What is the physics behind piercing a plastic piece?

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Discussion Overview

The discussion revolves around the physics of piercing a plastic material, specifically focusing on the factors that influence its tolerance to being pierced. Participants explore concepts related to material properties, stress states, and the complexities involved in understanding the failure mechanisms of plastics under concentrated loads.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the "tolerance" of plastic to piercing, suggesting a need to understand the failure criterion in terms of stress state.
  • Another participant emphasizes the importance of researching the modulus of low-density polyethylene (LDPE) but notes that this alone is insufficient due to the material's non-linear and inelastic behavior during large deformations.
  • A further contribution highlights the necessity of measuring strain and stress to determine the limits of the material's linear range, linking this to the contents of the bag being pierced.
  • One participant argues that the problem's complexity is underestimated, pointing out that the rheological behavior of the material requires more than just modulus and Poisson's ratio, and that the analysis would involve complicated mathematical modeling and potentially finite element software.
  • Another participant adds that factors such as mass per unit area of the bag, the velocity and shape of the penetrator, and friction must also be considered when analyzing the piercing process.

Areas of Agreement / Disagreement

Participants express differing views on the complexity of the problem and the factors that must be considered, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

Participants mention the need for experimental quantification of the material's rheological behavior and the challenges associated with analyzing deformation and stress under complex loading conditions, which remain unresolved in the discussion.

John Clement Husain
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I want to know an object's (in this case, the plastic) "tolerance" to being pierced until it gets pierced.

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What are your thoughts on this so far?
 
What's in the bag?
 
What is the failure criterion for the plastic in terms of the state of stress?
 
OP, you could research the modulus of LDPE.
 
osilmag said:
OP, you could research the modulus of LDPE.
Knowing the modulus is not good enough. LDPE behaves non-linearly and inelastically in large deformations, so the OP must be able to describe the large-deformation inelastic response of the material. In addition, knowing this response is not sufficient for defining the failure behavior of the material. That is an entirely different functionality involving the invariants of the stress tensor.
 
Right, and I think to do that he would have to measure the strain of the bag or the stress being applied to the bag in order to use the modulus. That would find the limit of its linear range. Which is why I asked the OP what was in the bag because that is applying stress to it.
 
osilmag said:
Right, and I think to do that he would have to measure the strain of the bag or the stress being applied to the bag in order to use the modulus. That would find the limit of its linear range. Which is why I asked the OP what was in the bag because that is applying stress to it.
In my judgment, you are grossly underestimating the complexity of this problem. This involves the large-deformational solid mechanics behavior of a highly non-linear material experiencing an extremely complex concentrated contact loading. The rheological behavior of the material must be described by much more than just a modulus, or even a modulus and Poisson's ratio, and the response will not even be purely elastic. Just the job of experimentally quantifying the rheological constitutive behavior of the material for an arbitrary general deformation (using simpler deformational kinematics) would be quite daunting. Even if this behavior were quantified experimentally using appropriate rheological constitutive equations, the analysis of the deformation and stresses in this very complicated loading problem would be mathematically complicated, and would require the use of finite element computational software to solve.
 
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What @Chestermiller said, plus the mass per unit area of the bag, the velocity of the penetrator, the shape of the penetrator, and friction between the penetrator and the bag. Think of poking a plastic bag with a pencil versus a high velocity bullet.
 
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