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Structural Testing of High-Strength Components

Strain gauges (SG) are critical when it comes to experimental stress analysis, as well as in the fatigue life analysis of critical components. The effects of real or simulated loads on assemblies or structures can be evaluated using SG-based chains, thus producing excellent results. New high-strength materials such as fiber composites are challenging to SG measuring equipment, especially when components are rated up to their mechanical performance limits. It sometimes happens that strain gauge oscillating for an extended period of time at a high load level weakens and fails earlier than the component under examination.

Resistance to alternating loads

The resistance of a mechanical component to fatigue is described by its resistance to alternating loads. This can be described and plotted by the material-specific S-N curve. In an S-N test, a material sample is loaded cyclically – usually sinusoidally – with a constant amplitude. The test runs until the material fails (e.g. breaks). If this test is now repeated with different amplitudes, and the number of cycles to failure noted in each case, representing the points in a graph produces the S-N curve. The number of cycles to material failure is entered logarithmically on the x axis, the associated amplitude is entered as mechanical stress or strain, on the y axis. The following diagram shows the S-N curves for different materials. The high resistance to alternating loads of fiber composites is clear to see.

 

The S-N curve of a strain gauge

The strain gauges themselves are subject to fatigue and thus have an S-N curve. This is affected by the materials (especially the grid material), the layout and the installation itself. SGs with measuring grids based on nickel-chromium alloys show a better resistance to alternating loads than typical SGs with Constantan measuring grids.  Curved geometries are preferred to angular ones in strain gauge design, to increase the resistance to alternating loads. Solder tabs with strain relief prevent the mechanical stress of the connected cable being transferred, and possibly causing a rupture between the solder tab and the measuring grid. Strain gauge developers have drawn on the experience gained from complex endurance testing to create the appropriate basis. By contrast, the installation is in the hands of the user. It is extremely important here for only thin coatings of adhesive to be applied, and for the solder to be used sparingly. This keeps the solder joints as flexible as possible, and avoids the need for predetermined breaking points. Testing to determine the S-N curve of a strain gauge, however, does not continue until the strain gauge fails completely, because an abort criterion is defined. Zero drift of more than 100 µm/m is usually the abort criterion applied. A typical value for the endurance strength of a standard SG with a Constantan measuring grid is 107 alternating loads at ±1400 µm/m. This is more than enough for metallic materials, but not for high-strength fiber composites (see graphic 1).

Increasing strain gauge resistance to alternating loads

The foil strain gauges of the so-called M series meet the stated criteria. The M series measuring grid consists of a special nickel-chromium alloy, the layout has been designed for optimum resistance to alternating loads. The solder tabs with strain relief are the optimum result of numerous tests with systematic improvements. High-strength materials can be tested in this way. The graphic below shows a direct comparison between the new M series and a universal strain gauge1 (e.g. Y series from HBM). 
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Extreme resistance to alternating loads with optical measurement technology

If the resistance to alternating loads continues to be increased and the S-N curve moves upwards, metal strain gauges can no longer be used. The obvious alternative is optical measurement technology, based on fiber Bragg grating technology (FBG). This technology is based on inscribing a Bragg grating into an optical fiber. The grating reflects a specific wavelength in the optical spectrum. Among other things, this wavelength is strain-dependent. This makes it possible to produce optical strain gauges. The optical fiber has isotropic mechanical properties and essentially does not know the meaning of fatigue, such as what is typically present in metallic materials. The optical fibers can be dynamically loaded up to an ultimate strength of approximately 30,000 µm/m. In fatigue life testing, tests at ±5,000 µm/m have already reached up to 107 load cycles without failure2. Optical fibers can also be embedded. In addition to the extreme, cyclic, high strain application already described, they can also be used where electrical strain gauges cannot be used in principle, such as in very high electromagnetic fields (transformers, high voltage switches, etc.). In fatigue life analysis, modern materials such as fiber composites demand high standards from the measurement technology with regard to resistance to alternating loads. By taking suitable action, HBM was able to increase the strain gauge resistance to alternating loads so that they can be used in most measurements. Optical measurement technology is also the appropriate tool for extreme alternating loads. 1Constantan measuring grid on a polyimide carrier foil2Measured with the "K-OL" optical strain gauge from HBM

About the author

Jens Boersch has been with HBM for 14 years as a product manager. He has worked with nearly all products, including strain gauges, amplifier systems and data acquisition software, in the test and measurement world of HBM. He is based in Darmstadt, Germany.

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