Laying strain gages correctly is critical for obtaining accurate and reliable test data in experimental stress analysis. Whether used in materials testing, structural health monitoring, or component validation, the quality of strain gage installation directly affects the quality of the data.
Here are some best practices for laying strain gages.
Surface Preparation: A clean, smooth surface is essential for proper gage adhesion and signal transmission. Begin by degreasing the area with a solvent like isopropyl alcohol or acetone to remove any oils or contaminants. Follow this by mechanically abrading the surface using fine sandpaper or emery cloth to remove oxidation and improve adhesion. After abrasion, clean the area again using a lint-free wipe and solvent. A properly prepared surface should be clean, dry, and slightly rough to the touch.
Handling: Strain gages can be easily damaged by improper handling. Debris or oils underneath or on the gauge will negatively impact the quality of the bond resulting in gauge drift and overheating. Use tweezers whenever possible to handle the gage and avoid touching the sensing grid or backing material with your fingers.
Store gages in a dry, dust-free environment until they are ready to be used. Static discharge can also damage some strain gages, so consider using an anti-static mat and wrist strap during handling and installation.
Adhesives: Choosing the best adhesive depends on the testing environment and application. Cyanoacrylate adhesives are commonly used for room-temperature testing and offer quick curing times. For high-temperature or long-term installations, epoxies may be more suitable.
Always follow the manufacturer’s instructions for mixing, application, and curing time. Apply the adhesive evenly and press the gage into place with consistent pressure to avoid air bubbles or uneven bonding.
Alignment: Strain gages must be aligned correctly to the principal stress direction. Use alignment marks and magnification tools to place the gage accurately. Misalignment can lead to measurement errors and compromise the results. A gage alignment fixture or template can improve accuracy, especially when installing multiple gages in the same orientation.
Wire Attachment: Solder the lead wires to the gage tabs using a temperature-controlled soldering iron and appropriate solder. Excessive heat can damage the gage, so use a heat sink if necessary.
After soldering, strain-relieve the wires by anchoring them to the test article with adhesive or tape. This prevents mechanical loading on the gage from wire movement or vibration.
Protection: Strain gages are sensitive to moisture, temperature changes, and other environmental factors. After installation, protect the gage with a suitable coating like silicone rubber, polyurethane, or a protective epoxy depending on the test conditions. This sealing helps ensure consistent readings and prolongs gage life.
Testing: Before beginning the actual test, verify the integrity of the strain gage installation. Use a resistance meter or a strain indicator to check for open circuits, short circuits, or unusual resistance values. A “null balance” test can confirm that the gage is properly zeroed.
Paying close attention to these details, test engineers can ensure that the strain gage installations provide accurate, repeatable, and meaningful data. This not only saves time and reduces errors but also increases confidence in the test results.
Want to learn more? Contact us if you have questions about this or other data collection procedures. We’ll be happy to offer some simple advice or discuss your test in more detail.