A 2D visualization of the stress function (for an elliptical tube with a = 3 and b = 2 with a typical pressure gradient) is given in the upper frame of Figure , while a 3D visualization of this stress function is given in the lower frame of this Figure. Now, on the x axis of an elliptical cross section of semi-axes a and b we have τ yz = 0 and hence τ = τ xz where τ xz is given by Eq. 2 (but we drop the minus sign as we deal with magnitude). Therefore, the rate of shear strain on the x axis is given by (see Eq. 1):