Abstract
Viscoelastic materials can be represented by models consisting of springs and dashpots because the material shows elastic and viscous effects. The two simple models that are commonly used to relate stress and strain are the Maxwell (relaxation) and the Kelvin model (creep). These 2-parameter models are available in commercial FE-codes. The measured relation between stress and strain is generally more complex. Thus combinations or modifications of these models are necessary. An appropriate 3-parameter model (Maxwell/Thomson) was programmed using FORTRAN. To use this model within the MARC FE-program the option user subroutine (HYPELA) was chosen. The parameters of the model are mathematically described as a function of the strain rate. The viscoelastic model is combined with a plastic element which is in active for stress below the actual yield stress of the material. In a first approach the von Mises yield condition and an isotropic “hardening” rule were applied. To consider the flexural recovery of polyoxymethylene (POM) the parameters of the model were also be varied. This model was verified by comparing measured and simulated results of simple mechanical tests and the snap-in and snap-out forces of a ball snap-fit. The modified material model can simulate the main effects of non-linear viscoelasticity and plasticity of POM.
Published Version
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