High-Elongation Strain Measurement
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 (>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. As shown in the following figure, it is best to use fine jumper wires (30-AWG) from the gage tabs to a bonded terminal, ensuring the wires are bent as shown to provide strain relief.
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.