On the Evaluation of the Blunting Line Equation at Low Temperatures as Used in the Determination of Ductile Fracture Toughness J_1c by J-R Curve

The blunting line equation recommended in the National Standard (BG 2038-80) for the determination of ductile fracture toughness using J integral techniques is discussed in this paper. In expounding its meaning we discovered that this equation is not appliccable to low temperatures (verified by experiments).Some work has been done to find equations of the blunting line at low temperatures.
We found that for the calculation of COD at low temperatures, the rotational factor r must be first determined for various low temperatures. Using this r we could determine the relation of J versus δ for the material tested under the specified low temperature. Then according to the specific δ-△a relation in the process of blunting of the tested material, we finally evaluate the J-△a relation, which is the very bluntig line equation we need at the corresponding low temperature.
The blunting line equation thus obtained has the form of J = a₀+β△a. The JQ given by the point of intersection of the J-R regression line and the blunting line well agrees with the J_1C value determined by the method of multiple specimens in combination with ultrasonic crack monitoring techniques.