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In order to understand the protective mechanism of helmet against shock wave, a head model under far-field blast is investigated experimentally and numerically. The distribution of overpressure on the surface of the head model are obtained and compared. The numerical simulations are carried out using the Multi-Material Arbitrary Lagrangian Eulerian (MM-ALE) technique with suitable material models for a Kevlar helmet and soft pads. The differences in the distributions of overpressure on the head models with and without the protection of the helmet are analyzed. The interaction of shock waves around the head and the helmet is investigated. Our experimental observations and numerical results indicate that, if protected by a helmet in the case of a shock wave acting from the front, the peaks of overpressure on the head are reduced at the top area, but increased at the rear area. The numerical simulations also show that the enhancement of the overpressure at the rear is mainly due to the collision of the shock wave. The underwash phenomenon also occurs when the shock wave acts from the side or the rear. [ABSTRACT FROM AUTHOR]