Contactless Interferometric Strain measurement

Fangjian Wang, M.Sc.

The sensor is being researched within a ZIM project in cooperation with SincoTec GmbH.

State of the art / Motivation

• Conventional strain measurement with strain gauges:
  • Measuring errors due to contact (adhesives)
  • Sensitive towards strain perpendicular to the measuring direction
  • Influences of temperature at the measuring location
  • Complex measuring set-up with several measuring points
• Current optical contactless strain sensors, e.g. Digital Image Correlation (DIC)
  • Low sampling rate
  • Higher noise
  • No electrical real-time measurement due to post-processing

Methods

• Sensor principle of in-plane Laser Doppler Vibrometry transferred to strain measurement
• A stripe pattern is generated by two laser beams crossing each other on the measuring surface. The movement of the scattering body through the stripe pattern forms the intensity modulation, whose frequency is proportional to the velocity perpendicular to the stripe. In-plane deflection of each measurement point is proportional to the integration of the frequency. The strain results from the difference of the deflection at two measuring points in relation to the distance.

Problem of speckle noise and signal dropouts:

• Low light power due to dark speckle - speckle noise
• Carrier-to-noise ratio (C/N) of the detector signal falls below the frequency modulation threshold (FM threshold) - Signal dropout
• In case of signal, dropout noise > signal power, strain not measurable

Solution of polarization diversity:

• For each measurement point two photodetector detects light in the orthogonal polarization states
• Stochastically independent detector signals
• Probability for simultaneous signal dropouts of both detectors is significantly lower than for a single dropout

• Design of the principle of a novel contactless strain sensor
• Simulation of the optimal sensor setup
• Realization of an experimental setup of the sensor
• Integration of strain sensor in resonance testing machine of SincoTec
• The optical sensor provides comparable measurement results to a strain gauge

Stand der Technik / Motivation

• Dehnungsmessung beim Hochgeschwindigkeitszerreißprüfung mit einer Geschwindigkeit von 30 m/s, Aufreißvorgang in nur wenigen ms oder µs
• Konventionelle Messverfahren wie Dehnungsmessstreifen (DMS) oder Speckle-Korrelation nicht geeignet
• Laser-Doppler-Dehnungssensor mit der hohen räumlichen und zeitlichen Auflösung, geeignet für Hochfrequenz- und Hochgeschwindigkeitsmessungen

Versuchsaufbau

• Laser-Doppler-Dehnungssensor mit Hochgeschwindigkeitszerreißprüfmaschine (von Sincotec GmbH

• Realisierung des Sensoraufbaus mit Signaldiversität. Einfluss von Signaldropout und Specklerauschen wird reduziert.
• Verbesserung des Signal-Rauschen-Verhältnis (SNR). Die hohe Rissgeschwindigkeit von 30 m/s erfordert eine sehr hohe Demodulationsbandbreite des Dehnungssensors. Das SNR sowie die Auflösung ist nicht schlechter als bei Dehnungsmessung in Betriebsfestigkeitsprüfung trotz der sehr hohen Bandbreite.

Publications

1. F. Wang, S. Krause, und C. Rembe:
   Signal diversity for the reduction of signal dropouts and speckle noise in a laser-Doppler extensometer
   ^{In: Measurement: Sensors, 2022. DOI: 10.1016/j.measen.2022.100377.}
2. F. Wang, S. Krause, J. Hug and C. Rembe:
   A Contactless Laser Doppler Strain Sensor for Fatigue Testing with Resonance-Testing Machine
   ^{In: Sensors 2021, 21(1), 319. DOI: 10.3390/s21010319.}
3. F. Wang and C. Rembe:
   Kontaktloser interferometrischer Dehnungssensor.
   ^{In: Tagungsband 3. Niedersächsisches Symposium Materialtechnik. pp. 51-65, 2019. DOI: 10.21268/20190312-3}
4. F. Wang and C. Rembe:
   Entwurf eines kontaktlosen interferometrischen Dehnungssensors.
   ^{In: tm - Technisches Messen. Vol. 85, Issue s1, pp. 117-123, Sept 2018. DOI: 10.1515/teme-2018-0045.}