Structural Health Monitoring on Structural Bonds Using Strain Gauges

How can the condition of a structural bond be permanently monitored in operation? Can damage be detected early and reliably by applying Structural Health Monitoring (SHM)?

Researchers at RWTH Aachen University’s Institute of Structural Mechanics and Lightweight Construction are investigating these questions. With the help of customer-specific strain gauges, production-related deviations in the calculated strain gauge position could be compensated for during post-processing.

Problem

In order to detect the structural condition of a bonded joint and to ensure reliable force transmission, a strain gauge that could compensate for even minor misalignments was needed.

Solution

The development of a custom-built strain gauge measurement grid provides a uniform strain distribution around the measurement position to increase measurement accuracy.

Results

By using the customized measuring grid, even small strain changes can be recorded over a short measuring distance. The exact arrangement of the measuring grids relative to each other allows conclusions to be drawn about the incorrect positioning of the strain gauge, so the measured values can be corrected to the desired position.

Permanent Monitoring of Bonding

Adhesive bonds cannot be tested completely non-destructively. This gives motivation to permanently monitor bonded joints to record their structural condition and to ensure reliable force transmission.

Such monitoring can already be accomplished very easily with a single strain gauge, which is bonded to the component in a particularly damage-sensitive position, the so-called zero strain point^[1]. This special position, a simple overlap bonding on the surface of the joined parts, is characterized by the fact that there are no strains if bonding is not damaged. As soon as damage occurs, the strain distribution shifts, and a clear measurement signal can be recorded.

The challenge of this promising SHM approach is to position the strain gauge as accurately as possible in the previously calculated position. However, both small misplacements (<200 µm) and variations in bond thickness cause a shift in the measurement signal. The resulting change in strain must then be corrected.

strain gauge measuring grids — Fig. 2: The measuring grids of the customized strain gauge

To measure the strain distribution at several defined points close to the measuring points, a customized strain gauge was designed and manufactured in cooperation with HBK. The measuring grids were offset longitudinally by 0.5 mm and arranged next to each other on the carrier foil (see Fig. 2).

Testing

To evaluate the suitability of use, the developed strain gauges were applied to both sides of the bond and the overlap bond was loaded to failure in a tensile test. As expected, the three measurement grids yielded three shifted measurement curves.

The use of three measuring grids, shifted relative to each other, made it possible to correct the measured values afterwards to the actual optimum position (see Fig. 3).

For the particularly sensitive method of damage assessment, this provides a great added value, since it already reacts sensitively to the slightest deviations from the optimum positioning.

The results of the first test run clearly show that remarkable results were achieved with HBK’s customized strain gauges and that the demanding measurement task could be completely fulfilled.

Fig. 3: The strain curve of the strain gauge and the corrected measured value

About the Institute for Structural Mechanics and Lightweight Design at RWTH Aachen

The Rheinisch-Westfälische Technische Hochschule Aachen (RWTH Aachen University) is the largest university for technical studies in Germany. The Institute of Lightweight Structures was founded in 1955 and cooperates closely with the German Research Institute for Aeronautics and Astronautics (Deutsche Versuchsanstalt für Luft- und Raumfahrt e.V.). In the recent past, teaching and research focuses have been expanded to include structural mechanics and structural health monitoring, which is also reflected in the name of the Institute for Structural Mechanics and Lightweight Design (SLA).

^{[1] Sadeghi, M. Z.; Weiland, J.; Preisler, A.; Zimmermann, J.; Schiebahn, A.; Reisgen, U.; Schroeder, K. U. Damage Detection in Adhesively Bonded Single Lap Joints by Using Backface Strain: Proposing a New Position for Backface Strain Gauges. Int. J. Adhes. Adhes. 2020, 97, 102494.}