Strain is typically measured using a strain gauge sensor bonded to the object of interest. As the object deforms under load, the strain gauge stretches or compresses, causing a small change in its electrical resistance. By wiring the gauge into a Wheatstone bridge circuit and providing a stable excitation voltage, one can measure the minute voltage imbalance that corresponds to the strain. In practice, the bridge output (in millivolts) is amplified and converted to strain (ε) using the gauge’s known gauge factor. This method allows extremely small deformations to be quantified (e.g., 500 µε causes only ~0.1% change in a 120 Ω gauge). Other methods, like optical digital image correlation or fiber optic gauges, exist, but the bonded electrical strain gauge is the most common for directly measuring material strain.

To measure strain with a strain gauge, you must bond the gauge to the test object and connect it to a measurement device (usually via a Wheatstone bridge). In a quarter-bridge setup, the strain gauge forms one arm of the bridge, with fixed resistors in the other arms. When the object experiences strain, the gauge’s resistance changes, producing a small differential voltage in the bridge. A strain gauge amplifier or DAQ system then amplifies this output. The amplifier is configured with the gauge’s specifications (gauge factor, bridge type, excitation voltage, etc.), so it can convert the measured millivolt-per-volt signal into strain units. In summary, install the gauge properly, wire it into a bridge, provide excitation, and use instrumentation to read the voltage change, which is proportional to strain.

Reading a strain gauge’s output involves converting the raw electrical signal to mechanical strain. First, measure the gauge’s output voltage (often in mV) via the Wheatstone bridge under no-load (for zero offset) and under applied load. The bridge equation and gauge factor give the relationship between the voltage and strain. For practical calibration, a known strain can be applied (or a shunt calibration resistor used) to produce a reference voltage change. Modern DAQ software simplifies this: you input the gauge factor and bridge type, and the system scales the mV/V reading into strain (microstrain) automatically. For example, if a quarter-bridge with gauge factor 2.0 outputs 1 mV/V under load, the software will calculate the strain as ε ≈ 500 µε (since 1 mV/V corresponds to 0.0005 strain in this case). In essence, you measure the bridge output and apply the gauge’s calibration factor to get strain.

A strain gauge is a sensor that converts mechanical deformation into a change in electrical resistance. It typically consists of a thin metallic foil pattern arranged in a grid and bonded to a flexible backing. When glued to a structure, the strain gauge deforms along with the material. Stretching the gauge makes its conductors longer and thinner, increasing resistance; compression does the opposite. By measuring this resistance change (usually with a Wheatstone bridge and instrumentation), one can determine the strain in the material. Strain gauges were invented in 1938 and are widely used in experimental stress analysis and sensor construction because of their accuracy and simplicity.

Strain is a dimensionless quantity because it is defined as the ratio of change in length to original length (ΔL/L). Therefore, it has no units. In practice, strain is often expressed in terms of a small unit like microstrain (µε), which is 1×10^(-6) (i.e., one part per million) of strain. For example, a strain of 500 µε means a 0.000500 (0.05%) change in length. Sometimes you will see strain written as “in/in” or “mm/mm” to emphasize it’s a ratio, but these are technically unity. Using microstrain makes the numbers more convenient since typical structural strains are on the order of tens to thousands of µε.

• Choosing the right strain gauge involves several considerations about your application:
  • Gauge pattern and geometry: Select a gauge configuration that matches the strain field. For instance, use linear gauges to measure uniaxial strain, or a rosette (with 2 or 3 gauges in known orientations) to measure strain in multiple directions.
  • Gauge length: The length of the strain-sensitive grid should suit the size of the strain region. A shorter gauge (e.g., 3 mm) measures strain at a more local point, whereas a longer gauge (e.g., 10 mm) will average the strain over a larger area – useful if the strain field is not uniform.
  • Resistance value: Common foil strain gauges come in 120 Ω or 350 Ω. A 120 Ω gauge draws more current (more self-heating) but is often more robust and cost-effective; a 350 Ω gauge gives a larger output signal and is better for use with long lead wires (less signal loss). For high-precision or battery-powered setups, higher resistance is preferred to minimize heating and power draw.
  • Temperature compensation: Choose a gauge alloy and backing that match the thermal expansion of your specimen. Strain gauges are available with self-temperature-compensation codes (e.g., matched to steel, aluminum, etc.) so that thermal expansion of the specimen doesn’t create spurious strain readings. Ensure the gauge’s stated compensation (often indicated by a code like “Matching Coefficient 13 for steel”) suits your material.
  • Environment and bonding: Consider operating temperature range and environmental conditions. Some gauges have special backing materials for high temperatures or are waterproofed for outdoor use. Likewise, select an appropriate adhesive and protective coating if needed.
  • Connections and installation: Gauges come with different leadwire options (solder pads, pre-attached leads, etc.). If your application faces vibration or repeated loading, gauges with flexible leads or solder tabs that relieve strain on the wire connections are beneficial.

In summary, match the gauge’s pattern, length, resistance, and material properties to the mechanics and environment of your test. For example, a 350 Ω linear gauge of moderate length with self-compensation for steel, and a 3-wire lead, might be chosen for measuring bending strain on a steel beam. Always consult the manufacturer’s selection guide – key criteria typically include geometry, grid material, backing, length, resistance, and temperature response.

“Carrier frequency” generally refers to a high-frequency signal that is used as a carrier for transmitting information or for excitation in measurement systems. In communications, for example, a radio station has a carrier frequency (say 100 MHz) which is modulated with audio signals. In the context of sensors and instrumentation, carrier frequency excitation is often used with bridge sensors (like strain gauges or load cells) to improve measurement stability. Instead of a DC excitation, an AC excitation at a fixed carrier frequency (e.g., a few kHz) is applied to the sensor bridge. The sensor’s resistance change then modulates this carrier signal (similar to AM modulation). The measurement device demodulates the returned signal to determine the resistance change corresponding to strain or force. This AC carrier technique offers advantages: it can minimize the effect of thermal EMFs and 1/f noise, and it allows using narrow-band filtering to reject noise at other frequencies.
For example, a carrier-frequency amplifier might drive a strain gauge bridge with a 5 kHz sine wave. When the strain gauge is stressed, it modulates that 5 kHz signal slightly. The amplifier only listens at 5 kHz (and not at DC), so it ignores drift and low-frequency noise, greatly improving stability and noise rejection. The output is then demodulated back to a DC value proportional to strain. In summary, the carrier frequency is the frequency of the excitation or carrier wave being used. In a measurement system context, it’s a known, fixed frequency at which the system operates to encode the measurement signal. For communications, the radio wave frequency carries the information. In both cases, it’s the central frequency of a system that is being modulated with the desired information.