Piezoelectric film sensor for measurement of peel stresses in bonded joints

Adhesively bonded composite patches employed for repairing fatigue cracks in metallic airframe structural components often fail under peel stress generated in the structure. The application of piezoelectric stress sensors embedded within the bonded joint for direct measurement of the peel stresses is reported here. Polyvinylidine fluoride (PVDF) film of about 28 micron thickness coated with nickel copper is employed to construct hin sensors embedded between the composite patch and the metallic surface of the crotch joint specimen. PVDF sensor with varying sizes were constructed and calibrated using polycarbonate test specimens subjected to uniaxial tension and compression. The sensors were then embedded between the composite patch and the metallic surface of the crotch specimen to monitor the peel stresses in the adhesive. The measurements are compare with stresses in the adhesive. The measurements are compared with stresses predicted by finite element modeling.