STRNADEL Bohumír VŠB-TU Ostrava

Spoluautoři BRUMEK Jan

The size and shape of test specimens, their microstructure and non-homogeneity, significantly influence the strength properties and plasticity of tested steels. Efforts to characterize small-scale tensile properties are driven by the need to reliably predict the performance of engineering parts during service. In this study it is clearly demonstrated that the tensile properties of the tested R7T steel depend on specimen size. Both the yield stress and ultimate tensile strength of cylindrical tested specimens with diameter between 2 and 10 mm and plate specimens with thickness of 1 to 5 mm are independent of size. The results show that the uniform elongation is independent of specimen size but post-necking elongation increases dramatically as specimen size increases. This is caused by localized stress distribution in the neck region. The stress localization around the neck was characterized by means of the Weibull stress, which is a measure of material resistance against hardening and fracture. The Weibull stress is determined by the sample volume and the shape parameter. Numerically calculated Weibull stress using FEM is given in relation to the uniform deformation of the sample. It has been proved that Weibull stress for a constant deformation grows with increasing sample size. Increasing the shape parameter, e.g. narrowing the local strength distribution, increases the Weibull stress. The flat specimens are much more sensitive to the change of tensile behaviors with increasing size than the cylindrical specimens. It has been clearly proved that the Weibull stress is a convenient parameter for describing the size effect of steels in both brittle and ductile regimes.

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