Lecture 7

Question 1

ductility

Answer 1

measure of degree of plastic deformation sustained at fracture under tensile load

Question 2

malleability

Answer 2

measure of degree of plastic deformation sustained at fracture under comprehensive load

Question 3

ductility and malleability important in what industry

Answer 3

metalworking materials that crack and break under stress cannot be manipulated using metal forming processes

Question 4

two ways of measuring ductility

Answer 4

elongation to fracture %EL

reduction of area %RA

Question 5

elongation to fracture

and reduction of area how to calculate

Answer 5

given as a percentage %EL (final length - initial length)/ initial length
same but with area

Question 6

ceramics and metal ductility

Answer 6

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)

Question 7

toughness is a

Answer 7

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

Question 8

catastrophic failure

Answer 8

material breaking quickly into two or more pieces

Question 9

brittle material vs tough material crack

Answer 9

crack present material and it grows quickly, tough material crack present but does not grow - stay stationary for a while

Question 10

material typical toughness

Answer 10

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

Question 11

stress strain curve of brittle and tough material

Answer 11

brittle material short yielding region - when they break catastrophically fail hear bang or snap crack rapidly propagates across material

Question 12

toughness can be linked to

Answer 12

area under true stress strain curve

Question 13

test for measuring fracture toughness

Answer 13

charpy impact tester

Question 14

charpy test

Answer 14

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

Question 15

charpy test atomic level

Answer 15

crack emanating from notch and spreading through material

Question 16

charpy test measure of

Answer 16

fracture toughness not toughness of material

Question 17

ductile and brittle are

Answer 17

relative terms depends what you comparing glass is more brittle than steel but may be ductile in comparison to other types of glass

Question 18

fracture is

Answer 18

separation of an object into two or more pieces most problematic when we dont want it to break

Question 19

cracks are dangerous as

Answer 19

weaken the material more than you expect from reduction of intact cross sectional area, stress reduces much faster than reduction in cross sectional areaa

Question 20

if have surface cracks in produce may want to

Answer 20

polish of cracks even though this reduces area it may increase the strength of the material

Question 21

ductile material cracks

Answer 21

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

Question 22

brittle material cracks

Answer 22

do not show signs of gross plastic deformation

crack propagation is fast and unstable once is starts cannot be stopped

Question 23

diagrams of ductile and brittle failure

Answer 23

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

Question 24

good toughness

Answer 24

good strength and ductility but increasing one normally decreases the other need good combination of both

Question 25

ductile failure steps and diagram

Answer 25

initial necking
small cavities form
coalescence of cavities to form crack
crack propogation until material can no longer support
final shear fracture

Question 26

initial necking

Answer 26

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

Question 27

small cavity formation

Answer 27

due to small defects vacancy interstitial atom or impurity atom, cause micro voids when localise stresses these grow

Question 28

microvoids likely to grow where

Answer 28

inter metallic
second phase particles
grain boundaries
particle matrix interface (where two materials meet)
where interfaces between two different phases of material become unbounded

Question 29

micro voids grow

Answer 29

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)

Question 30

crack propogation

Answer 30

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

Question 31

final shear fracture

Answer 31

material breaks in some cases get cup and cone shape but generally get a really irregular and fibrous appearance

Question 32

pure shear stress generate

Answer 32

microvoids elongated in one direction horse shoe shapes point in direction of origin

Question 33

if it was perfect tensile stress

Answer 33

equiaxed microvoids perfect circles on surface raised dimples

Question 34

brittle failure

Answer 34

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

Question 35

fracture can occur specific crystallographic planes ie

Answer 35

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

Question 36

also seperation can occur by

Answer 36

tensile forces not shear

Question 37

In polycrystals crack can

Answer 37

change direction at each grain boundary

Question 38

fracture on brittle material leads to

Answer 38

chevron like marks point to where crack started from

Question 39

crystallographic structure

Answer 39

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