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246 Mechanical Engineering Design
Figure 5–30 3.4
Cylinder subjected to internal
pressure p, having a radial a
crack in the longitudinal
3.0
direction of depth a. Use
p i
Eq. (4–51) for the tangential r i
stress at r = r 0 . r o
2.6
2.2
1.8
r r = 0.9
o
i
0.75
0.35
1.4
1.0
0 0.2 0.4 0.6 0.8
a (r – r ) ratio
i
o
One of the first problems facing the designer is that of deciding whether the condi-
tions exist, or not, for a brittle fracture. Low-temperature operation, that is, operation
below room temperature, is a key indicator that brittle fracture is a possible failure
mode. Tables of transition temperatures for various materials have not been published,
possibly because of the wide variation in values, even for a single material. Thus, in
many situations, laboratory testing may give the only clue to the possibility of a brittle
fracture. Another key indicator of the possibility of fracture is the ratio of the yield
strength to the ultimate strength. A high ratio of S y /S u indicates there is only a small
√
Table 5–1 Material K Ic, MPa m S y, MPa
Values of K Ic for Some Aluminum
Engineering Materials 2024 26 455
7075 24 495
at Room Temperature
7178 33 490
Titanium
Ti-6AL-4V 115 910
Ti-6AL-4V 55 1035
Steel
4340 99 860
4340 60 1515
52100 14 2070