THERMAL-SHOCK CRACKING OF LITHIUM-NIOBATE SINGLE-CRYSTAL

The quantitative estimation of failure stress of a lithium niobate (LN ) single crystal due to thermal shock was investigated. Cylindrical te st specimens were used in the thermal shock tests. The thermal stress of an LN test specimen under conditions of thermal shock cracking was calculated from a computer program which takes account of the crystal anisotropy, using the surface temperature measured in the thermal shoc k test. Four-point bending tests were also carried out to examine the relationship between the thermal shock cracking and the failure of a s mall test specimen due to mechanical load. LN single crystals fracture d at the cleavage planes {01 (1) over bar 2} in the thermal shock test and the four-point bending test. Although the failure stress data obt ained from both tests obey the Weibull distribution, the Weibull distr ibution depends not only on specimen size but also on loading type. Ac cording to the Weibull distribution of thermal shock test data, if the normal stress sigma(n) acting on the cleavage planes {01 (1) over bar 2} is lower than 10 MPa, the probability of thermal shock cracking be comes very small - less than 2%.