Linear analysis for plastic component - Below yield point 3

[ninjaz]

Will it be okay to perform a linear dynamic analysis for a plastic part?

The basic function of the plastic part is to support a weight in running vehicle. And the idea is to analyze the component within the yield point considering the analysis as linear. Is it okay to judge the part based on the stress values if it is under yield stress?

 

[johnbridge231]

Plastic is a non linear thing.
Linear dynamic is a linear thing.
Linear and non-linear things should not be mixed.


Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[rickfischer51]

Linear FEA can be and is used to design plastic parts, but you need to understand the true nature of what you are analyzing and think about the assumptions you would be making going in. Unfortunately, you dont really describe what you are doing. Some possible problem areas: How are you loading the model? Impact? Polymers generally get stiffer as the strain rate increases. If you use the data sheet value, you would be safe with regards to deflection. But elongation goes down as strain rate increases, so you better be well below yield. Cyclic? Fatigue of polymers is not straight forward. Cyclic loading produces heat which makes applying S/N data difficult. The type of loading (bending vs normal stress) can have a big influence on results. "Support a weight" sounds like a static component is present all the time, so you may need to consider creep. You can generate an isochronous stress strain curve from which you get an apparent modulus, but that can result in an overly conservative solution because the stress and creep will vary throughout the part, while your apparent modulus will be constant. And the apparent modulus will be lower than a data sheet Young's modulus, so you might get into static deflection issues. Then there is anisotropy, temperature effects, ageing effects, weld lines, etc, etc. You might want to read Structural Analysis of Thermoplastic Components by Trantina and Nimmer first. They deal with these and other issues. If you have access to nonlinear FEA, you should consider it.

 

[rb1957]

"Plastic is a non linear thing." ... I thought he meant "plastic" as a material rather than as a state of stress.

my 2c ... if you're below the proportional limit of the material than linear FEA should be ok. mind you, you'll have a very low gross section stress if you mean to avoid plasticity at geometric discontinuities.

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[inline6]

assuming plastic material (non the yielded kind) then also buckling could be a concern with low yound moduls

 

[ninjaz]

Thanks for your replies..

"Plastic is a non-linear thing" ..

In practical every material even steel is non linear thing beyond yield. Isn't it??

If we consider steel as linear material and used to do analysis well below the proportional limit , then why not plastic in the same way.??

 

[rb1957]

we're trying to clarify if you mean plastic material or plastic stress.

i think you're talking about using linear FEA in a linear stress domain, which is generally valid for any material.

it's been posted above that you need to be aware that not all failures are yielding ... crippling/buckling being an obvious one.

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