13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- hammer head will be significantly different to that experienced at the crack tip. The impact of the hammer on the specimen will generate longitudinal and traverse stress waves within the specimen and these stress waves will influence the stress intensity factor at the crack tip [4]. Experimental results from the Charpy test are therefore difficult to interpret, as large inertial oscillations are often present in the load-time histories. To overcome the limitations posed by traditional dynamic test methods much attention has recently been given to the study of dynamic behavior of materials under stress wave loading using a modified Kolsky bar, [5, 6, 7]. The Kolsky bar apparatus allows for fracture testing at rates greater the 106 MPa√m s-1 therefore simulating more realistic loading conditions experienced by super-hard materials during operation. Both the tensile and compressive loading configurations have been adopted for fracture testing. For the most part compressive stress pulse loading has been most popular and following on from the standard quasi-static setup the high rate-bending configuration has proliferated. Detailed descriptions of all Hopkinson bar fracture tests can be found in a review paper by Fengchun et al. [8]. Fracture toughness values presented in this paper should be considered to be measured or apparent fracture toughness’s, (K1db) as opposed to the critical dynamic fracture toughness, (Kd). This is due to the effect of notch tip blunting in dynamic fracture tests and the effect of finite notch root radii on the overestimation of the fracture toughness as outlined by Williams and Hodgkinson, [9]. 2. Experimental Set-up 2.1. One-Point Bend Fracture apparatus The one-point bend set-up is a modification of the traditional Kolsky bar apparatus [10], where the transmission bar has been removed. The apparatus consists of a single instrumented cylindrical bar of length 300 mm and diameter 3 mm, Fig. 1. In a typical test a striker impacts the leading edge of the incident bar setting up a longitudinal stress wave. This wave propagates down the incident bar towards the specimen and subsequently reflects at the bar/specimen interface. A portion of the wave is transmitted through the interface into the specimen causing fracture. The degree to which the wave is reflected or transmitted depends on the impedance of the bar and specimen material and will be discussed later. It is of the upmost importance that the striker contact with the incident bar be a planar one. This ensures a trapezoidal wave will be generated and avoids any spurious wave reflection at the impact interface. The compressive wave can be well controlled in terms of duration and amplitude by varying the length and velocity of the striker respectively. The striker must remain less then half the incident bar length so to allow complete unloading of the striker before the arrival of the reflected wave from the impact end. The striker and incident bar both must be made of the same material. In a one-point bend configuration the specimen maintains a single point of contact with the loading apparatus. That is, the specimen remains unsupported throughout the test with no supports to restrain the free-body motion of the specimen. In this way the specimen will fracture due to inertia alone.
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