Modal testing using high speed digital speckle pattern interferometry

Abstract

This thesis introduces the application of a high-speed speckle pattern interferometer (SPI) to perform quantified impact modal testing. The interferometer acts as a non-contact multipoint vibrometer that removes the mass-loading effects of contact transducers to improve measurement accuracy and decreases measurement time by the use of multiple measurement points. A temporal phase-stepped CMOS high-speed SPI system was used to capture the transient vibration response of two overlapping plates, the system had a maximum surface velocity of 1.4 mm/s and the results were compared to accelerometer data and correlated against a finite element model. To extend the surface velocity to 2.7 mm/s a spatial phase-stepped CMOS SPI system was used and compared to the finite element model. Both the temporal phase-stepped system and the spatial phase-stepped system showed high performance for quantified modal testing: showing high correlation for the natural frequencies and the modal assurance criterion correlated over 60% for the first six modes of vibration. The interferometer was improved by the application of spatial phase-stepping but was still limited by the maximum measurable velocity. The thesis also applied the SPI to the novel measurement of traveling waves on a centre-clamped disc. Traveling waves can be caused by structural problems or damage and are difficult to measure without the ability to capture the relative phase across multiple points. This was achieved through the spatial phase-stepped CMOS SPI system and the traveling waves were excited on the disc through a frequency modulated signal that excited the degenerate modes that occurred within 1 Hz of the disc’s second resonant frequency. The multipoint system identified the waveshape, direction and was able to show the ratio of standing wave to traveling wave in the measurements.