Damage Identification of Beams Using a Continuously Scanning Laser Doppler Vibrometer System

Abstract

A continuously scanning laser Doppler vibrometer (CSLDV) system is capable of rapidly obtaining spatially dense operating deflection shapes (ODSs) by continuously sweeping a laser spot from the system over a structure surface. This paper presents a new damage identification methodology for beams that uses their ODSs under sinusoidal excitation obtained by a CSLDV system, where baseline information of associated undamaged beams is not needed. A curvature damage index (CDI) is proposed to identify damage near a region with high values of the CDI at an excitation frequency. The CDI uses the difference between curvatures of ODSs (CODSs) associated with ODSs that are obtained by two different CSLDV measurement methods, i.e., demodulation and polynomial methods; the former provides rapid and spatially dense ODSs of beams, and the latter provides ODSs that can be considered as those of associated undamaged beams. Phase variables are introduced to the two methods for damage identification purposes. Effects of the order in the polynomial method on qualities of ODSs and CODSs are investigated. A convergence index and a criterion are proposed to determine a proper order in the polynomial method. Effects of scan and sampling frequencies of a CSLDV system on qualities of ODSs and CODSs from the two measurement methods are investigated. The proposed damage identification methodology was experimentally validated on a beam with damage in the form of machined thickness reduction. The damage and its region were successfully identified in neighborhoods of prominent peaks of CDIs at different excitation frequencies.

Damage identification of beam structures using free response shapes obtained by use of a continuously scanning laser Doppler vibrometer system

“…FRDIs in Eq. (27) associated with the first five modes of the structure were calculated and shown in Fig. 20(a) through (e) using the FRSs obtained by use of the CSLDV system with different scan frequencies in different numbers of half-scan periods, and the associated auxiliary FRDI in Eq.…”

“…2 (27) where N d is the number of FRSs to be included in the index with A polynomial that fitsφ h,i with an order r can be expressed bỹ…”

“…[26], where the demodulation method was used to obtain ODSs of various structures by scanning their cracked surfaces, and effects of cracks could be observed as local abnormalities in obtained ODSs. The demodulation and polynomial methods were synthesized to identify damage in beams, where damage can be identified by use of a CSLDV system that scans intact surfaces of damaged beam structures [27].…”

Non-Model-Based Identification of Delamination in Laminated Composite Plates Using a Continuously Scanning Laser Doppler Vibrometer System

“…and z Q ðxÞ ¼ zðxÞsinða þ hÞ (11) respectively [21]. Each obtained vibration shape from the demodulation method corresponds to a half-scan period.…”

“…A lifting method was proposed to obtain mode shapes of a structure under impact excitation by treating its free response measured by a continuously scanning laser Doppler vibrometer (CSLDV) system as that of a linear time-periodic system [20]. The demodulation and polynomial methods were synthesized to identify damage in beams, where damage behind an intact surface of a beam could be identified by use of a CSLDV system that scanned the intact surface [21]. A new type of vibration shapes called free-response shapes was developed and a corresponding damage identification method was proposed to identify damage in beams undergoing free vibration [22].…”