Dislocation structures adjacent to fatigue crack tips propagating at high rates in copper and 70/30 brass
- PDF / 2,442,210 Bytes
- 4 Pages / 594 x 774 pts Page_size
- 83 Downloads / 185 Views
K. KATAGIRI and K. KOYANAGI are Assistant Professor and Research Associate, Osaka University. J. AWATANI, Y. ONISHI and M. TSUJI, formerly Professor and Graduate Students, Osaka University, are now Professor at Aichi Institute of Technology, with the Kawasaki Heavy Industry Company, and Assistant Professor at Wakayama Technical College, respectively. Manuscript submitted January 28, 1980.
ill-defined cells in sizes of 0.5 to 2 ~tm are seen in a fan-shaped zone ahead of the crack with the vertex at its tip. The structure just beneath the crack sides is rather featureless, and fine contrast of indefinite shape is also seen in some cases. Numerous fine cells elongated in the directions approximately 60 deg to the crack growth are observed in the regions a little remote from the sides up to the tip of the crack although they are less obvious in the left region of the crack. Dislocation structures around the crack in copper growing at a relatively lower rate are shown in Fig. 4. As seen in the case of high rate propagation, cells observed are somewhat ill-defined. In this figure, a small distorted area where the dislocation density is high and the contrast of cells changes from the surroundings is seen just ahead of the crack. As mentioned above, cells ahead of the crack tips in the present experiment are more or less ill-defined as compared with those around the cracks growing at lower rates observed in the previous works? ,~ This appears to be due to either insufficient number of strain cycles or rearrangement of the structure in a certain volume ahead of the crack during rapid approach of the crack tip. Figure 5 shows substructures around the crack growing at a high rate in brass. The thin foil in this case was
Fig. l--Schematic illustration of thin foil preparation.
Fig. 2~Optical micrograph of the fatigue crack filled with electrodeposited copper in a thin foil specimen (brass).
ISSN 0360-2133/80/1211-2029500.75/0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 11A, DECEMBER 1980--2029
Fig. 3--Dislocation structure around the crack tip in copper: (a) shows the structure up to the area well remote from the crack and (b) the details j u s t ahead of it. (Three holes in (a) perforated during the electropolishing seem to be due to inclusions in this material, AK = 454 N / m m ~'~, d l / d n ~ 0.5 tsm/cycle, beam direction [110]).
Fig. 4--Dislocation structure around the crack tip in copper. (AK = 377 N / m m 3/2, d l / d n ~ 0.1 ~m/cycle, beam direction near
[i 101.
prepared from the test piece stopped at the tension phase in a cycle. A considerable amount of crack tip opening displacement appears to be preserved by the layer of electrodeposited copper. There are observed tangled dislocations of high density in the crack tip regions and many twin bands in the areas near the crack sides. A fairly distinct change in substructures is seen along the lines radiating from the crack tip at 60 deg or so to the crack growth direction. In some regions, the tw
Data Loading...
