Page 216 - Fiber Fracture

STRENGTH AND FRACTURE OF METALLIC FILAMENTS 20 I

0 4 8 12 16
Diameter [pml *

Fig. 16. Effect of the size on the tensile strength of Fe whiskers. (From Brenner, 1958a.)

strain rates that favor brittle fracture. Thicker whiskers often show a localized plastic
deformation in a single isolated region. This deformation begins on a line and then
expands much in the nature of a Luders band. The yield stress which is the stress to
nucleate plastic deformation is much higher than the stress necessary to maintain the
initiated flow. In Cu whiskers the ratio of the yield to flow stress may be as large as 90.
The value of the flow stress remains constant during the expansion of the plastic region.
Sometimes whiskers also show mechanical properties that deviate from this idealized
behavior. Two or more Luders bands may nucleate at the same time, the difference
between yield and flow stress can be much smaller, the flow stress may increase due to
obstructed flow and, finally, also the tensile strength vanes from sample to sample.
Fig. 16 shows the effect of the size of the diameter on the tensile strength of iron
whiskers (Brenner, 1958a). The size dependence and the scatter of this result have been
interpreted as indicating that whiskers contain a small number of defects that can cause
creep or initiate fracture. In fact, there is no sharp critical size that separates whiskers
from ordinary crystals. It is rather a continuous transition from real to more or less ideal
crystals and the smaller the size the smaller the probability to contain a defect. When,
as appears to be the case for the smaller whiskers, the number of defects capable to
produce plastic flow becomes a small integer value also experimental scattering will
become important. Moreover, the effectiveness of the defects is not unique. Plastic flow