Reliability of piezoceramic patch sensors under cyclic mechanical loading

Piezoceramic patch sensors have to withstand the primary stresses and strains of a structure during operation. In the leading project ‘Adaptronics’ a lifespan of 106 cycles at 0.1% strain was required for sensors applied on components of steel and carbon fibre reinforced plastic (CFRP). In order to test the reliability of the patches themselves and of their adhesion on the substrate, special four-point bending tests were carried out under quasistatic loading and under cyclic loading at different strain levels. The specimens consisted in sheets of steel and CFRP as substrates on which the newly developed patches with embedded piezoelectric foils and fibres were glued. In the quasistatic bending tests the performance of each sensor was characterized by measuring the sensor signal (charge) as a function of strain before and after cycling. Damage of the specimens would result in a decreasing slope of the charge–strain-curve after cycling. However, all the specimens tested survived 107 cycles up to 0.12% strain without marked loss of performance. (Some figures in this article are in colour only in the electronic version) 1. Piezoceramic patch sensors The structures of the two sensor types investigated are schematically shown in figures 1 and 2. The sensors with embedded piezoceramic foils and fibres of the PZT (lead–zirconate–titanate) system were developed and realized by DLR in Braunschweig and by Fraunhofer IFAM in Bremen [Ges00, Ges02], respectively. The manufacture of the PZT-foil sensors is described in more detail in an accompanying article of this issue [Wie03]. 2. Experimental investigations by four-point bending tests