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MEC132 JD > Lecture 5 elastic region > Flashcards

Lecture 5 elastic region Flashcards

1
Q

tensile testing is

A

fundamental test used to extract key properties of materials

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2
Q

specimens shape in tensile testing and why

A

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

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3
Q

how is tensile testing done

A

two dots placed on specimen distance measured this

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

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4
Q

gauge length is the

A

distance between the two dots during tensile testing

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5
Q

loading occurs

A

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

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6
Q

other properties measured in tensile testing

A

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

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7
Q

engineering stress strain graph regions

A

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

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8
Q

elastic region

A

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

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9
Q

yeilding region

A

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

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10
Q

necking region

A

localise stresses begin to occur failure

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11
Q

bond strength defines the

A

elastic region allows us to define anumber of elastic modula

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12
Q

what is an inset graph

A

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

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13
Q

stress

A

force /area

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14
Q

strain

A

change over length / length

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15
Q

if use original length of sample

A

engineering stress and strain inital length and cross sectional area

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16
Q

tensile stress vs compressive stress

A

just negative tensile materiel gets longer and thinner

compressive makes material get shorter and fatter

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17
Q

shear stress

A

applying force parallel to surface

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18
Q

labelled engineering stress stain curve

A

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

19
Q

How a material gets strength

A

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

20
Q

limit of propotionality

A

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

21
Q

Yield stress

A

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

22
Q

UTS

A

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

23
Q

fracture

A

point at which material breaks

24
Q

YMs is

A

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

25
Q

most materials follow

A

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

26
Q

steeper the elastic region

A

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

27
Q

Ashby diagram

A

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

28
Q

Ashby diagram

A

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

29
Q

poissons ratio

A

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

30
Q

poissons ratio equal to

A
  • lateral strain/normal strain (negative sign to indicate it is decreasing
31
Q

poissons ratio typically for metals

A

.33 for metals 0.25 to 0.35 for most

32
Q

poissons ratio close to zero

A

as you pull them their cross sectional area doesnt change much

33
Q

shear modulus

A

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

34
Q

bulk modulus

A

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

35
Q

4 key elastic moduli

A

YMs
poissons ratio
bulk modulus
shear modulus

36
Q

equations to link moduli together E G K/B v

A 
if have two can calculate other two
E = YMs
G = Shear modulus
K/B = bulk modulus
v = possions ratio
37
Q

why are all the moduli related

A

as all defined by bond between atoms

38
Q

bond stiffness or strength

A

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

39
Q

Bond stiffness equation

A

E = S/ao

40
Q

bond stiffness equation related to YMs

A

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

41
Q

YMs dependant on

A

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

42
Q

force acting on bond stiffness could also be

A

temperature due to thermal expansion

43
Q

youngs modulus against bond stiffness graph diagram

A

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

MEC132 JD (9 decks)

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