抄録
Brittle fracture strength of a mild steel under tensile loading was studied under different conditions of temperature, notch sharpness and deformation rate. The test temperature ranged from room temperature to liquid-nitrogen temperature and the deformation rate was varied from a quasistatic to a dynamic test with a impact velocity of 80m/s.
The results obtained are summurized as follows:
(1) Both strength reduction factor and deformation limit to fracture are decreased and notch strength is increased on sharpening the notch of the test piece. The main features in these changes are similar in both static and impact tests, but the latter gives more marked changes than the former.
(2) Increase of impact velocity, sharpening of the notch and a drop in testing temperature give the same effect on the appearance of brittle fracture. The notch strength shows a maximum when the fracture mode changes from a ductile to a completely brittle appearance when any one of the above three factors is varied. The maximum fracture notch strength ranges from 102 to 115kg/mm2.
(3) Change of tensile impact properties of notched specimens on testing at different temperatures shows a similar tendency to that of bending properties in Charpy impact test as observed by the authors in the previous experiment (Tetsu-to-Hagané Overseas, 1 (1961), p. 38). At the transition from a ductile to a fully brittle fracture condition, the tensile fracture notch strength shows a maximum. The higher impact velocity gives the higher temperature at which the fracture strength reveals a maximum. The maximum fracture notch strength in brittle fracture ranges from 104 to 110kg/mm2.
(4) In all the static or the impact tensile tests, reduction of area after fracture of the notched specimen drops discontinuously into brittle fracture region and shows more remarkable change on increasing the impact velocity or on lowering the testing temperature.
(5) Brittle fracture of a plain specimen without notch occurs at the impact velocity of 10m/s on testing at liquid-nitrogen temperature, giving the brittle fracture strength of 98kg/mm2.
(6) Some discussions have been made on the observed brittle fracture strength in relation to the testing conditions.
