Step into the world of precision measurement where materials tell their stories through subtle changes in resistance. Welcome to the realm of strain gauges – tiny sensors with an extraordinary ability to reveal the mysteries of deformation and strain. From towering bridges to cutting-edge spacecraft, these unassuming devices play a pivotal role in shaping our understanding of the physical world.

Imagine a technology that allows us to see the invisible forces that shape our structures and innovations. Strain gauges are the silent observers, delicately adhered to surfaces, waiting to transform mechanical strain into measurable data. With their ability to capture even the minutest deformations, they empower engineers, researchers, and enthusiasts alike to optimize designs, ensure safety, and unravel the secrets of material behavior.

In this journey, we'll delve into the principles behind strain gauges, explore their diverse types and applications, and uncover the art of precision installation and calibration. Join us as we unravel the intricate dance between forces and materials, guided by the unassuming yet indispensable strain gauge – a beacon of insight in mechanics.

Fig 1. Strain gauge

History of strain gauge

The history of the strain gauge dates back to the mid-19th century when scientists and engineers began to explore ways to measure and quantify mechanical strain, which is the deformation that occurs in an object when subjected to an applied force. The concept of strain measurement became increasingly important with the development of machinery, infrastructure, and other mechanical systems.

Here's a brief overview of the history of the strain gauge:

• Early Concepts and Experiments (19th Century): The concept of measuring strain started with the realization that materials change shape when subjected to external forces. Researchers like Lord Kelvin (William Thomson) and Sir George Stokes made early contributions by developing mathematical relationships to describe the behavior of materials under strain.
• Wire Resistance Strain Gauge (1930s): The modern strain gauge as we know it was developed in the 1930s by Edward E. Simmons and Arthur C. Ruge at the Massachusetts Institute of Technology (MIT). They introduced the concept of using a wire with electrical resistance as a strain sensor. When the wire is subjected to mechanical strain, its resistance changes due to the alteration in its length and cross-sectional area.
• Metal Foil Strain Gauges (1940s): During World War II, the demand for accurate strain measurements increased significantly for aircraft and other military equipment. The use of metal foil, such as Constantan, as the sensing element became common. These metal foil strain gauges were more robust and provided better performance compared to the earlier wire-based gauges.
• Introduction of Semiconductor Strain Gauges (1960s): The advent of semiconductor technology brought about a new type of strain gauge. Semiconductor strain gauges are made from materials like silicon and germanium. These gauges offer several advantages, including higher sensitivity, better linearity, and compatibility with integrated circuits.
• Modern Advances and Applications (Late 20th Century and Beyond): As technology advanced, strain gauges became more versatile and accurate. They found applications in various industries, including aerospace, automotive, civil engineering, and biomechanics. Additionally, techniques like thin-film deposition and microfabrication allowed for the creation of miniature and flexible strain sensors that could be applied to a wide range of surfaces.
• Digital and Wireless Strain Gauges: With the progression of digital signal processing and wireless communication, strain gauges started to incorporate these technologies. This allowed for real-time monitoring and remote data collection, further expanding their applications.
• Specialized Strain Gauges: Over time, strain gauges were developed for specific applications, such as high-temperature environments, corrosive conditions, and extreme pressures. These specialized strain gauges expanded their utility in diverse industries.

Today, strain gauges are an integral part of engineering and scientific research, used to measure and monitor mechanical deformation, stress, and strain in various structures and materials. They play a critical role in ensuring the safety, reliability, and performance of a wide range of products and systems.

What is a strain gauge?

A strain gauge is a device used to measure the deformation or strain of an object when subjected to an applied force or load. It operates on the principle that the electrical resistance of a material changes when it is stretched or compressed. Strain gauges are widely used in engineering, physics, and other fields to measure the amount of strain experienced by materials and structures, which helps in understanding their mechanical behavior and performance.

The basic working principle of a strain gauge involves attaching a small piece of wire or a thin strip of metal, usually made of materials like Constantan, to the surface of the object being tested. When the object experiences a force, it deforms slightly, causing the strain gauge to also stretch or compress. As the gauge undergoes this mechanical deformation, its electrical resistance changes in proportion to the strain it experiences.

The change in resistance of the strain gauge can be very small, so it is usually connected to a Wheatstone bridge circuit, which is a configuration of resistors that helps amplify the change in resistance and convert it into an easily measurable voltage output. This voltage change can then be calibrated to accurately quantify the strain applied to the object.

Strain gauges come in various forms, including

• Wire Strain Gauges: These consist of a thin wire with a known resistance value that is bonded to the surface of the object. When the object undergoes strain, the wire's resistance changes, and this change is measured.
• Metal Foil Strain Gauges: These are made from a thin strip of metal foil that is etched with a serpentine pattern to maximize its resistance. They are often used due to their durability and ease of application.
• Semiconductor Strain Gauges: These are made from semiconductor materials like silicon or germanium and offer higher sensitivity and accuracy. They are commonly used in microelectromechanical systems (MEMS) and integrated circuits.

Strain gauges are used in various applications, including structural engineering to monitor the stress and strain in buildings and bridges, aerospace to measure forces on aircraft components, automotive industry for crash testing and performance analysis, biomechanics for studying the movement of human and animal bodies, and many other areas where understanding the mechanical behavior of materials and structures is crucial.

Fig 2. Strain gauge in a circuit

What is the strain gauge internal parts?

A strain gauge consists of several internal components that work together to measure the deformation or strain of an object. The main components of a strain gauge include:

• Base Material: The base material is the physical structure of the strain gauge. It can be a wire, a thin strip of metal foil, or a semiconductor material. This material undergoes deformation when the object it's attached to experiences strain.
• Active Element: The active element is the part of the strain gauge that changes its electrical resistance in response to mechanical deformation. In wire strain gauges, the wire itself is the active element. In metal foil and semiconductor strain gauges, the etched or doped region that changes its properties under strain is the active element.
• Backing Material: The backing material provides support to the active element and helps protect it from external influences like temperature changes and moisture. It also aids in transferring the strain from the object being measured to the active element.
• Adhesive: An adhesive is used to attach the strain gauge to the surface of the object. The adhesive must be carefully chosen to ensure a secure bond while minimizing any interference with the strain measurements.
• Insulating Coating: An insulating coating is applied to the active element and the connecting wires to prevent electrical interference between the strain gauge and its surroundings.
• Wiring: Wiring connects the strain gauge to the measuring equipment. The electrical resistance changes in the strain gauge are transmitted through the wires to a Wheatstone bridge circuit or other measuring circuits.
• Protective Coating: A protective coating may be applied to the strain gauge to shield it from environmental factors such as moisture, dust, and chemicals. This coating helps prolong the lifespan of the strain gauge and maintain its accuracy.
• Lead Wires: Lead wires are the connections that extend from the strain gauge to the measuring instrument. These wires carry the electrical signals generated by the strain gauge's resistance changes to the measuring circuitry.
• Temperature Compensation Elements: Some strain gauges include additional components like temperature compensation elements. These elements help account for changes in resistance due to temperature fluctuations, ensuring accurate strain measurements even when the temperature varies.

It's important to note that the internal components of a strain gauge can vary based on the type and design of the gauge. Different applications may require specific materials and configurations to optimize accuracy and performance.

Fig 3. Strain gauge structure

How does the strain gauge work?

A strain gauge works based on the principle that the electrical resistance of a conductor changes when it is subjected to mechanical deformation or strain. The deformation causes a change in the dimensions of the conductor, leading to changes in its resistance, which can be measured and correlated to the amount of strain experienced by the object to which the strain gauge is attached.

Here's a step-by-step explanation of how a strain gauge works:

• Attachment: The strain gauge is attached to the surface of the object that you want to measure strain in. This is typically done using an adhesive that securely bonds the gauge to the object.
• Deformation: When an external force or load is applied to the object, it deforms slightly. This deformation causes the object's dimensions to change, resulting in both stretching and compressing of the material.
• Change in Length and Cross-Sectional Area: As the material of the object deforms, the length of the strain gauge changes. Additionally, the cross-sectional area of the strain gauge may change due to compression or expansion.
• Change in Resistance: The change in length and cross-sectional area leads to a change in the electrical resistance of the strain gauge. Most materials exhibit a positive or negative change in resistance when stretched or compressed. This change is often linear and proportional to the applied strain.
• Wheatstone Bridge Circuit: To accurately measure the small changes in resistance, strain gauges are typically connected to a Wheatstone bridge circuit. The Wheatstone bridge consists of four resistors arranged in a diamond shape, with the strain gauge as one of the resistors. The bridge is powered by a voltage source.
• Balancing the Bridge: Initially, when no strain is applied, the bridge is balanced, meaning the voltage across the middle point (junction of two pairs of resistors) is zero. When the strain gauge undergoes deformation and its resistance changes, the bridge becomes unbalanced.
• Output Voltage: The unbalanced bridge produces an output voltage that is proportional to the change in resistance of the strain gauge. This voltage can be measured and amplified to accurately quantify the strain experienced by the object.
• Calibration: Before using the strain gauge, it's necessary to calibrate it to relate the change in resistance to the actual strain. This involves applying known forces or strains to the object and recording the corresponding output voltages from the Wheatstone bridge.

By accurately measuring the changes in resistance of the strain gauge and converting them into electrical signals, the strain gauge provides a quantitative way to measure mechanical strain in various materials and structures. It is widely used in industries such as engineering, manufacturing, research, and quality control to ensure the reliability and performance of products and systems.

Fig 4. Quarter-bridge strain gauge circuit

Strain gauge formula

The strain gauge formula involves calculating the strain based on the change in resistance of the strain gauge and its sensitivity factor. The sensitivity factor is a constant that relates the change in resistance to the applied strain. The formula can vary slightly depending on the configuration of the strain gauge and the measuring circuit. Here's the basic formula:

Strain (ε) = ΔR / R₀ * S

Where:

• ε is the strain (dimensionless).
• ΔR is the change in resistance of the strain gauge (in ohms).
• R₀ is the initial resistance of the strain gauge (in ohms) when no strain is applied.
• S is the sensitivity factor of the strain gauge (in reciprocal units of strain, typically με⁻¹ or mm⁻¹).

In practice, the sensitivity factor (S) is often provided by the manufacturer or determined through calibration. It represents the change in resistance per unit strain and is specific to the type and design of the strain gauge.

To use this formula, you would need to know the initial resistance of the strain gauge, the change in resistance due to the applied strain, and the sensitivity factor. Once you calculate the strain, you can then interpret it in terms of the deformation or strain experienced by the object under test.

Keep in mind that strain can be positive (tensile strain, where the material is stretched) or negative (compressive strain, where the material is compressed). The formula's sign should be interpreted based on the type of strain you're measuring.

For more accurate strain measurements, you might need to consider temperature compensation and other factors that could affect the performance of the strain gauge in different conditions.

Fig 5. Strain formula

What are the strain gauge types?

There are several types of strain gauges available, each designed to suit specific applications and measurement requirements. The primary types of strain gauges include:

1. Wire Strain Gauges: These were one of the earliest types of strain gauges developed. They consist of a fine wire, usually made of an alloy such as Constantan, wound around a grid-like pattern. As the wire is subjected to strain, its length changes, causing a change in resistance that can be measured. Wire strain gauges are simple and inexpensive but are less sensitive compared to other types.
2. Metal Foil Strain Gauges: Metal foil strain gauges are made by attaching a thin strip of metal foil (often Constantan or Karma) to a backing material using an adhesive. The metal foil is chemically etched or mechanically formed into a pattern that maximizes the change in resistance due to strain. These gauges are more robust and offer higher sensitivity compared to wire gauges.
3. Semiconductor Strain Gauges: These gauges are made from semiconductor materials like silicon or germanium. Semiconductor strain gauges are highly sensitive and have a nearly linear response to strain. They are often integrated with microelectronics and can be quite small, making them suitable for applications where space is limited, such as in microelectromechanical systems (MEMS).
4. Bonded Resistance Strain Gauges: Bonded resistance strain gauges are bonded directly onto the surface of the object being tested. They can be a metal foil, semiconductor, or piezoresistive (using materials with inherent resistance changes due to strain). These gauges are popular for applications requiring high precision and where the object's surface is accessible.
5. Encapsulated or Enclosed Strain Gauges: These gauges are designed for harsh environments where protection from moisture, dust, and chemicals is essential. They are encapsulated within protective housings to ensure their durability and accuracy.
6. Piezoelectric Strain Gauges: While not strictly resistance-based, piezoelectric strain gauges use piezoelectric materials that generate an electric charge when subjected to mechanical strain. This charge can be measured and correlated to the applied strain.
7. Optical Strain Gauges: These gauges use optical methods to measure strain, such as changes in light intensity or wavelength. Fiber optic strain gauges are an example of this type and are particularly useful in applications where electrical interference is a concern.
8. Vibrating Wire Strain Gauges: These gauges rely on the change in frequency of a vibrating wire under strain. The wire's resonant frequency changes as it is stretched or compressed, providing a means to measure strain.

The choice of strain gauge type depends on factors like the application's requirements (sensitivity, accuracy, temperature range, environmental conditions, etc.), available space, and budget. Each type has its advantages and limitations, and selecting the appropriate strain gauge is crucial for obtaining accurate and reliable measurements.

Wire Strain Gauges:

• Working Principle: Wire strain gauges operate based on the change in the electrical resistance of a wire as it is stretched or compressed. The wire's length increases or decreases, altering its resistance.
• Pressure Range: Typically used for low to moderate pressure ranges.
• Applications: Simple applications where high accuracy is not required, such as structural health monitoring, material testing, and educational purposes.
• Pros: Inexpensive, straightforward to use, and suitable for basic applications.
• Cons: Lower sensitivity compared to other types, may not provide the accuracy needed for demanding applications.

Metal Foil Strain Gauges:

• Working Principle: Metal foil strain gauges involve attaching a thin metal foil to a backing material. The foil's resistance changes as it deforms due to strain.
• Pressure RangeThis can cover a broad range of pressure levels, depending on the specific design.
• Applications: Structural monitoring, load testing, stress analysis, and other applications requiring higher sensitivity and accuracy.
• Pros: Higher sensitivity and accuracy compared to wire gauges, more rugged construction, versatility in pressure ranges.
• Cons: Slightly more complex installation compared to wire gauges.

Semiconductor Strain Gauges:

• Working Principle: Semiconductor strain gauges utilize the piezoresistive effect, where the electrical resistance of a semiconductor material changes with strain.
• Pressure Range: Suitable for moderate pressure ranges, but can be used in specific applications where accuracy is paramount.
• Applications: MEMS sensors, microelectronics, aerospace, and medical devices where miniaturization and high sensitivity are critical.
• Pros: High sensitivity, nearly linear response, compatibility with integrated circuits and microelectronics.
• Cons: Sensitive to temperature changes, may require compensation techniques.

Bonded Resistance Strain Gauges:

• Working Principle: Similar to metal foil strain gauges, bonded resistance gauges are adhered directly to the object's surface and measure strain-induced resistance changes.
• Pressure Range: Depending on the specific design, can cover a wide range of pressure levels.
• Applications: Structural engineering, automotive testing, civil engineering, and wherever high precision is needed.
• Pros: High accuracy, versatility, and can be used in various pressure and temperature ranges.
• Cons: Installation can be more complex than some other types.

Piezoelectric Strain Gauges:

• Working Principle: Piezoelectric materials generate an electric charge in response to mechanical strain. The generated charge is proportional to the applied strain.
• Pressure Range: Used for dynamic applications where rapid strain changes occur.
• Applications: Dynamic stress analysis, impact testing, and other scenarios involving rapidly changing loads.
• Pros: Extremely fast response time, suitable for dynamic measurements.
• Cons: Limited to dynamic applications, and may require special equipment for signal conversion.

Optical Strain Gauges:

• Working Principle: Optical strain gauges use changes in light intensity or wavelength to measure strain.
• Pressure Range: Application-dependent, can cover various pressure ranges.
• Applications: Civil engineering, aerospace, and applications where electrical interference is a concern.
• Pros: Immune to electromagnetic interference, accurate in harsh environments.
• Cons: Equipment setup can be more complex, and may require specialized knowledge.

Vibrating Wire Strain Gauges:

• Working Principle: The frequency of a vibrating wire changes as it's subjected to strain, and this change is used to measure strain.
• Pressure Range: Primarily used for geotechnical applications in soil and rock mechanics.
• Applications: Geotechnical monitoring, civil engineering projects, tunnel construction.
• Pros: Suitable for long-term monitoring in difficult environments, robust design.
• Cons: Specialized applications, not as commonly used for other types of strain measurement.

The choice of strain gauge type depends on factors such as the required sensitivity, accuracy, pressure range, environmental conditions, and specific application demands. It's crucial to select the appropriate strain gauge type that best meets the needs of the project or experiment.

Fig 6. Bonded strain gauge

Comparing table

Here's a comparison table summarizing the main features of the different types of strain gauges:

Remember that the choice of strain gauge type depends on your specific application requirements, accuracy needs, pressure range, and environmental conditions. Always consider these factors when selecting the appropriate strain gauge for your project.

What is a strain gauge used for?

Strain gauges are used to measure and monitor mechanical strain, which is the deformation that occurs in an object when subjected to an applied force or load. They play a crucial role in various industries and applications where understanding the mechanical behavior of materials and structures is essential. Here are some common uses of strain gauges:

• Structural Engineering: Strain gauges are used to monitor the stress and strain in buildings, bridges, dams, and other civil engineering structures. This information helps ensure the structural integrity and safety of these constructions.
• Aerospace and Aviation: In aircraft and spacecraft, strain gauges are employed to measure the stress on critical components such as wings, fuselage, landing gear, and engine components. This data aids in designing safe and efficient aerospace systems.
• Automotive Industry: Strain gauges are used in crash tests, vehicle suspension analysis, and durability testing of automotive components. They help engineers evaluate the performance and safety of vehicles.
• Manufacturing and Quality Control: Strain gauges are utilized in quality control processes to ensure the consistency and reliability of manufactured products. They can be used to test the integrity of materials and verify that products meet the required specifications.
• Biomechanics: Strain gauges are employed in medical and sports research to study the movement and mechanics of the human body. They help analyze the strain on bones, muscles, tendons, and ligaments during various activities.
• Material Testing: Strain gauges are used to test the mechanical properties of materials under different conditions. They aid in characterizing materials' behavior, such as elasticity, stiffness, and ductility.
• Geotechnical Engineering: In geotechnical applications, strain gauges monitor the stability of slopes, tunnels, and other underground structures. They provide insights into the behavior of soil and rock masses.
• Industrial Machinery: Strain gauges are employed in machinery and equipment to monitor load distribution, vibrations, and stress levels. This helps in predictive maintenance and preventing equipment failure.
• Robotics: Strain gauges are used in robotics to measure the forces exerted by robotic arms and grippers. This information is critical for precise control and safe interaction with the environment.
• Research and Development: Strain gauges are commonly used in various research fields, including materials science, physics, and mechanical engineering, to conduct experiments and validate theoretical models.
• Environmental Monitoring: In environmental applications, strain gauges can be used to measure changes in geological formations, such as detecting shifts in rock structures or monitoring tectonic activities.

Overall, strain gauges are versatile tools that provide valuable insights into the behavior of materials, structures, and systems under different conditions. Their applications extend across a wide range of industries, helping engineers, researchers, and professionals ensure safety, optimize performance, and advance scientific understanding.

Fig 7. Strain gauge in the industry

Where can use the strain gauge?

Strain gauges can be used in a wide range of industries and applications to measure mechanical strain and monitor the deformation of materials and structures. Here are some specific areas where strain gauges are commonly used:

1. Structural Engineering:
   • Monitoring bridges, buildings, dams, and other civil structures for stress and strain analysis.
   • Assessing the impact of loads, vibrations, and environmental factors on structural integrity.
2. Aerospace and Aviation:
   • Measuring strain on aircraft wings, fuselage, landing gear, and other components to ensure safety and optimal performance.
   • Monitoring spacecraft structures and components during launch and in space environments.
3. Automotive Industry:
   • Evaluating stress and strain on vehicle components during crash tests and impact simulations.
   • Testing suspension systems, chassis components, and engine parts for durability and performance.
4. Biomechanics:
   • Studying human and animal movement by measuring strain in bones, muscles, and joints.
   • Monitoring sports equipment to optimize performance and prevent injuries.
5. Manufacturing and Quality Control:
   • Checking the quality of manufactured products by measuring strain on materials, welds, and joints.
   • Ensuring consistent product performance and adherence to specifications.
6. Material Testing:
   • Evaluating the mechanical properties of materials under different conditions, such as tension, compression, and bending tests.
   • Characterizing materials' behavior for research and development purposes.
7. Geotechnical Engineering:
   • Monitoring soil and rock masses for stability and deformation in tunnels, mines, and underground constructions.
   • Detecting shifts in geological formations and assessing potential risks.
8. Industrial Machinery:
   • Monitoring the stress and strain on machine components to ensure proper operation and prevent failures.
   • Predictive maintenance by identifying wear and tear on critical parts.
9. Robotics:
   • Measuring strain on robotic arms, grippers, and joints to ensure precision and safety during operation.
   • Enabling robots to interact with the environment while maintaining control.
10. Research and Development:
    • Conduct experiments and studies in various scientific and engineering disciplines to validate theories and models.
    • Innovating new products and technologies by understanding material behavior under different conditions.
11. Environmental Monitoring:
    • Detecting ground movements and shifts in geological structures to assess geological hazards.
    • Studying tectonic activities and the effects of natural events like earthquakes and landslides.

These are just a few examples of the many applications where strain gauges are used. The versatility of strain gauges allows them to be applied in diverse industries, contributing to safety, reliability, performance optimization, and advancements in scientific understanding.

Fig 8. Metal foil strain gauge

Installation and maintenance

How can use the strain gauge?

Using a strain gauge involves several steps, from proper installation to data collection. Here's a general guide on how to use a strain gauge effectively:

• Select the Right Strain Gauge: Choose the appropriate type of strain gauge based on your application's requirements, such as sensitivity, accuracy, pressure range, and environmental conditions.
• Prepare the Surface: Ensure that the surface where the strain gauge will be attached is clean, dry, and free from contaminants. Any irregularities can affect the gauge's performance.
• Attach the Strain Gauge: Follow the manufacturer's instructions for attaching the strain gauge to the surface. Generally, this involves using a suitable adhesive to secure the gauge in place.
• Wiring: Connect the strain gauge's lead wires to the measuring equipment. Follow the wiring diagram provided by the manufacturer to ensure proper connections.
• Calibration: Calibrate the strain gauge before taking measurements. Apply known forces or strains to the object and record the corresponding output from the measuring equipment. This helps establish the relationship between strain and output.
• Data Collection: Apply the load or force that you want to measure. The strain gauge will deform along with the material, changing its resistance. The change in resistance will be converted into an electrical signal by the measuring circuitry.
• Read and Record Data: Read the output signal from the measuring equipment. This could be in the form of voltage, current, or digital readings, depending on the measurement setup. Record the data for analysis.
• Data Analysis: Interpret the recorded data to understand the strain experienced by the material. Use the calibration data to convert the electrical signal into meaningful strain values.
• Consider Temperature Effects: Strain gauges can be sensitive to temperature changes. If accurate measurements are crucial, consider compensating for temperature effects using temperature sensors or compensation techniques.
• Maintenance and Validation: Regularly check the strain gauge setup to ensure that it remains securely attached and operational. Perform periodic validation to ensure accurate measurements over time.

Remember that each application may have specific requirements, and the steps might vary slightly. It's crucial to follow the manufacturer's guidelines, adhere to best practices, and consider factors like temperature compensation and calibration for accurate and reliable results. If you're unsure, consulting with experts in the field or seeking guidance from manufacturers can help ensure successful strain gauge usage.

Strain gauge calibration

Strain gauge calibration is a critical process to establish a relationship between the change in resistance of the strain gauge and the applied strain. Calibration ensures that the measurements obtained from the strain gauge accurately reflect the actual strain experienced by the object being tested. Here's a step-by-step guide on how to calibrate a strain gauge:

• Setup and Equipment: Set up the testing environment and equipment. You'll need the strain gauge, a known force or load source, measuring equipment (e.g., data acquisition system), and appropriate wiring.
• Select Calibration Points: Choose several calibration points that cover a range of strains you expect to encounter in your application. These points should include zero strain (no load) and other known strain values.
• Zero Point Calibration: Apply zero strain (no load) to the object. Record the output signal from the measuring equipment. This will serve as the baseline or zero point.
• Apply Known Strains: Apply known strains to the object using the load source. The known strains can be obtained from the load source's specifications or calculated using the material's properties and geometry.
• Record Measurements: Record the output signal from the measuring equipment for each applied strain. Ensure that the measurements are stable and consistent.
• Calculate Sensitivity: Calculate the sensitivity factor (S) of the strain gauge for each calibration point using the formula: S = (Change in Output Signal) / (Change in Applied Strain).
• Curve Fitting: Plot the sensitivity values against the known applied strains. Fit a curve to the data points. The curve can be linear or nonlinear, depending on the strain gauge's behavior.
• Interpolation: Interpolate the sensitivity values to find the factor for any given strain within the range of your calibration points. This factor will be used to convert future measurements into accurate strain values.
• Validation: Validate the calibration by applying strains that were not used during the calibration process. Compare the measured strains with the calculated strains to verify the accuracy of your calibration.
• Documentation: Document all the calibration points, sensitivity values, curve equations, and any relevant information about the strain gauge setup. This documentation is crucial for future reference and ensuring traceability.
• Re-Calibration (if needed): Depending on the application's requirements and environmental conditions, you might need to periodically re-calibrate the strain gauge to account for any changes in its behavior over time.

Remember that strain gauge calibration should be conducted meticulously and following best practices. Accuracy and consistency in calibration are essential for obtaining reliable measurements that accurately reflect the strain experienced by the object. If unsure, consulting experts or referring to industry standards can be beneficial during the calibration process.

Fig 9. Semiconductor strain gauge

Strain gauge accuracy

The accuracy of a strain gauge measurement is crucial for obtaining reliable and meaningful data about the deformation and strain experienced by an object. The accuracy of a strain gauge measurement depends on various factors and can be affected by both the strain gauge itself and the measurement setup. Here are some key considerations for achieving accurate strain gauge measurements:

• Calibration: Proper calibration is essential to establish the relationship between the change in resistance of the strain gauge and the applied strain. Calibrating the strain gauge at multiple points across the expected strain range helps ensure accuracy.
• Sensitivity: The sensitivity of the strain gauge, which relates the change in resistance to the applied strain, affects accuracy. Choose a strain gauge with appropriate sensitivity for your application's requirements.
• Installation: The proper installation of the strain gauge on the object's surface is critical. Any misalignment, uneven bonding, or contamination can introduce errors in measurements.
• Temperature Compensation: Strain gauges can be sensitive to temperature changes, which can impact accuracy. Employ temperature compensation techniques to minimize temperature-related errors.
• Measuring Equipment: The accuracy of the measuring equipment, including data acquisition systems and amplifiers, affects the overall measurement accuracy. Use high-quality equipment with suitable resolution and precision.
• Wiring and Connections: Proper wiring and connections between the strain gauge and measuring equipment are essential to prevent signal loss and interference.
• Environmental Conditions: Environmental factors such as humidity, electromagnetic interference, and vibrations can impact measurement accuracy. Minimize the effects of these factors as much as possible.
• Material and Object Properties: Understand the material properties of the object being measured, as well as its geometry and mechanical behavior, to ensure accurate strain calculations.
• Data Averaging: Taking multiple measurements and averaging them can help reduce random errors and enhance accuracy.
• Operator Skill: Proper training and skill of the operator conducting the measurement can significantly influence accuracy. Following best practices and guidelines is crucial.
• Verification and Validation: Regularly verify and validate the accuracy of your strain gauge measurements by comparing them to reference measurements or known values.

It's important to note that the achievable accuracy can vary based on the specific application, the quality of equipment used, the expertise of the operator, and the care taken during installation and calibration. In critical applications, consulting experts in strain measurement or adhering to industry standards can help ensure the highest level of accuracy.

Strain gauge pros and cons

Strain gauges offer numerous benefits and advantages, but they also come with certain limitations. Here are the pros and cons of using strain gauges:

Pros:

1. High Sensitivity: Strain gauges are capable of detecting even small amounts of deformation or strain, making them ideal for applications requiring high sensitivity.
2. Versatility: They can be applied to various materials, surfaces, and structures, making them versatile tools for different industries and applications.
3. Accurate Measurements: With proper calibration and setup, strain gauges can provide accurate measurements of strain, helping engineers and researchers make informed decisions.
4. Real-time Monitoring: Strain gauges can provide real-time data, allowing for immediate feedback on the structural behavior of objects under load.
5. Cost-Effective: Strain gauges are generally cost-effective compared to some other measurement methods like optical sensors or accelerometers.
6. Longevity: When properly installed and maintained, strain gauges can provide reliable and long-lasting performance.
7. Wide Range of Applications: They find applications in fields ranging from civil engineering and aerospace to biomechanics and materials science.
8. Integration with Systems: Strain gauges can be integrated into complex measurement systems, including data acquisition systems, for comprehensive data analysis.

Cons:

1. Installation Complexity: Properly installing strain gauges requires careful attention to detail, as any errors during installation can lead to inaccurate measurements.
2. Sensitive to Environmental Factors: Factors such as temperature changes, humidity, and vibrations can affect the accuracy of strain gauge measurements if not properly compensated for.
3. Calibration Required: Accurate measurements necessitate careful calibration, which can be time-consuming and may require specialized knowledge.
4. Surface Preparation: The surface where the strain gauge is attached needs to be properly prepared to ensure a secure bond, adding an extra step to the measurement process.
5. Wiring and Connections: Errors in wiring and connections can introduce noise or signal loss, affecting the accuracy of measurements.
6. Intrusive Measurement: Strain gauges are applied directly to the object's surface, which may slightly alter the object's mechanical properties.
7. Limited to Static Measurements: While strain gauges are suitable for dynamic applications, they are not always the best choice for measuring rapid, transient changes in strain.
8. Expertise Required: Proper use of strain gauges requires expertise in areas such as installation, calibration, and data analysis to achieve accurate and meaningful results.

It's important to weigh these pros and cons against your specific application requirements and constraints. While strain gauges have limitations, they remain a widely used and effective method for measuring strain and understanding the mechanical behavior of materials and structures.

Important factors to choose the right strain gauge

Choosing the right strain gauge for your application involves considering several important factors to ensure accurate and reliable measurements. Here are the key factors to keep in mind when selecting a strain gauge:

1. Type of Strain Gauge: Different types of strain gauges (wire, metal foil, semiconductor, etc.) have varying sensitivities, temperature characteristics, and suitability for specific applications. Choose the type that best matches your measurement needs.
2. Sensitivity Requirement: Determine the level of sensitivity required for your application. Some applications demand higher sensitivity to capture even small changes in strain accurately.
3. Pressure Range: Consider the range of strain you expect to encounter in your application. Choose a strain gauge that can accurately measure the strain within that range.
4. Environmental Conditions: Assess the environmental conditions in which the strain gauge will be used. Factors like temperature variations, humidity, and the presence of chemicals can impact the gauge's performance. Choose a gauge that is suitable for your specific environment or consider implementing compensation techniques.
5. Object Material and Geometry: The material properties and geometry of the object being measured can influence strain distribution. Choose a gauge that matches the material's mechanical properties and the object's geometry.
6. Accuracy and Precision: Determine the required level of accuracy for your application. Higher accuracy may require more sophisticated strain gauges and calibration procedures.
7. Installation Method: Consider the installation process. Some strain gauges require specialized techniques for proper bonding and alignment. Choose a gauge that you can install effectively for your specific object or surface.
8. Temperature Compensation: If your application involves temperature changes, consider strain gauges that come with temperature compensation features or implement compensation methods.
9. Frequency Range: For dynamic applications, ensure that the strain gauge can handle the frequency range of the expected strain variations.
10. Longevity and Durability: Evaluate the strain gauge's expected lifespan and durability under the conditions of use. Some applications might require gauges designed to withstand harsh environments.
11. Budget Constraints: Strain gauges vary in cost. Balance your measurement needs with your budget constraints to find an appropriate solution.
12. Manufacturer and Support: Choose a reputable manufacturer with a track record of producing high-quality strain gauges. Ensure they provide documentation, technical support, and calibration services.
13. Application Expertise: If you're unsure, consult experts in strain measurement or specialists in the specific field of your application. Their expertise can guide you toward the most suitable strain gauge.
14. Validation and Calibration: Ensure that the strain gauges you choose can be accurately calibrated and validated for your application's specific strain range.

By carefully considering these factors, you can make an informed decision when selecting a strain gauge that will meet your application's needs and provide accurate and reliable measurements.

Fig 10. Vibrating strain gauge

The best five brands manufacturing strain gauges

There are several reputable brands that manufacture high-quality strain gauges. The "best" brand can vary depending on your specific application requirements, budget, and geographic location. Here are five well-known brands that are recognized for their strain gauge products:

• Vishay Precision Group: Vishay is a prominent manufacturer of precision measurement equipment, including strain gauges. They offer a wide range of strain gauges designed for various applications and industries.
• HBM Test and Measurement: HBM is a leading global supplier of strain gauges and measurement solutions. They offer a comprehensive selection of strain gauges suitable for different measurement needs, from basic to advanced applications.
• Kyowa Electronic Instruments: Kyowa is known for its high-quality strain gauges and measurement systems. They provide strain gauges for diverse industries, including automotive, aerospace, civil engineering, and more.
• Omega Engineering: Omega offers a variety of sensors and instrumentation products, including strain gauges. They provide a range of strain gauge options suitable for different measurement requirements.
• Micro-Measurements (Vishay Precision Group): Micro-Measurements, a part of the Vishay Precision Group, specializes in strain gauges and related measurement solutions. They offer a wide selection of strain gauges optimized for accuracy and reliability.

When choosing a strain gauge brand, consider factors such as the brand's reputation, the specific strain gauge models they offer, the availability of technical support, calibration services, and budget constraints. It's also a good idea to review customer reviews, consult experts in the field, and evaluate the brand's presence in your geographic region.

Conclusion

Strain gauges are versatile and essential tools for measuring mechanical strain and deformation in various materials and structures. They find applications across industries such as civil engineering, aerospace, automotive, biomechanics, and more. By understanding the principles, types, installation, calibration, and factors influencing accuracy, you can make informed decisions about selecting and using strain gauges effectively.

When choosing a strain gauge, consider factors such as the application's sensitivity requirements, pressure range, environmental conditions, object material, accuracy needs, and installation complexity. By carefully evaluating these factors, you can select the right strain gauge for your specific application, ensuring accurate and reliable measurements.

Remember that proper installation, calibration, and maintenance are crucial for obtaining accurate data. Regular validation and adherence to best practices will help you achieve meaningful insights into material behavior, structural integrity, and mechanical performance. Whether you're monitoring bridges, testing materials, or optimizing vehicle components, strain gauges play a pivotal role in advancing engineering knowledge and ensuring safety across a wide range of applications.

To recap

1. What is a strain gauge?

A strain gauge is a sensor that measures mechanical strain or deformation in materials. It changes resistance as the material it's attached to undergoes strain, allowing for precise strain measurements.

2. How does a strain gauge work?

Strain gauges operate based on the principle that the electrical resistance of certain materials changes when they're stretched or compressed. This change in resistance is proportional to the strain experienced by the material.

3. What are the types of strain gauges?

Common types of strain gauges include wire, metal foil, semiconductor, bonded resistance, piezoelectric, and optical strain gauges. Each type has unique characteristics and applications.

4. How is a strain gauge installed?

A strain gauge is attached to the surface of the object using adhesive. The gauge's pattern follows the anticipated strain direction. Proper surface preparation, bonding, and alignment are crucial for accurate measurements.

5. Why is calibration important for strain gauges?

Calibration establishes the relationship between the strain-induced change in resistance and the actual applied strain. This process ensures accurate and reliable measurements.

6. What factors affect strain gauge accuracy?

Factors like installation quality, environmental conditions (temperature, humidity), calibration accuracy, equipment precision, and material properties can impact the accuracy of strain gauge measurements.

7. What are strain gauge bridges?

Strain gauge bridges are electrical circuits that measure changes in the resistance of strain gauges. The most common bridge configuration is the Wheatstone bridge, which balances resistance changes to amplify the strain signal.

8. What applications are strain gauges used for?

Strain gauges are used in structural engineering, aerospace, automotive, biomechanics, material testing, industrial machinery, geotechnical engineering, and more to measure and monitor mechanical strain.

9. Can strain gauges measure dynamic strain?

Yes, strain gauges can measure dynamic strain in applications involving rapid load changes or vibration. However, they may not be as suitable for high-frequency dynamic measurements compared to other sensor types.

10. How do I choose the right strain gauge for my application?

Consider factors like sensitivity, pressure range, environmental conditions, accuracy needs, installation complexity, and your budget. Consult experts, review manufacturer specifications, and choose a gauge that matches your application's requirements.

References

https://www.michsci.com/what-is-a-strain-gauge/

https://www.electrical4u.com/strain-gauge/

https://www.allaboutcircuits.com/textbook/direct-current/chpt-9/strain-gauges/

https://dewesoft.com/blog/strain-gauge-applications

https://tml.jp/e/knowledge/strain_gauge/about.html

https://www.allaboutcircuits.com/textbook/direct-current/chpt-9/strain-gauges/

https://www.bcmsensor.com/product-category/strain-measurement/metal-foil-strain-gauges-for-transducer-application/

https://www.bcmsensor.com/products/semiconductor-strain-gauges/semiconductor-strain-gauges/

https://www.geo-instruments.com/technology/strain-gauges-for-steel-concrete/

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