# What is the physics behind piercing a plastic piece?

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• John Clement Husain
In summary, the conversation discusses the complexity of determining the failure criterion for plastic material when subjected to piercing. This involves understanding the material's modulus and inelastic response, as well as experimental measurements of strain and stress. Additionally, factors such as the mass, velocity, shape, and friction of the penetrating object must also be taken into account. The use of finite element computational software may be necessary to solve this complicated problem.

#### John Clement Husain

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.

Monsterboy and jrmichler
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

## 1. What are the different types of forces involved in piercing a plastic piece?

When piercing a plastic piece, there are two main types of forces involved: compressive forces and shear forces. Compressive forces are responsible for pushing the piercing tool into the plastic, while shear forces are responsible for cutting through the plastic material.

## 2. How does the thickness of the plastic piece affect the piercing process?

The thickness of the plastic piece plays a significant role in the piercing process. Thicker plastic pieces require more force to pierce through, and the piercing tool may need to be sharper to achieve the desired result. Additionally, thicker plastic pieces may also require more heat to soften the material, making it easier to pierce through.

## 3. What is the role of temperature in piercing a plastic piece?

Temperature can greatly impact the physics behind piercing a plastic piece. Heating the plastic can make it more malleable, reducing the amount of force needed to pierce through it. On the other hand, cooling the plastic can make it more brittle, making it easier to fracture or break during the piercing process.

## 4. How does the shape of the piercing tool affect the physics behind piercing a plastic piece?

The shape of the piercing tool can greatly impact the physics behind piercing a plastic piece. A sharper and narrower tool will require less force to pierce through the plastic, while a duller and wider tool may require more force. Additionally, the shape of the tool can also affect the direction and angle at which the force is applied, which can impact the success of the piercing process.

## 5. What other factors should be considered when piercing a plastic piece?

In addition to the above factors, it is important to consider the type and properties of the plastic being pierced. For example, some plastics may be more brittle or harder to pierce through than others. It is also important to consider the size and location of the piercing, as well as the overall design and structure of the plastic piece.