The friction phenomenon produces elastic stress waves generated by the release of energy stored within a material, called acoustic emission (AE), that propagate through the material and can be detected by adequate sensors when they arrive at the material surface, provoking small displacements
The mechanical signal can be turned into an electrical signal of low amplitude and high frequency by a piezoelectric AE sensor
Electronically processed signals can be later analysed with mathematical and computational techniques
AE signals can be classified into two types:
continuous (signals are associated with plastic deformation in ductile materials)
burst (signals are observed during crack growth, impact or breakage)