Should I Use A Weldable Or Bondable Strain Gage?
Since weldable strain gages and temperature sensors were introduced, customers have regularly asked, “Should I use a weldable or a bondable gage?” The answer depends upon many factors concerning the test application and environment, although the bondable variety is normally the preferred choice when installation conditions permit.
In some test applications, weldable sensors can offer very significant advantages over bondable sensors. For example, they are ideal for use on large test structures when it is not practicable to apply clamping pressure to the gage installation during the adhesive cure process. Weldable sensors can also be an excellent substitute for bondable sensors when the temperature of the test surface is below the minimum required for proper cure of the boding adhesive, and cannot be easily raised.
Because they are spot-welded rather than adhesively bonded, weldable sensors can be readily installed in field application under a wide variety of weather conditions. The only requirements are that the installation area be properly cleaned and dried, and that the temperature be suitable for any protective coating system also used in the installation.
In certain applications having physical or time constraints, it may be desirable to preattach leadwires in the laboratory or at the test site prior to installation. This is possible with both bondable and weldable gages. However, clamping requirements can make this procedure difficult when certain sizes of soldered connections and instrumentation leadwires are present on bondable sensors. Weldable gages require no clamping and can then be easily installed – with or without preattached leadwires – to any material surface that can be readily spot-welded. The surface contour can range from flat, to a radius of curvature as small as one-half inch (13 mm), depending upon gage type.
When properly installed on a test part with a substantial cross-section, the measurement accuracy of weldable gages will likely be within a few percent of that obtainable with a bonded gage installation. The primary error source is the metal shim incorporated into the weldable gage. In addition to adding stiffness, it raises the gage grid above the surface of the test part, effectively shifting the neutral axis in bending applications. Performance of weldable gages is dependent to a significant degree upon the integrity of the spot-welds anchoring the gage metal shim to the test part surface. The welds must surround the foil sensing grid of the strain gage in order to reproduce in the weldable shim the same strain condition that exists in the test article.
Micro-Measurements standard weldable strain gages are available as single-element strain gages, and as two-element “T-rosettes”. The two CEA-Series patterns are available with a choice of either 120- or 350-ohm grids. Those in the LWK Series are available only with 350-ohm grids and preattached leadwires. LEA-Series weldable strain gages are both precabled and polymer-coated for water immersion and protection from weather. Micro-Measurements also offers a weldable temperature sensor to complement the gages.
For weldable strain gages, following is a list of materials needed and links to products: