Young’s modulus is the linear relationship between stress and strain in the elastic region, which does not exist for plastic parts (since the relationship is nonlinear and it’s mostly nonelastic aka plastic).
Plastic parts most certainly do have both an elastic region and a Young's modulus. Young's modulus is the slope of the curve during elastic deformation (the form of deformation where the part returns to its original shape after the force is removed) and is a measure of a material's inherent stiffness. All solids have a Young's modulus. Take a piece of plastic and bend it a little, then let go. It is elastic. Plastic deformation is the state passed the yielding where the part will not return to its original shape. Plastic the material and plastic the form of deformation are independently named from the greek plastikos: able to be shaped or molded.
There are undoubtedly elastic regions of plastic material but there are very few plastics with a linear relationship of stress strain (aka Young’s modulus). This is why for most FEA, the stress strain ratio is a lookup table rather a constant for plastic material. In addition, some plastic exhibit a non-Newtonian property. Long story short , Young’s modulus is not applicable for plastic parts. Also, it is not true all solids must have a one.
You have absolutely no idea what you are talking about.
The elastic region (there can only be one) is the range where a material has a linear relationship of tensile stress and strain. Every plastic and every solid has such a region, it is part of the definition of a solid. Again, the Young's modulus is just the slope of the curve at zero, it is mathematically impossible not to have one. All but the most brittle of materials have non linear stress strain relationships, and for FEA all materials use a look up table instead of using a constant value, because that's the point of FEA. Non-newtonian behavior is completely unrelated, instead dealing with a material's stress and time relationship, and again is not exclusive to plastics.
Again, the terms plastic as in deformation and plastic as in the material are an etymological coincidence and don't have anything to do with one another.
Check data sheets for any 3d printing filament, Young’s modulus is the main characteristic in them and that's how you can reason about filament abilities.
