Detecting Violations in Stacking of Fiber Layers in CFRPs Using Broadband Acoustic Spectroscopy

The possibility of detecting violations in stacking layers of reinforcing fabric in CFRPs, i.e., gaps between carbon fiber tapes, is demonstrated. Broadband acoustic spectroscopy with a laser ultrasound source is used for this task. The wide frequency band of the probing signal allows one to obtain frequency dependences for the attenuation coefficient of longitudinal ultrasonic waves in a CFRP in the spectral range of 1–11 MHz. Using the example of the investigated stringer panel, it is shown that a local minimum is observed in gap areas inside the resonance maximum of the attenuation coefficient caused by a violation of structure periodicity. Scanning the object along the surface allows one to determine the direction of these gaps in the plane of CFRP stacking.