On the Simulation of the Effects of the “Stone Chipping” on the Car Underbody - An Application of Montecarlo Methods

During the life of a vehicle its tyres project a considerable amount of particles against the car underbody as well as to car body visible parts: the “stone chipping” effect is a significant source of corrosion of an automotive underbody along with undesired unaesthetic effects. Deposition of protective films counteracts the effects of the particles but protective coating extension and layout is somewhat driven by the experience. An actual simulation of the stone impact is then useful in order to design both coating thickness and layout. Nevertheless, even if the simulation of a single particle is, with reasonable simplifications, a quite straightforward task, the simulation of an actual impact of a real set of particles is almost impossible to attack due to the very large number of particles involved in the process and their mutual interactions. A more feasible approach relies on defining a “random particle” as a set of random variables with suitable distributions of mass, velocity and direction as a function, among the others, of vehicle velocity and steering angle. The impacted part transforms the kinetic energy conveyed by a single particle, partly as residual (rebounding) kinetic energy partly as absorbed energy. The ratio of absorbed/initial energy is a measure of the damage the impacted part is suffering. The angle between the local normal to the impacted surface and the particle direction is another governing parameter. A definition of “random particle” was carried out and an impact detection algorithm was developed, along with an evaluation of surface energy absorbing properties driving the energy exchange between particles and hit surface. A large amount of “random particles” was generated to simulate the stone chipping effects during various given test track. The numerical results were compared against actual field test results carried out on a proving ground, giving very satisfactory results.