Material Mid Sem

Single phase alloys can be strengthened by

precipitation hardening

precipitation strengthening

grain size reduction

solid solution precipitation

solid solution precipitation

The stages of creep may be characterised by a number of regions plotted on a creep versus time curve. The secondary stage of creep deformation is characterized by a straight line with

transient, secondary or steady-state

transient creep

tertiary or acceleration creep

decreasing strain data

transient, secondary or steady-state

Fatigue of metal alloys may be considered to correspond to

the ductile fracture of the alloy after a number of cycles above the yield strength

the brittle fracture of the alloy after a number of cycles above the yield strength

the ductile fracture of the alloy after a number of cycles below the yield strength

the brittle fracture of the alloy after a number of cycles below the yield strength

the brittle fracture of the alloy after a number of cycles below the yield strength

The fatigue life of materials may be determined using Miner's rule. This approach does not take into account the

applied strain

applies stress

applied number of cycles

the type of material.

appliead strain?

Plastic deformation of metals

can be recovered when the load is removed

is permanent deformation when the load is removed

has a small viscoelastic component which is recovered when the load is removed

shows a yield strength when the load is removed

is permanent deformation when the load is removed

Hardness measurement of a metal is achieved using a 'hardness test', and can be explained as

the resistance of a metal to a round indenter

the resistance of a metal to a pointed indenter

the resistance of a metal to a plastic deformation

the resistance of a metal to elastic deformation

the resistance of a metal to a plastic deformation

A high impact energy is associated with:

large strain energy at yielding

cleavage failure mechanism

high surface energy

a fatigue failure

large strain energy at yielding, high surface energy

Which kind of fracture is associated with the intergranular crack propagation mechanism?

Ductile

Brittle

Torsion

Brittle or ductile

brittle or ductile

The major difference between the Izod and Charpy test is

temperature of the test

the geometry of the test pieces

the configuration of the test pieces

the Izod test piece does not have a notch

the configuration of the test pieces

A brittle material typically exhibits substantial plastic deformation with high energy absorption before fracture

True

False

false

The elastic modulus of a material is related to the shear modulus, for small strains by

shear stress

Hooke's modulus

R-ratio

one plus Poisson's ratio

one plus Poisson's ratio

The main mechanism for plastic deformation in metals is generally due to

the presence of vacancies

the presence of mixed dislocations

the presence of only edge dislocations

the presence of only screw dislocations

 

the presence of mixed dislocations

The _____________ temperature determines whether hot or cold working is occurring in metals.

recrystalization

Polycrystalline alloys can not be strengthened by

grain size reduction

solid solution precipitation

solid solution strengthening

precipitation hardening

solid solution precipitation

Annealing is

cold worked process in which ametal is heat treated and may experiencerecovery, recrystallization, and grain growth

both a cold worked and a hot worked process in which a metal is heat treated and may experience recovery, recrystallization, and grain growth

hot worked process in which a metal is heat treated and may experience recovery, recrystallization, and grain growth

working process in which a metal may experience recovery, recrystallization, and grain growth

hot worked process in which a metal is heat treated and may experience recovery, recrystallization, and grain growth

What are the three major factosof material selection?

properties, composition and processing

What types of bulk properties can material have

emchanical, electrical, optical and thermal

3 mechanical porperties

elastic modulus, shear modulus, hardness

3 eleectrical properties

conductivity,r esistivity, capacitance

3 optical porperties

reflectivity, absorbance, emission

3 thermal proeprties

thermal expansion, heat capacity, thermal conductivity

Metal properties

strong, malleable, good conductors, can be alloyed to improve properties

What is tensile strentgh

the maximum amount of pulling or stretching stress a material can withstand before breaking or failing

What is compressive strength

he maximum capacity of a material or structure to withstand loads that reduce its size (pushing together), measured as the stress applied just before the material breaks or deforms permanently.

Common mechanical test peformed in tension

stress-strain test

Whati is deformation

change n shape due to application of force

Strain is proportinal to applied stress only when

elastic deformation exists

tensi..e stress occures when the load is applied

perpendicula to the specimen cross-sectional area

In a material, what makes ahiher modulus of elasticity

The greate rthe force of attraction between atoms

What is a shearforce?

an internal or external force acting parallel to a material's cross-section, causing one part of the object to slide, cut, or deform diagonally past the other

What is torsion

the act, process, or state of twisting an object, typically created by applying opposite torques at either end. (a type of shear stress)

What is a moment

a measure of its tendency to cause a body to rotate about a specific point or axis.

what is a tensile test?

Applies increasing tensile force , until breaks, applicable to ductile materials

Effects of alloying

strength is affected, stiffness(modulus of elastciity) 9s not.

What is represented by E, 2, 3, 4 and the line at 4

Elastic region modulus, proportional limit, yield strentgh, yield strength at 0.2% offset, metals response when load is removed

What are anisotropic materials

substances whose physical properties—such as strength, conductivity, or elasticity—vary depending on the direction of measurement, rather than being uniform in all directions.

What does poisson’s ration describe

The deformation of a material in directions perpendicular to the direction of loading

What are isotropic materials

having a physical property which has the same value when measured in different directions.

What can we assume for polycrystalline materials with random grain orientation

assume isotropic

What is ductility

the general ability of a metal to be plastically deformed

Why do we generally want ductile materials

bends before it breaks and warns of something going wrong

What is a brittle material

a substance that breaks or shatters with little to no plastic deformation (bending or stretching) when subjected to stress, often failing abruptly without warning.

What is a flexural test

measures a material's behavior under bending loads, determining its ability to resist deformation and rupture, such as flexural strength and modulus.

Hardness test

measure of material’s resistance to localized plastic deformation

(e.g. dent or scratch)

Why check hardness

Machinability- Tool Life

Wear Properties

• Corrosion Resistance

• Fatigue/ Life Cycle

• Annealing

• Cost Savings

what type of tests

hardness tests

Hardness relation to tensile strength

Both show degree of resistance to plastic deformation, hardness proportional to tensile (depends on material)

Design/saftety factors

Because of design uncertainties allowances must be made to

protect against unanticipated failure

• For structural applications, to protect against possibility of failure—

use working stress, σw, and a factor of safety, N

Read this

Where does fracture typically initiate in a three-point bending (flexural) test

At the outer surface of the specimen that is under tensile stressrather than at the surface under compression

Ductile materials have:

Extensive plastic deformation and energy absorption (“toughnesss”) before fracture

Brittle materials have:

little plastic deformation and low wnergy absorption before fracture

What is the stress concentration factor

Dimensionless ratio of maximal stress to nominal stress which quantifies how much localised stresses increase due to abript geometric changes like holes, fillets or notches.

What is fracture toughness

Measure of a material’s resistance to brittle fracture when a crack is present

Low fracture resiistance leads to:

rapid crack propogation and brittle fracture

What are the charpy and izod tets

standardized pendulum-based impact tests used to determine a material's toughness and energy absorption (notch toughness) during sudden shock loading. Both measure brittle-to-ductile transition, with Charpy using a horizontal specimen (often for metals) and Izod using a vertical cantilevered specimen (often for plastics)

What is fatigue

time based failure that occurs due to cyclic loading at stresses below material’s yield strength

What are the two main methods of fatigue testing

rotary bending method, uniaxial method

revise this

What do S N curves measure

stress vs number of cycles to failure

What is low cycle fatigue (LCF)

small number of cycles, high loads, elastic and plastic deformation

What is high cycle fatigie (HCF)

large number of cycles (N > 10^5)

Low loads, elastic eformation

What is a fatigue limit

When stress amplitude is below which material never fells (section becomes flat)

What is fatige strenght

stress at which fracture occurs at a specified nmber of cycles

What are the stages of fatigue failuer

Crack initiaition, crack propagation, complete fractre

miner’s rule

an engineering principle used to predict fatigue failure in materials subjected to variable stress loading. It states that failure occurs when the sum of damage fractions—the ratio of applied cycles () to cycles causing failure () at various stress levels—equals or exceeds 1

Improving fatigue life

• modifying degree of stree variation, minimize or remove stress concentration points, generate compressive residual stress, regulate the operational envionment

Avoiding fatigue failure

1. Control stress risers like holes, sharp corners, grooves, threads, keyways,

and stamped markings.

2. Control surface finish, including scratches in critical areas.

3. Avoid corrosion and exposure to embrittling gases.

4. Be very careful with welds – spot welds and fillet welds.

5. Higher temperatures usually reduce fatigue strength.

6. Manage residual stresses from fabrication operations, including welding.

7. Make critical areas inspectable for cracks.

8. Design in stress margin.

9. Measure real load environment and test parts to determine actual fatigue life.

10. Design in redundant load paths.

What is creep

Deformation under static stress at elevated temperatures ( > 0.4 Tm)

at long periods of

What is the effect of increased temperature and stress on creep strain

Higher temperature and increased stress accelerate creep strain, resulting in faster deformation rates, shorter material life, and faster transition through creep stages

What is Solidification

result of casting of molten materia, nuclei of the solid phase form, crystals grow until their boundries meet crystaks become grrains

What are equiaxed and columnar grains

equiaxed: roughly same dimension in a lldirections

columnar: grains elongated in one direction

Types of imperfections

point defects, linear defects, interfacial or boundry defects

What are vacancy atmoic sites

Missing atim from an atomic site atoms around the vacancy displaced, the tensile stress field produced in the vicinity

What are self-interstitials

Host atoms positioned in interstitial positions between atoms

Hume rothery rules

Great way to predict if two elemensts will from a solid solution:

1. Δr (atomic radius) < 15%

2. Proximity in the periodic table; i.e.

similar electronegativities

3. Same crystal structure

4. Valences; All else being equal, a metal

will have a greater tendency to dissolve

a metal of higher valence than one of

lower valence

Describe in your own words the three strengthening mechanisms discussed in this chapter (i.e., grain size reduction, solid-solution strengthening, and strain hardening). Be sure to explain how dislocations are involved in each of the strengthening techniques.

Grain Size Reduction:
Reducing the grain size increases the number of grain boundaries in the material. Grain boundaries act as barriers to dislocation motion because dislocations must change direction or stop when they reach a boundary. Therefore, with more boundries, dislocations are less able to move, resulting in increased strength.

Solid-Solution Strengthening:
This method involves adding impurity atoms (either substitutional or interstitial) into the metal lattice. These atoms create lattice distortions due to size differences, producing strain fields. These strain fields interact with and hinder dislocation movement, meaning greater applied stress is required for deformation, thus increasing strength.

Strain Hardening (Work Hardening):
When a metal is plastically deformed, the number of dislocations increases significantly. As dislocation density rises, they begin to interact, tangle, and block each other’s motion. This makes further dislocation movement more difficult, thereby increasing the strength and hardness of the material.

Molecular weight slightly increases modulus due to greater chain entanglement. Higher crystallinity increases modulus because ordered crystalline regions restrict chain movement. Drawing increases modulus in the drawing direction by aligning polymer chains, making the material stiffer along that axis.

5 Factors that favour brittle fracture for thermoplastic polymers

1) reduction in temperature

2) Increase in strain rate

3) Presence of sharp notch

4) Increased specimen thickness

5) modification of the material (e.g. additio of hard fillers)

Describe the phenomenon of viscoelasticity

a material property exhibiting both viscous (time-dependent flow) and elastic (instantaneous, recoverable) behaviors when deformed

In stress relaxation, a constant strain is applied and the test is conducted by rapiidly straining the material ellastically in tension. decrease in stress as a function of time is measured. The parameter of interest is the relaxation modulus, found from stress divided by the fixed strain. In creep testing, a constant stress is applied and the increase in strain with time is measured. The parameter of interest is the creep modulus, found from the fixed stress divided by the time-dependent strain

Advantages and disadvantages of using transparent polymeric materials for eyeglass lenses

Advantages

• low densities, thus lightwieght

• relatively easy to gring to have desired contours

• Less likely to shatter than glass

• They filter out more UV radiation than glass

• Disadvantages

• Relatively soft so easuly scratched

• Not as mechanically stable so not as precise optically