THE EFFECT OF SIZE ON STRENGTH OF A BRITTLE ROCK

THE DECREASE OF STRENGTH WITH SIZE--THE SO CALLED SIZE EFFECT--HAS NOT USUALLY BEEN TAKEN INTO ACCOUNT IN ROCK ENGINEERING OR STRUCTURAL GEOLOGY. MOST SIZE-STRENGTH RELATIONS ARE BASED ON THE ASSUMPTION THAT ONLY THE STATISTICAL DISTRIBUTION OF THE FLAW POPULATION IS CRITICAL; THEY NEGLECT, HOWEVER, THE POSSIBILITY THAT THE MECHANISM ITSELF MAY BE AFFECTED. IN THIS PAPER AN ATTEMPT HAS BEEN MADE TO RELATE THE SIZE-EFFECT TO THE MECHANISM OF DEFORMATION. EXPERIMENTS HAVE BEEN PERFORMED ON SPECIMENS OF DIFFERENT SIZE, SOME OF THEM IN SERIES WITH A SPRING ELEMENT. THE RESULTS SHOW THAT THE START OF CRACK PROPAGATION IS ONLY INFLUENCED BY STATISTICAL EFFECTS, THAT THE RANGE OF STABLE CRACK PROPAGATION SEEMS TO BE INDEPENDENT OF SIZE, AND THAT THE RANGE OF UNSTABLE CRACK PROPAGATION IS AFFECTED BY STORED STRAIN ENERGY (WHICH IS A FUNCTION OF SPECIMEN SIZE). THESE CONCLUSIONS PROVIDE THE BASIS FOR ESTABLISHING NEW, MORE RATIONAL STRENGTH-SIZE RELATIONS. /AUTHOR/