Impact testing with a continuously-scanning LDV

The Scanning Laser Doppler Velocimeter (SLDV) has been used as a transducer in experimental modal analysis for many years, offering distinct advantages of non-contact, multiple-point measurement for structural vibration. Over the past decade, a variant of this basic method has been developed in which the laser is scanned continuously, rather than step-wise as is conventional. This approach gives significant further advantages over the point-point measurement technique by using spectral analysis of the scanned response signal to give spatial information in place of temporal i.e. mode shapes, instead of frequency content. Recently, the continuous scanning capability has been used to facilitate the measurement of steady-state vibration of rotating structures by combining area scanning with tracking the rotating structure. The continuously-scanning technique has been refined for steady-state, harmonic vibration and attention has recently been turned to explore the possibility of using the same approach to measure transient vibration. The basic concept here is to scan across the surface of a vibrating structure, as before, but this time during a period of transient free-response following an impact excitation. The resulting signal from the laser transducer is highly complex, containing in its spectrum a mixture of temporal information (the eigenvalues of the structure) and spatial information (the eigenvectors), all in a single time history. This paper presents some encouraging results from the application of this new concept to impact measurements on a simple beam and on a complex bladed disc structure.