In this study, T shaped tensile tests and 3-point tensile tests are conducted. The purpose of these experiments is to reveal the relationships between brittle fracture of spheroidal graphite cast iron and fracture critical stress in various stress conditions. Fig. 1 shows the application method of fracture critical stress for brittle fracture. Bending specimens are made of the T shaped castings shown in Fig. 2 by cutting. Specimens of round bar tensile tests is shown in the Fig. 3. The results are indicated in the Table1 and Fig. 4. Fig. 4 is Stress-strain relationships of T shaped castings.
Fig. 5 presents the T-shaped tensile bending test specimen and displacement measuring position. It is measured vertical displacement of the end of specimen, and calculated the deformation angle of the end. Fig. 6 presents the specimen case of three-point bending. PR shaped specimen has the protuberance. It is 5 mm. PL shaped specimen has no protuberances. All of these shaped specimens are composed of width 35 mm and 125 mm. The narrow width specimens are supposed to be plane stress condition, and the wide width specimens are supposed to be plane strain conditions.
Fig. 7 shows the Lord-deformation relationships and Bending stress-deformation relationships. Bending stress was calculated by the formula (1) and (2). The wide width specimens broke earlier deformation angle than the narrow width specimen. The list of bending test results are indicated in the table2.
Notch round bar tensile test was conducted to calculate fracture critical stress. The test specimen and analysis model are shown in the Fig. 9 and Fig. 10. The list of notch round bar tensile test is shown in the table3.
Fig. 12 shows the analysis model of T shaped specimen. As indicated in the Fig. 13, the Lord-deformation relationships are relative to experimental results. The difference between the maximum first principal stress of bending analysis and fracture critical stress was under 15%. It is shown in the Table4.
From the results of test and analysis, following findings were obtained.
(1) The wide width specimens were broken smaller deformation than the narrow width ones. That is because the wide width specimen is likely plane strain. This is why, when the same deformation with narrow width specimens, the wide ones has higher stress.
(2) The difference between the maximum first principal stress of bending analysis and fracture critical stress was under 15%. Therefore, in the various stress conditions, fracture critical stress is relative to brittle fracture of spheroidal graphite cast iron. It is necessary to set the safety factor of fracture critical stress when fracture critical stress is applied to actual structures.
(3) CS specimens (the tensile side is casting surface) were broken earlier than MS specimens (the tensile side is machining surface). However, it is necessary to consider the effects of cast iron surface for fracture, because the number of CS and MS specimens are not enough.
