Chaotic attractor of the normal form map for grazing bifurcations of impact oscillators

Highlights: • The topological structure of the chaotic attractor of the Nordmark map is studied. • The trapping region of the asymptotic dynamics of the map is constructed. • We prove that the closure of unstable manifold of the fixed point is the attractor. -- Abstract: Grazing bifurcations can cause impact oscillators to exhibit chaotic motions. Such dynamical behaviour can be described by a normal form map (called the Nordmark map). A main feature of the Nordmark map is that it has a square-root term. The purpose of this paper is to study the structure of the chaotic attractor of the Nordmark map from the topological point of view. First, the trapping region of the asymptotic dynamics of the map is constructed. It is then proven that, for some set of parameter values having positive Lebesgue measure, the $\omega$-limit set of each point of the trapping region is contained in a invariant set which is just the closure of the unstable manifold of the hyperbolic fixed point of the map. Besides, the dynamics on the invariant set is topologically mixing. Accordingly the invariant set is a chaotic attractor of the map.