Non-contact measurement of vibrations using deferred moiré patterns

Abstract

We measured vertical and horizontal displacements of distant objects based on the moiré effect. The grid attached to the object was photographed and recorded on video camera. The video was processed later in the lab. The method is self-calibrated and does not need special equipment in the field (except for a regular videographer’s apparatus). The objectives of this study were to create a method for non-contact moiré measurement based on camera images and to find a way to deal with anomalous vibrations of structures. The experimental measurements of the fundamental frequency are in accordance with the theory of square beams. Tests with broken models were also conducted. The influence of two opposite factors on the fundamental frequency in the beams with cracks was estimated. In determining the pre-failure condition, the decay rate seems to be a more promising indicator than the fundamental frequency; however, this was only observed when a crack was near the fixed edge. The results and findings can be applied in measuring displacement in various objects; in public safety, and particularly, in distinguishing between normal and abnormal vibration in bridges.