1984 年 33 巻 373 号 p. 1342-1347
Toughness of steels for automobile components is usually evaluated by Charpy impact tests using standard U-notch specimens. However, for components heat-treated to high hardness levels, it is desirable to use C-notch specimens having a similar notch depth but a larger notch root radius than the U-notch specimens.
In the present paper, the impact behaviour of C-notch specimens with notch radii of 2R, 5R and 10R was investigated in Charpy impact tests and compared with that of U-notch ones. Then, discussion was made about suitable notch radius for each impact specimen with different hardness, and the relation of impact properties between C-notch and U-notch specimens was examined.
The results obtained are as follows:
(1) It is recommended to select a suitable combination of notch radius and hardness of a specimen in such a way that the propagating crack may be formed at the maximum load in the load-time diagram during the impact test.
(2) The relationship of the total impact value between C-notch and U-notch specimens for various materials and testing conditions was approximately represented by a bent line having a knee. The straight line under the knee corresponds almost to the relationship of the impact values for crack formation, and the straight line above the knee to that of the impact values for crack propagation.
(3) A 1:1 relation was found to exist for the maximum plastic stress between C-notch and U-notch specimens except those heat-treated to high hardness levels above HV640.
(4) The relationship of time period preceding propagating crack formation between C-notch and U-notch specimens was similar to that of impact value for crack formation between the both types.
(5) With increase of thee notch radius of specimens, the ductile-brittle transition temperature shifted to lower levels, and the transition temperature range became broadened.