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Inflatable Softgoods for Next Generation Habitation Systems

Description:

Scope Title:

Structural Health Monitoring for Inflatable Softgoods

Scope Description:

Integrated sensing capabilities in crewed inflatable softgoods systems are critical to monitoring the performance of the structural restraint layer in situ over long-duration missions. This can include measuring load/strain, detecting damage and impacts, and predicting further degradation/potential failures. The ability to acquire, process, and make use of this data in real time is an important risk mitigation for potential structural failure modes. The current state of the art in this field are instrumentation systems such as high-resolution strain gages, fiber optics, accelerometers, and acoustic sensors using flexible electronics. However, there is a technology gap in developing a proven system that can integrate into a multilayer softgoods structure and continually monitor performance throughout its life. The proposed work should seek to demonstrate not just a sensor, but the approach and method to its robust integration into a high-strength synthetic webbing or cord (e.g., Vectran or Kevlar), with consideration of the ingress/egress of any wiring and connectors if required, and where/how the control electronics are attached and located. Innovative solutions to the following system properties and requirements are sought and should be considered in the specification and design of any proposed structural health monitoring (SHM) system:

Primary properties of interest for a sensor system (individual or combined sensing):

  • Strain, load, and/or impact detection (magnitude and location) in structural restraint layer:
    • For strain measurement (long-duration creep), sensors must be able to tolerate an initial strain of 2 to 5% while the inflatable deploys, then must be sensitive to 0 to 0.5% creep strain once in service, with enough resolution to track those changes over the mission life (i.e., there may only be 0.1% change over a year).
    • For load measurement, typical high-strength webbings and cordage used in these structures can have operational loads of up to 5,000 lb/in., i.e., a 2-in.-wide webbing could operate at 10,000 lb.
    • For damage or impact detection, the desire is to have enough coverage of sensors to localize where damage has occurred and be able to evaluate its severity. This would ideally be combined with or utilize the strain or load sensing capability.

Sensor System Softgoods Integration Focus:

  • There should be a strong focus on the method and vetting of integration of any sensor system with high-strength webbing and/or cordage that makes up the structural restraint layer.
  • The impact on the properties of the softgoods due to the integration of the sensor system should be addressed, i.e., bonding, coating, integration of new yarns, and integration, attachment and ingress/egress of any wiring or control hardware needed.
  • Complexity and additional work added to integrate and operate the sensor system should also be considered for its impact on the fabrication process of the inflatable structure and any additional work required on the mission to operate or read data from the system.
  • Desire is to have a system with broad applicability to different inflatable architectures.

Other Desired System Properties:

  • Minimize mass, power, and required auxiliary components where possible.
  • Automated system activation and data readout, i.e., does not require astronaut or external agent.
  • Launch and mission environment (consideration of path to flight):
    • Survive handling, integration, and packaging/deployment from a compressed state.
    • Survive launch environment and cold-vacuum prior to system deployment (once deployed the structural layer is near the interior, thus operation at close to room temperature is possible).
    • Mission life of up to 15 years without maintenance.
  • Ability to collate/unify distributed sensor system data to track structural health and predict further degradation/potential failures.

Design Notes:

  • The structural layer where the sensors are needed has multiple softgoods layers in front and behind it as part of a multilayer system; thus is nonaccessible or observable during the mission, and the layer is in close contact with the layers around it.
  • The outer layers typically have multilayer insulation (MLI), which incorporates thin metallic depositions. If wireless sensing equipment is to be used externally, this should be considered for any possible interference.
  • The interior of the inflatable structure will likely have a large amount of logistics deployed and installed once the structure is inflated, which could obscure direct access to large portions of the shell from the interior. In addition, these structures should be considered for any possible interference they may cause to wired or wireless sensor systems.

For this activity, a system concept that addresses the desired properties above and preliminary breadboard testing would be expected under Phase I on an applicable high-strength softgoods component(s) as a proof of concept. Integration into an inflatable softgoods structure or higher fidelity subcomponent test(s) is expected as part of Phase II to validate the feasibility of the approach and how it would be scaled to a full-scale crewed inflatable structure.

Expected TRL or TRL Range at completion of the Project: 3 to 4

Primary Technology Taxonomy:

  • Level 1 12 Materials, Structures, Mechanical Systems, and Manufacturing
  • Level 2 12.2 Structures

Desired Deliverables of Phase I and Phase II:

  • Research
  • Analysis
  • Prototype
  • Hardware

Desired Deliverables Description:

Phase I:

Approach to SHM for inflatable softgoods identified and a laboratory proof of concept to establish the efficacy of approach. Phase I demonstrates a system concept that addresses the desired properties specified in the scope and preliminary breadboard testing on an applicable high-strength softgoods component(s).


Phase II: 

Integration into an inflatable softgoods structure or higher fidelity subcomponent test(s) to validate the feasibility of the approach and how it would be scaled to a full-scale crewed inflatable structure.

State of the Art and Critical Gaps:

Approaches for SHM in inflatable softgoods are needed to track the performance of the material system in real time and identify when the structure has incurred damage or is at risk of failure. SHM typically uses strain gauges, digital image correlation, or accelerometers. SHM for inflatable softgoods requires novel approaches, as the material system is multilayerered and fundamentally different from typical rigid habitat structures. New techniques, such as flexible electronics, wireless systems, and fiber optics, are also generally unproven in a flight scenario for SHM and must be robust enough to integrate, package, and deploy with the inflatable structure.   

Relevance / Science Traceability:

Current work on inflatable softgoods is taking place under NASA's NextSTEP Habitat program and the CLD program. NextSTEP has been ongoing since 2016 and focuses on design of next generation habitat systems for cislunar space, the lunar surface, and Mars transit scenarios. CLD is focused on the next generation of orbiting space station. The work under this subtopic will strongly complement ongoing work under these programs and increase the potential for infusion of inflatable softgoods into future habitation concepts by reducing risks associated with understanding and predicting material behavior prior to and on a mission. Work could also serve to benefit entry/descent/landing systems that use inflatables, and terrestrial applications for integrated sensing and long-duration characterization of high strength softgoods materials that have wide use in industry and military.  

References:

Scope Title:

Accelerated Creep Testing for Softgoods

Scope Description:

In implementing inflatable softgoods in habitation structures, one of the primary long-term risks is failure of the structural material due to creep (deformation under sustained loading). Real-time creep testing at the component and subscale levels can take months to years, therefore new test methods need to be developed to reduce the time of long-duration creep testing of candidate webbing and cordage materials to help reduce the overall development time of softgoods structures for flight. Creep testing is currently a schedule driver for this class of structures. NASA has performed real-time creep testing of Vectran, and Kevlar webbings rated at 6,000 lb/in. and 12,500 lb/in. This data can be provided as a verification and validation data set to compare to an accelerated technique. These materials and load ratings are typical for inflatable habitats, but it is desirable that the proposed methodology could be applied to other high-strength synthetic webbings and cordage.

Typical creep acceleration methods using stepped isothermal or isostress approaches have had poor correlation to real-time results to date. Elevated temperatures affect the oils and sizing applied to the softgoods components, which may change the interfiber frictional properties and mechanical behavior, in parallel to the creep behavior. In addition, the stepped isostress approach is impacted by the nonlinear, stress-dependent behavior of these components that are often dominated by the architectural/constructional strain (decrimping) at stresses below ~50% of ultimate, versus being driven by the elastic strain of the fibers above that.

Properties of Proposed Approach:

  • May include novel test methods to reduce the duration of the test, while minimizing test effects on parameters outside of creep.
  • May include combination of test methods and modeling approaches to accelerate generation and capture of creep data that can accurately characterize the long-duration behavior of the softgoods.
  • If elevated temperature test approach is used: Addresses thermal effects on the material that may occur in parallel to the creep behavior, and incorporates that in the post-processing of the data.
  • If elevated stress test is used: Addresses impact of nonlinear mechanical behavior effects on the creep response and incorporates that in the post-processing of the data.
  • Can be performed in a typical mechanical load frame with environmental chamber (if needed).

It is understood that procuring and testing these specific materials under Phase I is challenging, thus demonstration and detailing of accelerated approaches can be performed on different materials and strengths of webbing and/or cordage if they address the issues mentioned and can be validated as doing so. Collaboration with industry or university partners is encouraged. Phase II work would be expected to apply the methodology and approach more generically to a series of materials of interest and demonstrate it via test and comparison to real-time testing.

Expected TRL or TRL Range at completion of the Project: 2 to 3

Primary Technology Taxonomy:

  • Level 1 12 Materials, Structures, Mechanical Systems, and Manufacturing
  • Level 2 12.1 Materials

Desired Deliverables of Phase I and Phase II:

  • Research
  • Analysis

Desired Deliverables Description:

Phase I: 

Demonstration and detailing of accelerated approaches; performance of testing on different materials and strengths of webbing and/or cordage. 

Phase II: 

Apply the methodology and approach more generically to a series of materials of interest and demonstrate it via test and comparison to real-time testing.

State of the Art and Critical Gaps:

Current state of the art for testing uses straps for real-time creep testing at the component level and subscale (or full-scale) inflatable softgoods test articles for (1) burst and (2) creep to failure testing. These tests are needed to understand the behavior of the inflatable softgoods over the mission lifetime and predict failure due to creep, which represents a catastrophic risk. Real-time testing takes months to years to collect data (depending on load level) and predictions require extrapolation from a limited number of data points. Accelerated testing techniques would enable higher fidelity characterization of the performance of the inflatable softgoods system over the entire mission scenario prior to flight, reduce development time, and reduce risk.

Relevance / Science Traceability:

Current work on inflatable softgoods is taking place under NASA's NextSTEP Habitat program and the Commercial LEO Destinations (CLD) program.  NextSTEP has been ongoing since 2016 and focuses on design of next generation habitat systems for cislunar space, the lunar surface, and Mars transit scenarios.  CLD is focused on the next generation of orbiting space station.  The work under this subtopic will strongly complement ongoing work under these programs and increase the potential for the infusion of inflatable softgoods into future habitation concepts by reducing risk associated with understanding and predicting material behavior prior to and on mission.  Work could also serve to benefit entry/descent/landing systems which use inflatables, and terrestrial applications for integrated sensing and long-duration characterization of high-strength softgoods materials that have wide use in industry and the military.  

References:

Jones, Tom and Litteken, Doug. "Certification Guidelines for Crewed Inflatable Softgoods Structures." JSC-67721. https://ntrs.nasa.gov/citations/20220011425 

 

Jones, T. C., Doggett, W. R., Stanfield, C., and Valverde, O. "Accelerated Creep Testing of High Strength Aramid Webbing," AIAA 2012-1771. 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference. April 2012.

 

Valle, Gerard, Litteken, Doug, and Jones, Tom. "Review of Habitable Softgoods Inflatable Design, Analysis, Testing and Potential Space Applications." AIAA SciTech. January 2019.

 

Le Boffe, Vincenzo, Jones,Tom, and Kenner, Winfred. "Development of a Compact, Low Cost Test Fixture to Evaluate Creep in High Strength Softgoods Materials Under Constant Environmental Control." https://ntrs.nasa.gov/citations/20205005004

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