A nonlinear Maxwell fluid model consisting of a linear dashpot in series wi
th a parallel arrangement of a linear spring and a second-order nonlinear s
pring, was developed. This configuration provides the flexibility necessary
to describe both the stiffening and the softening responses of some viscoe
lastic materials. A noteworthy feature of the model is that under constant
rate displacement, the force equation can be solved in closed form, thereby
providing a continuous, exact general solution.