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, 2
c/
D. σ
n, F in Region II can be approximately estimated from the experimental equation relating between σ
n, TR/σ
Y and 2
c/
D. Finally, a procedure is proposed to evaluate the brittle fracture strength of CNRB subjected to axial load.
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