Figure 1-7: Slip Lines and Fracture Plane in experimental tensile test specimen
tested by Materials Science and Engineering Department, UF. [8]
slip when the resolved shear stress on the slip plane reaches the critical resolved
shear stress for that material. To calculate the stress needed to exceed this level
(TCRSS) is calculated by
TCRSS
cos Q. cos A
a will be a minimum when = 45
Therefore when a load is applied slip systems get activated based on Schmid
factor, m. where
m = cos 4. cos A (1.12)
This behavior is known as Schmid's law.
The value of critical resolved shear stress depends chiefly on the material
composition and temperature. It is also a function of applied load and direction,
crystal structure and specimen geometry. During the application of a load to an
FCC single crystal specimen, the first planes to get activated are the planes of high
atomic densities and called the primary octahedral slip systems. Each octahedral
plane has six slip directions associated with it. Three of these are termed easy
slip or primary slip directions and the other three are secondary slip directions.
Thus, there are 12 primary and 12 secondary slip directions associated with the
four octahedral planes (Fig. 1-8 on page 14). In addition, there are six possible slip
directions in the three cube planes, as shown in Fig. 1-9 on page 15. There are 30
possible slip systems in an FCC crystal (Table 1-2 on page 16).