A new additive manufacturing process for producing composite materials with prescribed RF properties.

Currently small boat combatant design focuses primarily on speed and maneuverability. It would be advantageous to expand these capabilities to include reduced radar cross-section, and enhanced survivability to blast and ballistic threats for both the structure and warfighters.

Figure 1. Screen printing process

Investigators from the University of Delaware along with Navy partners at the Naval Surface Warfare Center developed the material building blocks necessary to realize hull materials for small boat combatants that combine structural properties with integrated radar absorption and enhanced ballistic protection. Specifically, additive manufacturing methodologies were used to develop new multifunctional materials that can be manufactured in a flexible, scalable and cost effective manner. These new materials and processing methods will be part of a library of core material building blocks that can be optimally combined in a stackable layup to produce the next generation of multifunctional hulls.

Most high speed combatant hulls are traditional sandwiched core designs constructed from a foam or balsa wood core sandwiched between two composite face sheets. Current methods to augment these structures with ballistic capabilities employ arrays of small panels, made typically from metal, carbon, glass or high strength polymer fibers, bolted onto the outer surface. One clear disadvantage of this approach is the large {>100%) increase in both size and weight. The use of outer panels constructed from high strength polymer fibers, such as Dyneema or Spectra, has shown the most promise in adding ballistic performance while minimizing additional weight. Similarly, most radar absorbing hulls are constructed by adding layers of radar absorbing materials (RAM) treatments to the outer surfaces of a traditional hull design. There are currently no examples of hull materials that combine structural, ballistic and radar absorbing functionalities.

The technical approach to solving that problem was to employ scalable screen printing to print patterns of functionalized custom inks and pastes to composite materials. The proposed effort was modeling and simulation driven guiding the selection of appropriate composite materials, inks, additives and printable patterns to create design methodologies that can be followed to produce composite materials with well-defined electromagnetic, structural and ballistic properties.

Figure 2. Micro dispensing system

To realize a multifunctional hull material that minimizes weight, a number of advanced composite materials were used in addition to standard woven glass and carbon based composites. These included woven glass fabrics and Kapton, a polyimide film that’s a common dielectric substrate used for printing electronics, as well as high strain rate polymer composites such as Dyneema HB26 or Spectra Shield.

Three types of electromagnetically functional inks were also explored: resistive inks, high dielectric constant inks, and magnetic inks.

To tailor structural, ballistic and electromagnetic properties within a composite required exploring new manufacturing methods, namely screen printing and micro-dispensing. Screen printing is an additive manufacturing method that utilizes a mesh screen, a flood, and a squeegee, as seen in Figure 1. The mesh screen that was used contains a desired geometrical pattern that was deposited onto the substrate. When the flood is actuated, it deposits ink across the surface of the screen, the screen is then lowered to a snap-off height above the substrate, and the squeegee actuates to deposit the desired pattern. The substrate is then heated to cure the deposited ink. This process allows for a designed pattern to be repeatedly deposited onto multiple substrates in a short period of time.

Micro dispensing is a process of additive manufacturing that utilizes a dispensing head to precisely deposit a material. The micro dispensing system that was used in this research was an nScrypt 3Dn-300, pictured in Figure 2. The system is capable of utilizing two different types of dispensing heads for depositing a wide variety of materials. The implementation of two printing heads allows for the printing system to toggle between printing two materials at once. These materials may also be swapped out mid-print in order to incorporate varying properties within one structure.

The results of this research are broken down into three categories based on the nature of the ink used. Specifically, detailed results are provided for high dielectric constant loaded composites, magnetically loaded composites, and resistively loaded composites.

This work was done by Mark S. Mirotznik of University of Delaware for the Office of Naval Research. ONR-0034

This Brief includes a Technical Support Package (TSP).

Advanced Multifunctional Materials for High Speed Combatant Hulls (reference ONR-0034) is currently available for download from the TSP library.

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