Analysis of a Three-Bar Kolsky Apparatus for High-Rate Three-Point Flexure

A Kolsky bar method is described that is suitable for measuring the flexural strength of ceramics under valid testing conditions in a high-rate three-point bending configuration. A three-bar arrangement is used—one input bar and two output bars—to provide measurements of force and displacement at each loading point. Small diameter bars are used to improve the force measurement made by the incident bar which in many cases would be inaccurate. An analytical model is formulated to predict the desired incident pulse based on the conditions of the experiment and the desire for a particular prescribed strain-rate. A finite element analysis is also conducted to determine suitable specimen geometries that will yield valid data at the desired rate. It is shown that stiff beam geometries improve specimen equilibrium and can reduce inertial effects. However, the use of small diameter bars necessitates unusually precise pulse-shaping. To achieve additional flexibility in pulse-shaping, a tapered striker is used in conjunction with a ductile wave-shaper to generate the desired incident pulse. A down-side of this approach is that a suitable taper geometry must be determined. It is shown that this geometry can be designed quickly and accurately using a simple one-dimensional finite element approach. The methods are demonstrated by measuring the dynamic flexural strength of α-SiC.