Bonding a strain gage on glass is a challenge, since the surface is extremely smooth and does not provide a good anchor for the adhesive to grip on. Normally the surface of a test part is abraded to get the adhesive to bond well, but abrading on glass is often not permitted. To improve the chance of a good bond, the surface of glass is degreased and cleaned repeatedly several times to get it absolutely clean, and M-Bond AE-10 from Micro-Measurements, which has superior bonding strength, is used .
M-Bond AE-10 is a two-component adhesive of resin and curing agent. A measured quantity of curing agent is added to the resin and stirred slowly but thoroughly, for 5 minutes. The mixed adhesive is applied to both the gage and the test part, then the gage is pressed down on the test part with s a spring-loaded clamp, with a silicone gum pad (SGP-1) and an aluminium backing plate between the clamp and the gage to ensure uniform pressure over the strain gage.
The adhesive cures at room temperature over a period of 24 hours. The cure time can be reduced if the temperature of the adhesive is elevated slightly.
When attaching lead wires to the straingage, it is better to avoid heavy lead wires and to keep the solder joints as small as possible on the straingage solder tabs. This will reduce the possibility of strain on the solder joint in the event of differential movement of the lead wire near the joint, that could cause the gage to be detached from the specimen. The heavy lead wires are taken from bondable terminals adjacent to the straingage, and the interconnection between gage and bondable terminals is made with thin flexible insulated wires type 134-AWP from Micro -Measurements. These wires have polyurethane insulation, which can be removed easily by dipping the end of the wire in a pool of solder. The insulation melts away and the copper wire which is exposed gets tinned with solder at the same time. The tip temperature of the solder should be set higher to speed up this process, but care should be taken to reduce the tip temperature back to normal, before proceeding to solder the wires to the straingage and bondable terminals.
Thorough cleaning of the solder joints and straingage with RSK rosin solvent from Micro Measurements is done to remove all traces of flux .
A layer of protective coating, such as M-/coat A from Micro Measurements, on the gage , solder joints and adjacent lead wires is desirable to prevent the effects of environment on the straingage.