Materials - Exam 3

Hookes law

stress= young modulus * strain

Significance of Hooke’s Law?

Stress in proportional to strain in the elastic region

Yield strength

stress where plastic deformation begins

Tensile strength

Maximum strength before necking

Ductility

% elongation

Resilience

Energy absorbed in elastic region

Toughness

total energy absorbed before fracture (entire area under curve)

Hardness

resistance to localized plastic deformation (indention, scratching)

How plastic deformation occurs

dislocation motion through slip systems

Slip system

Slip plane + slip direction

Strengthening mechanisms

grain size reduction, strain hardening (cold working), alloying

Grain size reduction

smaller grains = more boundaries = stronger

Cold working (strain hardening)

dislocation density increases, dislocation interactions are repulsive in nature, dislocation motion hindered by the presence of other dislocations

Effect of CW

Yield strength increases, tensile strength increases, ductility decreases

alloying

impurity atoms interfere with dislocations by pinning them

what are the two modes of failure

ductile and brittle

Ductile (mode of failure)

high energy absorption, slow crack propagation, predictable

Brittle (mode of failure)

low energy absorption, fast crack propagation, sudden failure

Ductile to brittle transition (DBTT)

seen in BCC metals, toughness drops sharply below a certain temperature

Fatigue 

failure under cyclic loading

S-N curve for Ferrous alloys

no fatigue failure if below asymptotic value

S-N curve for non ferrous alloys

will always have a fatigue failure

fracture toughness

ability of a material to withstand fracture in the prescence of a crack

cracks do not act as a stress amplifier

false

Ways to mitigate fatigue

reduce mean stress, surface treatments, avoid stress concentrators, galvanizing

sharp corners are a stress concentration point

true

Creep

failure mode that occurs in specimens at high temp (T> 0.4T)

Stages of creep

Primary (decreasing rate), secondary (linear, steady rate), tertiary (accelerating rate)

alloys melt over a range of temperatures

true

pure components tend to melt over a range of temperatures

false

Eutectic point

unique composition and temperature where the liquid transforms into two solid phases simultaneously

binary eutectic system

involves two components (like pb and sn) that have limited solubility in each others solid state, and form a eutectic point