During World War II, some welded transport ships suddenly fractured at around 40 degrees Celsius, despite room-temperature tests showing adequate ductility. Toughness tests determine if a material exhibits a ductile-to-brittle transition with decreasing temperature, represented by a curve showing decreasing impact energy absorption and increasing percentage of shear fracture over a temperature range. The ductile-to-brittle transition occurs as temperature decreases, with ductile fracture at high temperatures and brittle fracture below a certain temperature.

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DUCTILE-TO-BRITTLE TRANSITION
 During World War II when a number of welded transport
ships, away from combat, suddenly split in half. The vessels
were constructed of a steel alloy that possessed adequate
ductility according to room-temperature tensile tests….but the
brittle fractures occurred at relatively low ambient
temperatures, at about 40
C.
 One of the primary functions of Toughness tests is to
determine whether or not a material experiences a ductile-
to-brittle transition with decreasing temperature and, if so,
the range of temperatures over which it occurs.
 The ductile-to-brittle transition is related to the temperature
dependence of the measured impact energy absorption. This
transition is represented for steel by curve A in Figure 9.20.
Frequently, the percent shear fracture is plotted as a function
of temperature—curves B in Figure 9.20.

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 At higher temperatures the CVN energy is relatively large,
with a ductile mode of fracture.
 As the temperature is lowered, the impact energy drops
suddenly over a relatively narrow temperature range, below
which the energy has a constant but small value; that is, the
mode of fracture is brittle.

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 Appearance of the failure surface is indicative of the nature
of fracture, and may be used in transition temperature
determinations.
 For ductile fracture this surface appears fibrous or dull (or of
shear character); conversely, totally brittle surfaces have a
granular (shiny) texture (or cleavage character).
 Over the ductile-to-brittle transition, features of both types will
exist (Figure 9.21).
 For many alloys there is a range of temperatures over
which the ductile-to-brittle transition occurs (Figure 9.20);
 This presents some difficulty in specifying a single ductile-
to-brittle transition temperature.

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 T1 = FTP = Fracture transition plastic (temperature at which
the fracture changes from totally plastic to substantially
brittle).
 T5 = NDT = Nil ductility temperature ( temperature at which
fracture initiates with essentially no prior plastic deformation)

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 Not all metal alloys display a ductile-to-brittle transition.
Those having FCC crystal structures (including aluminum-
and copper-based alloys) remain ductile even at extremely
low temperatures.
 However, BCC and HCP alloys experience this transition.
 For these materials the transition temperature is sensitive to
both alloy composition and microstructure.
For example,
1) Decreasing the average grain size of steels results in a
lowering of the transition temperature. Hence, refining the
grain size both strengthens and toughens steels.
2) In contrast, increasing the carbon content, while increasing
the strength of steels, also raises the CVN transition of steels,
as indicated in Figure 9.22.

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