# What is the physics behind piercing a plastic piece?

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

#### Attachments

• maxresdefault.jpg
28.8 KB · Views: 653

Chestermiller
Mentor
What are your thoughts on this so far?

osilmag
Gold Member
What's in the bag?

Chestermiller
Mentor
What is the failure criterion for the plastic in terms of the state of stress?

osilmag
Gold Member
OP, you could research the modulus of LDPE.

Chestermiller
Mentor
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.

osilmag
Gold Member
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.

Chestermiller
Mentor
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.

Monsterboy and jrmichler
jrmichler
Mentor
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.

Chestermiller