Strain Gauges

[midsidenode]

Can anyone provide a recommendation as to where one could send a young engineer to learn the practical use of Strain Gauges? Specifically, we'd like him to learn the proper way to install and use strain gauges in order to obtain material allowables on composite samples, e.g. ASTM D 3039, ASTM D 5379, ASTM D 638 etc.

thanks
msn

 

[adfergusson]

Is there a local University with a composites research group? If so, your best bet might be to find a willing PhD student or technician there and slip them some cash to spend some time going through how its done.

Strain gauging composites is pretty similar to doing it on other materials; main differences will be the adhesives you use, the solvent you use to degrease and the fact that it is a bit easier than doing the same for metals as the heat from your soldering iron won't just flow into the bulk material (although a high powered electric or mini butane gas soldering iron is a nice thing to have).

There are videos on youtube showing how to apply strain gauges, e.g.

https://www.youtube.com/watch?v=EefxhKmJCy0

which should get you going on bonding strain gauges. The really fiddly bit is then soldering the wires that go to your signal conditioner/DAQ to the the terminal pads; after you've already soldered the connection from these pads to your strain gauges, one wire can easily come loose when you're soldering a wire to the DAQ to the same pad. Different people have different methods for dealing with this. One of my preferred methods when doing small test coupons is to clamp a coin over the wires on one side of the terminal pad, to act as a local heat sink and keep the wires in place, while I solder the other side (this is where the high power soldering iron is a nice thing to have).

As a final note, I'd suggest that way you end tab your samples and the type of end tab/adhesive bond you use can play a very large part in how your samples fail and is worth some serious consideration, especially if you're going to be testing composites with NCF or woven fibre reinforcement. The ASTM standards were largely developed for UD prepreg composites; getting valid failures for other types of composites presents more problems that are not addressed in these methods (which partially explains the massive differences you some times see in results reported in by different research groups, even in well regarded publications).