Science in progress part I: My experimental set up

Recently I made a post about the objectives of my research. In other words, talking about what I do in the abstract sense.  All well and good you might say, but what is it that I actually do all day? Today I thought I’d give a more concrete example of my work, by discussing the experiments I’m currently running.

At the moment I’m performing a set of so-called double cantilever beam (DCB) tests. A regular cantilever beam is just a beam that is completely fixed on one end, but free to move on the other end; like the one in the picture below.

Schematic example of a cantilever beam.
Schematic example of a cantilever beam.
Image credit: Wikimedia Commons / Vladsinger.
Licensed under Creative Commons Attribution-Share Alike 3.0 Unported.
http://creativecommons.org/licenses/by-sa/3.0/deed.en

To make a double cantilever beam specimen, you stick together two pieces of material. Then you pull them apart, pulling perpendicular to the interface between the two pieces, creating two ‘arms’. In my case I’m using two aluminium alloy beams stuck together with an epoxy resin. As you can see in the picture I attach a hinge block to each side of the specimen. Then I attach a grip plate to each hinge block. This grip plate fits into the grips of a testing machine, allowing me to apply force to the specimen.

A close-up of a DCB specimen in the test machine
A close-up of a DCB specimen in the test machine

To help matters along a bit I didn’t glue the entire length of the beams, but I inserted some Teflon tape in order to make sure a short length of the beams wasn’t stuck together. In effect you then have two beams that are free to move at one end (the end you are pulling on), and are fixed at the other (the end where they are stuck together). In other words, you have two cantilever beams. Hence the name: double cantilever beam.

Overview of my test set-up showing the test machine, a camera, and a lamp.
Overview of my test set-up showing the test machine, a camera, and a lamp.

Once the specimen is installed in the testing machine it’s time to start running the experiment! I’m doing what are called displacement controlled fatigue crack growth tests. That means that I’m commanding the machine to move (displace) the two beams apart a set distance, then bring them not-quite back together and then pull them apart again. This is repeated five times a second, until I have enough data. Generally that is after about 200,000-300,000 cycles, which is equivalent to one to two and a half days of continuous running.

Remember that we had a portion of the specimen where the two arms are not bonded together? That forms a kind of initial ‘pre-crack’. All the pulling and pushing (well, pulling mainly) of the fatigue machine will cause an actual crack to slowly start growing in the adhesive; beginning at the pre-crack. Each time there is a new fatigue cycle, the crack will grow a little further. What I’m trying to measure in these experiments is how fast the crack will grow.

Fortunately I don’t have to sit next to my specimen with a pen, a notebook, and a ruler all day. Instead I have a camera set up next to the specimen (you can see it in the picture). Every 1,000 cycles the test machine will go to the maximum displacement I’ve set for the test. It will hold the specimen there for a short while. Then the camera will take a picture like the one below. After that the load cycles will continue again.

Example picture used to measure crack length.
Example picture used to measure crack length.

Those of you who are good at mental arithmetic will have realised I have about 200 to 300 of these pictures per specimen. Fortunately I don’t have to go through those by hand. Curious what I do do? Stay tuned for part II of this series…

(hint: it involves something Instagram is famous for)

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