Adhesives in Very High Cycle Fatigue testing

The versatility of adhesive bonding creates new opportunities also for the experimental activities. One example is Very High Cycle Fatigue (lives up to 1E+10 cycles) [1] testing, in which the axial vibration generated by a piezoelectric transducer in the range of kHz is applied to the specimen by means of a horn-shaped adapter, which increases the vibration amplitude. Continuity between horn and specimen is ensured by a butt joint, which must be both capable to transmit the motion without disturbing wave propagation and suitable of being assembled/disassembled easily each time the specimen is replaced. In the experimental campaigns carried out in Politecnico over the last years [2], this goal was successfully accomplished by means of adhesive bonding. Cyanoacrylate was chosen to minimize the thickness of the adhesive layer and ease the bonding/debonding procedures. As the strength of the metal specimen is more than one order of magnitude higher than that of the adhesive, to prevent failure in the joint the key issue was to ensure that the joint section represents a maximum in the displacement mode shape and, therefore, a node (i.e. nil value) in the stress mode shape. To do so, a detailed finite element analysis was carried out to tune the horn-specimen setup. Indeed, no failure in the joints occurred during the tests, although the alternating stresses in the specimens were in excess of 500 MPa. In a different perspective, a controlled deviation from the above mentioned condition can represent a technique to test the adhesive under fatigue: by offsetting the bond from the node of the stress mode shape, the adhesive undergoes a stress cycle. As the general stress level is high, the offset must be controlled precisely. Again, the problem was investigated by finite element modelling, which enabled to assess the desired condition and identify the material parameters (elastic constants, damping) governing the case. References [1] C. Bathias, P.C. Paris. Gigacycle fatigue in mechanical practice (CRC Dekker, New York, 2005). [2] D.S. Paolino, A. Tridello, G. Chiandussi, M. Rossetto. Fatigue Fract. Eng. Mater. Struct. 37:570–579 (2014)