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In this paper is reported a study on the behaviour of ES mild steel sheets subjected to perforation by hemispherical projectiles. Experiments have been conducted using a pneumatic cannon within the range of impact velocities 5m/s<=V0<=60m/s. The experimental setup allowed evaluating initial velocity, failure mode and post-mortem deflection of the plates. The tests have been recorded using high speed infrared camera. It made possible to obtain temperature contours of the specimen during impact. Thus, special attention is focussed on the thermal softening of the material which is responsible for instabilities and failure. Assuming adiabatic conditions of deformation, the increase of temperature may be related to the plastic deformation. The critical strain leading to target-failure is evaluated coupling temperature measurements with numerical simulations and with analytical predictions obtained by means of the Rusinek-Klepaczko constitutive relation [Rusinek, A., Klepaczko, J.R. Shear testing of sheet steel at wide range of strain rates and a constitutive relation with strain rate and temperature dependence of the flow stress. Int J Plasticity. 2001; 17, 87-115]. It has been estimated that the process of localization of plastic deformation which leads to target-failure involves local values close to for the boundary value problem approached. Subsequently, this failure strain level has been applied to simulate the perforation process and the numerical results obtained show satisfactory agreement with the experiments in terms of ballistic limit, temperature increase and failure mode of the target.