Perforation of thin plate targets by laser impact
A basic ballistic problem, the perforation of thin plate targets , is analyzed for laser impact by a continuous wave solid state fiber laser in order to identify the basic process parameters. For this purpose, thin plate targets made of mild steel with thicknesses between 1 mm and 5 mm have been exposed to an intense laser beam. Overall intensity and beam diameter have been varied systematically. As a key performance parameter, the time to perforation as well as a method for its assessment based on rear side infrared video imaging is proposed. The analysis of the infrared video data reveals different stages of the interaction, including heating of the solid sample, transition from the solid to the liquid state, material flow, and finally the perforation of the sample. The observed time scales are in good agreement with computer simulations calculating the energy transfer, heat conduction and phase transitions in the laser impacted metal plates. In addition, an idealized engineering model is presented which allows the prediction of the target response up to the point of melting very well. The experiments clearly indicate that the perforation of the sample occurs with a significant delay after melting of the material in the region of the laser beam. This observation is related to the melted material flow dynamics. Since the laser does not transfer a significant momentum to the sample, the flow dynamics is governed by surface forces and gravitational forces for the static target arrangement used in the experiments. Hence, a three-phase-model for perforation of thin metal plates by laser impact is proposed with Phase I: heating of plate area exposed to laser impact, Phase II: transition from the solid to the liquid state, and Phase III: a melt flow dynamics phase giving rise to perforation.