Low-T, high-P rheology of a Nalgene water bottle

Callan’s post on the rheology of an overheated water bottle reminded me of a little experiment from before I started this blog. I had not thought to share the results with y’all until now!

This post in a nutshell: undeformed and deformed Nalgene water bottles

This is, of course, all John’s idea. “John” is in the engineering side of our department, and he does a lot of work with concrete and steel to accomplish undoubtedly ingenious engineering…schemes. One piece to the puzzle lies in designing buildings and bridges that do not break or fall down at the drop of a hat. This involves much testing of tension and compression yield strengths of industrial materials; tests which utilize the awesomely named universal testing machine (UTM) and other implements of destruction. Our UTM can apply a force up to 600,000 pounds (300 tons; about the weight of a Boeing 747).

The UTM applies an incrementally increasing force to an object, and real-time sensors record the load pressure. When a material fails (i.e., when the load equals yield strength), it drops its load *ahem* and the resistance pressure decreases. For example, braided steel cables snap in twain when they reach yield strength. Snapping steel cables is a fine demonstration of brittle deformation, so enjoy this video of a UTM in action (not of our machine):

It looks dangerous – and thus fun – but one day John found himself with dozens of steel cables to destroy. When he needed a breather, I suggested we test something about which he had professed a curiosity: How would a Nalgene water bottle fare under extreme pressure? Would it break brittly like the steel cables, or would it behave…well, plastically? Nalgenes are, after all, considered unbreakable. John and I figured it would crack up at some point, but from the photo above you can see the results were not what we expected!

Positioning the Nalgene for compression testing. The UTM is normally used on much larger structures, and so we had to prop it up with some wood blocks and steel plates. John has several years experience with the UTM and is fully trained in its safety and usage limitations. My contributions were limited to providing the water bottle, and documenting and sharing the test.

We used a 1-liter Nalgene water bottle from my undergrad years, since replaced by a smaller stainless steel model. The UTM is capable of testing compression or tension at multiple angles; limited to a single test subject, John made the executive decision to keep it simple and test vertical compression.

(Left) Nalgene after main body failure, and (Right) my sketch of time-resolved applied load.

The Nalgene experienced two load failures, appearing as peaks in the time-resolved graph above. The first load failure at ~700 lbs reflects the failure of the weaker threaded top, which compressed straight downward into the main body. The main body of the Nalgene then held out to a load of ~1400 lbs (0.7 tons).

The top failure compressed inward and resulted in a pentagon. The bottom is triangular. Weird?

What does it mean? Well, you could probably support an average sized car with these Nalgenes (one under each tire), although the tops would likely fail. You, certainly, should have no problems standing on one of these – balancing is another matter. But keep in mind the load was applied over a relatively long time (2-5 minutes, I think). The results would probably be different if you suddenly smacked a Nalgene with 700 lbs of force! That is the difference between load (pressure exerted on object) and impulse (pressure exerted on object over a certain time interval).

The flattening dashed line at the right of the graph reflects when the Nalgene was nearing maximum compression (or minimum volume). At this point, the UTM was essentially measuring the strength of the plastic instead of the structure EDIT: After the bottle structure had failed, further compression reached a point where we began plastic-on-plastic compression and we stopped the test. (Thanks to jaspevacek for pointing out sloppy language.) We heard crackling from the water bottle as we approached max compression. A water test revealed a slow-dripping leak, but the source is hidden behind the folds of a sticker.

Nalgene back-lit to highlight en echelon cracks (center). Force was exerted vertically relative to picture.

There are some areas with sequences of minor cracks that do not leak. To me, they have the appearance of en echelon fractures, which are tensional (pull-apart) features. The fractures point in the relative direction of stress, so the plastic above the cracks was being pulled to the right relative to the area below (see annotated version below). This is because the outside of a curve experiences tension while the inside compresses.

Markup of previous photo showing in red the tensile stresses. The net relative movement of plastic over the cracked area is indicated in orange.

And here are a couple photos of sticker deformation if you care to compare with Callan’s. Compression was vertical in both photos.

The Nalgene compressed from its original height of ~20cm to ~10cm, but we can see (below) that we compressed it down to probably ~5cm. Also note that at maximum compression the E and O were essentially touching. Now the letters are separated by ~3cm. The rebound is a partially elastic response after compression ended.

Nalgene at maximum compression compared with current state. Difference between E and O at max is ~0cm, now it is ~3cm.

In this simple experiment we have evidence for plastic (irreversible) and elastic (reversible) deformation as a result of compression. Brittle deformation is also recorded in the fractures from both tension (en echelon) and compression (the hidden leak).

I recently unearthed another 1-liter Nalgene of the same make, so maybe someday we will get around to testing horizontal rheology. I would also love to stretch a Nalgene, but I am not sure we would be able to perform tension tests on something this small and awkwardly shaped compared with the machine. What do you think? Any suggestions for future tests? Or have you performed a time-killing experiments with old water bottles or other field equipment?


7 thoughts on “Low-T, high-P rheology of a Nalgene water bottle

  1. “At this point, the UTM was essentially measuring the strength of the plastic instead of the structure.” ???

    I would propose that you were measuring the resistance of a seriously deformed bottle and nothing more.

    Measuring the strength of the plastic means that you could back of this fundamental property. For your experiment, you would need extensive computer modeling to get that property, and I wouldn’t trust the results in this highly nonlinear regime without lots of experimental verification.

    And as you noted, rheology is very much frequency dependent.

    1. What? You’re saying my one data point isn’t good enough?! (Just kidding) I re-wrote that sentence a few times and looks like it still ended up sloppy.
      I meant that after the bottle structure had failed, further compression reached a point where it was compressing plastic on plastic. “Strength” implies too much here, so I’ll edit that sentence accordingly.

  2. Nice to see! Have this Nalgene be one of the new Tritan ones? Or still an old HDPE one?
    Thanks for testing

  3. Cool thread! Thx for the investigation. How does the modulus of elasticity of Nalgene (Tritan) compare with that of PET? You guys seemed really smart. I thought you might just know. The Nalgene brand seems popular for many applications. Dunno about this one: http://www.thegrowlerguys.com/phssssh/

    Yeah, I guess you’re right. In the final analysis, eventually, everything boils down to beer, huh?

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