Problem: Estimating pressure of compressive garment

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Discussion Overview

The discussion revolves around estimating the pressure generated by active compression garments utilizing materials that can actuate with recoverable strains. Participants explore the mechanics of how these materials, particularly shape memory alloys, contribute to compression through various mechanisms and the modeling required to estimate pressure output.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes using active compression garments that actuate to create pressure based on the recoverable strain of the materials involved.
  • Another participant questions whether the radial growth and contraction of the fibers is the primary mechanism for generating compression, suggesting the need for clarification on the material structure.
  • A participant mentions the use of Nitinol, a shape memory alloy, which can recover strains of approximately 6% and cites its application in generating high stresses in specific conditions.
  • Concerns are raised about the accuracy of using hoop stress equations to model the pressure, given the complexities of the fabric weave and the arrangement of fibers.
  • One participant suggests conducting biaxial stress-strain experiments or developing theoretical models to predict the stress-strain properties based on fiber and weave geometry.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms of compression and the modeling approaches needed, indicating that multiple competing perspectives remain without consensus on the best method to estimate pressure.

Contextual Notes

Limitations include the potential inaccuracies in modeling due to the complex interactions of fibers in a woven fabric and the need for empirical data to support theoretical predictions.

qwertypie
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Basically, I'm working on a project to make active compression garments instead of passive. There are a few materials that can actuate with large recoverable strains, and I want to use them to create the compression. I need to estimate how much pressure they could create based on the recoverable strain they are capable of producing though. This would be either by constricting upon actuation or expanding upon actuation and the pressure being generated upon release of stimulus as the fibers try to return to normal, like with regular passive compression garments. I just need a rough estimate, or just the idea of how I would model this.
 
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Do you really think it's the radial growth and contraction of the fibers that causes the compression?

Are the fibers embedded in a matrix, or is it a fabric? Are the fibers single ply? What is the nature of the fabric weave>
 
Chestermiller said:
Do you really think it's the radial growth and contraction of the fibers that causes the compression?

Are the fibers embedded in a matrix, or is it a fabric? Are the fibers single ply? What is the nature of the fabric weave>
It's using shape memory alloys. Just to have some numbers, an alloy called Nitinol can recover strains up to ~6% after a phase transition. Nasa developed a device for rock splitting that use modified nitinol expanding elements that were capable of generating stresses in excess of 1.5 GPa at only 2-3 percent corresponding strain. In the time waiting, I found an equation for hoop stress, equated the stress to the young's modulus of the material x the strain, and solved for pressure. But I imagine that's wildly inaccurate to the real situation, which would be a cross-hatched weave where the lateral fibers contain the SMA wires as a core and some cladding.
 
You need to do biaxial stress-strain experiments on the woven fabric, or you need some sort of theoretical development to predict the biaxial stress-strain properties from the fiber geometry, the weave geometry, and the frictional characteristics of the fibers.
 

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