Finite Element Analysis for Plastics and Rubber Components
Posted by Bhaumik Dave on February 3rd, 2015
The use of linear analysis is limited for materials that do not undergo large deformations and retain their original shape and size, once the loads are removed. However, in practicality, every material available in nature possesses some sort of non-linear behavior. As such, it is very much obvious to choose non-linear analysis to ensure accuracy in results.
The need of non-linear analysis is even more prominent when dealing with highly deformable products such as rubbers and plastics. The use of rubbers and plastics is extensive in various applications; some of the common examples include seals, diaphragms, tubing, mounts and many more.
Compared to metals, FEA for plastics and rubber materials is much more complex as it requires capable software and a good understanding about the material behavior and testing requirements. Usually, a metal or a plastic is termed as linear when the stress-strain relationship follows Hooke’s Law, which states that the stress and strain remain proportional to each other within the elastic limit.
However, beyond the elastic range, the metals and plastics are known to behave non-linear, referred as plasticity. However, the non-linearity of rubber is different from metals and plastics, which requires the use of special non-linear material models while performing Finite Element Analysis.
Commonly used nonlinear material models in FEA are elasto-plastic for metals and plastics, while hyperelastic models are usually utilized for rubber. Rubber being highly stretchable, flexible and resilient, its properties are highly distinguishable from other materials. However, its properties also vary dramatically with varying temperature.
High temperature making rubber to reach a glassy state, its resilience is lost significantly. As such, in FEA the rubber must be modeled as a non linear and pure elastic hyperelastic material. Most commonly used models are Mooney-Rivlin or Ogden formulations and Hyperelastic Blatz-K formulations (for compressible polyurethane foam type rubbers).
To perform nonlinear analysis, it is also required to know the plastic stress-strain curve, which can be determined experimentally. However, the accuracy of FEA results for highly nonlinear materials such as plastics and rubbers is largely dependent on material modeling. This is due to the fact that rubber modeling involves number of assumptions made with material modeling, testing and constant determination.
Moreover, there is a high possibility of change in mechanical and physical properties of rubber from batch-to-batch. As such, FEA alone cannot be used to produce accurate results. Comparing the FEA results with experimental tests and then improving the material model further can help in improving the accuracy of FEA results.
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