All bonded resistance strain gages are limited by characteristics of the grid alloy, the matrix, or the adhesive in the maximum strain level that can be applied before failure. Bonded, rolled foil strain gages have the ability to make measurements at very high strains. Properly installed and wired polyimide-backed gages, Micro-Measurements EA-Series, in larger gage lengths more than 3 mm, can easily operate at strains exceeding 10% ,100,000 µε. Post yield strain gages, Micro-Measurements EP-Series, can achieve even higher strains, often operating in excess of 20% before the foil grid fails. Achieving such high elongations requires attention to detail and the correct installation materials, techniques.
The bonding surface must be free of gross imperfections, but must also have a slightly rougher than normal finish to support gripping the high-elongation adhesive. M-Bond A-12 is the adhesive of choice for high strain measurements. The surface finish should be 250 micro-inches RMS, or slightly rougher. For aluminum, 200-grit to 300-grit sandpaper works well, and for steel 60-grit to 80-grit sandpaper is best. The surface should be abraded in a crosshatch pattern at 45° to the axis of primary elongation. Solder connections to the gage tabs must be kept as small as practicable. Use a soft solder, like 361A-20R.
Minimal protective coating, if any, should be used over the gage installation. Use a thin layer of a soft, pliable coating like M-Coat C.
Using the right gage and installation materials/techniques provides for successful high-elongation strain measurement exceeding 20% strain.
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