In this paper, a linear analysis is carried out for partial differential equations (PDEs) describing granular flow. Regarding constitutive effects, elasticity and shear hardening with a nonassociative flow rule are included. The paper shows that flutter illposedness is an essential feature of many such constitutive models. In the paper, a readily applicable criterion for the occurrence of flutter illposedness is developed. When parameters characterizing the material reach this criterion, the governing PDEs exhibit flutter illposedness over a range of values of the hardening modulus. Asymptotic estimates for this range are given. In some cases, flutter illposedness is derived by a perturbation argument (perturbing the constitutive law, not the solution). The paper demonstrates this material with a deviatorically associative flow rule at small strains; although flutter illposedness does not occur, a generic small perturbation will cause flutter illposedness. For example, in the kinematic hardening model, flutter illposedness occurs when the objectivity of the rate of change of stress is included. In other cases, flutter illposedness may be derived by straightforward analysis without perturbation theory. For example, this is the case for the yield vertex model in which the flow rule essentially satisfies nondeviatoric associativity.
