Strain Gaging Carbon Fiber Shaft

We received  several questions from a customer who wants to gage a carbon fiber shaft and measure the torque.

Will strain gages work for this application? If so, what do you recommend?

Yes, you can use strain gages on carbon fiber to measurements of load and torque.  You may find that the composite will show more non-zero return and creep but that is the nature of testing fiber reinforced materials when compared to metals such as steel or aluminum.  Our CEA-06-187UV-350 gages would be a good choice or the standard CEA-06-250US-350 if there is no bending strain expected. See sketch below for 187UV wiring.

Can a gage on a carbon fiber shaft be coated M-Coat JA? If not, what coating would you recommend?

Yes.  Assuming there is no mold release agent on the composite, the M-Coat JA should bond quite well to the surface.  Make sure that as your are prepping the surface that you clean enough area to include the environmental coating. 

You might also consider using an epoxy coating such as the M-Bond AE-10 if the shaft is spinning at a high speed. 

Tutorial: How to Apply a Strain Gage to Steel Rebar using M-Coat J (M-Coat JA) (Video)

What other considerations should we have for this application?

When applying strain gages to composites, we normally recommend the following:

a.         Keep the resistance at least 350 ohms to reduce the current and therefor heating effect from the excitation

b.         Use larger size gages (>0.125 inch active length) for better heat dissipation and averaging

c.         Use a two part epoxy such as the M-Bond AE-10 for bonding; AE-10 is a better filler for what is sometimes a textured or rough surface

d.         Preattach the leadwires on  the strain gages to avoid damage to the adhesive layer or the strain gage since the composite insulates rather than conducts heat away during soldering.  This is particularly important if you have used M-Bond 200 to bond the strain gages. 

Strain Gage Installation On A Shaft For Torque Measurement (Video)

Measuring Torque On A Shaft - Theory Of Operation (Podcast)

Strain Gage for Shear Strain and Torque (Landing Page)