24G 
PROFESSOR J. A. EWIXC4 AXI) .^IR. J. C. AV. HUAIFREY OX THE 
170,000 reversals of a stress of 12‘3 tons per sq. inch, at Avhich it broke. The spot 
shown (magnified 150 diameters) is a little way further from the grips than the crack 
through which the specimen actually broke, hut another severe crack is seen running 
across the centre of the figure. Comparing this wfith fig. 5, it will he at once seen 
that there are far fewer lines due to slip upon each individual crystal than in the 
former specimen. Fig. 8 shows the same spot after re-polishing and etching. 
Comparing this wdth fig. G it is seen that a greater proportion of the slip-lines appear 
in this instance as cracks after re-polishing. The maximum stress is less here than 
in the former example, and more than twice as many reversals were required to bring 
about fracture. This agrees with wliat has been found for all other specimens 
I'jroken, viz., the lower the sti’ess the fewer the slip-lines upon each crystal, but the 
greater proportion of these actually develoj:) into cracks under the .more numerous 
reversals to wdiich the less severely stressed specime:i is exposed. 
The ])hotograplis described al)ove are on rattier too small a scale for the actual 
changes in the slip-lines themselves to be clear!}’' seen, and these are better illustrated 
by the next series, figs. 9-12. These show with a magnification of 1000 diameters 
a small part of the surface of another specimen of the same iron which was subjected to 
reversals of 12 A tons per sq. inch. Fig. 9 is taken after 1000 reversals. The slip-hands 
wdiich have formed are very faintly visible as fine lines upon the surface of the ciystal. 
Fig. 10 is after 2000 reversals; the slip-bands seen in fig. 9 are now more distinct, and 
some new ones on the right of the crystal are fairly strongly defined. Fig. 11 is after 
10,000 reversals; some of the slip-bands now show a decided tendency to broaden out 
and those upon the right have extended further across the crystal. Fig. 12, taken 
after 40,000 reversals of stress, shows further broadening out and spreading of the 
slip-bands. At this stage it could be seen 
by the focussing that this lu’oadening was 
crystal. It is to be conjectured that the 
action is of the kind indicated in the 
sketch (fig. 13) wdiere («) represents an 
ordinaiy slip-band seen in section at 
right angles to its length, and (h) repre¬ 
sents the efiect of I'eversals of stress 
upon it. Very little further change took 
place in the particular ciystal of figs. 
9-12 as further reversals were applied, the siDecimen breaking elsewhere after 
160,000 reversals. It has been noticed in this respect that wdien once an incipient 
crack heghis to form across a certain set of crystals, tlie effect of further reversals 
due to a heaving-up of the surface of the 
ciystal in the neighbourhood of each 
slip-band, the markings being decidedly 
above the level of the other parts of the 
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Fig. 13. 
