Investigation into the mechanisms leading to explosion of pétanque balls

Abstract

Pétanque is a game originating in Provence, France, which has become common in many European countries. It is played using balls with a diameter of 70–80 mm and a weight of 650–800 g. The weight can either be adjusted by the thickness of the metallic shell or, mainly in case of cheap balls, by a filling. Pétanque balls are commonly available at supermarkets. After an explosion of a pétanque ball stored in a cupboard, thousands of balls sets had to be recollected in Switzerland under strong safety measures. Several balls, including the remains of the exploded one, where subsequently investigated. The balls were made of a metallic shell, completely filled with a sand-like material. Up to 1 l of a gas, mainly composed of hydrogen, was found within intact balls, compressed in the pore space of the sand. This indicated a systematic cause of the explosion. To elucidate the mechanisms leading to the explosion of the ball, the materials of the shell and the filling were characterised (composition, porosity, moisture content, strength, etc.). Furthermore, the fracture surface and the welding of the two shell halves were analyzed. Based on these results, the pressure in the ball was calculated and compared to the theoretical resistance of the shell. Due to a high moisture content, small metallic particles in the filling corroded under anaerobic conditions in the completely sealed ball, leading to a formation of hydrogen gas. The resistance of the shell was considerably reduced in the region of the weld due to notch effects. The explosion could be reasonably explained by comparing the stresses due to internal pressure with the reduced resistance of the shell.