Lecture 7 Flashcards
ductility
measure of degree of plastic deformation sustained at fracture under tensile load
malleability
measure of degree of plastic deformation sustained at fracture under comprehensive load
ductility and malleability important in what industry
metalworking materials that crack and break under stress cannot be manipulated using metal forming processes
two ways of measuring ductility
elongation to fracture %EL
reduction of area %RA
elongation to fracture
and reduction of area how to calculate
given as a percentage %EL (final length - initial length)/ initial length
same but with area
ceramics and metal ductility
ceramics less than 1% very small plastic deformation - very brittle
metals moderate elongation to failure 1 to 50% ductile
(strengthing a metal reduce this making a metal more brittle)
toughness is a
resistance to fracture MPa *m^0.5
also good way of looking at how well a material can support a crack or small void within them
amount of energy a material absorbs before it breaks
lot of energy high toughness
small amount low toughness or birttle
catastrophic failure
material breaking quickly into two or more pieces
brittle material vs tough material crack
crack present material and it grows quickly, tough material crack present but does not grow - stay stationary for a while
material typical toughness
steel 50 Mpa*m^0.5 can support small void in material tough
glass 0.5 MPa*m^0.5 can no support work break brittle
stress strain curve of brittle and tough material
brittle material short yielding region - when they break catastrophically fail hear bang or snap crack rapidly propagates across material
toughness can be linked to
area under true stress strain curve
test for measuring fracture toughness
charpy impact tester
charpy test
take material make notch
raise pendulum - has potential energy
release energy becomes kinetic
if sample doesnt break has higher toughness than energy put in
hope to have pendulum swing through and break material
if compare potential energy before and after can see energy required to break
if brittle pendulum will swing along way after (little energy to break)
if tough pendulum wont swing far requires alot of energy to break
charpy test atomic level
crack emanating from notch and spreading through material
charpy test measure of
fracture toughness not toughness of material
ductile and brittle are
relative terms depends what you comparing glass is more brittle than steel but may be ductile in comparison to other types of glass
fracture is
separation of an object into two or more pieces most problematic when we dont want it to break
cracks are dangerous as
weaken the material more than you expect from reduction of intact cross sectional area, stress reduces much faster than reduction in cross sectional areaa
if have surface cracks in produce may want to
polish of cracks even though this reduces area it may increase the strength of the material
ductile material cracks
gross plastic deformation %RA and %EL are large
crack propagation is slow, resists propagation unless more force is applied leak before break
form stable cracks in material
brittle material cracks
do not show signs of gross plastic deformation
crack propagation is fast and unstable once is starts cannot be stopped
diagrams of ductile and brittle failure
100% ductile 100% RA comes to point
moderate ductile some necking before fracture some reduction in area break not smooth
brittle no plastic deformation crack
good toughness
good strength and ductility but increasing one normally decreases the other need good combination of both
ductile failure steps and diagram
initial necking small cavities form coalescence of cavities to form crack crack propogation until material can no longer support final shear fracture
initial necking
first reached after UTS
at and past this point plastic instability
relative large amount of strain in disproportional amount of material ie when pulling material not all under same strain small region experiencing larger amounts
small cavity formation
due to small defects vacancy interstitial atom or impurity atom, cause micro voids when localise stresses these grow
microvoids likely to grow where
inter metallic
second phase particles
grain boundaries
particle matrix interface (where two materials meet)
where interfaces between two different phases of material become unbounded
micro voids grow
many cavities started off coalesce to form one large crack
as cavities expanded individually before coming together get dimpled rough surface
crack grows transverse to the major axis (axis through which force is applied)
crack propogation
crack grows until material cannot withstand it then material acts and behaves more like brittle material - crack growing rapidly at 45 degree to tension axis and occurs due to shearing deformation
final shear fracture
material breaks in some cases get cup and cone shape but generally get a really irregular and fibrous appearance
pure shear stress generate
microvoids elongated in one direction horse shoe shapes point in direction of origin
if it was perfect tensile stress
equiaxed microvoids perfect circles on surface raised dimples
brittle failure
crack propagation is spontaneous and rapid perpendicular to applied stress perpendicular to major axis forming almost flat surface
unstable crack begins to move will grow without further stress being applied
little or no plastic deformation little or low amount of energy
cracks initiate at some point of stress concentration scratches fillets threads and dents
fracture can occur specific crystallographic planes ie
FCC (111)
specific grain orientation crack wants to move through certain crystallographic plane therefore through at each grain may follow this plane even if orientated differently but is easiest path for it
also seperation can occur by
tensile forces not shear
In polycrystals crack can
change direction at each grain boundary
fracture on brittle material leads to
chevron like marks point to where crack started from
crystallographic structure
fracture normally occurs by tensile not shear loads and usually ends with fracture surface that is bright shiny and flat (quite smooth) as material has cleaved
