Errors in shearography measurements due to the creep of the PZT shearing actuator

Abstract

Shearography is a modern optical interferometric measurement technique. It uses interferometric properties of coherent laser light to measure deformation gradients on the µm/m level. In the most common shearography setups, the ones employing Michelson interferometer, the deformation gradients in both x-and y -direction can be identified by setting angles on the shearing mirror. One of the mechanisms for setting the desired shearing angles in the Michelson interferometer is using the PZT actuators. This paper will reveal that the time-dependent creep behaviour of the PZT actuators is a major source of measurement errors. Measurements at long time spans suffer severely from this creep behaviour. Even for short time spans, which are typical for shearographic experiments, the creep behaviour of the PZT shear actuator induces considerable deviation in the measured response. In this paper the mechanism and the effect of the PZT creep is explored and demonstrated with measurements. For long time-span measurements in shearography, noise is a limiting factor. Thus, the time-dependent evolution of noise is considered in this paper, with particular interest in the influence of the external vibrations. Measurements with and without the external vibration isolation are conducted and the difference between the two setups is analyzed. At the end of the paper some recommendations are given for minimizing and correcting the here studied time-dependent effects.

Calibration and correction procedure for quantitative out-of-plane shearography

“…Thus the shearography measurements are affected by the air-currents, especially the warm air rising between the camera, the laser and the observed surface. The environmental disturbances can be considered random and their influence is increased with the duration of the experiments [21]. The high-frequency noise is related to the laser speckle and the speckle decorrelation and the influence of it can be reduced by filtering.…”

“…The PZT material can exhibit creep behaviour, which can affect the measurement. Research by our group [21] showed that the PZT creep can create a considerable error in the measurements. This error is more pronounced with increasing duration of the measurement, so it is important mostly for long time-span measurements.…”

“…Shearography is generally considered to be less susceptible to environmental vibrations than the other interferometric techniques, as the gradient measurement is not affected by the rigidbody translations. However, our previous work [13,14,21] has shown that vibration isolation can greatly benefit noise reduction in the measurements. The refraction between zones of air with different temperatures can cause transient optical distortion.…”

“…In the shearography camera, the change in the shearing distance can be performed either mechanically or by piezo-electric elements. It was shown previously [21] that using the piezo-elements to control the shearing distance can cause measurement errors due to the creep response of the piezo-electric material. Thus, for the purpose of the experiments in this paper, the shearing distance was induced mechanically.…”