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Prelims > Mechanical Properties > Flashcards

Mechanical Properties Flashcards

1
Q

What is the definition of stress? (equation)

A

stress = Force / Area. Units are Pa (N/m^2)

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2
Q

What is the definition of strain? (equation)

A

strain = change in length / original length. Units are dimensionless

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3
Q

What is Hooke’s Law?

A

Stress = Young’s Modulus * strain
sigma = E epsilon

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4
Q

What is Poisson’s Ratio?

A
  • lateral strain / axial strain
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5
Q

What is stress concentration factor?

A

Stresses near points of applications of concentrated loads reach values larger than average stress in the member

Stress concentration factor = maximum stress over average stress

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6
Q

What does viscoelastic mean?

A

Has both liquid and solid characteristics. Material response depends on magnitude of stress and strain rate

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7
Q

Difference in response when apply oscillating strain for solids, liquids, and viscoelastic materials?

A

Solid: in phase response, scales with applied force
Liquid: 90deg out of phase
Viscoelastic: offset of cyclic response

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8
Q

What is storage modulus and loss modulus? How does it relate to phase angle?

A

Storage modulus = “solid” part of viscoelastic material
Loss modulus = “liquid” part of viscoelastic material

Phase angle - phase shift of response, related to storage and loss modulus

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9
Q

What is time-temperature superposition?

A

higher strain rate is equivalent to low temperatures and vice versa

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10
Q

Explain “Entropy Elasticity”

A

When you have a highly entangled polymer, there is high disorder. When you stretch it, it alligns the chains. The polymer will want to return to its original entanglement to increase the entropy. The higher molecular weight you have, the higher the degree of entanglement. This corresponds to “rubbery plateau”

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11
Q

What are the 5 regions of temperature vs. Modulus plot for polymers? What are the different factors that impact them?

A
  1. Glassy, Hard, Brittle
  2. Molecular motion activated, less stiff
  3. Rubbery Plateau - long range elasticity, entropy driven
  4. Rubbery flow
  5. Liquid Flow

higher MW = more entanglements, delays rubbery flow (longer rubbery plateau)
cross-links makes flow less likely
more crystalline = higher Tg, glassy and brittle for longer

cross-linking prevents chain movement, and entanglements delay chain movements

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12
Q

What are elastomers?

A

High degree of cross-linking, can stretch more, increasing stiffness and Tg. Makes them less processible since they can’t melt or flow as easily. Maintained above Tg

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13
Q

What is a stress tensor?

A

sigma_i,j

i = surface normal direction
j = direction of force

[11 12 13
21 22 23
31 32 33]

11, 22, 33 are normal stresses
Others are shear stresses

12 = 21 – matrix is symmetrical

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14
Q

What is the stiffness and compliance matrices?

A

Stiffness = relates stress tensor to strain tensor
Compliance = relates strain tensor to stress tensor

Reduces values to be a 6x6 matrix, symmetry depends on crystal structure

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15
Q

Engineering Stress / Engineering Strain

A

Pressure over original area
Change in length over original area

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16
Q

Ultimate Tensile Strength

A

Maximum stress from Stress strain curve

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17
Q

True Stress / True strain

A

stress and strain calculated over actual area of sample

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18
Q

Yield Stress

A

Stress required to generate a plastic strain of 0.2%

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19
Q

What is critically resolved shear stress?

A

When the resolved shear stress is larger than the critically resolved shear stress, there will be an onset of permanent deformations

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20
Q

Temperature dependence of strain rate?

A

Plastic flow is governed by thermally activated defect motion
Higher temp = higher dislocation velocity, strain rate lower
Higher temp = lower polymer stiffness = higher chain motion

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21
Q

Ductility

A

Degree of deformation before some event (necking, failure) - strain at failure usually

22
Q

Toughness

A

Energy required to break or fracture a specimen - integral of entire stress-strain curve

23
Q

What is twinning?

A

A strain accomodation mechanism that doesn’t involve dislocation motion. Happens with lower stacking fault energies and more difficult cross-slips

24
Q

How do dislocations govern mechanical properties?

A

Plastic flow is governed by dislocations. Velocity of dislocation is correlated to strength.

25
Q

Hardness

A

Resistance to plastic deformation. Measure with Rockwell, Brinell, Knoop, Vickers

26
Q

What is a burger’s vector and how do different dislocations relate to it?

A

burgers vector is how much a crystal will shear when a dislocation goes through. If parallel to dislocation line, it is a screw dislocation, if perpendicular, it is an edge dislocation

27
Q

What kind of stresses do the area around screw and edge dislocation experience?

A

Screw = shear stresses only
Edge = tensile and shear stresses

28
Q

Why is FCC softer than BCC despite having less slip systems?

A

Able to form stacking folds

29
Q

What is the Ductile-to-Brittle Transition Temperature?

A

In BCC only, at low tempertures, very brittle but strong, at high temperatures, very ductile but weak

Due to BCC having mostly screw dislocations that are thermally activated as more energy is needed to move them

30
Q

What are the 4 hardening mechanisms?

A
  1. Work Hardening
  2. Solid Solution
  3. Precipitation
  4. Grain Boundary
31
Q

How does solid solution hardning work?

A

interstitial or substitutional atoms migrate to compressive / tensile regions of edge dislocations, makes them harder to move and thus plastically deform

extra stress is proportional to square root of atomic concentration

32
Q

How does precipitation strengthening work?

A

Particles (oxides, carbides) impede dislocation motion

extra stress proportional to 1/length between particles

33
Q

How does work hardening / strain hardening work?

A

During plastic deformation, dislocation density increases. Dislocations interact with each other and impede each other’s motion

extra stress proportional to sqrt(dislocation density)

34
Q

Why do we need to anneal cold-worked metals? What are the three stages?

A

As we work harden, dislocations entangle, multiply, and dislocation motion becomes difficult. This makes the material become brittle, and there is a need to recover.

Recovery: dislocations of opposite signs annilate by cross-slip and climb

Recrystallization: new grains form that have low dislocation density, small, consume cold-worked grains

Grain growth: larger grains consume smaller ones, grain boundary area reduced

35
Q

How does grain boundary strengthening work?

A

Grain boundaries are another barrier impeding dislocation motion.

extra strengthening proportional to D^-(1/2) –> D = grain size

36
Q

What is martensitic strengthening?

A

Cool austenitic steel at high rate, creates martensite phase that is BCT and metastable.

Carbon retained in solid solution –> solid solution strengthening

Original austenite retained, lots of grain boundaries present

High dislocation density to relieve misfit stress

Carbide cementite particles present - particle strengthening

Very brittle! need to temper to relieve brittleness – heterogeneous nucleation of carbide particles by removing carbon in martensite, decompose retained austenite

37
Q

What are high entropy alloys and why are they so hard?

A

Lots of components, brings solid solution strengthening to its maximum

38
Q

What is creep?

A

Time dependent continued plastic deformation at constant load or stress

39
Q

What are the three regions of creep? Draw curve

A

Draw curve! (see notes)
Stage one: strain rate high, gradually decreases
Stage two: strain rate constant, steady state creep
Stage three: strain rate high

40
Q

What temperature range is creep significant?

A

T/Tm > 0.5 the closer the temperature is to Tm, the more creep it will experience

41
Q

What are the effects of stress on creep?

A

Higher stress means less fracture time, higher strain rate

42
Q

What are the different creep mechanisms? Why are they affected by temperature?

A

Dislocation movement (cross-slip, climb), diffusion, grain boundary sliding

Dislocation motion may be impeded by precipitates in the way. At higher temperature, cross-slip and dislocation climb to avoid these particles is a lot easier

Grain boundaries may elongate to accommodate the stress by diffusing atoms from the sides of the grain to the top and bottom of the grains. This is assisted at high temperatures, especially since it is vacancy mediated (need high T)

at high T, grain boundaries become weaker, so can slide against each other

Depending on temperature and stress applied, different types of mechanisms will occur

43
Q

What is Griffith’s Criterion?

A

There is a critical crack size that is required for the crack to grow, or else nothing will happen. This depends on the elastic modulus, the surface energy of the material, and the applied stress.

44
Q

Difference between ductile and brittle fracture?

A

Fracture is crack formation and propogation due to stress

Ductile fracture - plastic deformation with high energy absorbance before fracture, stable and preferred

Brittle fracture - no deformation, rapid crack propagation perpendicular to applied tensile stress

45
Q

What are stress concentrators?

A

There will always be microscopic flaws, and the stress in those areas will be a lot larger due to the radius of curvature

46
Q

What is Fracture Toughness Equation?

A

K_IC = Y /sigma_C sqrt(\pi a)
Y = dimensionless, relates to specimen, crack sizes, geometries
\sigma_C = critical stress for crack propagation
a = size of crack
K_IC related to resistance to brittle fracture when crack is present [MPa sqrt(m)]

We want a higher K value

47
Q

What variables impact fracture toughness?`

A

lower fracture toughness from lower temperature, higher strain rate, more solid solution hardening, increased grain size

48
Q

How do we test for fracture toughness?

A

Impact testing - use low temp, high strain rate, (charpy test)

Testing for lower level of KIC so matl works better at higher T

49
Q

What is Fatigue?

A

Dynamic and fluctuating stresses - failure possible at lower loads than at static

Brittle-like in nature even in ductile, very little plastic deformation associated with failure

50
Q

Why do BCC and FCC fatigue S-N curves look different?

A

Has to do with the number of available slip systems for plastic deformation

51
Q

How does fatigue failure work? How can we improve fatigue resistance?

A

Comes from crack initiation, propagation, and failure - mostly from surface defects. We can make this better by designing the shape of the metal better, getting rid of surface markings, case hardening (diffuse carbon to outer layer), shot peening (plastically deform outer layer) to improve fatigue behavior

52
Q

What is the Hall-Petch Equation?

A

Strengthening related to grain size - larger number of grains means more dislocations pule up at boundaries, dislocations multiply

sigma_y = sigma_0 + K d^(-1/2)

Prelims (5 decks)

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