Growth of Crack in Steel by Impact Tension

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This paper describes the critical crack growth condition of cracked steel specimens at 77 K under impact tension. Impact tension was imposed on a specimen under a static tension σ_s. In the fracture tests, impact stress σ_d did not depend on σ_s for a lower range of σ_s but it decreased with an increase of σ_s for a higher range of σ_s. On the other hand, critical impact stress, σ_<cd>, to growth of a crack was smaller for a short specimen than for a large one. Impact or static fracture surfaces were observed by SEM and TEM for the crack tip region. In the case of impact fracture, twin did not always appear and dislocation distribution was very complicated at the crack tip. And also, microscopic deformations of the specimen surface near the crack tip were measured by X-Ray micro beam diffraction method. In the case of impact fracture, micro lattice strain Δd/d was large and it extended far from the crack tip.
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