Simmulating pullout forces in particleboard

Detta är en Master-uppsats från Linköpings universitet/Mekanik och hållfasthetslära; Linköpings universitet/Mekanik och hållfasthetslära

Sammanfattning: Pull-out forces of screws and connectors in particleboard are particularly important when designing for strength and, or assembly in the furniture industry. The aim of this thesis was to develop a simulation model of particleboard that can predict pull-out forces of screws better than the currently used simulation model at IKEA of Sweden. The developed model was validated against experimental results from pull-out tests carried out by IKEA. Material properties for particleboard are determined from experiments previously carried out at IKEA test labs unless stated otherwise in the report. The authors did not carry out any tests. In order to simulate the pull-out forces of a screw, the finite element method was employed, simulation models from three different material models selected from the Finite Element Analysis (FEA) software LS-DYNA. The first material model considered Mat_143, a wood material model based on the Hashin failure criterion and used to develop simulation model 1. The second material considered, Mat_122 3D, based on Hill’s plasticity theory, was used with Mat_Add_Generalised_Damage to allow for incremental damage accumulation and failure. This was simulation model 2. The third one, Mat_221 which represents an orthotropic material with simplified damage, was used to develop simulation model 3. Experimental test results for tensile test, bending test, shear test and, finally, the screw pull-out test was used to validate the simulation models. Before the scale validation, a single hexahedral element was simulated to evaluate the accuracy of the simulation models and to get a better understanding of the limitations of the material models. A mesh type and convergence study was carried out where it was concluded that the first- and second-order hexahedral and tetrahedral elements could be employed for the full model simulations, giving a sufficiently accurate result, i.e. matching the experimental results by at least 86%. An element size range of 1-3 mm was enough for quasi-static load cases and while an element size range 0.15-3 mm was relevant for dynamic load cases. Model 1 was abandoned when it was discovered the material model was not suitable for predicting material behaviour other than that of wood for a large mesh. The remaining two simulation models were evaluated for tensile, shear and bending load cases. Simulation model 2 predicted both tensile and bending forces with an accuracy of 99% or higher, and predicted shear forces with an accuracy of 86%. Simulation model 3 predicts both tensile and shear forces with an accuracy of 98% or higher and predicted bending forces with an accuracy of 86%. For the screw pull-out test, simulation models 2 and 3 under-predicted the pull-out forces significantly due to premature shear failure.iiiThe currently used particleboard simulation model is referred to as simulation model 4 throughout the report. Simulation model 4 was the most accurate simulation model for predicting pull-out forces in particleboard.

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