Piezoelectric ceramic prestress
Piezoelectric ceramics, like all ceramics, are weak in tension. Therefore, in order to prevent damage to the ceramics as they vibrate ultrasonically, they must be prevented from experiencing any net tensile stress. Thus, a static compressive stress (prestress) must be applied to the ceramic. This static compressive stress must be larger than the largest expected ultrasonic tensile stress in the ceramic. This prestress is commonly applied by stack bolt(s) that compress the ceramics between the back driver and front driver.
Also called preload or bias load.