It is obvious that the lifetime of exposed material is substantially lower than that of the base material both for stress symmetrical cycling and pulsating cycling in tension. The fatigue limit defined on 107 cycles for loading with the stress ratio R = –1 is 240 MPa for the base 21 material and 140 MPa for material after 20 yearlong weather exposition. The ratio of fatigue limits is 1.71. The fatigue limits for pulsating loading in tension (R = 0) are 190 MPa for the base material and 120 MPa for material after exposition. The ratio of fatigue limits is slightly lower than that for the case of symmetrical cycling.
The qualitative explanation of the deterioration of fatigue properties due to corrosion, which can be found in literature, is based either on the influence of corrosion cracks developing in material and thus facilitating the fatigue crack initiation or on the decrease of fatigue limit due to the development of surface roughening. In the case studied no corrosion cracks were observed in material below the surface rust layer. The cracks observed in the rust layer did not penetrate into the base material. This discharges the effect of corrosion cracks or cracks which appear in the corrosion layer as starters of fatigue cracks,
i. e. the damage mechanism reported . This finding, however, is related to the particular environment in which the material under examination was exposed. The basic information on corrosion conditions in the Czech Republic based on measurements, which has been performed since the eighties of the last century.
The decrease of fatigue strength of weathering steels in relation to the time of exposure has been determined experimentally on specimens exposed to atmospheric corrosion. Results of this work, obtained on material detracted from parts of transmission towers used for a long time in Bohemia confirm the strong negative influence of exposition on fatigue performance of weathering Atmofix 52A steel.
It is well known tat the quality of surface influences the fatigue strength. That is why the experimentally
observed decrease of fatigue life with the time of exposure is possible to correlate with increasing roughness expressed in terms of maximum height of the profile or arithmetic mean deviation Ra. However, as far as to the author’s knowledge, no attempt to predict quantitatively the influence of surface roughness on the decrease of fatigue limit of corroded weathering steels was made up to
now. The fact that the corrosion dimples are sites of the crack initiation indicates that their stress concentration factor should be responsible for the observed decrease of the fatigue strength. The fractographic observation of failed specimens reveals the largest dimples, which initiated fatigue cracks resulting in final fatigue failure.
This enables, provided the dimensions of a dimple are known, to evaluate its fatigue notch factor Kf ,
characterizing the decrease of fatigue limit due to presence of a notch. The fatigue notch factor is defined as the ratio of the fatigue limit of a smooth specimen to the fatigue limit of a notched specimen.
The dimple on material surface, which resulted in crack initiation and finally fatigue failure. To
evaluate the dimple dimensions a sphere was fitted to the dimple. Is diameter, as can be seen, is in this particular case equal to 970 µm. The depth of the dimple below the surrounding surface is 210 µm.
The stress concentration effect of a notch can be simply calculated by means of finite element method. The result of the numerical determination of the stress distribution around the dimple with the above-evaluated dimensions on the specimen gauge length loaded by uniaxial tension is shown. The calculation,
yields the elastic stress concentration factor Kt = 1.63. The knowledge of the theoretical stress concentration factor Kt and the notch size enables to predict the fatigue notch factor Kf .