2 During a tensile test of a polyethylene dumbbell sample sa

2. During a tensile test of a polyethylene “dumbbell” sample (same shape that you tested in lab) at room temperature using a strain rate of 10 mm/min, at what position relative to the sample is failure most likely to occur?

a) Exactly at the mid-point between the jaws of the tensile tester

b) Exactly in the middle of the neck

c) Equally likely at either end of the dumbbell, where the neck meets the amorphous region

d) At one end of the dumbbell, furthest from the point at which the necking originated

Solution

ans D

The polymer chains in the neck are highly aligned, and material is drawn out from the amorphous regions at either end of the dumbbell (notice that the polymer in the neck itself does not stretch, but the neck simply extends as more amorphous material is aligned). The end furthest from the origin point of the neck contains the most amorphous material, and therefore will therefore extend until no more material can be draw out, at which point brittle failure occurs at the junction between the neck and the remaining material around the jaws.