Hooke's Law and stretched fabric

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    Fabric Hooke's law Law
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Discussion Overview

The discussion revolves around the application of Hooke's Law to a piece of fabric when it is stretched in multiple directions, specifically whether it can continue to obey Hooke's Law when simultaneously stretched at right angles to the initial direction. The scope includes theoretical considerations of material behavior and properties, particularly in relation to isotropic and anisotropic materials.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that a piece of fabric can obey Hooke's Law when stretched in one direction, but questions whether this holds true when stretched in another direction at right angles.
  • Another participant agrees that it is possible to some extent, noting that every minimum resembles a parabola on some scale.
  • A different viewpoint emphasizes that the material's properties are crucial; isotropic materials may not exhibit directional dependence, while anisotropic materials would have different spring constants for different directions.
  • One participant provides a resource link that may assist in modeling the behavior of materials under stress.
  • It is noted that linear or nonlinear behavior is independent of isotropy or anisotropy, with anisotropic materials potentially exhibiting different stiffness in different directions, which does not contradict Hooke's Law as long as the forces and stretches remain small enough for linearity.
  • A further elaboration indicates that woven fabrics are typically anisotropic and nonlinear, with initial stretching affecting the stiffness of the fibers involved.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Hooke's Law to fabrics under multi-directional stress, with some asserting it is possible under certain conditions while others highlight the complexities introduced by material properties. No consensus is reached on the general applicability of Hooke's Law in this context.

Contextual Notes

Limitations include the dependence on material definitions, the need for small forces and stretches to maintain linear behavior, and the complexities introduced by anisotropic versus isotropic properties.

verdigris
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A piece of fabric obeys Hooke's Law - force is proportional to extension - when stretched in one direction.Is it possible for the fabric to continue to obey Hooke's Law if it is simultaneously stretched in another direction at right angles to the first direction?
 
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Basically yes (to some extent), since every minimum looks like a parabola on some scale.
 
It depends on the material. If the material is isotropic the direction in which you stretch doesn't matter. For some materials, it will stretch more easily in one direction than in another (and different for the 3rd independent direction still). You'd have 3 spring constants (one for each direction).
 
Linear or nonlinear material behaviour is indepedent of whether the material is isotropic or anisotropic.

For an anisotropic materal the stiffness is different in different directions, and if you pull in one direction you might get stretching or bending in a different direction!. That doesn't contradict Hooke's law, provided twice the force gives twice the stretch.

The general answer to the OP is "yes, provided the forces and stretches are small enough so the material remains linear".

If by "fabric" you mean something like a piece of woven cloth, then it is highly anisotropic, and also highly nonlinear even if you pull in just one direction. The initial pull tends to straighten out the fibers, so the stiffness is low and the stretch is quite large. As the fibers become straight the stiffness becomes much higher because you have to stretch the fibers themselves.
 
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