Selecting Adhesives for Strain Gauge Installation
Task and Function of the Bonding Material
The bonding materials have the task of firmly attaching the strain gauge to the surface of the measurement object and transfer the deformation of the object without loss to the strain gauge. Various conditions and influences, and the application options, require different bonding materials and application methods. Bonding is the most important factor. The particular advantages of this connection method, with regards to strain gauge installations, are:
- The possibility of connecting different material, even dissimilar ones. Depending on the type of adhesive, connection is implemented at room temperature or higher temperatures.
- No influence on the materials to be connected (some restrictions may apply for plastics).
- Chemically-hardening adhesives (only these are still used in strain gauge technology) are characterized by low moisture absorption.
- Control of working speed through selection of different adhesive types or curing conditions (hot or cold curing)
- Higher specific electrical resistance contributes to higher insulation resistance between strain gauge and component
Types of Bonding Materials
Both the working conditions at the installation site and the various requirements for the bonding material performance, particularly with regards to operating temperature, have led to various types of bonding materials being available.
It is essential not to use any adhesive other than the recommended adhesive. Strain gauge adhesives must fulfill different requirements than general adhesives. This is why they are generally based on special developments or modifications of commercial adhesives. That an strain gauge simply adheres to an object is not a sufficient criterion to evaluate the suitability of the adhesive for measurement purpose, it must also ensure a faultless transmission of the object strain. This requires more in-depth investigations (strain gauge tests according to VDI/VDE 2635 or comparable standards automatically include the adhesive.
Bonding materials can be differentiated as follows regarding the application technology:
These can be easily applied and do not require much effort. There are single-component adhesives that start curing when e.g. air is excluded (“anaerobic”) and two-component adhesives that must be mixed before application. Adhesive with very short reaction times are also called “superglues”. Their preferred field of applications is in experimental test.
These adhesives can only be used where the test object can be brought to the required curing temperature. This is generally possible in the manufacture of transducers, but also where strain gauges can be installed before machine assembly or where parts can be dismounted for strain gauge installation. In contrast to cold-curing adhesive, the hot-curing adhesive offer a wider application range at higher temperatures and are suitable for meeting the generally higher accuracy requirements in transducer production.
- With zero-point related measurement, the measured values are referenced to the zero point (usually static measurements)
- With non zero-point related measurement, the zero point can fluctuate, only the dynamic part is important (dynamic measurement)
- The specified temperature limits are fluid and depend on strain gauges being used, on the expected measurement accuracy and on the curing process.