AN ENCAPSULATED SPLIT HOPKINSON PRESSURE BAR FOR TESTING OF WOOD AT ELEVATED STRAIN RATE, TEMPERATURE, AND PRESSURE

This paper presents a device, based on a split Hopkinson pressure bar (SHPB) setup, by which it is possible to obtain stress vs. strain for a wood specimen at high deformation rate, high temperature and high steam pressure. The need for determining the mechanical properties of wood not only at high deformation rate but also at high temperature and pressure is motivated by the need to model the wood chip refining process in mechanical pulp-ing. At mechanical chip refining, e.g. in thermomechanical pulping processes, preheated wood chips together with added water are fed into the centre of a refiner which in essence consists of two circular discs. Most often one disc is stationary and the other is rotating. The wood chips are transported radially between the discs due to inertia. On their way, due to impacts from radial bars on the discs, they are eventually broken down to individual fibres and fibre fragments. The device presented here is an SHPB set-up, modified so that the bars and the specimen are encapsulated in a pressure vessel within which the temperature is constant. In this way effects of temperature gradients in the bars are avoided. Pilot tests have been carried out which verify the intended per-formance of the device.