UNDER PRESSURE

All strain gage bonding adhesives have a “recommended” pressure to apply during adhesive curing. This can be as simple as “firm thumb pressure” for cyanoacrylate adhesives (M-Bond 200) to a specific 40-50 psi for M-Bond 610 on a transducer element. What is it that drives these recommendations?

M-Bond 200 is a thin-setting adhesive, meaning that it won’t cure properly- or at all- if no pressure is applied. Fortunately the cure only takes a minute and thumb pressure is readily available, convenient and as precise as needed.

For solvent thinned adhesives like M-Bond 600, 610, 43-B, and 450 the need for pressure during bonding is more complex.

1. The final adhesive thickness under the gage is controlled by clamping pressure during adhesive cure. Too little clamping will result in an adhesive layer that is thicker than optimum and this will increase errors due to adhesive creep. Too much clamping and the adhesive could be too thin and may not even hold the strain gage in place, let alone work well. Transducer manufacturers regularly monitor clamping pressure during cure to keep it close to the desired target.
2. For general purpose stress analysis use the adhesive layer thickness is not as critical as transducers so a wider clamping range can be used. 10-70 psi is recommended. The lower end is particularly helpful for vacuum pad clamping applications.
3. Solvent thinned adhesives contain volatiles that must be liberated during curing. Without clamping pressure these volatiles would cause the adhesive to “boil” during heating and this could result in bubbles under the installed strain gage.
4. Clamping under pressure and curing the adhesive at elevated temperatures can result in locked-in (residual) stress in the adhesive bond-line. These residuals will gradually relax over time and this will result in small shifts in gage resistance as well. To reduce these residuals it is always recommended to “post-cure” the gage installations to accelerate the relaxation. Generally 2 hours, unclamped, at a temperature 50 °F above the curing temperature is recommended. Post curing is not required for good adhesive performance. Any application where rebalancing the zero can be done easily would not have to be post-cured.

Click here to access Micro-Measurements Strain Gage Installation Accessories – Adhesives.