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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
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