Delamination-induced relative natural frequency change curve and its use for delamination localization in laminated composite beams

Delamination is one common type of damage in composite laminates. To avoid structural failure and catastrophic events, detecting delamination is vital. Vibration-based delamination detection methods, which employ changes in structural dynamic properties such as natural frequencies, have been extensively investigated. However, natural frequency is generally considered as a global dynamic property and as such cannot identify local locations of delamination. This study analyzes delamination-induced relative natural frequency change (RNFC) curves in laminated composite beams and finds the relationship between RNFC curves and mode shapes, which can be used for the approximation of RNFC curves. Based on RNFC curves and measured RNFCs, a novel delamination indicator is developed that features the capability of locating delamination simply. Proof of this method is undertaken numerically on a finite element beam model. Its applicability is experimentally validated on glass fiber-reinforced composite beams. Numerical and experimental results show that the method is capable of locating both mid-plane and off-mid-plane delamination in laminated composite beams under different boundary conditions.