Validation of An Iteration Free Material Model for Paperboard (NEO)

Abstract: The NEO model, developed by Lindström (2019) for paperboard, is a non-linear material model that utilizes empirically based constitutive relations with switches to mimic elastic-plastic responses. Due to the explicit definition of the constitutive relations, the NEO model does not require a constitutive equilibrium loop. This, in addition to modeling flexibility from the switches, poses the NEO model as more efficient and flexible than traditional material models. However, the validity of this model is unproven. This is the primary motivation for this thesis project which aims to determine the validity and scope of use of the NEO model for in-plane elastic-plastic responses. This was achieved by implementing a version of the NEO model in LS-Dyna®and comparing it with conventional models developed by Xia (2002) (XIA) and Borgqvist et al. (2014) (EBT), and experimental results collected by Alzweighi et al. (2022). Both uniaxial and biaxial responses for various material directions were extensively investigated. All models were calibrated for a 260mN paperboard material. The loading conditions investigated aimed to replicate the tests from Alzweighi et al. (2022) to control for geometric and calibration differences between the simulated responses and experimental responses. In general, the results from all simulations were promising. The NEO model showed a strong correlation with experimental results for uniaxial responses; how- ever, during the biaxial tests Alzweighi et al. (2022) experienced significant slippage, which limited the comparisons between the biaxially loaded NEO simulations and experimental results. For all biaxial loading regimes, the NEO model did show a correlation with the EBT model and differences between the two models could be attributed to calibration. It is therefore concluded that, given appropriate calibration, the NEO model and EBT models are somewhat equivalent for plane-strain in-plane loading conditions. Furthermore, loading in the off-axis material direction showed little difference from loading in the calibration direction (11, 22 and 12 shear), suggesting that the NEO model is valid for general loading cases and the material direction has little influence on the performance of the model. All differences between the NEO model and the EBT and XIA models could be attributed to calibration, highlighting the NEO model’s sensitivity to calibration. Further limitations of the model stem from assumptions about the expansion of the yield surface; however, this was not seen to have a major effect for the loading regimes investigated. These assumptions will become invalid for highly directionally coupled materials, limiting the NEO model to orthotropic materials such as paperboard. Due to time constraints, the computational performance of the model was not investigated. As this is theoretically a key advantage of the NEO model, it is highly recommended that further work on this model focus on these areas.