Applying bandgap defect modes to crack detection in beams using periodic concentrated masses

In this paper, a method for crack detection in beams is proposed by applying the concept of defect modes in band gaps of imperfect phononic crystals (PCs). In the proposed crack detection method, a periodic array of concentrated masses is attached on a straight beam to generate the Bragg band gaps. Then, a crack in the beam might induce the defect modes in some of the band gaps, whose frequency depends on the periodicity of the concentrated masses or the location and size of the crack. With the spectral element method formulation, we first study the influence of the concentrated masses on the bandgap formation. Then, we study the influences of the lattice constant or the size and location of the crack on the bandgap defect modes in the displacement transmission of the beam. Furthermore, we show that when the beam is excited at the frequency of the defect mode, the flexural waves will be sensitively localized around the crack even inside the band gap and this effect can be used to locate the crack. To experimentally demonstrate the proposed crack identification method, we periodically attach steel balls as concentrated masses on a cracked beam and the crack locating is realized by bonding polyvinylidene fluoride (PVDF) films in a line along the cracked beam segment. Our analytical and experimental results indicate that the phenomenon of the defect modes in imperfect phononic crystals can be applied on crack detection in beams. (C) 2020 Published by Elsevier Ltd.