Ductile and brittle failure behaviors of semi-crystalline polymers such as high density polyethylene (HDPE) were investigated under uni-axial tensile loading at various test conditions.

The first part of the thesis examines the influence of aspect ratio of rectangular cross-section on the tri-axial stress state developed by necking in tensile specimens of HDPE. The approach included both experimental and numerical simulation, and identified that anisotropy is involved in the deformation process during necking. The study shows that with increasing aspect ratio anisotropy in the deformation process increases.

A new phenomenological model is then developed in an endeavor to reproduce the experimental work to identify the tri-axial stress state during the large deformation. The innovation lies in the unique technique used to develop the simulation model. The study clearly shows the advantages of this model over previously developed models by other researchers in this field. The results show that the technique is capable of considering non-linear and creep deformation during the necking and has the potential to mimic accurately the stress-strain relationship along with lateral dimensional deformation obtained from the experimental testing.

Deformation of HDPE in tension was also analyzed at various crosshead speeds, to quantify the corresponding strain rate and strain rate variation during the necking process. The study clearly states with evidence that the common practice to evaluate the strain rate effect based on the measured total strain is acceptable in spite of the involvement of the creep strain.

Another new test methodology developed by using cylindrical specimens with a gauge section design to generate and evaluate bulk cavitation in HDPE. This design is unique, as it has the potential of generating bulk cavitation without the presence of any sharp notch, whereas all the works in literature to study bulk cavitation in polymers involve sharp notch and crack growth. Since no sharp notch is used in the new specimen design, its deformation does not involve crack growth, and therefore, is purely governed by the cavitation-induced rupture process. The FEM results indicate that hydrostatic stress level for bulk cavitation is about three times of that for necking at the same strain level. As a result, the new specimen design has bulk cavitation replace necking as the dominant deformation mechanism in HDPE. The thesis shows that by changing the gauge section geometry, deformation of HDPE specimen under tension can be dominated by either bulk cavitation or the commonly observed necking. The FE results also suggest that the hydrostatic stress at the peak load needs to reach a level similar to the uniaxial yield strength in order to replace the necking by the bulk cavitation.