The Department of Defense, specifically the Navy, is seeking innovative solutions for the development of an acoustically transparent underwater curing adhesive aimed at repairing defects in submarine sonar conformal flow surfaces. The objective is to create a material that can be applied and cured underwater, maintaining acoustic performance while simplifying maintenance processes that currently require dry-docking. This technology is critical for enhancing the longevity and effectiveness of sonar systems embedded in submarine hulls, thereby reducing repair costs and downtime. Interested parties should note that the solicitation will open on January 8, 2025, with proposals due by February 5, 2025, and can find more information at the official SBIR website.

## Description

Current and developing sonar technologies rely on conformal structures that serve dual purpose as a fastening component and as an acoustically enhancing material. These structures may suffer damage and degradation over time and decrease acoustic performance of the sensors due to flow obstructions which may cause unwanted noise. Current repair methods require the submarine to be dry-docked, with high time and cost expenditures. This SBIR topic seeks to repair these flaws using a material with properties which allow it to be applied, cured, and used underwater and make it acoustically compatible with array sensing capabilities. Existing consumer gap fill material is used for bonding to metals for pipe repairs, ceramics, and some rigid plastics. In the naval environment, currently approved materials per S0600-AA-PRO-200 include HYCOTE 461 and HYCOTE 151, which are an underwater curing epoxy and paint, respectively. These materials are used mostly on metal surfaces, with their focus on functioning as an anti-corrosion coating and contouring smooth surfaces after repair work has been performed. Another material in use has been PR-944F, which is a two-part, elastomeric, chemically curing adhesive for metals, plastics, woods, and ceramic surfaces. This adhesive has been shown to be used underwater as a seam filling material when performing sonar structure repairs. Sonar sensors embedded in conformal polymeric materials (NGD-09) are being incorporated in existing and future submarine hulls. These structures interact with incoming sound waves to assist in detection. Current adhesives and fill epoxies do not contain the full set of properties that would allow them to effectively function as an underwater repair solution for emerging structural imperfections in acoustic embedding material. A product with the strength properties of existing adhesives, underwater curability, and acoustic transmissibility under system working frequencies which is compatible with the undersea environment would simplify efforts to maintain conformal sonar systems without affecting their acoustic performance. A non-toxic compound would be desirable. To produce such a material, several research requirements must be addressed: - Material must be able to be applied, cured, and faired underwater under expected pier side working environments. - Material will also be evaluated based on ease of application and use by working personnel in typical shipyard conditions. - Once set, material must demonstrate acoustic transparency and produce no altering or detrimental effects on the acoustic performance of the sensors under broadband frequencies under use conditions. - Once set, the material must maintain its adhesive strength to bonded material (i.e., metals, urethane, glass-reinforced plastic (GRP), NGD-09, and rubber) under expected environmental conditions experienced throughout the submarine&rsquo;s lifetime (i.e., salinity, pressure, and temperature fluctuations). - Mechanical properties must be compatible with and equivalent to those of existing conformal acoustic material. - Material should not have toxic effects on the environment or user during application, curing, and use. - Anti-fouling capabilities must be equivalent to or improve upon that of current array coatings. - Material must be able to be applied on a sufficiently large flaw area determined by an analysis of current and expected flaw sizes. Additional uses exist which fall outside the requirement of acoustic transmissibility for underwater curing adhesives. An adhesive could effectively function to repair, fill, or seal any non-acoustic structure that would benefit from underwater repair. A specific application is repair of damage to the surface of polymer-based fairing structures that surround the main sonar system to ensure they effectively contour the hull, and no turbulent flow develops which can hinder sonar performance. Contending material solutions will be evaluated against standards for applicability in repairs, and physical, chemical, and mechanical properties similar to those used on PR-944F and existing sensor conformal material. As for acoustic transmissibility, the standards to be used will fall within those of sensor conformal material. The development of this technology can provide the Navy with an alternative to perform repairs which would remove the labor, cost, and time associated with the dry-docking process. This technology will have use in existing and future hulls due to the increasing amount of sonar systems which rely on conformal sensor embedding structures. Advanced developments in future materials may eventually produce a product that is incompatible with this technology, however this is unlikely during the short-term due to the current state of conformal structures as being newly introduced into the fleet. Work produced in Phase II may become classified. Note: The prospective contractor(s) must be U.S. owned and operated with no foreign influence as defined by 32 U.S.C. &sect; 2004.20 et seq., National Industrial Security Program Executive Agent and Operating Manual, unless acceptable mitigating procedures can and have been implemented and approved by the Defense Counterintelligence and Security Agency (DCSA) formerly Defense Security Service (DSS). The selected contractor must be able to acquire and maintain a secret level facility and Personnel Security Clearances. This will allow contractor personnel to perform on advanced phases of this project as set forth by DCSA and NAVSEA in order to gain access to classified information pertaining to the national defense of the United States and its allies; this will be an inherent requirement. The selected company will be required to safeguard classified material during the advanced phases of this contract IAW the National Industrial Security Program Operating Manual (NISPOM), which can be found at Title 32, Part 2004.20 of the Code of Federal Regulations.

## Objective

Development of an underwater applicable, curable, and acoustically transparent adhesive for the mitigation of defects in submarine sonar conformal flow surfaces.

## Phase I

Define and develop a concept for an underwater acoustically transparent adhesive that can meet the performance constraints listed in the Description. Feasibility for the material is expected to be theoretically demonstrated, however any further preliminary physical artifacts may be produced to support its validation (e.g., any preliminary lab samples that demonstrate fulfillment of the previously stated capabilities). Outline an approach to validate the proposed solution. Describe a production and test plan geared toward generating a prototype for Phase II. The Phase I Option, if exercised, will include the initial design specifications and capabilities description to build a prototype solution in Phase II. Verify contending solutions for feasibility and fulfillment with the main requirements and capabilities stated under the identified problem in the Description.

## Phase II

Determine if the solution can meet Navy requirements. Due to the physical nature of an underwater adhesive, these developments will take the form of a prototype material to be applied in a series of simulated environments and conditions. Perform a series of prototype evaluations. Develop a plan to transition the technology into a system that can be acquired by the Navy. A prototype should be delivered at the end of the Phase II. Conduct tests that involve a facet for non-acoustic, physical material performance and another for testing of acoustic transmissibility. The setup to simulate the working environment will likely contain sensors embedded in conformal material as well as other structures to which the adhesive may want to be tested on. This mockup will also have manufactured or induced flaws on the surface of the conformal material for repair. These evaluations specific to the prototype will be performed to determine if the adhesive material at a minimum meet the performance constraints stated under the identified problem in the Description. It is probable that the work under this effort will be classified under Phase II (see the Description section for details).

## Phase III: Dual Use Applications

A successful demonstration of the prototype in Phase II will concretize the process of transitioning the solution into a technology for Navy use. This process is expected to be supported by the Phase III awardee. Building on test results and further discussions on final performance requirements, develop a production plan to meet the needs of the specific application. This plan will consider the compatibility of the solution for use on legacy, existing, and future platforms. It will also consider the manufacturability of the product given current and future resources and lifecycle requirements. Support will be expected for the transition of the technology into use on Navy submarines under existing schemes for maintenance and repair processes. Further validation of the product for Navy use will take place using identified material property and performance standards as a guide to any further testing and qualification processes that must take place. Depending on the material developed, it may need to be presented to the Navy non-metallic Technical Warrant Holder for the military standard it would be evaluated against and approval. The final adhesive material will be used in platforms which contain conformal acoustic structures. Given the final performance of the adhesive underwater, it may be used to repair non-conformal structures whose underwater repair would bring cost and schedule reductions to the Navy. These are most of the submarine platforms under the VA and OHIO programs, as well as the upcoming CLB class sonar systems. This technology will meet needs for allowing a continuous performance across a wide range of submarine sonar systems with a reduction on the cost and schedule to perform repairs. Outside of the Navy, and dependent on potential classification of material properties, this product may be streamlined to commercial and recreational boating applications which would require underwater repair of the hull.

Acoustically Transparent Underwater Curing Adhesive

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