Experimental investigation of dynamic effects in a two-bar/three-point bend fracture test.

A Hopkinson pressure bar has been modified to measure the dynamic fracture properties of materials at loading rates greater than approximately 10(6) MPa ms. Some fundamental dynamic effects associated with the incident stress pulse, such as stress wave propagation characteristics along the Hopkinson bar and within the cracked specimen, the specimen's dynamic response excited by the stress pulse, and the specimen contact situations with the impactor and supports, need to be understood. To better comprehend these fundamental issues, an experimental investigation of these dynamic effects with the emphasis on "loss of contact" was first performed on a two-bar/three-point dynamic bend fracture test setup using a voltage measurement circuit across the specimen/loading-pin interfaces and high-speed photographs. It was demonstrated here that the three-point bend specimen employed with the current two-bar/three-point bend test setup remains in contact with the impactor and supports throughout the first loading duration and that "loss of contact" does not occur. A further improvement using a pulse-shaping technique was employed for achieving a tailored incident pulse. The effect of pulse shaper on the rise time and duration of the incident pulse as well as the dynamic stress equilibrium in the cracked three-point bend has been investigated, for the first time here, with profound implications for significantly improved dynamic three-point bend fracture testing.