PRO-R
Poolc [l] expressed concern about two potential shortcomings of our new technique for measuring fracture toughness [2]. We wish to thank him for expressing these concerns. However, neither represents a significant issue that could detract from the usefulness of the new technique.
Professor Pook's first concern is that the loading rate is not s~cien~y controlled when using the hand operated testing device, However, the loading rates typically imparted on a sample using the toughness tool are, in fact, within the guidelines recornrnended by the American Society for Testing and Materials. Since the measured fracture toughness is only a weak function of loading rate, the range of permissible loading rates is quite broad.7 As a result, very little operator skill is needed to stay within the guideline. As a matter of fact, the required loading rates are applied instinctively.
Professor Pook's second concern regards the shape of the chevron notch used by the four-point bend specimens. Our experience has shown that a crack develops at loads well below the maximum load. This is evident from the cracking sounds that can be heard when testing most materials, and from the change in compliance of unloading lines. (UnIoading compliances were measured using a four-point bend fixture mounted in a servohydraulic load frame 121.) Indeed, cracks can be found [S] in sectioned samples that were not loaded all the way to failure.
Our experience has also shown that the straight sided chevron notch geometry is more adaptable to different machining methods than a plunge cut geometry such as that suggested by Pook. For example, the chevron notch in the four-point bend samples can be prepared using a conventional milling machine, wire EDM, or diamond abrasive sawing. The restrictions on the final notch geometry are similar to those in the existing ASTM standards [3.4].