Strain Gage Sensors Always Keep Your Glass Half Full, Never Half Empty
Bonding Strain Gage Sensors to Glass or Similar Non-Porous Surfaces
The application of bonded foil strain gage sensors to glass can be problematic due to the smooth surface and the fact that surface abrasion is generally not allowed on glass or glass-like test articles (stress concentrations). That, along with glass’ poor heat-sink rating can contribute to excessive gage failures during the installation. Surface preparation is generally limited to chemical rather than chemical/mechanical, which is typical of most metals.
The application of gages to glass-like specimens can be divided into two parts:
1 — Surface Preparation:
- Degrease the glass surface with CSM-3 or GC-6 (reagent grade of IPA). This removes any gross organic contamination (oils, greases, etc.).
- Generally, no surface abrasions are allowed on glass. Always consult with the right people to see if abrasion is allowed. Abrasion will increase peel strength of the adhesive. If abrasion is allowed, a grit blast is preferred, as it produces a more uniform surface roughness. The next best method would be an acid treatment to “frost” the glass (usually a high concentration of sulfuric acid). Both increase the surface roughness uniformly.
- Scrub with Conditioner A (MCA-1) using a cotton-tipped applicator. Dry with a clean gauze sponge.
- Scrub with Neutralizer 5A (MN5A-1) using a cotton-tipped applicator. Dry with a clean gauze sponge.
2 — Adhesive/Gage Application:
- Apply the strain gage sensor with the adhesive best suited for the application/test requirements. Follow the instruction bulletin for mixing/using the selected adhesive, which is usually M-Bond 200 or M-Bond AE-10.
- Care must be taken when removing the gage handling tape, as the peel strength on glass is usually poor. Pull the gage handling tape back 180° onto itself; brushing rosin solvent at the tape/glass interface will reduce the “peel” by breaking down the tape mastic as the tape is pulled back.
- The use of pre-wired gages (C2A series or options P or P2) is preferred, as tinning the tabs and installing the leads would typically overheat the tab/backing and gage/adhesive interfaces, causing delamination. Glass is a very poor conductor of heat.
- If leaded gages cannot be used, then the following steps should be adhered to:
- Make sure to maintain the soldering iron tip. Keep it clean and coated with fresh solder at all times to avoid oxidation.
- Use a chisel or screwdriver tip. The flat portion of a chisel/screwdriver tip in contact with the strain gage solder pad will ensure uniform heating. Avoid a conical tip.
- Use a temperature-controlled soldering iron. Set the tip temperature to the melt point of the solder plus 150°F.
- Avoid using a wet sponge to clean the tip. Instead, clean it by wiping on a clean, dry gauze sponge.
- Use a solder with the lowest melting point allowed for the application.
- Keep the soldering iron on the solder pad for no more than three seconds. Reworking of the solder connection is OK, but only after allowing it to cool for several seconds.
- Use a rosin-core solder or liquid flux.
- Be sure to remove all residual flux after soldering.