Abstract
To observe the effect of stress triaxiality upon brittle fracture surface, we performed two types of experiments which differ in stress triaxiality. As a result, crack branch starting speed changes in two specimens and the speed was affected by stress triaxiality. In bending condition, branch starting speed is around 0.86 cr (cr: Rayleigh wave speed), which is higher than that in tensile condition, 0.59 cr. It was realized that in higher stress triaxiality, branching is easy to occur because in bending condition stress triaxiality is said to be lower. On the other hand, mirror-mist transition speed is not affected by stress triaxiality. By fracture surface observation, we proposed that branch occurs when microbranch grew. This proposition was supported by FEM calculation with microbranch model, it was proved that in bending condition microbranch is difficult to grow. Additionally, we proposed a qualitative explanation that microbranch is easy to grow when stress triaxiality is higher because growth of microbranch is affected by T-stress. It is since the phenomena is not on the main crack propagating plane.
Highlights
1.1 BackgroundAs the result of the growth of demands for international transportation and energy, recent ships are in the higher risk of brittle fracture of their structural steel
Since it had been proved that stress triaxiality differs in two types of specimens[4], effect of stress triaxiality upon fracture surface roughness can be observed by comparing results from these experiments
We checked two boundary crack speeds which clearly changing fracture surface roughness, mirror-mist transition speed and branch starting speed. If these boundary crack speed differs between two types of specimens in the same material, it can be concluded that stress triaxiality affects fracture surface roughness
Summary
As the result of the growth of demands for international transportation and energy, recent ships are in the higher risk of brittle fracture of their structural steel. TMCP method, it is a technology of make steel stronger and tougher by controlling cooling rate in the stage of steel extension. This method has a limit of maximum thickness around 100 mm, and some parts of structure in enormous container ships have already reached 100 mm. This method can improve brittle toughness without technical difficulties, price of structural steel gets higher and unstable. If we could make the fracture surface rougher intendedly, we could make brittle crack controlled and prevent structures from fatal collapse
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