Evaluation of Brittle Fracture Strength of Circumferentially Notched Round Bar under Axial Load

Abstract

The brittle fracture initiation characteristics of circumferentially notched round bar, CNRB, subjected to the axial load have been investigated theoretically and experimentally using JIS-SCM 440 and SMn 443 steel bars. The curve showing the relationship between net section stress at fracture, a and temperature, T, has two regions macroscopically. One is called Region I where σ_{n, F} diminishes with decreasing T, and the other is Region II where σ_{n, F} is nearly constant irrespectively of T. The effect of diameter, D, and notch depth, c, on σ_{n, F} in Region I can be evaluated on the basis of the linear elastic fracture mechanics. σ_{n, F} at the transition temperature from Region I to II, σ_{n, TR}, is a little higher than the general yieding stress level of CNRB analyzed with the aid of finite element method. σ_{n, TR}/σ_Y (σ_Y: yield stress) shows little change with D and strength of materials, and rises with an increase in relative notch depth, 2c/D. σ_{n, F} in Region II can be approximately estimated from the experimental equation relating between σ_{n, TR}/σ_Y and 2c/D. Finally, a procedure is proposed to evaluate the brittle fracture strength of CNRB subjected to axial load.