Deformation Behaviour

Deformation Behaviour

1. types of deformation

   • Elastic
     • proportionality btw stress and strain (expression)
     • return to original shape when load is removed
     • for metalstypically up to 0.5% strain, similar to ceramics
       • polymers lower due to dif types atomic bonding
       • decrease when increasing tmp
   • plastic
     • do not return to original shape
     • from atomic perspective = breaking of bonds with original atom neighbors and re-forming of new ones also large # of atoms & mol move relative to one another
     • diff in diff materials
       • crystalline: by slip (motion of dislocations)
       • noncrystalline solids: by viscous flow mechanism

2. considerations

   • Hooke´s Law: relationship btw engine stress and strain (eq)
   • cte of proporcionality = modulus of elacticity or Young´s modulus
   • Elastic deformation = deformation in which stress and strain are proportional (linear relationship)
   • atomis scale : macroscopic strain = small changes in the interatomic spacing and the stretching of interatomic bonds
     • >> mag of modulus of elasticity is a measure of the resistance ti separation of adjacent atoms (interatomic bonding forces)
     • Modulus also proportional to slope of the interatomic force-separation curve at equilibrium spacing ( eq)

3. Yield

   • Definition: stress at which a material begins to deform plastically
   • Proportional limit= yield point : initial departure from linearity of stress-strain curve // deformation beggins
     • difficult to measure> convention= parallel line at 0.2%)
   • depending on transition (draw)
     • yield point phenomenon: elastic-plastic transition well defined and abruptly
       • Upper Yield Point: plastic def initiated + apparent decrease in eng stress
       • Low "": stress rise with increasing strain
         • associated to average stress bc well defined and relatively insensitive to testing procedure
       • once material has yielded at one point easier to continue deforming at that point
     • With gradual yield once have yield at one point it starts to work harden so easier to yield elsewhere
   • regions (draw): linear elastic-work hardening - necking & failure
     • explanation notes

4. slip system

   • formed by the slip plane (preferred plane where dislocations move) and slip direction (direction of movement > direction most closely packed with atoms = high linear density)
   • depends on
     • crystal structure of metal
     • minimize atomic disortion that accompanies the motion of a dislocation
   • #
     • FCC: 12
       • BCC:48
         • HCP:3
     • + slip>> easier to deform more ductile
   • yield strenght of a single crystal dependence on the critical resolved shear stress and the orientation of the most favorably oriented slip system

5. Yield and dislocations

   • while material is yielding dislocations move through he material
   • Grain boundaries provide a stop for dislocation movement
   • Equation given # dislocations that can form a pile

6. Ductility: measure of deformation failure (from total length of fractured specimen) - temperature dependent

   • equations