A Practical Guide to Choosing the Right Strain Gauge for Testing

1. Design and Configuration

Strain gauges come in various configurations for different structural layouts and force directions. Based on the strain direction, loading type, and the object’s surface, you can choose from the following variants:

• Linear: Ideal for assessing strain in a single direction
• Double Linear: Dual strain gauges for measuring strain from opposite surfaces
• Shear: Gauges with 45-degree grids to measure shear or torsional strain, typically seen on rotating shafts
• Full-Bridge: Ideal for high-sensitivity applications like automotive testing, torque measurement, aerospace testing, and force measurement
• T-Rosette and Stacked Rosette: Gauges with specific angles (45° or 90°) to measure from unknown strain directions

2. Environmental Conditions

The type of gauge you choose will also depend on the temperatures where you’ll use it. Here are the common types of gauges to consider, based on the testing temperature and conditions:

• Cryogenic gauges for liquid nitrogen testing (temperatures between -269°C and -196°C)
• Encapsulated (waterproof) gauges for testing in marine areas with high humidity.
• High-temperature gauges for testing in temperatures up to 800°C.
• Low-elastic gauges for testing in chemical plants

3. Gauge Length

Another key consideration is the strain gauge’s length, which influences the spatial averaging and resolution during testing. Generally, you’ll need to pick between two lengths:

• Short Length: Gauges 1mm or shorter, used for testing stress concentration areas like notches and narrow surfaces where fine spatial resolution data is required.
• Long Length: Gauges 5mm or longer, ideal for concrete, composites, and other inhomogeneous materials.

4. Electrical Resistance

Strain gauges are generally available with electrical resistance values of 60 Ω, 120 Ω, 350 Ω, and 1,000 Ω, which affects power consumption, signal stability, and testing performance. Based on the resistance, there are two gauge options to choose from:

• High-Resistance: These gauges are for 350 Ω and above. They reduce the strain’s self-heating, require less power, and are ideal for longer cable runs.
• Low-Resistance: These gauges are for resistance between 50 Ω and 120 Ω. They aren’t highly susceptible to electrical noise and aren’t prone to impact from insulation resistance fluctuations.

5. Accounting for Testing Temperatures

Changes in the testing temperatures often affect both the gauge and the test specimen. Ensuring measurement integrity is key to getting accurate readings, and for this, follow these tips:

• Opt for self-temperature-compensated (STC) strain gauges, which are calibrated to align with the thermal expansion of various materials, such as titanium, steel, or plastics.
• Use temperature-matched adhesives with the strain gauge and check the adhesive rating for minimum or maximum temperature exposure.

6. Mounting Method

Choosing the right welding or bonding method is critical for accurately mounting the gauge, and this will depend on the testing materials and applications. Here’s what you should follow:

• Choose adhesive bonding if you need the gauge to test metals, plastics, and composites. Remember, gauge performance may decrease at higher temperatures due to adhesive leakage.
• Choose welding for long-term or high-temperature testing conditions where adhesive failure can be risky.