Characterization of Fretting Scar Cross-sections
Nurmi, Verner (2018)
2018
Materiaalitekniikka
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2018-04-04
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201803201403
https://urn.fi/URN:NBN:fi:tty-201803201403
Tiivistelmä
Fretting movement is dangerous for machines, because it can cause cracking and surface degradation. Idea of this thesis was to study fretting test pieces from material scientific point of view. This kind of research finds out what happens in the material during fretting test, which can provide fundamental insight to failure mechanisms and also explain anomalies in test data. This thesis concentrates to test pieces obtained in previous studies, using so called annular flat-on-flat fretting device. Circular shape of contact surface makes sure that there has not been edge effect in movement direction. Studied material was 34CrNiMo6 quenched and tempered steel.
Main focus on the thesis was to study fretting scar cross-sections. Fretting scar means area of severe surface degradation due to fretting. Cross-sections from fretting scar area revealed lot of anomalies: cracks and material transformations. Main tools used to study cross-sections were optical microscope, Vickers hardness test device and scanning electron microscope. Chemical microanalysis was performed with energy dispersive X-ray spectrometry in scanning electron microscope. Electron backscattering diffraction maps were also created to study orientation of microstructure.
Longest cracks appeared in pairs, and initiated to edges of fretting scar. Crack propagation continued under the scar, roughly in 26° angle. Crack pairs are very regular, as crack length, depth and distance between two initiating points of crack pair have linear correlation. It seems that cracks initiate from edge of fretting scar, and propagate from there in specific angle, until contact induced stresses fade. Slip amplitude was found to have effect on fretting crack length. Nominal normal pressure seems not to have major effect on fretting crack formation.
Plastic deformation took place in fretting scar area, deformation level being higher near contact surface. Formation of tribologically transformed structure was observed in immediate presence of fretting scar. Plastic deformation occurs under this structure. Oxidized third body layer was generally found in gross-sliding samples, usually as relatively thin layers. Third body layer occurred always as uppermost surface layer, regardless of presence of other degradation layers. Conclusion is that all fundamental cracking and material degradation takes place in fretting scars.
Main focus on the thesis was to study fretting scar cross-sections. Fretting scar means area of severe surface degradation due to fretting. Cross-sections from fretting scar area revealed lot of anomalies: cracks and material transformations. Main tools used to study cross-sections were optical microscope, Vickers hardness test device and scanning electron microscope. Chemical microanalysis was performed with energy dispersive X-ray spectrometry in scanning electron microscope. Electron backscattering diffraction maps were also created to study orientation of microstructure.
Longest cracks appeared in pairs, and initiated to edges of fretting scar. Crack propagation continued under the scar, roughly in 26° angle. Crack pairs are very regular, as crack length, depth and distance between two initiating points of crack pair have linear correlation. It seems that cracks initiate from edge of fretting scar, and propagate from there in specific angle, until contact induced stresses fade. Slip amplitude was found to have effect on fretting crack length. Nominal normal pressure seems not to have major effect on fretting crack formation.
Plastic deformation took place in fretting scar area, deformation level being higher near contact surface. Formation of tribologically transformed structure was observed in immediate presence of fretting scar. Plastic deformation occurs under this structure. Oxidized third body layer was generally found in gross-sliding samples, usually as relatively thin layers. Third body layer occurred always as uppermost surface layer, regardless of presence of other degradation layers. Conclusion is that all fundamental cracking and material degradation takes place in fretting scars.