Impact monitoring of CFRP composites with acoustic emission and laser Doppler vibrometry

The use of Acoustic Emission (AE) to detect impacts is of interest within industries where vital components are prone to impact damage, in particular where Carbon Fibre Reinforced Polymers (CFRP) are used, as damage can often go un-noticed within them. For AE monitoring of impacts piezoelectric sensors are used to detect the ultrasonic wave produced by an impact. Classification is also possible of these waves enabling a distinction between damaging and non-damaging impacts. These sensors do however have resonance, so do not give an accurate picture of how the waves propagate, better knowledge would enable better selection of sensors. Laser Doppler Vibrometry is a non-contact and non-resonant method of analysing the surface displacement on a structure. In this study, a vibrometer was used to monitor CFRP plates during impact to assess its applicability for distinguishing between damaging and non-damaging impacts, compared with a surface mounted AE sensor. The vibrometer was able to detect both low frequency flexural modes due to the impact process and the higher frequency extensional modes, initiated by damage. When compared to the AE sensor the vibrometer was comparable in its results, and unlike the sensor, not susceptible to resonance or decoupling. For the tested material the vibrometer identified frequencies greater than 20 kHz to be associated with damaging impacts.