Real life seemingly not matching material properties 1

[Marmight]

I was doing some informal "testing" of two material samples. They were material sample cards from protolabs, one of Delrin homopolyer and one of Valox 420 PBT with 30% glass fill. Each card is about 3" x 6" and 1/8" thick.

The Delrin has a rated yield strength of 71MPa on prospector, and some other sources listing a break strength of ~90ish MPa. The Valox with 30 glass is listed as having a yield strength of 120MPa.

With that being said, in my informal desk side strength test, I was able to break the Valox card one handed bending it with my thumb on the desk. (fingers on top edge, thumb pressing in the middle and bottom edge on the desk)

With the delrin card, I was unable to make the card break with this method. Nor with that method two handed. Indeed it wasn't until I brought it close to my chest and compressed it between my palms with lots of leverage was I able to make it break.

To me this doesn't fit the story told by the material properties sheets.

What am I missing?

 

[rb1957]

"I had a certain expectation based on my understanding of the theory.
I observed a different outcome than expected in a rough, poorly controlled test.
I am seeking to rectify the discrepancy in my understanding of the theory."

i'm guessing your expectation was that the delrin should've failed first (as it is weaker); a not unreasonable expectation.

in the light of the results, i suggest that you need to refine your expectation to investigate the stresses in the test samples. I'd hope that the PBT is failing at it's spec stress, and that the delrin was over-strength. It is of course possible that the stress concentration of the lettering and the ribs is creating a hot spot in the PBT, particularly if it is more brittle than the delrin.

i was looking at your post of stress/strain curves and assumed that you were showing both materials (to have similar E).

another day in paradise, or is paradise one day closer ?