Optical strain measurements on fast moving fiber reinforced polymer rotors using diffraction gratings

In-situ measurements of the deformation and of the structural dynamical behavior of moving composite structures, such as rotors made of glass fiber reinforced polymers (GFRP), are necessary in order to validate newly developed simulation models. Local methods like strain gauges and fiber Bragg gratings lack spatial resolution, while contactless optical methods like image correlation or speckle interferometry suffer from noise effects in the presence of fast rigid body movements. A novel compact sensor-based on the diffraction grating method-is introduced for spatially and temporally resolved strain measurement. The use of a line camera allows the measurement of vibrations up to several tens of kHz. With a scanning movement, strain fields at submillimeter resolution can be recorded. The use of two diffraction orders and an objective lens reduces cross sensitivities to rigid body movements on the strain measurement by two to three orders of magnitude. A validation on a GFRP probe was conducted in a quasi-static tensile test with an optical extensometer up to 14500 mu epsilon. Furthermore, a strain measurement on a moving rotor at surface speeds up to 75 m/s was performed and the results were compared with those of strain gauges as a gold standard. The statistical standard deviation was around 10 mu epsilon and independent of the rotational speed.