Strain rate-dependent mechanical properties of high-density polyethylene(HDPE)

Sammanfattning: In today’s packaging industry HDPE is widely used and correct understanding of itsproperties and how to model them is of vital importance. HDPE is a semi-crystallinepolymer with a known strain rate dependence, that is a higher yield and lower strainto failure at higher strain rates. HDPE does also exhibit the phenomena of cold-drawing, together with other polymers. Cold-drawing is where after the specimenhas necked, the necking stabilizes and starts to pull material above and below intothe neck, effectively elongating the neck while maintaining its width. The objective of the study is to look at the local strain rates as the specimen necksand if a simple Abaqus model can capture those effects. The effect of strain rate onthe shape of the neck was also studied. The work was to test HDPE in uniaxial tension with different strain rates (∼10-3 s-1to ∼10-1 s-1) and measure the local strain rates with 2D-DIC. A decent amount oftime was used to make sure the camera setup gave the best quality possible for theequipment available. The videos produced was used for the DIC analysis as well asfor the image analysis to measure the width of the neck. After the tests a calibrationscheme was used to create a material model that matched the force-displacementfrom the physical 100 mm/min test data. Studying the force displacement the strain rate effects noted in previous researchare present. The results from the DIC show a very high local strain rate as the spec-imen necks, between 11-65 times higher than the global (grip-to-grip) strain rate.From the measurement of the width there are some rate effects as well. The slowerspeeds (5 and 10 mm/min) shows a continually reducing width while the 50 and 100mm/min shows a more stable neck and the 500 mm/min test does not have any sig-nificant neck propagation. The simple elastic-plastic model show similar local strainrates as the experiment however does show a noticeable thinner neck.