Understanding the response of granular matter to intrusion of solid objects is key to modelling many aspects of behaviour of granular matter, including plastic flow. Here we report a general model for such a quasistatic process. Using a range of experiments, we first show that the relation between the penetration depth and the force resisting it, transiently nonlinear and then linear, is scalable to a universal form. We show that the gradient of the steady-state part, K _ϕ , depends only on the medium’s internal friction angle, ϕ, and that it is nonlinear in μ = tan ϕ, in contrast to an existing conjecture. We further show that the intrusion of any convex solid shape satisfies a modified Archimedes’ law and use this to: relate the zero-depth intercept of the linear part to K _ϕ and the intruder’s cross-section; explain the curve’s nonlinear part in terms of the stagnant zone’s development.

A new method is proposed that can deal with multi-impact problems and produce energetically consistent and unique post-impact velocities. A distributing law related to the energy dispersion is discovered by mapping the time scale into the impulsive scale for bodies composed of rate-independent materials. It indicates that the evolution of the kinetic energy during the impacts is closely associated with the relative contact stiffness and the relative potential energy stored at the contact points. This distributing law is combined with the Darboux–Keller method of taking the normal impulse as an independent ‘time-like’ variable, which obeys a guideline for the selection of an independent normal impulse. Local energy losses are modelled with energetic coefficients of restitution at each contact point. Theoretical developments are presented in the first part in this paper. The second part is dedicated to numerical simulations where numerous and accurate results prove the validity of the approach.

The spherical joint with clearances can be modeled as an axi-symmetric quasi-static contact of a sphere in a cavity. The Hertz theory based on the assumption of nonconformal contact is often used to represent the normal force-displacement relationship for the spherical joint with clearances. This assumption limits the application of the theory, especially in the case of occurring large deformation in contact area. The Steuermann theory is effective to solve the conformal contact problem in some cases, but it is strictly dependent on the selecting index $n$ of the polynomial, which is used to represent the contact profiles. In this paper, an approximate contact model of the spherical joints with clearances is developed that is based on using the distributed elastic forces to model the compliant of the surfaces in contact. The new formulation is simple and straightforward, and its validity has been tested by comparison to the finite element results.