Precision Crack-Off of Ceramics

Abstract

The “pinch-off” or crack-off of brittle materials was discovered in the early 1900’s by P. W. Bridgman (Bridgman, 1912; 1949) while examining the rupture of solids at high pressure. In Bridgman’s experiments, a glass rod was exposed to lateral fluid pressure while the two ends were kept free from pressure by using packing seals as in figure 1. At a certain pressure the rod broke and the divided parts were pushed out of the pressure chamber violently. The fracture surface formed was smooth and perpendicular to the axis of the rod. For brittle materials such as glass, the rupture was found to occur on a “perfectly” clean plane perpendicular to the axis. On the other hand, for ductile materials such as soft steel, copper, or aluminum, there was a significant contraction at the plane of breakage similar to the “necking” instability observed in tensile test specimens. The pressure at which this instability occurred under lateral pressure was found to be exactly the same as the stress at which necking occurred in a uniaxial tension test. The general features of the rupture phenomenon are well described by Bridgman who describes the failure during “pinching-off” as arising due to the axial tensile strain created in the solid by the lateral pressure on the curved surface. However, this does not address the question of dependence of the pressure at fracture on the surface state of a brittle solid, a fact pointed out by Bridgman himself. After Bridgman’s discovery, the crack-off effect was little investigated until the 60’s when Jaeger and Cook (1963) investigated this phenomenon in the context of the drilling of rocks. In the mid-70’s and 80’s there were several studies of this effect mainly directed at developing it as a process for slicing brittle materials, see for example Sato and Naoyuki (1975), Chandrasekar and Shaw (1988) and Santhanam (1989). Some Russian studies into this effect are briefly summarized in Cherepanov (1979).