Lecture 5 elastic region

Question 1

tensile testing is

Answer 1

fundamental test used to extract key properties of materials

Question 2

specimens shape in tensile testing and why

Answer 2

dogbone shape - ends larger than middle - localize stress into the centre of the piece and not be affected by the gripping of the materiel

Question 3

how is tensile testing done

Answer 3

two dots placed on specimen distance measured this

record the extension in the specimen gives the strain measure forced required gives stress

Question 4

gauge length is the

Answer 4

distance between the two dots during tensile testing

Question 5

loading occurs

Answer 5

via load cell with moving cross head - move top head up specimen clamped between two clamps

Question 6

other properties measured in tensile testing

Answer 6

elongation after fraction - tells us about plastic deformation
gauge length at failure - another measure of ductility

Question 7

engineering stress strain graph regions

Answer 7

elastic region - underneath straight line
uniform plastic region yielding - before UTS
nonuniform plastic region necking after UTS

Question 8

elastic region

Answer 8

pulling bonds slightly then letting material go so it returns to its original shape

Question 9

yeilding region

Answer 9

begin to break bonds between material such that it begins to flow - plastic deformation

Question 10

necking region

Answer 10

localise stresses begin to occur failure

Question 11

bond strength defines the

Answer 11

elastic region allows us to define anumber of elastic modula

Question 12

what is an inset graph

Answer 12

due to sharp rise in stresses with little increase in strain during the elastic region on a normal stress strain graph you can not see the elastic region therefore have smaller graph showing just that region

Question 13

stress

Answer 13

force /area

Question 14

strain

Answer 14

change over length / length

Question 15

if use original length of sample

Answer 15

engineering stress and strain inital length and cross sectional area

Question 16

tensile stress vs compressive stress

Answer 16

just negative tensile materiel gets longer and thinner

compressive makes material get shorter and fatter

Question 17

shear stress

Answer 17

applying force parallel to surface

Question 18

labelled engineering stress stain curve

Answer 18

YMS limit of proportionality (hookes law obeyed to)
offset yield strength UTS
failure strength

Question 19

How a material gets strength

Answer 19

all material becomes stressed and strained
eventually once you pull past UTS
local region will exhibit large amounts of strain

Question 20

limit of propotionality

Answer 20

point beyond which hookes law is no longer obeyed - force and strain directly proportional straight line

Question 21

Yield stress

Answer 21

stress at which yeild stress occurs hard to pinpoint typically use offset yield strength

Question 22

UTS

Answer 22

point at which plastic deformation becomes unstable and necking begins peak value of engineering stress strain curve

Question 23

fracture

Answer 23

point at which material breaks

Question 24

YMs is

Answer 24

stiffness of material (not just YMs affected by shape) makes a material difficult to deform
steels typically 100Gpa
within protional region change in stress/strain can be in comprehension constant for a material

Question 25

most materials follow

Answer 25

linear elastically hookes law region follows same line in loading and unloading some materials follow non linear elasticity hookes law not obeyed but loading and unloading still follow same line - use tangential moduli dont worry not on course

Question 26

steeper the elastic region

Answer 26

greater the YMs stiffer the material diamond very stiff would be steepest 100GPa aluminium 69 GPa

Question 27

Ashby diagram

Answer 27

takes two mechanical properties of a material and plots them against each other

Question 28

Ashby diagram

Answer 28

takes two mechanical properties of a material and plots them against each other typically as you increase density increase YMs - elastomers vary from this density moderate but YMS low

Question 29

poissons ratio

Answer 29

materials change cross section area with length elastic region only normal stress is along length of material goes from lo to lo + x cross section goes from A to smaller value lateral direction

Question 30

poissons ratio equal to

Answer 30

- lateral strain/normal strain (negative sign to indicate it is decreasing

Question 31

poissons ratio typically for metals

Answer 31

.33 for metals 0.25 to 0.35 for most

Question 32

poissons ratio close to zero

Answer 32

as you pull them their cross sectional area doesnt change much

Question 33

shear modulus

Answer 33

resistance to shear stress shearing stress to material to produce shear strain symbol G shear stress = G * change in length parrallel to force / length perpendicular to force stress = modulus * strain

Question 34

bulk modulus

Answer 34

resistance to change in volume due to hydro static loading stress = - Bulk modulus * change in volume/original volume bulk modulus equal to K or B

Question 35

4 key elastic moduli

Answer 35

YMs poissons ratio bulk modulus shear modulus

Question 36

equations to link moduli together E G K/B v

Answer 36

``` if have two can calculate other two E = YMs G = Shear modulus K/B = bulk modulus v = possions ratio ```

Question 37

why are all the moduli related

Answer 37

as all defined by bond between atoms

Question 38

bond stiffness or strength

Answer 38

S can be realted to force that is pulling atoms apart and displacement created

Question 39

Bond stiffness equation

Answer 39

E = S/ao

Question 40

bond stiffness equation related to YMs

Answer 40

F = S *delta(a) Stress = force/area = force* change in atomic spacing/atomic spacing ^2 strain = Change in atomic spacing/original atomic spacing Youngs modulus = Stress/strain = Bond stiffness/original atomic spacing E = S/ao see power point for better explanation

Question 41

YMs dependant on

Answer 41

atomic spacing and bond stiffness, all elastic moduli dependent on each other therefore all dependent on these can relate them all

Question 42

force acting on bond stiffness could also be

Answer 42

temperature due to thermal expansion

Question 43

youngs modulus against bond stiffness graph diagram

Answer 43

covalent > metallic > ionic > hydrogen > van der waals bond covalent bond stiffness high YMs metallic high bond stiffness primary bonds have good bond stiffness and good mechanical properties secondary bonds bond stiffness lower and mechanical properties not as good - may be useful