Ultrasonic Very High Cycle Fatigue (VHCF) tests on a cyanoacrylate and on a high strength epoxy adhesive

In the present work, the VHCF response (at 10^9 cycles) of a cyanoacrylate adhesive and of an Araldite 2011 epoxy adhesive were experimentally assessed. Fully reversed tension-compression tests (stress ratio R=-1) at ultrasonic frequencies (loading frequency of 20 kHz) were carried out by using a testing equipment developed at the Politecnico di Torino. In particular, the Ultrasonic Fatigue Testing Machine (UFTM) commonly used for VHCF tests on metallic materials was adapted to perform accelerated tests on adhesive butt-joints. The tested adhesive butt-joint was obtained by bonding together two bars in Ti6Al4V alloy with calibrated length. The bar obtained by bonding together the adherends was designed to have the same resonance frequency of the horn (mechanical amplifier) used in the UFTM. The length of the adherends was designed so to have the desired range of applied stress amplitude within the adhesive layer. Finite Element Analyses were performed to define the length of the adherends. The proposed testing equipment was experimentally validated. A strain gage calibration was performed to validate the Finite element results. VHCF tests up to 10^9 cycles were performed on a commercial cyanoacrylate adhesive to validate the proposed testing equipment and, finally, preliminary ultrasonic tests were carried out on the Araldite butt-joints. Interfacial-cohesive and cohesive failures were found experimentally for the cyanoacrylate adhesive, with the former showing a shorter fatigue life, as expected. The Araldite adhesive, which showed only a cohesive failure mode, was characterized by a significantly larger VHCF response, with all the failure data above the failure data for the cyanoacrylate adhesive.