Thickness effect on cleavage fracture of ferritic sheet during in-plane impact tests

Abstract

The in-plane impact test on non-standard samples limited by original materials is essential in assessing crack resistance due to its emersion of realistic stress conditions. The thickness effect on fracturing behaviors and crack resistances was investigated, based on Fe-3Si-0.9Al non-oriented silicon steel, one kind of brittle ferritic sheets. The “V-notch” impact samples with different thicknesses were obtained alternately with uniform microstructure. The crack resistances of samples deviated from the forecast curve based on thick samples when the samples were thinner than about 4 mm. On the fracture surfaces, each sector area following initiation points at the “V-notch” could be regarded as their separation coverages. As the reduced thickness, the low-frequency coverages increased obviously in proportion, and the crack resistance deviated from and over the original forecast values. The extra failure behavior appeared with the thickness decreasing. Meanwhile, the factor of incomplete coverage was dominant at impact moments, resulting in serious differences of instantaneous strain in the thickness ranges of impact points. The micro-crevices twinned between both layers vertically to the impact fractures. It is speculated that there were such micro-crevices above or under the observed fracture surfaces. In addition, one triangle range with special lattice plane on the cleavage fracture was discovered at the impact point, similarly symmetrically distributing by the center line of fracture surface. Thus, the thickness of impact samples affects crack resistance forecasts, stress subregions at impact moments, strain differences at impact points. The present work not only reveals the thickness influence on impact behaviors, but also proposes the relationship between the impact energy of the samples with different thicknesses and depths.