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I had an idea to teach my son (almost 4) about bridges and large structures from an engineering point of view. I thought that it would be cool to build a large bridge and put a strain gauge on it. One of those foil like strain gauges. Then I would drive some heavy LEGO cars filled w/ pennies over it and we could watch the resistance change on the strain gauge.

My question is, would it be possible to get some meaningful results out of this? I see a number of problems.

1) I've never used a strain gauge before, and I'm wondering if the pulling apart of the bridge due to the various forces is an appropriate use case for a real strain gauge. The deformations would get rather large quickly since we're working with bricks and could possibly render the strain gauge useless.

2) The introduction of the strain gauge itself, glued onto bricks radically would change the way that the forces are distributed. The strain gauge now acts as a physical support holding two bricks together.

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    Why not use a pressure sensitive adhesive such as Blu-tack instead of glue? That way you can still re-use your bricks and strain gauge. – Ambo100 Oct 7 '14 at 19:00
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Lego bricks can withstand a fairly high pressure before they deform, but Lego creations tend to deform more easily at the various connection points. Therefore it's possible to build a relatively easily deformed bridge using long pieces, such as technic beams. You could build a bridge that deforms easily and then show how it flexes under the various weights.

As for using a real strain gauge, that might be overkill, but if your bridge is fairly bendy you could probably compensate for whatever structural rigidity you'll get from the guage itself. Also I'd go with putting the gauge on a separate, flexible piece (such as paper or plastic) and just pin it to the bridge using normal lego parts. Presumably you don't need super-accurate measurements for this demo?

Lego also has some springs and shock absorbers which can possibly be used to help with your design. Either use them to make the bridge flexible, or use the compression of the shock absorber as your measurement of how deformed the bridge is.

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    A pure mechanical strain gauge seems more in the spirit of Lego. And more likely to work and be robust. I suspect you could make a simple "spring balance" type gauge quite easily using a rack piece and an 8T gear, with either rubber bands or suspension print parts. Add a cardboard circle as backing and "calibration" just consists of adding weights and marking points on the dial. – Móż Oct 6 '14 at 21:31
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I have designed some special measuring tooling for my previous employer that uses strain gauges, but is is not always that easy. You need for sure:

  • an amplifier,
  • glue them on the construction in the correct orientation
  • 2 strain gauges on both sides for temperture compesation
  • a way to calibrate the measuring device or construction

this resulted for us that we most of the time we were using commercial available tooling like load cells and only make our own if commercial was not an option.

I don't know what will happen if you glue them over the joint between 2 brick, but the measurement will not correct. These strain gauges have a liniear behavior when glued to a surface. When we had a strain gauge that was not correctly glued or got loose we noticed that in the measurents.

I thing I was wondering about, for who are you planning to do these measurement, your 4 year old son, or for your self. I think if its for your son, that he is to young to fully understand this, for the effort you need to get it working.

If its for your son, I would just let the brigde break, when the load is to high. If you want to visualize the behavior you can also use 1 of the bridge construction games that are available for example on Steam. These show with colors the stresses in the different beams.

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    I know that he's young, however even if he misses the point, it'll still be worth it since it's something we'll do together. He thinks that "inflation" is a bad guy we need to put in jail :). – mj_ Oct 5 '14 at 19:40

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