Delamination monitoring using low frequency stress waves for laminated composites

Owing to the anisotropy of composite laminates, local responses happens at periphery of the drilled hole even under the same loads. The strain measurement is probably the most sensitive and better option to detect and quantify delamination inside composite materials. In the paper, dynamic strain of the composite laminates was online monitored using fiber Bragg grating (FBG) and strain gauges. Two experiments with different sensors configurations on the specimen made of glass fiber reinforced polymers (GFRP) were conducted: mounted on the entrance and the exit side of the drilled hole. The low frequency stress wave signals contained more drilling details have been extracted from those sensors at different locations. The wavelet transforms shows the time-frequency spectrum corresponds to the delamination behaviour, especially when the drill tip contacted the first ply and pierced the last plies of the GFRP laminates.