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1994-He Shiyi ((-) male Ph.D. student S. Elecfroric Publi used the main sample g (d = m.), the test frequency is underaged state 2A12 alloy age hardening curve S-N fatigue curve 22 macro fatigue The fracture analysis is a macroscopic fatigue fracture scanning electron microscope (SEM) photograph.
It can be seen that the fatigue fractures in both air and vacuum are transgranular fractures. With the increase of cyclic stress, the proportion of the extended area gradually decreased. Under high cyclic stress, the terraced topography appears on the fracture, as shown in (c) and (d). This shows that as the level of cyclic stress increases, the number of fatigue sources increases.
23 Micro fatigue fracture analysis The analysis of the fatigue fracture in the air environment shows that the tire pattern can be seen in the local area. This is due to fatigue cracks in the expansion process of the oxide film on the matching surface under the action of cyclic stress on the surface of the fracture leaving a row of parallel indentation and an equal interval of indentation. At low cyclic stresses, such as 220 MPa, no tire pattern was observed. In addition, there is no dimple around the tire pattern, indicating that it is generally formed in the fatigue expansion zone.
Under the vacuum environment, no tire pattern was observed on the fatigue fracture. This indicates that the formation of the tire pattern may be related to the movement of the air flow during the fatigue crack propagation, or even to the oxide film formed on the crack tip. The formed oxide film is crushed and indented within a half cycle of fatigue crack closure, resulting in the formation of a tire pattern.
Hz, sinusoidal waveform, cyclic stress symmetry coefficient R and 400 MPa. After the fatigue test, the S*N curve was plotted, and the fatigue fracture morphology was observed using a CAMSCAN*4 scanning electron microscope.
2 Test results 21S-N Fatigue curve Under-aged state 2A12 alloy fatigue test results in room temperature atmosphere and vacuum environment. It can be seen that at higher levels of cyclic stress, the number of fatigue fracture cycles in vacuum is comparable to that in air. As the cyclic stress gradually decreases, the difference between the two gradually increases. When =250 MPa, the former can be up to 10 to 20 times higher than the latter. Overall, the fatigue life in vacuum is longer and the fatigue curve is clearly shifted to the right. The cyclic stress is reduced and the number of fatigue fracture cycles is significantly increased, indicating that the vacuum environment has a greater impact on the fatigue performance.
Under-aged 2AL2 alloy fatigue fracture macroscopic photograph (SEM) Ny=6374(b)a=250MPaNf=42353 Fatigue fractures appearing in air under fatigue conditions (SEM) Observation of fatigue fracture under air conditions, but also in local Find fatigue stripes in the area, see. As the cyclic stress decreases, many areas of fatigue streak can be found. If you can see a lot of clear fatigue fringes, the fringes have a certain degree of curvature, parallel to each other. Fatigue fringes generally bypass the point of inclusions, but sometimes also cut through the point of inclusions. Due to the influence of inclusions, fatigue fringes in the inclusions The surrounding density is high, and the fatigue fringes are parallel to each other in a small plane, but not necessarily in the same direction in the adjacent facets, and the two intersecting facets form a fatigue step at the intersection, as shown in (a). At the bottom of the valley, there are also secondary cracks perpendicular to the section, generally about 5 long.
24293 Underaged State 2A12 Alloy Fatigue Streaks in Air Conditions Fatigue streaks are generally not seen in vacuum fatigue fractures. Only some fatigue fringes were found when the cyclic stress was 250 MPa. See. However, the fatigue fringes are not as obvious and regular as those seen in air conditions. The fringe spacing is approximately 1 to 2 and the fringe chambers are ambiguous. A empty room wet. Atmospheric, the fatigue state of the underaged state 2A12 alloy under vacuum conditions 3A Discussion In comparison with room temperature atmosphere, the under fatigue state of 2A12 alloy under room temperature vacuum conditions has greatly improved the fatigue performance. The vacuum shifts the fatigue curve to the right, and the fatigue life is greatly improved under the same cyclic stress conditions. Especially at lower cyclic stresses, vacuum increases the fatigue life considerably. The reason for the above results may be related to the effect of vacuum on the fatigue crack propagation process.
The fatigue crack propagation is the result of repeated opening and closing of the crack front under cyclic stress. See. Fatigue fringes appearing on fatigue fractures are actually traces of repeated opening and closing of the crack front. In general, one stripe corresponds to one stress cycle. The fringe spacing is a measure of the crack propagation length at each stress cycle. In the process of fatigue crack propagation, a humid atmosphere can oxidize the newly formed fracture surface SLIVIK M. Beijing: National Defense Industry Press 1999. (edited by Zhang Jibin). This phenomenon may be related to the vacuum d to make the number of stress cycle breaks significantly.
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