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materials engineering lab > Test 1: hardness, charpy, tensile > Flashcards

Test 1: hardness, charpy, tensile Flashcards

1
Q

What is Hardness?

A

The resistance of a material to localized plastic deformation. Resistance to scratching, abrasion, or cutting. No units

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2
Q

What are advantages of Hardness Tests?

A

Simple and inexpensive. Nondestructive. tensile strength can be estimated from data obtained from hardness test

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3
Q

What is the relationship between a material’s hardness and its wear properties?

A

The higher the hardness, the better the wear properties.

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4
Q

What is an example of an application of a hard material?

A

Rock crushers have hard plates that make contact with the rock to reduce wear. The hard material is inserted at the points of stress of the machine in order to improve life of parts

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5
Q

What are two mechanical properties that are a indicator of a metal’s resistance to plastic deformation?

A

Hardness and Tensile strength, they are roughly proportional.
units: psi or MPa

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6
Q

Compare hardness of Cast Iron, Plastics, and Diamond

A

Diamond >cast iron> plastics

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7
Q

What is a static Indentation Test?

A

a test where a perpendicular indentor is forced against a material. dimensions of the deformation zone are used to obtain a parameter. Ex: Vickers, Rockwell, Brinell, Knoop

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8
Q

What is a Scratch Test?

A

a test where the ability of materials to produce scratches on each other is measured. Tool used is a Sclerometer.

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9
Q

What is a Rebound Test?

A

A certain size object is dropped from a height onto a material, height of rebound is measured. Method used by “shore scleroscope.”
Harder material = higher rebound
Softer material =more kinetic energy absorbed, lower rebound

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10
Q

What is a plowing test?

A

Blunt tool is moved across a surface. the width of groove measure hardness

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11
Q

What is a damping test?

A

pendulum with a hard pivot resting on test surface will be damped in proportion to surface hardness

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12
Q

What is a cutting test?

A

Sharp tool removes a chip of predetermined dimensions at low speed. The force required divided by the cross-sectional area of the chip provides hardness

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13
Q

What is an abrasion test?

A

a specimen is loaded against a rotating disk and the rate of wear is measured

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14
Q

What is an erosion test?

A

sand or another abrasive material impacts the test sample and the rate of surface loss is measured

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15
Q

What are the classifications of hardness testing?

A

macrohardness, microhardness

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16
Q

What are characteristics of macrohardness tests?

A

Quick, simple method of getting property data for bulk material from a small sample.
Require application of loads above 1 kgf.
too large to determine coating and surface hardness properties.
examples: Brinell and Rockwell

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17
Q

What are characteristics of microhardness tests?

A

Determined by forcing an indentor into the surface of material using a load smaller than 1kgf.
Indentations measured with microscope
Ex. Vickers and Knoop

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18
Q

Brinell Hardness Test

disadvantages, nml load amount, indentor

A

Requires 10 mm diameter of hardened steel or tungston carbide ball as indentor.
nml load is 3000kg for varied times based on material.
disadvantageous bc errors may occur bc data must be observed.

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19
Q

Rockwell Hardness Test

A

Does not require visual measurements, uses direct depth gauge scale. Determines hardness by measuring .
2 types: rockwell and superficial rockwell
Uses many types of scales.

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20
Q

What are the minor and major loads for rockwell and superficial rockwell tests?

A

Rockwell Minor: 10kg
Major: 60,100,150kg

Superficial Minor: 3kg
major: 15, 30, 45kg

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21
Q

What is the difference between Brinell and Rockwell hardness tests?

A

Rockwell: IMPRESSION DEPTH OF INDENTATION is measured.
Initial minor load followed by major load.

Brinell:DIAMETER OF INDENTATION LEFT IS MEASURED.

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22
Q

How is rockwell hardness specified?

A

hardness number and scale symbol

“80 HRB”

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23
Q

What is elastic deformation?

A

temporary change in shape when load is applied.

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24
Q

What is plastic deformation?

A

Irreversable deformation.

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25
Q

What is characteristic of ductile materials?

A

experience significant plastic deformation before fracture

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26
Q

WHat is characteristic of brittle materials?

A

Experience little of no plastic deformation before fracture

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27
Q

What is normal stress?

A

force per unit area

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28
Q

WHat is shear stress?

A

force per unit area concerning stress that acts parallel to surface

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29
Q

What is strain?

A

change in length per unit area

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30
Q

What is toughness?

A

ability of a metal to rapidly distribute within itself both stress and strain caused by a shock load.opposite of brittle

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31
Q

What is a ductile fracture?

A

Cracks move slowly and are accompanied by a lot of plastic deformation.
Cup/cone shape.

32
Q

What is a Brittle Fracture?

A

Cracks move rapidly through a stressed material. usually fx without warning
Granular & shiny surface.

33
Q

What are the mechanisms of crack propagation for ductile and brittle materials?

A

Ductile: Microvoids
Brittle: Transgranular/intergranular fx

34
Q

What is a microvoid?

A

small voids that grow together to form a crack

35
Q

What is a transgranular fx?

A

fx travels through grain of material

36
Q

What is a intergranular fx?

A

crack travels along grain boundaries

37
Q

What do impact tests measure?

A

Object’s ability to resist high-rate loading.

38
Q

What is the Charpy Impact test?

impact v. absorbed energy

A

standardized high strain rate test which determines the amount of energy absorbed by a material during fracture.

impact energy=work done to fx specimen

absorbed energy=material toughness measurement

39
Q

Describe how a Charpy test is set up

A

A notch is loacted at center of test sample and the test sample is horizontally supported. The sample recieves impact from a pendulum of a specific weight on side opposite to notch.

40
Q

What are some factors that affect impact energy?

A

temperature, ductility/brittleness, strain rate, notches, heat treatment, microstructure, fracture mechanism

41
Q

What is temperature-dependent brittle-ductile transition?

A

can be determined by Charpy Impact test.
Shown on temperature v. impact energy graph.

At higher temperature, impact energy absorbed is large and fx is ductile

At lower temperature, impact energy absorbed is less and fx is more brittle.

42
Q

What is axial strain?

A

change in length per unit length along direction of loading.

43
Q

WHat is lateral strain?

A

Change in length per unit length along perpendicular direction of loading.

44
Q

What is Poisson’s ratio?

A

Ration of lateral strain over axial strain.

45
Q

Describe how a tensile test is performed

A

A test specimen of circular/rectangular/or square cross-section is prepared and then deformed until tracture. Tensile load is gradually applied uniaxilly along the long axis of the specimen

46
Q

What are values that can be obtained by the tensile test?

A 
load and displacement can be used to calculate stress and strain. 
Young's modulus
Yield strength
failure strength
UTS
reduction in area at fx.
true stress/strain
47
Q

What is Young’s Modulus?

A

Deformation in which stress and strain are proportional. Follows Hook’s Law

48
Q

What is Yield Strength?

A

Stress at which a material just starts to undergo permanent plastic deformation. typically taken at 0.2% strain

49
Q

What is the Ultimate Tensile strength?

A

Highest value of stress on the stress-strain curve

50
Q

Why is it beneficial to use true stress-true strain?

A

It shows that the material is increasing strength beyond the maximum point bc it takes into account the rapidly decreasing cross sectional area in the neck region.

51
Q

How is true stress defined?

A

load divided by instantaneous cross-section area over which deformation is occuring

52
Q

How is true strain defined?

A

Instantaneous strain

53
Q

What is a polymer?

A

an organic substance composed of molecules repeating structural units.
Natural V. Synthetic

54
Q

What is a plastic?

A

materials that have some structural rigidity under load.

55
Q

What are 2 (3) types of polymers/plastics?

A

Thermoset
Thermoplastic
(elastomer)

56
Q

What are properties of a thermoset polymer?

A

solidify when heated.
irreversable process
cannot be recycled
little to no cross linkage

57
Q

What are properties of a thermoplastic polymer?

A

melt when heated and harden when cooled.
recyclable
complex 3D structure with cross linkage

58
Q

What are factors that influence properties of polymers/plastics?

A

molecular weight, degree of polymerization, molecular structure, degree of crystallinity

59
Q

What are 5 processing methods of plastics?

A

Injection molding, compression molding, extrusion, blow molding, casting

60
Q

What happens to plastics during tensile testing?

A

Plastics yield at end of elastic region and then start necking. Plastics can endure lots of strain after onset of necking. (molecule chains stretch from agglomerated polymers to aligned polymers)

61
Q

What is a Craze?

A

Regions of very localized plastic deformation, lead to formation of small and interconnected microvoids.

62
Q

What are 3 advantages and disadvantages of plastics?

A

Advantages: can be recycled, low density, high specific strength
Disadvantages: low toughness, can’t be applied in high temp, can produce harmful fumes when burned.

63
Q

What are some factors that determine material selection?

A

application, working conditions, avaliability, cost, ease of manufacturing, service life

64
Q

What is a composite material?

A

materials consisting of two or more constituents (phases) that are combined microscopically and not soluble in each other.
Natural v. Synthetic

65
Q

What are common phases of composites?

A 
bulk phase (matrix)
Dispersed phase (reinforcement)
66
Q

What are 2 examples of composites. Name their matrix and reinforcement

A

Reinforced Concrete

matrix: concrete
reinforcement: steel bars

Carbon FIber Composite

matrix: epoxy resin
reinforcement: steel bars, carbon fiber

67
Q

What are some classifications based on reinforcement type?

A

Long fiber, flake, short fiber, particulate, filler

68
Q

What are classifications based on reinforcement dispersion?

A

Continuous: unidirectional, crossed, multidirectional, strand

Particulate (whiskers, short fibers): aligned oriented, randomly oriented, randomly distributed particulates

69
Q

What are special properties of metal and ceramic matrix composites?

A

Metal: high strength and stiffness
Ceramic: high thermal stability and good stiffness

70
Q

What are special properties of polymer matrix composite?

A

wide range of applications,

low price, ease of processing,

71
Q

What is the role of the matrix of composites?

A

Transfer load to reinforement.
Protect reinforcement
Bind reinforcement in proper orientations
may provide insulation, surface finish, reduced stress concentration

72
Q

What is role of reinforcement of composite?

A

Provide strength
Increase toughness
provide framework for matrix
May enhance electrical or thermal properties

73
Q

What is lamina and laminate?

A

Lamina: materials are formed into layers
Laminate: lamina with different/same fiber orientation stacked together.

74
Q

How does fiber orientation affect tensile strength?

A

If FIbers are oriented up and down, 0 degrees, they have highest tensile strength

If fibers are oriented horizontally, 90 degree to stress load, they have least tensile strength.

75
Q

What do mechanical properties depend on in composite materials?

A

orientation of fibers
fiber concentration
distribution
amount of defect

76
Q

What is better about composites than metal?

A

higher specific strength

higher specific stiffness

77
Q

What are some advantages and disadvantages of composites?

A

Advantages: high mechanical properties, flexibility of design, ease of fabrication, light weight, environmental durability, impact resistance, good fatigue strength

Disadvantages: high material cost, high manufacturing cost, temperature limitations, brittle failure mechanism, properties are process dependent

materials engineering lab (2 decks)

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