Development of an Equipment for Conducting Fatigue Tests in the VHCF Regime with Stress Ratios other than -1

Margarida Barroso Paiva Gonçalves Verdade

Key information

Authors:

Margarida Barroso Paiva Gonçalves Verdade (Margarida Barroso Paiva Gonçalves Verdade)

Supervisors:

Pedro Ferreira Rodrigues da Costa; Luís Filipe Galrão dos Reis (Luís Filipe Galrão dos Reis)

Published in

11/28/2022

Abstract

This dissertation details the development of an ultrasonic system and support structure which allow for conducting fatigue tests under axial conditions in the very high cycle regime with a superimposed mean stress. The overall mechanical system was designed to fit into an electromechanical machine which applies and controls the tensile static load imposed onto the system. The mechanical components of the ultrasonic system were designed through an iterative process using the finite-element method. The ultrasonic system was designed in such a way that the resonance frequency of the required axial mode of vibration is of 20 kHz. Experimental analyses were then conducted to evaluate the mechanical behaviour of the developed ultrasonic system under different loading conditions. The stress-displacement relation was evaluated (i) analytically using the equations provided by the available literature, (ii) numerically resorting to a finite-element analysis, and (iii) experimentally using as instruments a strain gauge and a laser vibrometer. Additionally, a methodology for establishing the desired ultrasonic conditions for conducting the fatigue tests with a superimposed mean stress was drawn. Finally, fatigue tests were conducted for identical specimens using the developed mechanical system, and the fatigue results and fracture surfaces analyses were presented. The obtained results demonstrate that the developed system and support structure allow for experimentally conduct fatigue tests with stress ratios different from -1, and that one may resort to the analytical methods rather than the time consuming numerical ones to determine the stress-displacement relation, since these two methods have proven to lead to similar results.