Conventional use technology of resistance strain gauge

Conventional use technology of resistance strain gauges
When measuring strain, only by correctly selecting and installing strain gauges can the measurement accuracy and reliability be guaranteed and the expected test purpose be achieved.
2.7.1 Selection of resistance strain gauges
There are many types of strain gauges. The selection should be based on the test environment conditions, the strain state of the measured component, the material properties of the measured component, the size and resistance value of the strain gauge, and the measurement accuracy.
The general selection principle is:
1. According to the test environment conditions
(1) When measuring the ambient temperature, the appropriate strain gauge should be selected according to the temperature of the component so that the strain gauge can work normally within the given test temperature range.
(2) Ambient humidity Humidity has a great impact on the performance of the strain gauge, resulting in reduced insulation resistance and decreased bonding strength. In severe cases, it is impossible to measure. For this reason, in a humid environment, a film strain gauge with good moisture-proof performance, such as phenolic-acetal, polyester film strain gauge, etc., should be selected, and appropriate moisture-proof measures should be taken.
(3) Magnetic field environment Under the action of a strong magnetic field, the sensitive grid of the strain gauge will stretch or shorten, causing the strain gauge to produce output. Therefore, the sensitive grid material should be made of nickel-chromium alloy or platinum-tungsten alloy with small magnetostrictive effect.
2. According to the strain state of the measured component

(1) Strain distribution The strain value measured by the gradient strain gauge is the average strain value within the range of the strain gauge grid length. Therefore, when the strain is uniformly distributed along the axial direction of the specimen, a strain gauge with any grid length can be selected without directly affecting the test accuracy. The strain gauge with a long grid length has a small lateral effect coefficient and is easier to paste. If the component with a large strain gradient is tested, a strain gauge with a small grid length should be selected according to the specific situation.
(2) Strain properties For static strain measurement, temperature change is an important cause of error. If conditions permit, a temperature self-compensating strain gauge can be selected for the specific specimen material.
For dynamic strain measurement, a strain gauge with a high fatigue life should be selected, such as a foil strain gauge.
3. According to the material properties of the component to be measured
(1) If the material of the component to be measured is a homogeneous material with a high elastic modulus (such as metal material), there is no special requirement for the strain gauge.
(2) If the material of the component to be measured is a non-homogeneous material (such as wood, concrete, etc.), a strain gauge with a larger grid length should be selected to eliminate the influence caused by material non-uniformity. The grid length of the strain gauge used for measuring the surface strain of concrete should generally be more than four times larger than the diameter of the particles.
4. According to the size of the strain gauge
The selection of the size of the strain gauge is based on the material and stress state of the specimen, as well as the area allowed for the strain gauge to be pasted. For example, for materials with rough and uneven surfaces such as concrete, cast iron, and wood, a strain gauge with a larger grid length is selected. For materials with smooth and uniform surfaces, a strain gauge with a smaller grid length is selected. For cases where the stress distribution on the surface of the specimen is uniform or does not change much, and a larger pasting surface is allowed, a strain gauge with a larger grid length is selected. If it is in the stress concentration area of ​​the specimen, or the pasting area is allowed to be very small, a strain gauge with a grid length of ≤1mm is selected. For materials with poor thermal conductivity such as plastics, a strain gauge with a longer grid length is generally selected. The smaller the size of the strain gauge, the higher the requirements for pasting quality. Therefore, on the premise of ensuring measurement accuracy and sufficient installation area, it is advisable to select a strain gauge with a larger grid length.
If the strain gauge is used for dynamic strain measurement, the requirements such as the strain gauge's response to frequency should also be considered when selecting the strain gauge's grid length.
5. According to the resistance value of the strain gauge
The selection of the resistance value of the strain gauge is generally based on the requirements of the test instrument for the strain resistance value and the strain sensitivity of the measured strain, as well as the test conditions. For example, the resistance strain gauge commonly used in stress analysis tests is usually designed based on the strain gauge resistance value of 120+5Ω. Therefore, strain gauges with a resistance value of 120Ω are generally selected for stress analysis tests. Strain gauges with high resistance values ​​(such as 350Ω, 500Ω, 1000Ω, or even 5000Ω) are usually selected for sensors because this can improve their stability or output sensitivity. Sometimes, in order to reduce the attenuation effect of the resistance of the strain gauge leads and connecting wires on the strain gauge strain sensitivity, or to improve the signal-to-noise ratio of dynamic strain measurement, strain gauges with high resistance values ​​are also selected.
6. According to test accuracy
It is generally believed that strain gauges with adhesive films as substrates and copper-nickel alloys and nickel-chromium alloy materials as sensitive grids have better performance. They have the advantages of high accuracy, good long-term stability, and good moisture resistance.
2.4.2 Pasting of resistance strain gauges
The installation of normal temperature strain gauges usually adopts the pasting method. Therefore, the bonding process is a very important link in strain testing. The quality of strain gauge bonding directly affects whether the strain on the surface of the component can be correctly and reliably transmitted to the sensitive grid, affecting the accuracy of the test. The following is a brief introduction to the operation process of strain gauge pasting.
1. Inspection and sorting of strain gauges
The strain gauge should be visually inspected and the resistance measured before patching. Check whether the sensitive grid of the strain gauge has rust spots, whether the base and cover layer are damaged, whether the lead is firm, etc. The purpose of resistance measurement is to check whether the strain gauge has open circuit or short circuit, and sort it according to resistance value to ensure that the resistance value of a group of strain gauges using the same temperature compensation sheet does not differ by more than 0.1Ω.
2. Preparation of pasting surface
First, remove the oil, paint, rust, electroplating layer, etc. on the pasting surface of the component, cross-grind with sandpaper to create fine lines to increase the bonding force, then scrub with gauze or cotton balls soaked in alcohol (or propylene copper), and draw the patch positioning line with a steel drawing needle. Finally, scrub once more until there is no stain on the gauze or cotton ball.
3. Patching
Apply a thin and even layer of glue on the bottom surface of the strain gauge and the treated adhesive surface, place the strain gauge with tweezers and adjust the position, then cover it with fluoroplastic film, rub and roll it with your fingers, squeeze out the excess glue, and remove the bubbles under the strain gauge to make the strain gauge and the specimen fit completely. After a suitable time, peel off the fluoroplastic film from the end of the strain gauge without leads to the end with leads, and try to make the force direction parallel to the bonding surface.
4. Curing
The most commonly used adhesive for patching is cyanoacrylate adhesive (such as 502 glue, 501 glue adhesive). After using it for patching, it can be fully cured as long as it is left at room temperature for several hours, and it has strong bonding ability. For adhesives that need to be heated and cured, they should be strictly followed. Generally, it is baked with an infrared lamp, but the heating speed should not be too fast to avoid bubbles.
5. Welding and fixing of measuring wires
After the adhesive is initially cured, the wires can be welded. When measuring static strain at room temperature, the conductor can be a single-filament yarn-wrapped copper wire or a multi-strand copper core plastic soft wire with a diameter of ф0.1~0.3mm.
It is best to use a terminal block between the conductor and the strain gauge lead, as shown in Figure 2-14. It is made by corroding a copper-clad plate. The terminal block should be pasted near the strain gauge, and the conductor and the strain gauge lead are soldered to the terminal block. Strain gauges at room temperature are all soldered. In order to prevent false soldering, the oxide scale and insulation at the welding end must be removed, and then cleaned with solvents such as alcohol and acetone. The welding should be accurate and quick.
The soldered wire should be fixed along the way on the test piece. The fixing method includes sticking with tape, gluing (such as 502 glue), etc.

Figure 2-14 Schematic diagram of the terminal block fixing the wire
6. Inspection
For strain gauges that have been fully cured and connected with wires, quality inspection must be carried out before formal use. In addition to the appearance inspection of the strain gauge, it is also necessary to check whether the strain gauge is well pasted, whether the patch orientation is correct, whether there is a short circuit and open circuit, whether the insulation resistance meets the requirements (generally not less than 100MΩ), etc.
2.7.3 Protection of resistance strain gauges
For the installed strain gauges, appropriate moisture-proof measures should be taken. The choice of protection method depends on the working conditions, working period and required measurement accuracy of the strain gauge. For normal temperature strain gauges, silicone rubber sealant protection method is often used. This method is to apply silicone rubber directly around the strain gauge that has been generally cleaned. It can be bonded and cured after 12 to 24 hours at room temperature. The longer the storage time, the better the bonding effect. Silicone rubber is easy to use, has good moisture-proof performance, strong adhesion, long storage period, high and low temperature resistance, and no corrosion to the strain gauge, but the strength is relatively low. In addition, epoxy resin, paraffin or vaseline can also be used as moisture-proof protection materials.