Design, modeling, and testing of a piezoelectric impact compressive kinetic (PICK) tool for crack-stop hole treatment

This paper outlines the design, modeling and testing of a new class of tool intended for the treatment of crack-arrest holes to improve fatigue life. By integrating a stack of high-power piezoelectric elements in a compression caliper, this Piezoelectric Impact Compressive Kinetic (PICK) tool can be used to clamp very tightly on either side of an aluminum plug, which is inserted in a crack-arrest hole. Ultrasonic vibrations at high compression loads applied by the piezoelectric stack dynamically cold work both the aluminum plug and the inside of the crack-arrest hole. This paper describes the overall design of the tool, the configuration of the aluminum plug, and the effect of dynamic vibrations on the plug and on the surface of the crack-arrest hole. The system was driven at various resonance modes during the coldworking process. Several 3.2-mm (1/8-in.) thick steel specimens with 3.2-mm (1/8-in.) diameter crack-arrest holes were treated ultrasonically with the PICK tool. Dynamic fatigue tests showed that fatigue lives of the specimens was increased substantially as a result of the ultrasonic treatment. Microhardness and neutron diffraction testing confirmed that the tool induced high levels of cold working at the edge of the hole and increased the grain density, with a regular decay as a function of distance from the edge of the hole.