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Mat > Mechanical > Flashcards

Mechanical Flashcards

1
Q

Deform

A

change shape

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2
Q

Fail

A

break

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3
Q

Mechanical Properties

A

Stiffness
Strength
Toughness

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4
Q

Materials can gradually degrade and fail over time as a result of

A

Wear
Creep
Fatigue

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5
Q

The Tensile Test

A
  • Take sample of material
  • Pull ends to stretch it
  • Measure force F
  • Measure stretch L-Lₒ

-know diagram

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6
Q

Tensile Test practicalities

A
  • any size, but parallel sides
  • cross section same throughout
  • can make ends of specimen bigger so its easy to grip in testing machine
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7
Q

Stress/Strain curve

A
  • to see how sample stretches for given applied force
  • x-axis: stress
  • y-axis: strain

-X on curve indicates point which sample breaks

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8
Q

Stress formula

A

σ = Force/Area

N/m² or Pa

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9
Q

Strain formula

A

ε = (L-Lₒ)/Lₒ

No units

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10
Q

Cross sectional area

A

For rectangular A = width x thickness

For circular, A = πr²

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11
Q

Types of stress

A

Tensile stress (pulling)
Compression (pushing)
Shear
Pressure

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12
Q

Compression and Tensile stress

A

Pushing is still known as tensile stress, but has negative value

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13
Q

Shear

A

A type of stress that causes sliding

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14
Q

Shear stress formula

A

τ = Force/Area

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15
Q

Shear Strain formula

A

γ = L/Lₒ = Tanθ

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16
Q

Pressure

A

created by having same force acting in all directions eg. hydrostatic pressure underwater

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17
Q

Pressure formula

A

P = F/A, same as tension but written as positive when compressive

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18
Q

Strain due to pressure

A

A change in volume, called dilatation

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19
Q

Dilatation formula

A

△=-(V-Vₒ)/Vₒ

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20
Q

Any type of stress

A

can be expressed as a mixture of these three:

tension, shear, pressure

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21
Q

Stress/Strain curve: First Stage

A
  • Elastic Deformation
  • When line is straight at start
  • material behaves like spring
  • remove stress, strain goes back to zero
  • stress ∝ strain
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22
Q

Stiffness

A
  • Slope of stress/strain curve in elastic region
  • called Young’s Modulus (or elastic modulus), E
  • if line is straight E=stress/strain at any point in line ie E = σ/ε

-we measure E when applied stress is tensile, most materials have same E value in compression as in tension

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23
Q

Yield Strength and Plastic Deformation

A

Above certain stress, σᵧ, line becomes flatter and curved,

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24
Q

σᵧ

A

This point where line stops being straight is called yield stress or yield strength of the material

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25
Q

Deformation can be

A

Temporary (elasticity)

Permanent (plasticity)

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26
Q

Bending

A

Creates tension on one surface, compression on the other, no stress or strain in middle

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27
Q

Common types of bending

A
  • Cantilever Bending
  • Three-point bending
  • Four-point bending

diagrams in lecture 2 slides

28
Q

Testing for bending

A

We test for deflection (d) of the loading point as a function of the applied force F

29
Q

Bending Equations

A

will be given on exam, know what the letters stand for

30
Q

Other Types of Stiffness

A 
-If material loaded in shear or with a pressure, different elastic modulii can be calculated:
Shear Modulus (G)
Bulk Modulus (k)
31
Q

Poisson’s Ratio

A

v = εₜ / ε

  • ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force
  • There will be some strain in two directions perpendicular to loading direction.
  • If vol of piece stays constant, transverse strain εₜ will be half longitudinal strain ε
32
Q

Tensile and compressive deformation

A

Tensile deformation is considered positive and compressive deformation is considered negative.

33
Q

True Stress and True Strain

A
  • During test specimen gets thinner, so cross section area decreases
  • But Stress = F/A so stress bigger than we thought
34
Q

Nominal Stress/Engineering Stress

A

F/Aₒ

Aₒ = area of original, unstretched piece

-We plot nominal stress on stress strain curves usually

35
Q

True Stress

A

The correct value, F/A

36
Q

Nominal Strain

A

(L-Lₒ)/Lₒ

True strain will be smaller

37
Q

Elastic Energy

A
  • If you load up material in elastic region to some stress σ, area under line is measure of energy used in doing it.
  • Energy per unit volume of material in sample
  • This energy is stored in material, will be released if you unload it
38
Q

Hysteresis

A
  • Non-linear Elasticity
  • When stress/strain line curved in elastic region, sometimes loading and unloading lines are different
  • E is into constant
  • Some energy is lost (given by area between lines)
39
Q

buckling

A

what happens when you have a long, thing, structure loaded in compression

40
Q

stiffness

A
  • How much material deforms under load when deforming elastically
  • quantified by property called Young’s Modulus
41
Q

Above yield strength

A
  • Stress/strain non-linear relationship
  • Two things can be happening:
    a. plastic deformation
    b. damage
42
Q

Plastic Deformation

A

The permanent distortion that occurs when a material is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength and cause it to elongate, compress, buckle, bend, or twist

-diagram in lecture slides 3

43
Q

Finding plastic strain

A

Distance between Origin and new point on x-axis on stress/strain graph

44
Q

Damage

A

If, when you come down to zero after loading and unloading, there is no plastic strain

  • material becomes less stiff
  • due to internal damage
45
Q

Damage form

A

Usually takes form of cracks

46
Q

Ultimate Tensile Strength (UTS)

A

The stress at the maximum point and the strain where it finally breaks ε(subscript f)

47
Q

Ductility

A

The breaking strain

48
Q

Energy under whole stress/strain curve

A

energy per unit volume needed to make it fail (sometimes called toughness)

49
Q

Elastic Recoil

A
  • Black triangle in stress/strain curve (see lecture slide 3 diagram)
  • You get some energy back in elastic recoil
50
Q

Brittle Materials

A
  • In brittle materials, failure may occur before any change in slope of line
  • Yield Strength and Ultimate Tensile Strength the same
51
Q

Compression - young’s modulus and yield strength

A
  • Young’s modulus normally same as in tension

- Yield strength usually higher when damage happens

52
Q

Shear - yield strength

A

-Yield strength half of what it is in tension

τᵧ = σᵧ/2

53
Q

Importance of strength

A
  • can’t use material above its strength, it will break or become permanently deformed or damaged
  • σᵧ is maximum allowable stress
  • Designers usually apply safety factor, allowing material to be used only up to fraction of its measured strength
54
Q

Factor of Safety

A

Factor of safety = ultimate stress/actual stress

55
Q

When we want yield strength to be low

A
  • In manufacturing operations like forging, wire-drawing, sheet rolling
  • If σᵧ low and ε(subscript f) high, can deform material into new shape using little energy and avoiding failure
56
Q

applications of strength

A
  • sets upper limit to stress we can use material

- sets lower limit to stress at which we can plastically deform it

57
Q

How does glass berak?

A
  • breaks without yielding

- no plastic deformation, no damage before failure - happens without warning

58
Q

Yield strength of glass

A

-much higher than stress it normally fails at

59
Q

Brittle Fracture

A

the sudden, very rapid cracking of equipment under stress where the material exhibited little or no evidence of ductility or plastic degradation before the fracture occurs.

60
Q

Materials that experience brittle fracture

A
  • Ceramics
  • Many Polymers
  • Metals under certain circumstances
  • rubber
61
Q

tough vs brittle materials

A
  • tough materials fail after absorbing lot of energy, strain a lot before failing
  • brittle materials fail with little energy absorption at low strains, fail by cracking
62
Q

Definition of Toughness 1

A
  • Area under stress/strain curve
  • energy per unit volume needed for failure
  • j/m^3
  • brittle materials have low values bc no yield point , UTS low even tho E may be high
63
Q

Definition of Toughness 2

A
  • energy absorbed in fast fracture using specimen containing a notch
  • Material notched to create flaw, struck with swinging heavy pendulum
  • fracture energy is measured by how high pendulum rises after impact
64
Q

Definition of toughness 3 - used in Mechanical Engineering

A

-The fracture toughness

65
Q

Fracture Toughness Test

A

like a normal tensile test except specimen already contains a crack

Mat (5 decks)

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