2024-12-13 GeoSitter 0
What is a Strain Gauge?
A strain gauge is a sensor used to measure the strain on the surface of an object. It is widely used in areas such as geotechnical engineering and mechanical engineering. This article focuses on the use of strain gauges in the geotechnical engineering field.
In geotechnical engineering, strain gauges are monitoring sensors that are used to detect deformation in structures caused by loads, temperature changes, or other external factors. Based on their installation methods, strain gauges can be divided into surface strain gauges, embedded strain gauges, and stress-free strain gauges. In terms of working principles, they can be classified into differential resistance strain gauges, vibrating wire strain gauges, differential capacitance strain gauges, and resistance strain gauges.
Compared to resistance strain gauges, vibrating wire strain gauges offer several distinct advantages. They provide frequency-based output signals that are unaffected by the cable length, allowing for long-distance transmission without signal attenuation. These gauges also excel in accuracy, sensitivity, and long-term stability. In recent years, vibrating wire strain gauges have become more prevalent in civil engineering projects. Below, we delve deeper into the specifics of vibrating wire strain gauges.
In modern engineering monitoring, vibrating wire strain gauges have become essential tools for ensuring the safety of critical structures, such as hydraulic structures, bridges, tunnels, rail transportation, and water conservancy. Renowned for their high precision, stability, and reliability, these gauges measure internal strain within structures.
By providing key data on structural deformation, vibrating wire strain gauges play a vital role in structural health monitoring. They offer critical insights for preventing structural safety incidents and ensuring public safety, making them indispensable in monitoring and maintaining large-scale infrastructure projects.
The surface strain gauge is primarily used for measuring strain on the surfaces of steel structures, reinforced concrete, and concrete. Its standout feature is its simple and quick installation process, allowing it to be easily set up just before testing begins. This minimizes the risk of damage during earlier construction stages, resulting in a high survival rate for the device.
The embedded strain gauge is designed for long-term strain monitoring in underground engineering. It can be directly embedded into concrete structures during the pouring process. Fully encased within the concrete, this type of strain gauge is shielded from external construction activities, offering excellent stability and durability. Its robust design significantly extends its service life.
Concrete undergoes "free volume deformation" due to factors such as temperature, humidity, and cement hydration. The stress-free strain gauge, also known as a "stress-free meter," is specifically designed to measure this type of deformation. The strain values it records are caused by intrinsic factors like temperature and humidity rather than external stress, making it a reliable tool for assessing free strain in concrete structures. The use of this type of strain gauge is currently relatively uncommon in the market.
The working principle of a vibrating wire strain gauge is based on the vibration theory of a wire under tension. When an external force is applied, the wire vibrates with a specific frequency, and this frequency is proportional to the magnitude of the applied force. By measuring the vibration frequency, the strain on the object can be accurately determined.
A typical vibrating wire strain gauge consists of a tensioned wire, a pair of anchors, and a measurement system. The wire is attached to the surface or internal structure of the object being measured, with both ends fixed by the anchors. As the object deforms, the wire stretches or compresses accordingly, causing changes in its vibration frequency. The measurement system detects and analyzes these frequency changes to calculate the strain value.
This precise and reliable mechanism makes vibrating wire strain gauges highly effective for monitoring deformation in various engineering applications.
Proper installation is critical to ensure the accuracy and reliability of strain gauge readings. Below are step-by-step instructions for installing the GeoSitter GS-SG02 Strain Gauge:
Before installation, inspect the strain gauge for any damage and verify its initial readings using a compatible readout device.
For standard models, the initial readings are within the modulus range of 2200-2500 (frequency range: 1500-1600), unless custom specifications are provided.
Note: Since the ends of the instrument are not fixed, the readings of the sensor in its free state may appear unstable. This is normal, and the readings will stabilize once the instrument is properly installed and fixed.
Step 2: Prepare the Mounting Position and Mount
When installed on steel structures
Figure 2: Welding sequence and position of mounting blocks
During welding, care should be taken to avoid overheating, and the flat end faces should not be welded, as this may affect the instrument’s assembly and disassembly. After welding, use an appropriate method to cool down the mounting blocks, remove welding slag, and check and adjust the alignment of the two blocks. Remove the mounting rod and install the instrument, as shown in Figure 3:
Figure 3: Installation schematic
Start by fixing the end of the strain gauge with the V-shaped groove using screws (the conical-point screws should be tightened into the groove of the block with the groove). Slightly press on the other end to achieve the expected initial modulus of 1600-1800 (or frequency 1250-1350), and then tighten the screws to secure the sensor.
If the instrument needs protection, a fixed bolt can be welded prior to the instrument installation for attaching a protective cover. The distance between the fixed bolt and the strain gauge should be ≥ 150mm.
When installed on concrete surfaces.
Weld anchor (which can be made using φ8–10mm rebar, with a length of 60–80mm) onto the mounting blocks. Use a mounting rod to position the blocks and drill two holes, 70–90mm deep, at suitable locations. The minimum diameter of the holes should be 12mm. The anchor heads are then fixed in the drilled holes using fast-setting grout or epoxy, as shown in Figure 4:
The standard mounting blocks can also be directly bonded to the concrete surface using a special adhesive. If this method is used, all sand, debris, and contaminants at the installation site must be removed and the surface cleaned.
Once the mounting blocks are securely fixed, remove the mounting rod, install the instrument, and first fix the end of the strain gauge with the V-shaped groove using screws (the conical-point screws should be tightened into the groove of the block with the groove). Slight press on the other end to achieve the expected initial readings (modulus 1800–1900), and then tighten the screws to secure the sensor.
Step 3 Cable Extension by Welding
Before welding, strip the outer insulation of the cable at the ends for approximately 8 cm to expose the core wires. Roughen the remaining cable insulation (about 3cm) using sandpaper or a sanding cloth. A φ12mm heat shrink tubing (approximately 14 cm in length) is then placed over the outer cable insulation. Use wire strippers to remove 0.5–0.8cm of insulation from the core wires, and slide φ2mm heat shrink tubing onto the core wires. Match the core wires by their respective colors, twist them together, and solder using a soldering iron. During soldering, avoid cold joints and remove any burrs. All five core wires should be welded, and the following precautions should be observed during the welding process:
Stagger the connections of the core wires.
Ensure all core wire lengths are consistent to allow for even force distribution when the cable is under tension.
After welding, the exposed core wire length should be about 7cm. Once the welding is complete, slide the φ2mm heat shrink tubing over the soldered joints and use a hot air gun to shrink it onto the connection. Then, slide the φ12mm heat shrink tubing over the cable connection and use the hot air gun to shrink it onto the joint. The φ12 mm heat shrink tubing should cover approximately 3 cm of the cable’s outer insulation at both ends.
When using the hot air gun to shrink the tubing, control the temperature carefully. Ensure the hot-melt adhesive inside the heat shrink tubing melts to a transparent, fluid state and completely fills the joint. If the temperature is too high, the insulation on the core wire may melt, causing short circuits, and the heat shrink tubing may become carbonized and brittle.
Notes:
After welding the core wires, the readings should be checked using a readout. Additionally, use a multimeter to measure the resistance between the core wires to ensure there are no short circuits or open circuits at the joint caused by the welding process.
Figure 5 Schematic diagram of cable welding
Turn on the readout
Turn on the readout and select the appropriate range based on the frequency range (the frequencies of strain gauges ranging from 1000 to 1800 Hz, select the “B” range). Set the readout to display both frequency modulus and Celsius temperature.
When taking readings, the last digit may fluctuate by one or two digits. Record the displayed value.
Read and calculate
When installed on steel surface, the strain calculation formula is as follows:
ϵ (strain)=G×C×(R1−R0) (μϵ), Where:
G is the standard coefficient, in microstrain/modulus.
C is the average correction factor (usually between 0.95 and 1.05, provided by the calibration table).
R1 is the current reading (modulus).
R0 is the initial reading (modulus).
Modulus R= Hz²/1000, where Hz is the frequency.
Refer to the calibration table for temperature coefficient and temperature coefficient correction.
When installed on concrete surface, the strain is calculated as follows:
ε（microstrain）＝G×C×(R1 R0)＋(Y1－Y2)×(T1－T0) (με), where
Y1 is the expansion coefficient of the steel wire (12.2 µε/°C).
Y2 is the expansion coefficient of concrete, which varies depending on the type of concrete. Generally, use the measured or design value.
T1 is the current temperature.
T0 is the initial temperature.
Upon receiving the instrument, please first check the quantity of the instrument (including accessories) and verify that the factory inspection certificate and other documents match the packing list.
The instrument should be stored in a dry and well-ventilated room to ensure proper maintenance and functionality.
Fully stainless steel, integrated design: Features include anti-rotation, anti-bending, impact-resistant, drop-resistant, grounding and lightning protection.
The design of the vibrating wire strain gauge is sophisticated, enabling it to be permanently embedded in various concrete structures to monitor internal strain in real-time.
This strain gauge can be used independently or, with the addition of accessories such as multi-axis strain gauges, stress-free gauges, and rock strain gauges, it can provide a more comprehensive and multidimensional monitoring solution.
The vibrating wire strain gauge features high sensitivity, high precision, and excellent linearity and stability. These characteristics allow it to accurately measure strain in various complex operating conditions, providing reliable data support for structural health monitoring.
The vibrating wire strain gauge also offers convenient data transmission capabilities. When used in combination with a vibrating wire sensor data collector, it can quickly capture real-time data from field sensors and convert it into corresponding physical quantities. This data can then be uploaded to GeoSitter self-developed data software through various data transmission methods, enabling real-time monitoring and data analysis. This functionality not only improves monitoring efficiency but also provides project managers with more intuitive and accurate monitoring data.
Conclusion:
The vibrating wire strain gauges have widely applied in the field of geotechnical monitoring. By accurately measuring the strain within a structure, potential safety hazards can be identified in a timely manner, allowing for the implementation of corrective measures to ensure the structural health monitoring and safe use of the engineering structure.
For more information about GeoSitter monitoring instruments or to discuss your project needs, contact us today at +86-13868064820 or email info@geositter.com.
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