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MSE Prelim > Mechanics > Flashcards

Mechanics Flashcards

1
Q

How is martensite formed?

A

Quenching austenite in order to create the metastable formation martensite

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2
Q

How do the mechanical properties of normal materials and superalloys at high temperatures differ and why?

A

Normal materials become weaker at elevated temperatures while superalloys get stronger due to Kear-Wilsdorf locks (stacking fault, anti-phase boundary, stacking fault)

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3
Q

What is creep, and what is a more practical way of testing it in a lab?

A

Creep is the deformation of a material under static stress at an elevated temperature
Larson-Miller parameter is used to predict material behavior at one temperature while testing it at a higher temperature to accelerate the process

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4
Q

Write a stress tensor what what the components are

A

(σxx τxy τxz)
(τyx σyy τyz)
(τzx τzy σzz)

σ: stress components
τ: shear components

Hydrostatic stress is isotropic stress, meaning that the compression/expansion is uniform in all directions

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5
Q

What are the formulas for stress, strain, and shear stress? What is the relationship between engineering and real stress and strain?

A

σ=F⊥/A
ε=Δl/l
τ=F∥/A

Perpendicular and parallel are in reference to the cross section of the material, which is perpendicular to the tensile axis

εt = ln(1+εe)
σt = σe(1+εe)

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6
Q

What is Peierls-Nabarro stress?

A

It is the stress required to move a dislocation within a plane of atoms in the unit cell

τPN ∝ Ge^2piW/b
W = d/(1-v)

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7
Q

What is elastomer strain-time, and what are the three models for it?

A

Strain time shows how stress increases under the initial strain, and then decreases as the elastomer shows its viscous properties
Maxwell: spring and dashpot in series; elastic response dominating at short times and the viscous response dominating at longer times
Kevin-Voight: in parallel; viscous response dominating at short times and the elastic response dominating at longer times
Standard Linear Solid Model: spring and dashpot in series with a spring and dashpot in parallel; both elastic and viscous behavior simultaneously, with the viscous response dominating at short times and the elastic response dominating at intermediate and longer times

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8
Q

What are some ways to achieve a smaller grain size?

A

Milling, controlled cooling, or introducing particles to provide grain size pinning

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9
Q

What are the different kinds of strengthening mechanisms? What are the sections of annealing process?

A

Solid solution strengthening, precipitation hardening, strain hardening, and grain size reduction
Recovery, recrystallization, and grain growth

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10
Q

What formula is used to calculate the increase in strength from solid solution strengthening?

A

Δσs=ks sqrt(c)

ks is the solid solution strengthening coefficient
c is the concentration in atomic fraction

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11
Q

What is the strain offset method for finding yield strength?

A

Start at (.002, 0) and go up following the slope of elastic deformation, yield strength is where it crosses the curve

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12
Q

Identify the important parts of the tensile stress strain curve

A

Elastic region: linear part at the beginning, used to find Young’s modulus
Plastic region: everything else
Yield strength: local maxima right after elastic regime
Ultimate strength: absolute maxima
Strain hardening: after yield strength before ultimate strength
Necking: after ultimate strength

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13
Q

What are the three main kinds of polymeric stress strain curves?

A

Brittle, Plastic, Elastic

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14
Q

What are the two kinds of dislocations and how can you identify them with their Burger’s vector?

A

Edge dislocation: Burger’s vector is perpendicular to dislocation line vector, a row of atoms was added/removed,
Screw dislocation: Burger’s vector is parallel to dislocation line vector, part of a plane of atoms is pushed down

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15
Q

What is twinning?

A

Mechanical twinning occurs is BCC and HCP metals at low temperatures under shock loading; either side of the twin will have identical planes; low bulk plastic deformation, but good increase in strength

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16
Q

How can you strengthen a polymer?

A

Drawing, orienting the crystals of a semicrystalline polymer along the tensile axis

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17
Q

What is the difference between ductile and brittle fracture?

A

Brittle fracture has cleavage, ductile fracture has necking, and in between likely has cupping

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18
Q

What are the different regions within a ceramic fracture site?

A

In order of increasing radii from origin:
Mirror region: flat, no noticeable texture
Mist region: small dust/mist like imperfections
Hackle region: larger cracks that propagate through the material

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19
Q

What are different kinds of cyclic stressing, and what is the fatigue limit? What are some other ways to fatigue a sample?

A

The three kinds of cyclic stressing are symmetric, asymmetric, and random cycling (single amplitude symmetric over x axis, like the previous, but shifted up/down, and varying amplitude)
The fatigue limit is a stress threshold for some materials, below which the sample can withstand a near infinite number of stress cycles
Thermal fatigue or corrosion/chemical fatigue

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20
Q

How does crystal structure influence mechanical properties?

A

Having more close packed planes like FCC and HCP makes it easier for dislocations to move, this is because the Burger’s vector is smallest in a CPP

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21
Q

What is the difference between engineering and real stress/strain? How does this influence stress strain curves?

A

Engineering stress and strain is divided by the initial area or length, while true stress and strain accounts for previous area and length changes
For a tensile stress strain curve, engineering stress goes down during necking while true stress has a fairly linear slope through to failure

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22
Q

What is Hooke’s Law?

A

σ = Eε: stress = Young’s modulus * strain
So E = σ/ε, Young’s modulus is stress over strain

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23
Q

What are some ways to increase the corrosion resistance of a metal or metallic alloy?

A

Surface treatment: A common method for increasing corrosion resistance is to modify the surface of the alloy. This can be done through the application of coatings, such as paint, varnish, or polymer coatings, that create a barrier between the alloy and the environment.

Alloy composition: The composition of the alloy itself can also be modified to increase corrosion resistance. For example, adding chromium to steel can create stainless steel, which is highly corrosion-resistant.

Passivation: Passivation is the process of creating a passive layer on the surface of an alloy that makes it less reactive with the environment. This is often done by treating the surface of the alloy with a chemical solution, such as an acid or alkaline solution.

Galvanization: Galvanization involves coating an alloy with a layer of zinc, which is highly corrosion-resistant. This is commonly done with steel to create galvanized steel, which is often used in construction.

Cathodic protection: This involves connecting the alloy to a sacrificial anode, which corrodes preferentially to the alloy. This protects the alloy from corrosion by providing an alternative source of electrons for the electrochemical reaction that causes corrosion.

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24
Q

What happens when rubber is exposed to the air for a long time, are there ways to prevent/slow down this change?

A

The air will oxidize the rubber causing increased crosslinking, making it more brittle
Adding antioxidants or UV stabilizers can help prevent this

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25
Q

In polymers, what happens at the glass transition temperature and melting temperature?
What are the three aspects of polymers that influence chain flexibility and Tg?

A

Polymer goes from liquid to rubber to rigid solid as the temperature decreases; this represents the polymer having enough thermal energy to overcome intermolecular forces (Tg) and crosslinking (Tm) that hold it together
Chain flexibility goes down and Tg increases with
1. Bulky side groups
2. Polar groups
3. Double bonds and aromatic groups

26
Q

How is work hardening measured?

A

It is measured in percent work
Change in area / initial area x 100%

27
Q

What is one way of representing toughness of a material?

A

Area under the stress strain curve

28
Q

What is Poisson’s ratio?

A

Poisson’s ratio (v) is the negative ratio of transverse and longitudinal strains, typically .25-.35 for metals
v= -εx/εz = -εy/εz
Represents the change in widths of the sample when it gets stretched

29
Q

What is hardness, and what are two common ways to measure it?

A

Hardness is a measure of a material’s resistance to localized plastic deformation

It is normally measured with a micro-indentation test, like Vickers or Rockwell
Rockwell tests use a diamond cone or hardened steel ball to measure penetration depth, while Vickers uses a diamond pyramid and measures indentation area

30
Q

What is the formula for grain boundary strengthening?

A

The Hall-Petch equation gives strength as a function of grain size
σy = σ0 + ky d^-1/2
σ0 and ky are material constants

31
Q

How is a sample precipitation hardened?

A

Alloy is heated to form a solid solution a then quenched
Then heated to a lower temperature in the miscibility gap so that precipitates will form as it ages, then cooled

32
Q

How is a sample solid solution strengthened?

A

Solid solutions can be created by melting metals together and quenching them
Carburizing, or case hardening, is completed by heating a sample in a carbon rich atmosphere to harden the outside of the sample

33
Q

How can a sample be work hardened?

A

Forging, rolling, extruding, and drawing
Can be hot or cold working depending if it is above or below recrystallization temperature
- Hot working: large deformations, retention of ductility, but surface oxidation leads to material loss and poor finish
- Cold working: strain hardening and loss of ductility, good finish and great dimensional control

34
Q

How do shearable and non-shearable precipitates affect the movement of dislocations and the strength of a material?

A

Non-deforming particles, or non-shearable precipitates impede dislocation motion and induce Orowan strengthening, where the dislocations bow and can create Frank-Read sources
Shearable particles are sheared as dislocations move through them, creating antiphase boundaries which further strengthen the material more than non-deforming particles

35
Q

What are partially stabilized zirconia and how are they used?

A

Zirconia (ZrO2) is a ceramic material that has three distinct phases that are dependent on temperature: monoclinic, tetragonal, and cubic (1137 °C, 2370 °C, and 2690 °C respectively)
It experiences volume increases as it transitions from cubic to monoclinic (martensitic transformation
Yttria (Y2O3) is used as a stabilizer to create partially stabilized zirconia (PSZ) which is stable in its cubic form at room temperature
Small cracks allow the phase transformation to occur, increasing volume, adding stress and stopping crack propagation

36
Q

How can you estimate the theoretical strength of a material?

A

σth ~ E/10

It is approximately one tenth of the Young’s modulus

37
Q

What is fatigue, and how does it lead to failure?

A

Fatigue is the repeated straining of a material, usually until failure
The sample strain hardens until it becomes brittle enough for the given load to initiate a crack
Crack propagation continues as the stress is cycled, often branching due to the changes in loading
Eventually the crack reaches a critical length and the sample fails

38
Q

How does crystal structure affect the fatigue limit of a material?

A

FCC materials have slip systems that can operate at very low stresses, resulting in fatigue even without high stress, and an inability to withstand an infinite number of cycles
BCC on the other hand does not have an activated slip system until high stresses and therefore have a fatigue limit, under which it can withstand an infinite number of cycles

39
Q

How do you test brittle materials, and what is the formula associated with that method?

A

Three point bend test, quantified using the modulus of rupture (MOR)
MOR=3PL/(2b(d^2))
P: force of fracture
L: distance between supporting pins
b: width of sample
d: thickness of sample

40
Q

What are some ways to avoid fatigue, or help remedy it fatigue cracking has already begun?

A

Issues caused by fatigue can largely be avoided in different design requirements by having the regular stress of use be below the fatigue stress of the material
Once fatigue cracks have already been initiated, it can be relieved by shot peening, boring a small hole at the crack tip, or drilling an oversized hole over an existing one and using a bushing to reduce it to its original size

41
Q

What is the DBTT, what causes it, and what materials is it most noticeable in?

A

The ductile to brittle transition temperature below which a material stops displaying ductile properties and instead shows brittle properties
This is mostly noticeable in BCC materials, and is due to the Peierls-Nabarro stress, which is dependent on shear modulus, which is temperature dependent

42
Q

What does a creep curve look like?

A

Strain over time, similar to an x cubed curve; steep, then levels out to a linear section, then gets steep till failure
These are the primary/transient, secondary/steady-state, and tertiary regions
Primary: increase in creep resistance or strain hardening
Secondary: balancing strain hardening and recovery
Tertiary: accelerated creep rate associated with necking and grain boundary voids leading to failure

43
Q

What are three different creep mechanisms?

A

Dislocation Creep: predominant at intermediate temperatures with mid to high stresses; modes include glide, climb, and cross slipping
Grain Boundary Creep: prominent at low temperatures under high stress, grains elongate and slide perpendicular to tensile loading to reduce the formation of voids in the material
Diffusion Creep: most prevalent at high temperatures under low stress
- Nabarro-Herring Creep: volume diffusion creep, atoms self-diffuse from areas of high stress to areas of low stress, occurs in bulk materials and is most prominent in single crystal or aligned grains
- Coble Creep: boundary diffusion creep, atoms diffuse along grain boundaries, prominent in materials with fine grains

44
Q

What are some ways to prevent the effects of creep on a material?

A

Choose a material with a higher melting point; create a sample with larger grains or with a single grain, use alloying to create a superalloy

45
Q

What is the difference between stress amplification and stress intensification?

A

Stress amplification is due to geometric features like a notch, hole, or fillet
Stress intensification is due to a difference in local material properties, like a welding joint

46
Q

What is a dislocation, and why do they exist?

A

Dislocations are planar defects in a crystal lattice, and are a response to stress in the system

47
Q

What is resolved shear stress?

A

Resolved shear stress is the stress parallel to the slip direction within a slip plane
τR= σcosΦcosλ
σ: applied stress
Φ: angle between tensile axis and direction normal to slip plane
λ: angle between tensile axis and slip direction

48
Q

How is dislocation movement different in ionic materials?

A

The forces needed to move a dislocation are greater because of the attraction between anions and cations, and the Burger’s vector is longer because a dislocation typically needs to move two lattice sites in order for an anion to move into another anion location

49
Q

What is a Frank-Read source?

A

When a dislocation is pinned between two obstacles (like another dislocation or precipitate) and further stress bows the dislocation until it creates a loop that separates from the original dislocation

50
Q

What are the two kinds of brittle fracture?

A

Transgranular: breaks along crystallographic planes through grains, called cleavage
Intergranular: breaks along grain boundaries

51
Q

What is the equation for critical stress for crack propagation in a brittle material?

A

σc = (2Eγs/πa)^1/2
γs: specific surface energy
a: 1/2 the length of an internal crack

52
Q

What is the fracture toughness formula?

A

Kc = Yσc (πa)^1/2 = Y(2Eγs)^1/2
Y: dimensionless parameter that depends on crack and specimen geometry and applied load

53
Q

What is the general relationship between grain size, fracture toughness, and strength?

A

As grain size decreases strength tends to go up, and fracture toughness tends to go down because the material endures less plastic deformation and has less area under the curve

54
Q

What is zone refinement?

A

Zone melting, or zone refinement is the process of melting a small part of an ingot at a time in order to accumulate impurities within the liquid phase, which then solidify a the end of the ingot and can be removed
This can also be used to create a single crystal

55
Q

How does surface quality affect crack formation?

A

Samples with smooth surfaces are less likely to develop a crack than samples with rough surface because a rough surface can include stress amplifiers

56
Q

What are some common ways to test ceramics, why is a tensile test not normally used?

A

Tensile testing is not usually used because attaching it to the testing device can damage the sample and compromise the test
Three or four point bending tests are used as well a cantilever tests
Testing multiple samples (>3) is always required as any defects can concentrate stress and greatly reduce the performance of a sample

57
Q

What is an S-N curve?

A

An S-N curve is a fatigue curve, and is stress over log number of cycles
The two main kinds of S-N curves are for materials with and without a fatigue limit

58
Q

What is cross slip?

A

Cross slip is when a screw dislocation moves into second plane due to local stress
This is unique to pure screw dislocations as they have a parallel dislocation line and Burger’s vector, meaning they have an infinite amount of slip planes

59
Q

What is the difference between slip and twinning?

A

Slip is when a dislocation moves along a plane under applied stress
Twinning is a specific pattern of matching planes with a thin, mismatched one in the middle, and is usually created by shock loading at low temperatures

60
Q

What is a stacking fault?

A

A stacking fault is when a layer of atoms is shifted slightly from where it is supposed to be, most easily demonstrated as a wrong layer in FCC or HCP stacking

61
Q

How can you get tensile strength from hardness?

A

Brinell multiplier
TS(psi) = 500HB
TS(MPa) = 3.45HB

MSE Prelim (5 decks)

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