other shielding techniques, the developed multifunctional material generates shielding levels minimally greater than 90 dB with certain frequencies exceeding 110 dB, thereby extending the application and operational environment capabilities. In addition, with the use of the additive manufacturing process, there is the potential to increase these levels even further with discrete placement and quantity of mesh. Figure 13 illustrates the shielding effectiveness capability of the developed material and how it reduces the existing protection gap with current technology.
The integration methodology of the structural graphite reinforcement and Cumesh has minimal impact to the structural characteristics of the base composite material. While this result is largely dictated by the amount of Cumesh introduced into the structure, as well as its location in the laminate, any concern over structural degradation is mitigated in the design process. With in-situ tape placement, various types of simple and complex structures are possible due to precision placement of the mesh within the laminate and a reduced need for dedicated enclosure boxes/structure.
Application of this technology to an aircraft structure is illustrated in Figure 14, where the material provides the structural load-carrying capability for the design and the added Cumesh provides EM shielding levels required to protect internal electronics in the electronics bay. The mesh is added only to those areas that require the EM shielding, and the composite is transitioned in and out of these regions to tailor both structural and EM properties. These properties can be tailored to provide windows for sensor transmission while providing protection for other areas from EM energy levels between 80 and 120 dB across various frequencies.
Due to the multifunctional aspects for the developed material form (structural and EM shielding), and the manufacturing integration process, custom designed structures can be developed with shielding integrated into those areas necessary to protect contained electronics.
In summary, the multifunctional composite material provides the following features:
- A 30–70% weight reduction over traditional metal and composite structural configurations where EM interference shielding is required.
- Equivalent structural/operational characteristics to metallic and composite materials, including excellent durability and tailorable thermal conductivity.
- Sufficient shielding effectiveness to support all applicable MIL-STD-461, -464, and -2169B requirements.
- Resistance to chemicals/fluids (air and ground), CBRN effects, and decontamination exposure, as well as producibility and scalability to large and small structures using a fabrication methodology that is compatible to standard manufacturing processes.
- Compatibility with highly automated manufacturing and assembly processes.
- Ability to be tailored to individual requirements.
- Ability to seamlessly integrate continuous metallic bond for gaskets and connectors, which is achievable due to Cumesh layers.
- Adaptability to aircraft structural designs by providing a capability to integrate EM shielding into a structure that also provides structural efficiency (high strength-to-weight ratio), damage tolerance, durability, and environmental resistance.
- Supportive of EM shielding signature reduction from a structural integration design perspective.
ABOUT THE AUTHOR
Mr. Harry R. (“Rick”) Luzetsky is currently a composites and aircraft survivability subject-matter expert at the SURVICE Engineering Company. With nearly 40 years of experience in design, test, research, and development, including 30 years with the Boeing Company, Mr. Luzetsky has helped develop and assess survivability features for numerous aircraft and has been active in composite design for vehicle performance and survivability improvements. He is the lead engineer for SURVICE’s role in the development of the EMI multifunctional material for a composite electronics enclosure, as well as a thermoplastic drive shaft. He also is a coauthor of two patents on an advanced fuel containment technology. Mr. Luzetsky holds a B.S. in materials engineering from Drexel University.
 U.S. Department of Defense Interface Standard. “Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment.” Mil-Std-461F, Washington, DC, 1 December 2007.
 U.S. Department of Defense Interface Standard. “Electromagnetic Environmental Effects Requirements for Systems.” Mil-Std- 464C, Washington, DC, 1 December 2010.
 U.S. Department of Defense Interface Standard. “High-Altitude Electromagnetic Pulse (HEMP) Environment.” Mil-Std-2169B, Notice 1, Washington, DC, 19 January 2012.
 Dexmet Applications. “EMI/RFI Shielding.” www.dexmet.fr/en/applications/emi-rfi-shielding/, accessed 1 December 2018.