Jobu Awatani

Early stage crack tip dislocation morphology in fatigued copper

Observations of dislocation structures around the fatigue cracks in the early stage in polycrystalline copper were made through an ultrahigh voltage electron microscope operating at 2000 kV. Ladder like structures and initial cracks within them were fre-quently observed. These dislocation structures were formed prior to the crack initia-tion, and the crack was initiated and propagated within these structures accompanied by no appreciable dislocation entanglements in the earliest stage. On the other hand, cell structures were observed around the cracks which appeared to have grown into the sub-sequent stage (the stage between Stages I and II). Cell size ahead of the crack tip was not so small, and it seemed that the crack was able to propagate across the cells. On the basis of these observations, the mechanisms of fatigue crack initiation and propagation in the early stage in copper bulk-specimens are briefly discussed.

Dislocation structures around propagating fatigue cracks in iron

Dislocation structures around the fatigue cracks for known stress intensity factor ranges were observed through an electron microscope operating at 2000 kV. To prepare thin foils for this purpose, a jet electropolishing technique has been developed, and regions in the vicinity of crack tips, as close as 0.1 μm from the sides, could be preserved in these foils. There were found 1) in front of the crack tip, substructures characterized by considerably dense dislocations in the cells just ahead of the tip and the formation of a lot of cells elongated in one direction, and 2) in the vicinity of the crack sides, a very fine cell structure with the interior of the cells showing relatively low density of dislocations. Besides cracks growing along the boundaries of cells or grains, some cracks penetrating into cells were observed. However, any crack supposed to be formed by linking of voids was not found. The misorientation between adjacent cells formed near the crack was generally a few minutes of arc, but the value as high as about 10° was sometimes observed immediately ahead of the crack tips. From the difference between the structures ahead of the tip and those in the vicinity of the sides, a brief discussion is made about structural changes produced when the crack tip has passed.

Microstructures around the tips of fatigue

Thin foils containing sharp fatigue cracks were prepared from bulk specimens of iron by means of jet electropolishing under a special condition, and microstructures both immediately ahead of the crack tips and adjacent to the crack sides were observed through an electron microscope operating at 2000 kV. It was found that dislocation tangles of high density were developed around the crack tips, and a fine cell structure in the areas near the sides of the cracks. The dislocations of high density ahead of the crack tips seemed to rearrange into cell walls to form small cells, in a relatively short span of time after the crack tip has passed.

A study on the effect of stacking fault energy on fatigue crack propagation as deduced from dislocation patterns

Fatigue tests of crack growth rates and transmission electron microscopy in the immediate vicinity of crack tips were performed on specimens of copper and 70/30 brass. In brass, very dense dislocations accumulated on the primary and the conjugate slip planes were observed just ahead of the crack tips, and they appeared to be not easy to move because of the difficulty of cross slip. In copper, well-defined cells were observed around the crack tips and there were frequently found cracks propagating along or across cell boundaries. On the basis of these observation results, an interpretation is put on the effect of stacking fault energy on fatigue crack growth.

A study of the fatigue limit of copper

This paper shows that, in fatigue tests on copper, a great number of cycles beyond expectation is necessary for determining the fatigue limit. An ultrahigh frequency fatigue testing machine was used andS-N curves covering 1010 cycles were obtained for annealed specimens and for stretched ones. In annealed copper, the fatigue limit appeared at about 9.8 × 109 cycles. Observations were made on the development of slip bands and substructures in the course of a test at the strain level (3.8 × 10-4) of the fatigue limit. The slip bands continued to develop up to about 9 × 109 cycles, but remained unchanged if further cycled. On a searching examination of the behavior of microcracks, one end of which stayed in a grain without propagation, it was proved that fatigue hardening was responsible for the existence of the fatigue limit. Contrary to expectation, cell structures were found after 1010 cycles in such a low strain fatigue. In stretched copper, however, microcracks continued to grow even in the stage beyond 1010 cycles when fatigued at the strain level of the fatigue limit inferred from itsS-N curve.