Clamping Strain Gages by Vacuum
Bonding a foil strain gage is a laminating process and requires pressure to be applied until the adhesive is cured. Uniform pressure applied over the strain gage ensures that the layer of adhesive between the strain gage and the test material is very thin and that, when cured, the adhesive will cause the strain gage to become a part of the surface, thus duplicating the surface deformations within the foil of the strain gage so that strain magnitude and, in the case of rosettes, direction can be accurately measured.
Since cyanoacrylate adhesives (M-Bond 200) cure very quickly, pressure is applied using your thumb. For adhesives that require cure time (M-Bond AE-10/AE-15, M-Bond A-12, M-Bond GA-2, RTC-2 Epoxy, etc.), pressure is typically applied using a point-force on a backing plate, with a soft, silicone rubber pad between the backing plate and the strain gage so that the force is distributed evenly over the area of the strain gage. There are occasional applications that require a strain gage to be bonded on a curved surface or otherwise in a location where applying a point-force is not possible are at least very inconvenient. For such applications, consider using a vacuum pad to clamp the strain gage.
Commercially available systems for vacuum-clamping of strain gages are available, and work extremely well. Often, such systems are available with built-in heaters to permit curing certain adhesives, or speeding up the cure of other adhesives. Micro-Measurements TechTip TT-610 “Strain Gage Clamping Techniques” illustrates how a vacuum clamping fixture can be constructed from simple materials; see the figure below.
This type of constructed vacuum clamp works very well and may even have an advantage over the commercially available suction cups in that it will form a seal on rough surfaces, such as parts with a textured surface as produced by a sand casting process. Depending on altitude above sea level, this type of clamp should easily produce the minimum 10 psi required by many strain gage adhesives.
For more information about strain gage clamping techniques, contact a Micro-Measurements Applications Engineer, Technical Sales Manager, or independent Sales Representative at firstname.lastname@example.org.