Stoja Rešković

Dependence of thermoelastic effect on volume change by elastic deformation

Thermo-graphic investigations were carried out on niobium microalloyed steel. During tensile testing temperature changes of samples, due to elastic and plastic deformation, were measured using infrared camera. The results of temperature measurements indicate that the specimen’s temperature is lowered during the elastic deformation. This temperature drop is in other studies associated with so-called thermo-elastic effect which occurs in the metals during elastic deformation. Parallel with the infrared camera, surface of samples was recorded with digital imaging camera. Analysis of recorded images was analyzed using digital image correlation method. This is used to determine the deformation that occurs during stretching. The results show that during elastic deformation of samples the volume of samples is slightly increased. The volume change of samples and temperature drop, due to thermo-elastic effect, are compared for all samples. The analysis of experimental results explains that the thermo-elastic effect is directly related to the change of specimen volume during the elastic deformation of steel.

Influence Of Niobium On The Beginning Of The Plastic Flow Of Material During Cold Deformation

Investigations were conducted on low-carbon steel and the steel with same chemical composition with addition of microalloying element niobium. While tensile testing was carried out, the thermographic measurement was tacking place simultaneously. A specific behavior of niobium microalloyed steel was noticed. Test results have shown that, in the elastic deformation region, thermoelastic effect occurs, which is more pronounced in niobium microalloyed steel. Start of plastic flow in steel which is not microalloyed with niobium begins later in comparison to the microalloyed steel, and it is conducted so that, at the point of maximum stress, deformation zone is formed within which stresses grow. In steel microalloyed with niobium after proportionality limit, comes the occurrence of the localized increase in temperature and the occurrence of Lüders band, which propagate along the sample forming a deformation zone.

Distribution of Stress in Deformation Zone of Niobium Microalloyed Steel

Microalloyed steels today represent a significant part of total world production and processing of steel. Although widely used, there are scarce data on the stress distribution in the deformation zone of these steels. Research was carried out on two steel grades, both low-carbon structural steels with the same basic chemical composition, with one of them additionally microalloyed with niobium. Differences in the stress distribution in the deformation zone between two tested steels were continuously observed and measured using the methods of digital image correlation and thermography. It has been found out that niobium microalloyed steel has significantly more complex material flow and stress distribution in the deformation zone when compared to the plain low carbon steel.