the maximum-tensile-stress criterion of rankine,dating from 1876,is generally accepted today to determine whether a tensile failure has occurred for a brittle material. According to this criterion. brittle failure takes place when the maximum principal stress at a point inside the materials reaches a value equal to the tensile strength 0 as found in a simple tension test,regardless of the normal or shearing stress that occur on other planes through this point.which result in three planes perpendicular to 1,2 and 3 axes,respectively as shown in fig20. this surface will be referred to as the tension-failure surface or simple tension cutoff.