A strain gauge can only perform as expected when the measurements are transferred faultlessly and free of loss. To ensure that, a strong connection between the strain gauge and the test specimen needs to be established. An improper or incorrect installation can degrade or completely ruin the validity of a test. In this article, we are going to discuss the required steps for a strain gauge installation, along with the techniques for the protection of gauge installation. We will be covering this using some scenarios for installing strain gauges on different surface materials and the connection technique being used for the installation.
Case 1: Strain Gauge Installation on Metals using Adhesives
Preparation
The very first step is to ensure that all the items required in the process are available. These items include: strain gauges, bonding adhesive, connecting terminals, test specimens, solvent, cleaning tissue for industrial use, soldering iron, solder, abrasive paper (120 -320 grit), marking pencil, scale, tweezers, extension lead wire, polyethylene sheet, and nippers.
Now, before beginning with the installation process, it is important that the surface of the test specimen on which the gauge is to be bonded should be cleaned. To do so, remove all grease, rust, paint, etc., from the bonding area to provide a shiny metallic surface. Use abrasive paper to abrade an area somewhat larger than the bonding area uniformly and finely. The abraded area should be fine cleaned by soaking an industrial tissue or cloth in a small quantity of chemical solvent such as acetone until it is completely free of contamination.
Adhesive Curing and Pressing
Drop a small amount of adhesive on the back of the strain gauge, depending on the size of the gauge and spread the adhesive uniformly using a nozzle. Now, place the gauge at the position on the test specimen. It should be noted that this attaching part needs to be done as soon as the surface is decontaminated, to avoid bonding on a dirty surface. Place a polythene sheet or a translucent tape onto the gauge and press down on the gauge constantly using your thumb or a gauge clamp.
Bonding Connecting Terminals
After the adhesive beneath the polyethene sheet is cured, raise the gauge leads up to a bit inside the gauge base. Bond the terminal close to the gauge leads carefully using tweezers and apply the solder to cover the metal foil of the terminal. Twist off the excess gauge leads using tweezers. Now, solder the end of the lead wire to the terminals, while avoiding excessively heating up the terminal that might peel off the metal foil.
Case 2: Strain Gauge Installation on a Concrete Surface
Preparation
Firstly, remove all the laitance, paint or other soiling from the installation area to get a clean surface using a surface preparation agent to remove any moisture from the surface. If the surface is still wet, the adhesive may not harden properly. Next, sand an area of about 20mm to 30mm larger than the installation area with an abrasive paper. Now, clean the abraded surface with an industrial tissue or cloth dampened with a small amount of a solvent such as acetone.
Pre-coating
Before we bond the strain gauge, it is required that we prepare the surface to act as a barrier against any dampness that is released from the surface of the concrete or mortar. This is done to prevent the absorption of moisture by the underside of the strain gauge.
Now, cut the gauge binder supplied with the strain gauge about 5mm in from the fold. Apply a packing tape on the borders of the binder so as to mask the area roughly 10mm larger than the binder on each side. Then, apply the adhesive fully into the mortar or concrete in such a way that a layer 0.5mm to 1mm thick is formed on the installation surface.
Bonding the Strain Gauge
Place the cut piece of the gauge binder over the applied adhesive pressing it into place gradually from one end so that no air bubbles are trapped under the binder. Once the adhesive has hardened, peel off the piece of gauge binder and install the strain gauge.
The successful installation of a strain gauge on a concrete or mortar mainly depends on the gauge length of the strain. Such structures are classified as inhomogeneous materials. This means that if we use strain gauges with short gauge lengths, partial strains in pebbles or individual sections of the hardened cement paste are more likely to be measured. With larger gauge lengths, it is possible to span several pieces of aggregate in order to measure the representative strain in the structure. Therefore, it is recommended to use a gauge length that is at least five times as long as the largest grain size.
Use adhesive to attach the strain gauge to the mortar specimen surface pre-coated with adhesive. It is recommended to use a strong, pore-filling adhesive that forms reliable bonds even if some amount of residual moisture is still present in the concrete. For gauge installation, the same steps need to be followed for bonding the connecting terminals as mentioned above for installation on metal surfaces.
Case 3: Installing Weldable Strain Gauges
A weldable strain gauge is spot welded to a metallic surface, while impacting the structure only in a minor way. The installation doesn’t require stringent preparing conditions as in case of adhesive bonding.
Preparation
For preparing the installation surface, simply remove all the dirt and oil to get a clean surface using a surface preparation agent.
A weldable strain gauge is supplied with a metal ribbon for trial welding along with a securing sleeve and an MI cable. The trial welding is done to adjust the welding power of the spot welder. If cracks or a hole appear in the ribbon, the welding power is reduced and if the ribbon is unmarked, the power is increased.
Welding Process
To begin with the welding, align the strain gauge on the centre of the installation area. Now, using a spot welder, press along with sides of the gauge together with the metal ribbon. The number and order of the welding points need to be carefully arranged during the installation. It should be checked that the welding points are not forming a crisscross pattern. Doing this ensures that there are no mechanical stresses included in the steel substrate.
To avoid load being placed on the secured sleeve, secure the MI cable with the metal ribbon. Furthermore, the cable between the gauge and connecting terminal can be bent in a gentle curve to avoid any undue strain on the MI cable.
These are the different types of strain gauge installations varying according to the connection technique and the installation surface properties. We offer a wide range of foil strain gauges and optical strain gauges that can be installed using the above-mentioned techniques. We also have a variety of adhesives for different operating conditions and a spot welder to enable the installation of weldable strain gauges.
