Friction Between Steel and a Confined Inert Material Representative of Explosives Under Severe Loadings

The ignition of a confined explosive submitted to an impact strongly depends on the friction conditions between the explosive and the confinement material (generally steel). A test has been developed to study the friction between steel and a material mechanically representative of an explosive. The scope of interest is that of high pressures and high relative velocities (respectively 20 MPa and 10 m/s). The friction device consists of making a cylinder, formed of the material, slide through a steel tube. Axial prestress enabling the steel-material contact stress to be generated is performed by means of a screw-nut system. This confinement state avoids any fracture of the material from occurring throughout the test. Two kinds of tests are carried out: low-velocity (around 1 mm/min) and high-velocity (around 10 m/s). The relative displacement is obtained using a testing machine during the low-velocity tests, and thanks to a Hopkinson bars system during the high-velocity tests. Examination of the measurements obtained during high-velocity tests shows that a workable steady state of equilibrium has been reached. As the interface stresses cannot be measured, the friction coefficient must be determined using indirect data: force measurements obtained from the machine or from the Hopkinson bars and strain measurements made on the exterior of the tube. The procedure to identify the steel-material friction coefficient from these measurements entails analytical modelling and finite element simulations of the mechanical behaviour of the tube-specimen assembly. The friction coefficient identified during the high-velocity tests is far higher than the coefficient identified during the low-velocity tests.