Behavior and modeling of injection molded PP
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 models
ability to describe the behavior of an injection molded polypropylene, provided by TOYOTA, has
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 its 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 materials 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
Experimental tests were also performed to determine how the materials molding thickness would
affect its 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 materials molding thickness. The tests suggested that the difference was larger for
the materials skin than for its core, which may be due to the distribution of particles.
To evaluate SIMLab polymer models ability to describe the materials 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 its 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 SIMLabs polymer model could predict the materials
behavior well, but that improving the formulation of the plastic flow will yield better results. The
current formulation is not able to capture the materials transverse behavior and the materials
behavior in compression.
Place, publisher, year, edition, pages
Institutt for konstruksjonsteknikk , 2014. , 347 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-24512Local ID: ntnudaim:10475OAI: oai:DiVA.org:ntnu-24512DiVA: diva2:712626
Clausen, Arild Holm, ProfessorMorin, DavidMottola, ErnestoNgueveu, Yann Claude