International Society for Rock Mechanics (ISRM) proposed in 1995 a new type of rock fracture toughness specimen—cracked chevron notched Brazilian disc (CCNBD); however

the calibration of the dimensionless stress intensity factor (SIF)

which is an important mechanical parameter for fracture toughness calculation

is still in question. In authors previous paper

the new and more accurate wide range calibration values

listed as tables

are obtained by using a new slice synthesis method combined with finite element method. In order to guarantee the consistency of the table with ISRM

linear regression is adopted to obtain the lists of calibration with exponent function. The results show that the error of the new formula

compared with the values of calibration

is very small; and that it allows application for a wide range of specimen dimensions and further theoretical analysis. Based on this new formula of dimensionless stress intensity factor

size effect law of rock fracture toughness test is further discussed. The result of analysis with application of the new formula is proved to be reliable.