Kvantitativní fraktografie trhlin typu rybí oko při únavě v ohybu a krutu

Quantitative fractography of fish-eye crack formation under bending-torsion fatigue

journal article in Web of Science

en

The paper presents results of the fractographical analysis of fish-eye cracks that were formed in plasmanitrided steel specimens under symmetrical bending, symmetrical torsion and biaxial in-phase bending-torsion loading combinations. Due to higher strength and compressive residual stresses introduced by plasma nitriding procedure, the subsurface fish-eye cracks were initiated inside the specimen bulk as a dominant failure mechanism. The geometrical characteristics of the fish-eye cracks (the radial asymmetry, the average size and the elliptical coefficient) were studied in the relation to the fish-eye crack location, the loading regime and the fatigue life. Both the ompressive residual stresses and the bending-torsion loading gradients were taken into account when discussing the results. Besides the well known fact that the nitrided surface layer reduces the crack growth rate, the study revealed that the average size of the fish-eye crack increases with both the distance from the free surface and the number of cycles to failure. The change of the fish-eye shape with the loading regime was also observed. A simple method for an estimation of the residual stress level within the nitrided layer was proposed. This method is based on the difference in the Wöhler curves of virgin and nitrided specimens under pure bending in combination with fractographical analysis.

Článek prezentuje výsledky fraktografické analýzy rybích ok, které vznikly za symetrického ohybu, krutu a jejich kombinace ve vzorcích nitridovaných plazmou. Vzhledem ke zpevnění a tlakovým zbytkovým pnutím způsobeným při plazmové nitridaci byla iniciována podpovrchová rybí oka jako dominantní lomový mechanismus. Geometricaké characteristiky rybích ok (radialní asymetrie, průměrná velikost a eliptický koeficient) byly studovány v relaci k pozici rybího oka, zátěžného režimu a únavové životnosti. V diskusi výsledků byla uvažována jak tlaková pnutí tak zátěžné gradienty. Kromě potvrzení známých skutečností studie ukázala, že střední velikost rybího oka roste se vzdáleností od povrchu a počtem cyklů do lomu. Byla rovněž pozorována změna tvaru rybího oka v závislosti na režimu zatěžování. Byla navržena jdnoduchá metoda odhadu vnitřních pnutí uvnitř nitrodované vrstvy založená na rozdílu Wöhlerových křivek panenských a nitridovaných vzorlů porušených symetrickým ohybem, v kombinaci s fraktografickou analýzou.

The paper presents results of the fractographical analysis of fish-eye cracks that were formed in plasmanitrided steel specimens under symmetrical bending, symmetrical torsion and biaxial in-phase bending-torsion loading combinations. Due to higher strength and compressive residual stresses introduced by plasma nitriding procedure, the subsurface fish-eye cracks were initiated inside the specimen bulk as a dominant failure mechanism. The geometrical characteristics of the fish-eye cracks (the radial asymmetry, the average size and the elliptical coefficient) were studied in the relation to the fish-eye crack location, the loading regime and the fatigue life. Both the ompressive residual stresses and the bending-torsion loading gradients were taken into account when discussing the results. Besides the well known fact that the nitrided surface layer reduces the crack growth rate, the study revealed that the average size of the fish-eye crack increases with both the distance from the free surface and the number of cycles to failure. The change of the fish-eye shape with the loading regime was also observed. A simple method for an estimation of the residual stress level within the nitrided layer was proposed. This method is based on the difference in the Wöhler curves of virgin and nitrided specimens under pure bending in combination with fractographical analysis.

kvantitativní fraktografie, rybí oka, únava při ohybu a krutu, plasmová nitridace, zbytková napětí

quantitative fractography, fish-eye cracks, bending-torsion fatigue, plasma nitriding, residual stress

2010

20.02.2010

Elsevier

0142-1123

INTERNATIONAL JOURNAL OF FATIGUE

32

6

921–928

8