STRAIN GAGES AND STRUCTURAL HEALTH MONITORING

The safety of bridges, buildings, tunnels, and other structures depends upon the expertise of engineers and the accuracy of the instruments they use. The foil strain gauges produced at Micro-Measurements—though tiny—have a big impact on the overall health of structures. In this blog, we explain how to properly install a foil strain gauge for accurate Structural Health Monitoring (SHM).

 

What is structural health monitoring? 

 

Structural Health Monitoring (SHM) is a catch phrase for the monitoring of any infrastructure. SHM can be applied to bridges, buildings, dams, tunnels, wind turbines, industrial facilities, and even light poles. SHM systems provide accurate and near-real-time information regarding the performance and condition of structures.

Through SHM systems will never replace visual inspection and human judgment, when properly deployed they can be used to keep a remote eye on critical structures. These systems minimize inspection cycles, alerts engineers to any changes, and aid in planning and designing maintenance activities. Such protocols increase safety, reduce uncertainty, and lower operating expenses.

 

Foil Strain Gauges for Structural Health Monitoring

 

One of the most common SHM sensors in use today is the foil strain gage. Typically the smallest instrument on site, the foil strain gage in both bondable and weldable form, is the cornerstone of many SHM systems. Bridge Diagnostics, Inc. (BDI) is a leading provider of diagnostic testing, monitoring, and nondestructive evaluation services around the world. BDI’s strain gauge of choice for field application is the Micro-Measurements weldable foil strain gauge. Traditional adhesively bonded foil strain gages are extremely difficult to properly install in outdoor heavy civil environments. In contrast, the Micro-Measurements weldable foil strain gauge minimizes the potential for error and improves the survivability of these field-installed strain gages.

 

Structural Health Monitoring: Tricks for Weldable Sensors

 

In collaboration with BDI, Micro-Measurements has outlined the key steps to properly completing strain gage installations. These tips and tricks have been developed through nearly 30 years of installing weldable foil strain gauges. Here’s an overview:

  1. Be prepared for accessing the instrumented location.

Use lanyards and a pre-assembled toolbox. Also, be sure the installer has practiced the installation and is comfortable with the access method, be it ropes, boat, manlift, or ladder. The more comfortable the installer is, the safer and more effective the installation.

  1. Measure and mark out the gauge location.

Mark out the location with two perpendicular and intersecting lines; this will pinpoint gage location as well as orientation. Be sure to write the dimensions, orientation, and gauge location designation on the surface near the instrumented location. If you forget to document your installation the first time, this makes it easy to recover the information. Also, don’t forget to take a picture to document the installation.

  1. Carefully prepare surface where you will be welding the foil strain gauge.

Ensure the installation surface is dry and free of contaminants like grease or oil. You can use a coarse sanding wheel (80-120 grit) to grind the gauge location and surrounding area to remove paint, rust, pitting, and other surface imperfections. With a finer sander (220+ grit), regrind at the gage location until a smooth, uniform, finish is achieved.

  1. Re-mark the gauge location. 

Using the marked lines outside the sanded area, connect the collinear segments into a single line. The point of intersection between each set of perpendicular lines represents the same gage location that was marked out before surface preparation.

  1. Install the weldable foil strain gauge sensor. 

Tape the gage down to the surface so that the two marked lines run down the middle of the length and width of the gage. After completing the first part of the welding pattern, inspect that the strain gauge is positioned properly. Remove the tape. Complete welding per the provided Micro-Measurements welding pattern.

  1. Attach strain gauge lead wires.

Like adhesive bonding, an environment anything less than ideal makes lead wire attachment difficult. The same care given to weldable gages and procedures should be taken with soldering.

  1. Manage all strain gauge related cables. 

Bundle, zip-tie, or tie cables to the structure for a more professional look. For temporary installations, cables should be run at no more than five-foot intervals. Cables should be secured in conduit for all permanent installations.

  1. Apply protective coating over the gauging area. 

Be sure you are selecting an appropriate proactive coating for the strain gauge location. Also, this protective coating must be adhered to the base mental, not the paint.

 

Weldable Strain Gages (High Temperature).

 Weldable Strain Gages (Standard).

Portable Strain Gage Welding Unit.

 

For more extensive tips and tricks for installing weldable foil strain gauges for SHM, check out the full case study.

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Yuval Hernik

StrainBlog Editor in Chief