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Behavior and modeling of injection molded PP
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Structural Engineering.
2014 (English)MasteroppgaveStudent thesis
Abstract [en]

The structural impact laboratory (SIMLab) at NTNU have developed a constitutive material model to represent the behavior of brittle and ductile polymeric materials. The material model can be implemented into finite element solvers, e.g. LS-Dyna and Abaqus. In this thesis, the material model’s ability to describe the behavior of an injection molded polypropylene, provided by TOYOTA, has been studied. The material had during previous experimental tests shown an inhomogeneous material behavior. As the material was loaded in uniaxial tensile, the core would fracture prior to it’s outer section. This ”skin-core” behavior has been observed for many injection molded polypropylenes, but the effect varies substantially due to differences in the molding process and the material additives. To determine how the material parameters would vary through the material’s cross section, the ”skincore” effect had to be better understood. To be able to separate the core from the skin, experimental tests were conducted using specimens with and without a reduction in its section thickness. The difference between the material parameters of the specimens with a full thickness and with a reduced thickness, was suggested to be the material parameters of the skin. The experimental tests showed that the main difference was linked to the specimens’ yield stress and damage propagation. The specimens with a full section thickness had a higher yield stress and a lower damage propagation than the specimens with a reduced sections thickness. Experimental tests were also performed to determine how the material’s molding thickness would affect it’s material properties. Three different molding thicknesses were used; 2mm, 3mm and 4mm. The tests indicated that the molding thickness mainly affected the damage propagation and yield stress of the material, the damage propagation increased and the yield stress decreased with an increase in the material’s molding thickness. The tests suggested that the difference was larger for the material’s skin than for it’s core, which may be due to the distribution of particles. To evaluate SIMLab polymer model’s ability to describe the material’s behavior, numerical simulations of the experimental tests were conducted. The numerical simulations ability to reproduce the behavior of the experimental tests would be used to determine the accuracy of the material model. The numerical models were composed of two sections; one for the core and one for the skin. Where the behavior of each section was controlled by it’s own material card. The thickness of the skin was determined to be 0.2mm, based on visual evidence using an optical microscope. Drop-tower tests and tensile loaded notched specimens were used for validation of the material model. The simulation results, indicated that SIMLab’s polymer model could predict the material’s behavior well, but that improving the formulation of the plastic flow will yield better results. The current formulation is not able to capture the material’s transverse behavior and the material’s behavior in compression.

Place, publisher, year, edition, pages
Institutt for konstruksjonsteknikk , 2014. , 347 p.
URN: urn:nbn:no:ntnu:diva-24512Local ID: ntnudaim:10475OAI: diva2:712626
Available from: 2014-04-15 Created: 2014-04-15 Last updated: 2014-04-15Bibliographically approved

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