Do I Need to Remove Anodizing From Aluminum Prior to Bonding a Strain Gage?

A customer recently asked the following question:

Can you tell me if strain gages can be installed on anodized surfaces using M-Bond 200?  I’m concerned about aluminum anodized and sealed per MIL-A-8625, both Type II and Type III.  What surface prep is required before bonding the strain gages, and are the steps given in B-127-14 still applicable?

Adhesion of the strain gage to an anodized surface is no problem, it is strain transfer through anodizing that is questionable.  Ideally, you want the deformation that is occurring in the structural aluminum to be duplicated through the anodized coating and into the strain gage so that the strain gage is deformed identically to the aluminum.  Clear anodizing is considered okay in the elastic region of aluminum, but black and colored anodizing may detach on a microscopic level and cause loss of strain transfer.  The same thing can happen to clear anodized surfaces during plastic deformation.

Since the anodized coating is not considered to add strength, strain measurement on the aluminum is the most reliable.

Given that you have a clear anodized coating, which is considered reliable in the elastic region, abrading the surface sufficient to bond the strain gage is okay.

Per Bulletin B-129-8, “Surface Preparation for Strain Gage Bonding”, page 5 of 7, my recommendation is the following steps:

1.       Strip the sealer at the gage location.

2.       Degrease using CSM degreaser.

3.       Abrade using 400-grit silicon carbide paper and Conditioner A as the wetting agent, wipe dry with gauze pads.

4.       Complete the burnish marks for gage alignment on the aluminum, if required.

5.       Scrub with Conditioner A and a cotton-tipped applicator, wipe dry with gauze pads.

6.       Scrub with Neutralizer 5a and a cotton-tipped applicator, wipe dry with gauze pads.

The surface is then ready for gage bonding.  The abrasive step (step 3 above) will remove the anodized surface treatment.