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 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!
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.
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).
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.
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.
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.
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?