DYNAMIC FRACTURE-TOUGHNESS DETERMINED FROM LOAD-POINT DISPLACEMENT

The paper presents a method to determine dynamic fracture toughness us ing a notched three-point bend specimen. With dynamic loading of a spe cimen there is a complex relation between the stress-intensity factor and the force applied to the specimen. This is due to effects of inert ia, which have to be accounted for to evaluate a correct value of the stress-intensity factor. However, the stress-intensity factor is propo rtional to the load-point displacement if the fundamental mode of vibr ation is predominant in the specimen. The proportionality constant dep ends only on the geometry and stiffness of the specimen. In the presen t method we have measured the applied force and load-point displacemen t by a modified Hopkinson pressure bar, where two-point strain measure ment has been used to evaluate force and displacement for times greate r than the transit time for elastic waves in the Hopkinson bar. We hav e compared the method with the stress-intensity factor derived from st rain measurement near the notch tip and good agreement was obtained. T he method is well suited for high-temperature testing and results from fracture toughness tests of brittle materials at ambient and elevated temperatures are presented.