A typical cross-head speed in a tensile testing machine is 0.2 in./min. (a) What is the nominal/engineering strain rate imposed by this cross-head speed on a typical engineering tensile specimen with a 2 inch gauge length? (b) Estimate the average dislocation velocity that would be obtained at this strain rate in an alpha-iron specimen with a total dislocation density of 1010 cm-2 of which only half is mobile. In your calculation assume Burgers vector of perfect dislocations and take the lattice constant of alpha-iron as 0.248 nm at room temperature. (Hint: Orowan’s equation)
4. Johnston and Gilman reported that in a grown LiF crystal that has been subjected to a constant
stress of 10.8 MPa, the dislocation velocity at 249.1 K was 6×10-3 cm/s and at 227.3 K the
velocity was 10-6 cm/s. They also observed that their data suggested an Arrhenius relationship
between the dislocation velocity and the absolute temperature so that we can write
? = ?exp ( ― ?/??)
where v is the dislocation velocity, A is a constant of proportionality, Q an effective activation
energy in J/mol, and R is the universal gas constant (8.314 J/mol.K). Determine Q and A using
the given data and the above equation.
5. A typical cross-head speed in a tensile testing machine is 0.2 in./min.
(a) What is the
nominal/engineering strain rate imposed by this cross-head speed on a typical engineering tensile
specimen with a 2 inch gauge length?
(b) Estimate the average dislocation velocity that would be obtained at this strain rate in an
alpha-iron specimen with a total dislocation density of 1010 cm-2 of which only half is mobile. In
your calculation assume Burgers
alpha-iron as 0.248 nm at room temperature. (Hint: Orowan’s equation)
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