Corroboration and extension of analysis of c-axis sapphire filament fractures from pores

摘要

Newcomb's thesis [1] and subsequent publications with Tressler [2, 3] provide very useful data on the tensile fracture of c-axis sapphire filaments (Saphi-kon, Inc., Milford, NH) between 22 and (mostly 800) 1500 °C at strain rates of 4.8 X 10-1 to 9.6 X lO-5. While this data (and most other referenced data) is generally higher in strength than earlier data of Shahinian [4], it is consistent with the general trend of his data, i.e. a rapid tensile strength drop from 22 °C to a minimum at -400 °C, then a maximum at 800-1000 QC before again decreasing. Major observations and conclusions of their work [1-3] were as follows. Internal pores, singly or as pairs closely spaced or partially joined, were about half of the sources of failure identified fractographically [2] (feasible primarily from -800 °C and above). Fracture features over the mirror region on both fracture halves, rather than being mirror images of each other, matched each other, as previously suggested by Rice and Becher [5], indicating propagation of a sharp crack at all temperatures, and did not reflect changing to a finger crack growth mechanism as proposed by Heuer and Firestone [6, 7], Until ~1400 °C (or lower temperatures at higher strain rates) pores at origins were the only discernible feature there with dimensions consistent with the flaw size, indicating that the pores acted as sharp flaws. Above this strain-rate-dependent transition temperature range, peripheral cracks around the pores were found, whose size was consistent with the flaw size for brittle fracture. Fracture toughness was approximately constant at 1.4MPam'/2 for fracture of c-axis filaments (mostly 1200-1500 °C), reasonably consistent with the data of Iwasa and Bradt [8].