Modelling the effects of superimposed ultrasonic vibrations on tension and compression tests of aluminium

This paper presents a study of the effects of superimposing ultrasonic vibrations on the lower platen in tension and compression tests of aluminium. By measuring the oscillating force response as well as the static force, it is shown that the experimentally derived stress-strain data from these tests does not satisfy the description of a simple oscillatory stress superposition model. Finite element models of tension and compression tests are created and a description of the contact friction condition is included for the compression test model. By incorporating ultrasonic vibration of the lower platen for an interval during plastic deformation, the finite element model predicts that the stress-strain relationship satisfies a simple oscillatory stress superposition model. The finite element models are then developed further to investigate the predicted stress-strain relationship if a softer material model is incorporated only during the interval of ultrasonic excitation. For the tension test model, this allows the predicted stress-strain data to match the experimentally derived data. For the compression test model, by combining a softer material model description with a change in the coefficient of friction at the contact surface, only during the interval of ultrasonic excitation, the finite element model predicted stress-strain data matched the experimentally derived stress-strain data. The study indicates that it is not sufficient to explain the effects of ultrasonic excitation in metal forming processes only in terms of oscillatory stress superposition and contact friction. (c) 2007 Elsevier B.V. All rights reserved.