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Unformatted text preview: ME 382 Lecture 12 3/ii/06 1 M ECHANISMS OF PLASTIC DEFORMATION Plastic deformation occurs by shear Loading parameter for yield: ! S , ! H Material property for yield: Y Yield criteria: ! S " ! Y or ! H " ! Y Three major mechanisms to change shape by shear Twinning Martensitic transformations (discussed in a later lecture) Slip Slip is dominant mechanism Occurs by motion of dislocations lines along glide (or slip) planes These are generally the close-packed planes Useful to think of dislocation lines as having physical characteristics Energy of dislocation Atoms in core of dislocation are displaced from equilibrium Regions of very high stress near core of dislocation Very high strain energy in region of core of dislocation Strain energy per length of dislocation given by U = G b 2 /2 J m-1 G = shear modulus, b is Burgers vector ME 382 Lecture 12 3/ii/06 2 For copper: G = 48.3 GPa b = 0.26 nm U = 1.6 x 10-9 J m-1 Dislocations try to shorten their length to minimize energy Curved dislocations will try to become straight Line tension of a dislocation Effect of dislocations trying to shorten their length looks like a line tension 1-D analogue to surface tension of a liquid (minimize surface energy)...
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