A prior test is recommended in all cases for applications on plastic. Due to the large number of diverse materials, it is not always possible to make a prediction.
Strain gauges (SG) are usually applied to measuring bodies using adhesives. This article looks at the selection of the correct adhesive dependent on the material and temperature range. It also provides tips for preparing a measuring point to ensure good bonding for strain transfer.
Application of Strain Gauges
The most common way to bond strain gauges to the measurement object is to glue them on. The quality of the adhesive bond must be suitably high to ensure good transfer of the strain from the measurement object into the strain gauges.
The following elements are required to ensure a high-quality bond:
- Good bonding substrate
- Selection of the proper adhesive
- Careful execution of the bond
- Suitable protection of the measuring point
Good Bonding Substrate
The components being bonded must be carefully cleaned to eliminate dirt, grease, and oxide layers. Thorough preparation is a basic requirement to achieve good adhesion. The cleaning agent RMS1 has been proven for this purpose. RMS1 is also available as a spray making it quick and easy to use.
Roughening of the surface is important in addition to the cleanliness of the measuring point. This increases the active surface available for bonding at a microscopic level. Sand-blasting is typically used for this purpose, as is simple emery paper with an appropriate grain size between 180 and 300.
The following working steps are used during the preparation of the measuring point:
- Coarse cleaning
- Fine cleaning and degreasing
The cleanliness of the measuring point, and therefore the necessity of several carefully implemented cleaning steps, is of obvious importance.
These working steps are explained in great detail in the HBM reference book "Practical Hints for the Installation of Strain Gauges" written by Karl Hoffmann, and available for download here.
Strain gauges can be applied using cold or hot curing adhesives. In experimental stress analysis, fast-acting adhesives (superglues) are commonly used as they cure very rapidly at room temperature. The typical superglues Z70 and X60 can be used up to 120°C and 80°C respectively.
However, if the subsequent temperature range is higher than this, then the choice must lie either with the cold-curing, heat-resistant, two-component adhesive X280, or with hot-curing adhesives such as EP310S or EP150. Careful observance of the heating curves and times for the hot-curing adhesives is essential here to achieve a good result during subsequent measurements at higher temperatures.
In addition to the temperature range, the surface properties of the test object are also important in selecting the correct adhesive. This means that low-viscosity adhesives, such as Z70, which is used to create light, thin adhesive layers, are only suitable for smooth surfaces. More pasty adhesives such as X60 must be used for uneven and absorbent surfaces in particular.
Single-component adhesives such as Z70 are, in principle, somewhat easier to apply as they do not need to be mixed before processing. However, it must be noted that Z70 requires a relative humidity greater than 30% for application. This must be taken into account in cold and/or very dry ambient conditions.
The following tables show which adhesives are suitable for which materials and what specific recommendations can be given for gluing SG onto the corresponding material. For a better overview, the table is subdivided into metals, plastics, and other materials.