A case study describing a strain gauge DAQ system used to validate several new designs of a heavy lift lattice boom crane to comply with SAE J987 standards.
When it comes to measuring structural stresses and load, every application is unique, and sometimes that means using a different solution. In some cases where strain gauges are applied, it can be advantageous or even necessary to use weldable strain gauges instead of chemical bonding the strain gauges to the specimen.
Since many epoxies used to chemically bond strain gauges require specific heat and pressure for curing, it can be difficult or even impossible to use this method on large or irregular structures. Weldable strain gauges offer an advantage in this situation as they eliminate the need to clamp and cure any epoxies for bonding. In addition, weldable gauges can be installed in a variety of environments and weather conditions which offers additional advantages over traditionally bonded strain gauges. Although weldable strain gauges are applied differently, they function in much the same way as their bondable counterparts.
To properly install a weldable strain gauge, you will need:
- Proper PPE
- Weldable Gauges
- Straight Edge
- Scribe, Pen or Marker
- Grinding Pads
- Spot Welder
Always remember to have the proper safety equipment on hand, such as eye protection and gloves, as well as any PPE required by your environment.
When installing a weldable strain gauge, you must first prep the area. Unlike bondable gauges, the area does not need to be polished to a fine degree. Simply degrease the gauge area, sand down any paint, coatings or excess debris and ensuring the area is purely metallic and free of chemicals is enough. A clean metal surface is important to the welding process.
After the part has been cleaned, you can position the gauge. Most weldable gauges come marked so you can align the grid properly. When the gauge is properly aligned, spot weld the gauge on either side of the gauge along the centerlines, to hold it firmly in position and prevent shifting as the gauge is welded further. Once secure, the gauge should be welded all around the carrier surface, as illustrated below.
- Clean the area to be bonded (Remove all paints, coatings, residue and debris until the surface is smooth).
- Mark center-lines for the gauge location with scribe and straight edge.
- Place the gauge on the part, lining up the center-lines.
- Spot weld the gauge on each line, to secure it in place.
- Weld the entire perimeter of the gauge in the pattern shown above.
When troubleshooting structural failures or validating FEA models through testing, strain gauge rosettes are used to find the full state of strain at areas of concern around the structure. iTestSystem’s Rosette Analysis tool is used to calculate the principal strain, principal strain angle, shear strain, principal stress, and other values from strain gauge rosette data. This video shows how to use the Rosette analysis tool.
For questions about using the Rosette Analysis tool or other iTestSystem analysis tools contact Chase Petzinger.
Download your free version of iTestSystem today.
Image1: Shaft torque strain gauge installation example for field testing
Our engineers and technicians have epoxied, soldered and spot welded strain gauges for applications ranging from high temperature exhaust systems to miniature load cell measurements. Every application requires a unique understanding of the strain measurement requirements including installation environment.
If the strain gauge installation is to survive in the field you must plan for the conditions it will undergo. Three important variables that you should account for are temperature range, liquid exposure, and potential impact forces. These variables determine the type of strain gauge, epoxy, solder, wiring, coating, and impact/wear protection to use in the application. The table below shows which variables affect your installation choices.
Table1: Strain gauge installation variables
For more information about ITM’s strain gauging services contact Ryan Welker at email: firstname.lastname@example.org or phone: 1.844.837.8797 x702
When making strain measurements it is important to perform a shunt calibration both before and after the actual measurements are acquired. Shunt calibrations ensure accurate strain measurements by adjusting the sensitivity or gain of the data acquisition equipment to compensate for leadwire resistance and other scaling errors.
iTestSystem takes advantage of the shunt calibration circuits included in the National Instruments (NI) cDAQ strain modules. The NI-9235, NI-9236, and NI-9237 strain modules contain an internal shunt resistor that when switched on “shunts” across one leg of the strain circuit’s wheatstone bridge. When active, the shunt resistor offsets the strain measurement by a constant strain which is calculated using the equivalent shunt calculation. The equivalent strain/shunt value is dependent on the strain gauge configuration, gauge resistance, shunt resistance, gauge factor, and material properties.
In the latest version of iTestSystem, we added a built-in strain gauge shunt equivalent calculator that can be accessed from the strain configuration page. This calculator has allowed us to speed up the calibration process and eliminate hand calculation errors.
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ITM provides software development, structural and mechanical testing services, industrial monitoring, strain gauging, and data analysis solutions to clients on six continents. ITM is a recognized National Instruments Gold Alliance Partner.