In this article, a non-contact laser Doppler strain sensor designed for fatigue testing with the resonance-testing machine is presented. The compact sensor measures in-plane displacements simultaneously from two adjacent points using the principle of in-plane, laser-Doppler vibrometry. The strain is computed from the relative displacements divided by the distance between these two points. The optical design, the mathematical model for estimating noise-limited resolution, the simulation results of this model, and the first measurement results are presented. The comparison of the measurement results of our sensor with the results of a conventional strain gauge shows that our design meets the measurement requirements. The maximum strain deviation compared to conventional strain gauges of the laser-Doppler extensometer is below 4×10−5 in all performed experiments.

ZusammenfassungIn diesem Beitrag stellen wir einen Lasersensor vor, der die mechanische Dehnung bei der Betriebsfestigkeitsprüfung in Schwingprüfmaschinen kontaktlos misst. Die Auslenkungen an zwei nebeneinanderliegenden Messpunkten in der Dehnungsrichtung werden mit dem Sensor interferometrisch erfasst. Die Differenzauslenkung bezogen auf den Abstand der beiden Messstellen entspricht der Dehnung. Im Vergleich zum Stand der Wissenschaft zeigen wir einen verbesserten Sensoraufbau und ein erweitertes mathematisches Modell. Außerdem werden erstmals Testmessungen mit unserem Versuchsaufbau durchgeführt. Schließlich bestimmen wir den Beitrag des Rauschens, der sich mit unserer derzeitigen Signalverarbeitung ergibt. Unsere jetzige Realisierung erlaubt eine Auflösung der Dehnungsmessung von 0,02 ‰ bei einer Bandbreite von 1,25 kHz.

The uncertainty in fatigue lifetime calculations brings up the necessity of experimental verification. Several efforts have been undertaken in the past to improve and introduce new test methods. An alternative method for variable amplitude loading using resonance test facilities is introduced and discussed. A comparison of test results on a resonance test facility with a new control system and a servo hydraulic test facility shows no significant difference in the test results. The compatibility of the test results to those results of accepted fatigue life tests is proven for three test series of different specimens. The introduced resonance test control offers a new way for fatigue life tests, reducing the cost and time, as well as increasing the safety of products because it permits testing to much longer lives in a relatively short time.

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