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Next Generation Buoyancy Material

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: Fabricate high-resilience, high-stiffness, and high-strength, low-density buoyancy material for submersibles. 

DESCRIPTION: The utility of a submersible is enhanced by maximizing speed, range, and endurance/payload. Increasing a submersible’s payload at its maximum operating depth, without compromising speed or range, is of high interest to the U.S. Navy. Currently used buoyancy materials are costly and lack the technical characteristics (e.g. high strength, resilience, stiffness, and low density) to increase payload at maximum depth and speed. A buoyancy material that achieves increased technical performance; and can be additively manufactured will increase operational capacity, decrease manufacturing costs and material waste, while streamlining maintenance actions during the operations and sustainment phase. Increased payload improves operational flexibility and mission capacity as additional warfighters and/or equipment can be delivered to a target area. Materials possessing high resilience, high stiffness, and low density are rare but necessary for buoyancy applications. Currently used materials (polymethacrylimid, phenolic, and polyvinylchloride foams) lack sufficient strength, resilience, and stiffness at a density of 0.05 grams per cubic centimeter (g/cm3). Lack of resilience is found in currently used materials which exhibit creep (permanent deformation) under sustained loading at temperatures in the range 4ºC to 30ºC. Creep is likely observed due to low glass transition temperatures of the foamed polymers. The threshold (minimum) requirement is a material with low density defined to be less than or equal to 0.05g/cm3 with high strength defined to be uniaxial compression failure stress exceeding 2MPa (in the lowest strength loading direction), high stiffness defined to be a Young’s modulus exceeding 100MPa, and bulk modulus exceeding 30MPa. Strength measurements must be conducted in uniaxial compression using a sample with 1:1 aspect ratio at a strain rate between 10-3, and 100 in the lowest strength loading direction. Strength must be calculated as the first fracture event or onset of plastic deformation depending on material type developed. Young’s modulus must be calculated from the slope of the stress strain curve in the elastic region. Bulk modulus must be calculated by measuring the hydrostatic pressure required to decrease the initial volume by 0.5% and dividing the measured pressure by 0.005. The material must be able to sustain damage and remain watertight (less than 1% by weight water infiltration after being fully submerged for one week at 0.33MPa hydrostatic pressures). Resistance to water intrusion after damage requires a closed cell microstructure or self-healing properties. Water infiltration after damage must be measured by removing core samples from the material representing 10% of the original volume, measuring the dry mass of the material after cores are removed, submerging the material (with cores removed) in water for one week at 0.33MPa hydrostatic pressure, removing material from water, blotting exterior with absorbent cloth, allowing to drip dry for one hour, measuring mass, and comparing to dry mass. Ideally, the material should be additively manufactured to minimize material waste in machining complex geometries. Resilience is a measure of the material returning to its original volume after compression. High resilience is required and is defined by a creep rate less than 3x10-11 s-1 for uniaxial loads of 1MPa (in the lowest stiffness loading direction) applied in the temperature range 4ºC to 30ºC. Creep must be measured by applying a load of 1MPa for 25 hours and determining the permanent plastic deformation induced in the sample after unloading. The material must be produced in dimensions at least 30cm x 30cm x 5cm. The production method must be capable of producing 0.1m3 of material per day at a cost less than $50,000/m3. The objective (maximum) requirement is maximum strength, resilience and stiffness at minimum density. For example, a foam with 0.01g/cm3 density that could withstand 110MPa hydrostatic loading (pressure at bottom of the deepest part of the ocean) with no detectable decrease in volume at that pressure would be ideal but likely impossible with known materials. A difficult but more realistic objective requirement is a material with density of 0.01g/cm3, uniaxial compression failure stress of 5MPa, Young’s modulus of 200MPa, bulk modulus of 100MPa, and water infiltration and creep rates as defined for the threshold requirement with a production rate of 1m3 of material per day at a cost less than $20,000/m3. The description is not intended to restrict proposed solutions, but rather provide references and possible avenues for exploration. Open cell materials are unlikely to meet the Navy requirements due to water infiltration into the pores. Stochastic foams will likely require high specific strength and high specific stiffness base material due to the exponential decrease in foam strength and stiffness with relative density. Pressurization of cells may improve strength and stiffness based on cellular solid theory. Closed cell ordered cellular solids with sufficient nodal connectivity show a linear decrease in cellular solid strength and stiffness with relative density. Currently, few solutions exist for fabricating large dimension samples with sufficient nodal connectivity. Most syntactic foams use polymeric matrix materials and may lack resilience (creep rate too high). Syntactic foams using metal as the matrix material may not meet the density requirements due to the high density of most metals. Current additive manufacturing methods could be adapted to include extrusion of suitable syntactic foams into desired geometry, powder bed bonding of hollow spheres, or possibly a droplet/hollow sphere deposition method. A 30% cost savings in the acquisition phase is expected if additive manufacturing methods are developed. Due to the high cost of buoyancy materials, machining losses drive up cost considerably. Net shape additive manufacturing would reduce/eliminate most of those costs. Additionally, maintenance actions can be streamlined and repair costs would be reduced as damaged sections of buoyancy material could be repaired using additive manufacturing instead of having to procure and machine additional buoyancy material to replace the damaged section(s). A successful project will include an iterative approach of testing and process refinement to produce a material meeting Navy requirements. Projects progressing to Phase II shall provide samples at intervals discussed in section Phase II: for independent Navy testing. If an additive manufacturing method has been used for production of material, a hemisphere with 8cm diameter and a toroid with 10cm inner diameter and 13cm outer diameter and square cross section shall be produced and delivered at the end of Phase II. The final material production method shall produce material with less than ±10% variability in material properties including density, strength, and stiffness. This will be verified by lot testing as described in section Phase III: Testing per SS800-AG-MAN-010/P-9290 System Certification Procedures and Criteria Manual for Deep Submergence Systems will also be required for projects that progress to Phase III. 

PHASE I: Define and develop a concept to produce a material with the properties listed in the description and demonstrate feasibility of said concept. The concept must be described in a report, which includes either modeling results or mathematical analysis of the expected cellular material properties as a function of the base material properties. Density, strength, stiffness, resilience, microstructure, water intrusion after damage, and attainable dimensions must be analyzed, included in the report, and compared to values discussed in the description. Cost estimates and additive manufacturing methods must also be included and compared to values discussed in the description. The Phase I Option, if awarded, will include the initial design specifications and capabilities description to build a prototype solution in Phase II. The Phase I option must include engineering drawings of additive manufacturing apparatus and any hardware required for material fabrication. Develop a Phase II plan. 

PHASE II: Based on the results of Phase I and the Phase II Statement of Work (SOW), develop and deliver a prototype for evaluation. This includes fabrication of any Phase I option hardware and production of material for testing. Preliminary test results of material must be presented 18 months after receipt of Phase II funding. Test results must include measurement of density, strength, stiffness (Young’s modulus and bulk modulus), creep at 1MPa uniaxial loading (in the lowest stiffness loading direction), water infiltration after material damage, and microscopy to evaluate microstructure. Additionally, three blocks of material with minimum dimensions of 30cm x 30cm x 5cm must be provided for independent testing. Based on test results, perform process refinement to meet Navy requirements defined in the description or to improve material properties further if Navy requirements have already been met. Additional test results must be provided at 24 months along with three additional blocks of material with minimum dimensions of 30cm x 30cm x 5cm. If an additive manufacturing method has been used for production of material, a hemisphere with 8cm diameter and a toroid with 10cm inner diameter and 13cm outer diameter and square cross section must be produced and delivered at 24 months. 

PHASE III: Support the Navy in transitioning the technology for Navy use, including scaling the material production technology to support Navy requirements. PMS 340 will use said material to increase mission capability including carrying greater payloads at greater depth. A minimum production rate of 0.1m3 of material per day is required. The process must produce material with less than ±10% variability in material properties including density, strength, and stiffness. A total of 100 samples from at least 10 blocks of material must be subjected to the following measurements to determine if the material meets Navy requirements. (1) Density measurements of material samples must be conducted by measuring mass of the sample and dividing by the volume calculated from exterior dimensions of the sample. (2) Strength measurements must be conducted in uniaxial compression using a sample with 1:1 aspect ratio at a strain rate between 10-3 and 100 in the lowest strength loading direction. (3) Strength must be calculated as the first fracture event or onset of plastic deformation depending on material type developed. (4) Young’s modulus must be calculated from the slope of the stress strain curve in the elastic region. (5) Bulk modulus must be calculated by measuring the hydrostatic pressure required to decrease the initial volume by 0.5% and dividing the measured pressure by the 0.005. (6) Water infiltration after damage must be measured by removing core samples from the material representing 10% of the original volume, measuring the dry mass of the material after cores are removed, submerging the material (with cores removed) in water for one week at 0.33MPa hydrostatic pressure, removing material from water, blotting exterior with absorbent cloth, allowing to drip dry for one hour, measuring mass, and comparing to dry mass. (7) Creep must be measured by applying a load of 1MPa for 25 hours and determining the permanent plastic deformation induced in the sample after unloading. Testing per SS800-AG-MAN-010/P-9290 System Certification Procedures and Criteria Manual for Deep Submergence Systems will also be required. If the material developed in this topic has tensile ductility, the company should consider aviation applications. Ductile low-density, high-strength, high-stiffness material would be ideal as a stiffening material in sandwich panels such as wings and core filled geometries such as struts. High performance automobiles will also likely benefit from foam core structures. If the material developed is a ceramic cellular structure with poor tensile ductility, electrical, acoustic, and thermal insulation applications are plentiful. The material will be useful for applications requiring high buoyancy such as unmanned underwater vehicles (UUVs), submarines, mobility platforms, and buoys. The material may also be useful in energy absorbing applications such as crash mitigation where the material is sacrificed to limit force transmitted from the impact. 

REFERENCES: 

1: Gibson, Lorna J. "Cellular Solids Structure and Properties – 2nd Edition." Cambridge University Press, 2001. https://ocw.mit.edu/courses/materials-science-and-engineering/3-054-cellular-solids-structure-properties-and-applications-spring-2015/lecture-notes/

2:  Deshpande, Vikram S. "Effective properties of the octet-truss lattice material." Journal of the Mechanics and Physics of Solids 49 2001: 1747-1769. https://doi.org/10.1016/S0022-5096(01)00010-2

3:  Sanders, Wynn S. "Mechanics of hollow sphere foams." Materials Science and Engineering A 347 2003: 70-85. https://doi.org/10.1016/S0921-5093(02)00583-X

4:  Ashby, Michael F. "Metal Foams: A Design Guide." Boston: Butterworth-Heinemann, 2000. https://www.researchgate.net/profile/Norman_Fleck/publication/248464569_Metal_Foams_a_Design_Guide/links/5400a7af0cf23d9765a3fe61/Metal-Foams-a-Design-Guide.pdf

5:  Kendall, James M. "Metal shell technology based upon hollow jet instability." Journal of Vacuum Science and Technology 20 1982: 1091-1093. http://dx.doi.org/10.1116/1.571574

KEYWORDS: Syntactic Foam; Additive Manufacturing; Creep; Cellular Material; Buoyancy; Submersible 

CONTACT(S): 

Dr. Aaron Wiest 

(951) 393-4819 

aaron.wiest@navy.mil 

Sam Pratt 

(301) 227-5036 

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