The installation of a strain gauge is a critical stage in strain measurement. While a perfectly installed strain gauge helps in providing accurate measurements, improper installation may seriously degrade or mess up the validity of a test. The quality and success of a strain gauge installation are greatly influenced by the precision of the installation process and the choice of connecting technique. Two basic methods that are mainly used to install a strain gauge are: bonding and welding. Both of these methods involve using a material to attach the strain gauge to the measurement object in order to facilitate the transfer of the object’s deformation to the strain gauge without loss. The choice between these two approaches depends on the factors concerning the test application and the environment.
Weldable Strain Gauge
A welding-based connection approach is feasible when the environment for these applications is rough and rugged, for instance in construction sites or production plants. In such environments, usually cleanliness of the bonded material cannot be guaranteed and it is more likely to be damaged. The connection process involves attaching a weldable strain gauge to a metallic surface using a spot resistance welder. Here, the surface of the elements to be connected is molten in such a way that it impacts the structure only in a minor way. The elements are generally connected with 0.5mm stranded connections wires.
Weldable strain gauges are ideal for use on large test structures when it is not practicable to apply clamping pressure to the gauge installation during the bonding process. These strain sensors are an excellent substitute for a bondable strain sensor when the temperature of the test surface is below the minimum temperature required for proper cure of the bonding adhesive, and cannot be easily raised. The spot-welded connection of strain sensors allows them to be readily installed in field applications under a wide variety of weather conditions. Applications that employ a weldable strain gauge include mechanical load analysis on bridges, piles, tunnels, buildings, pipelines, wind power plants, and tanks.
Bonded Strain Gauge
In this approach, the bonding materials or adhesives are used to firmly attach the strain gauge to the measured object’s surface. The adhesive is simply applied over the rear face of the strain gauge and the gauge is immediately attached to the bonding site. Furthermore, depending on the type of adhesive used, the connection is made. This type of strain gauge connection is implemented at room temperature or higher temperatures, depending on the type of adhesive. The bonding can be done by using differential materials, even dissimilar ones. The adhesive based connection method offers quick and flexible installation on tricky areas such as radial surfaces or small-dimensioned areas.
The bonding material and installation method are employed only after considering the various conditions and influences, and application options. These factors include the working conditions at the installation site, and the various requirements for the bonding material performance, particularly in regard to the operating temperature. Due to the requirement of these many considerations for bonding a strain gauge to the test specimen, a wide variety of adhesives are available to choose from. These include both cold- and hot-curing adhesives for performing experimental stress analysis and transducer manufacturing.
Cold-Curing Adhesives
The cold-curing adhesives are single element adhesives generally made from materials such as Methacrylate, Cyanoacrylate and Epoxy resin. They can be easily applied and processed at minimum cost and effort as they harden under normal ambient conditions. They start curing with appropriate humidity as well as two-component adhesives that must be mixed before application. Usually, the time required for these adhesives to cure is very short and therefore they are sometimes referred to as “superglues”. The preferred field of application of cold curing adhesives is in experimental stress analysis. However, these kinds of adhesives are only to be used when the operating temperature is up to 80°C. For temperatures higher than that, hot curing adhesives are recommended.
Hot-Curing Adhesives
The hot-curing adhesives require certain curing temperatures to successfully bond the strain gauge to the test specimen. They are generally made from materials such as Epoxy resin and Phenolic resin. This type of bonding technique can be employed where strain gauges can be installed before machine assembly, or where parts can be dismounted for strain gauge installation. One such possible application is transducer manufacturing. The operating temperature range offered by these adhesives is wider than that offered by cold-curing adhesives, which is ideal for meeting the generally higher accuracy requirements in transducer production.
Other Considerations
The above-mentioned methods are widely used by many industries for various applications. Apart from these two installation methods, a strain gauge can also be pressed against the specimen surface by magnetic force without actually gluing it onto the surface of the specimen. Here, the strain is measured by friction caused between the contact surface of the strain gauge and the measurement surface of the metal specimen. This installation technique allows the reusability of the strain gauge and can only be operated under a temperature of 60°C. For special measurement requirements that involve internal strain measurement such as axial strain measurement in a bolt, embedment type strain gauges are employed. They can be used to measure the internal strain of concrete, mortar or asphalt by being embedded into the material before its hardening.
