A proposal has been made by Ferguson and Hudson (fur. Polym. J., 29 (1
993) 141-147; J. Non-Newtonian Fluid Mech., 52 (1994) 121-135) that th
ree-dimensional representation of extensional flow data can be used to
resolve apparent disagreements among the results from a variety of ex
tensional flow experiments. A theoretical investigation of the procedu
re, which involves plotting a transient extensional viscosity against
strain and time is carried out in this paper. We then seek to draw som
e conclusions about the validity and limitations of the approach. The
method does not work for purely viscous non-newtonian liquids or for s
imple (co-rotational and upper convected) Maxwell models. However, the
failure of results to lie close to a unique surface (for any particul
ar material) is most marked in situations where our theoretical models
are least reliable. More work, both experimental and theoretical, is
required. (C) 1997 Elsevier Science B.V.