A parametric-experimental study of void growth in superplastic deformation

Substantial void growth in metals constitutes a problem in many industrial operations that utilize superplastic deformation. This is because of the li kelihood of material failure due to such growth. Hence, there is a need to study void growth mechanisms in an effort to understand the parameters gove rning it. In this work, numerical and experimental studies of Void growth, and the parameters that affect it, in a superplastically deforming (SPD) me tal have been performed. In the numerical studies, using the finite-element method, a 1x2 sized thin plate (i.e. plane stress conditions) of a viscopl astic material with pre-existing holes has been subjected to a constant ext ension rate. The experimental studies were performed under similar conditio ns to the numerical ones and provided for qualitative comparison. The param eters affecting void growth in SPD are: m (the strain-rate sensitivity), vo id size (i.e. diameter) and the number (density) of existing voids. The res ults showed that increased m values produced strengthening and decreased th e rate of void growth. In addition, larger initial void size (or, equivalen tly, a larger initial void fraction) had the effect of weakening the specim en through causing accelerated void growth. Finally, multiple holes had the effect of increasing the metal ductility by reducing the extent of necking and its onset. This was realized through diffusing the plastic deformation at the different hole sites and reducing the stress concentration. The num erical results were in good qualitative agreement with the experiment and s uggested the need to refine existing phenomenological void growth models to include the dependence on the void fraction. (C) 2001 Elsevier Science Ltd . All rights reserved.