MAT E 202 MIDTERM

Metals

Atoms are surrounded by a "sea of free e-"
non-directional bonding

Ceramics and Glasses

Can have covalent or ionic bonding

Covalent bonding

Valence electrons are shared
directional bond
electronegativity determines the amount of covalent bonding

Ionic Bonding

Occurs between a cation and an anion
Requires transfer of electrons
Non-directional bond

Polymers

two types of bonding (intra-chain bonding and secondary bonding)

Intra-chain bonding

bonding between atoms in polymer chain

Secondary bonding (polymers)

Van der Waals bonding between chains on polymers

What does bonding affect in a material

-strength
-elastic modulus
-ductility
-performance
-thermal and electrical properties

Different types of stress

-tension
-compression
-shear

Uniaxial tension

loading occurs in one direction

Biaxial tension

loading occurs in multiple directions

Hydrostatic compression

compression due to water pressures
highly complex

For metals, what is assumed about it's tensile properties

tensile properties are the same as the compressive
(commonly not true)
(commonly stronger in compression)

Bauschinger effect

effect that the tensile and compressive properties of a material are not the same

Uniaxial tensile test

-most common mechanical property tested
-must obtain a representative section of material
-specimens can be rectangular of cylindrical

Standard tensile test conditions

- room temperature
-loaded at a very slow rate
-strain rate of (5.75*10^-5 - 5.75*10^-6)

Strain rate formula

True stress

True strain

When are true stress/strain values used

When large strains are present

Ceramic stress strain curve

Metal stress strain curve

Polymer stress strain curve

Poisson's ratio

Ratio of lateral strain over vertical strain

Poisson's ratio meaning

The larger the Poisson's ratio the larger the elastic deformation of a material

Atomic bonding

forces that bind two atoms together

Repulsive forces

negative electron shells repel each other

Attractive force

positive nucleus with negative shell of neighboring atom attract

Force-distance graph

Top has a high modulus of elasticity and is strongly bonded. Bottom is opposite

Which has a higher bond strength and modulus of elasticity

Energy-distance graph

Blue has a lower elastic modulus and bonding strength

Which has a higher bond strength and modulus of elasticity

R_0 on energy/force-distance graph

equilibrium separation
where attractive and repulsive forces are balanced

Bond Energy (E_0) on energy/force-distance graph

energy at r_0
energy required to separate two atoms an infinite distance apart

Melting temperature

melting is the separation of atoms to an infinite distance
temperature is energy
the larger the E_0 value the more energy must be added to melt

Elastic modulus

amount of elastic deformation

What does a high elastic modulus mean

Little elastic deformation for high stress

What does a low elastic modulus mean

High elastic deformation for low stress

Plastic deformation

Permanent deformation of a material (will not return back to original shape)

Yield stress

stress at which plastic deformation begins

Proportional limit

Idealized transition between elastic behavior and onset of plastic deformation

How to determine Yield stress

used a 0.2% offset parallel to elastic deformation

Ultimate tensile strength (UTS)

Maximum stress on the engineering stress/strain curve

Ductility

The amount of plastic deformation a material undergoes in a tensile test (% elongation)

% reduction in area

Determines how brittle a material is

Brittle material

Low %RA (

Ductile material

High %RA (>5%)

How is yield strength and ductility related

as yield strength increases ductility decreases

Importance of TRIP steels

have alot of plastic deformation and strength

Energy absorbed by a material

Area underneath a stress-strain curve

Distribution of tensile strength in metals

narrow distribution in metals (ductile)

Distribution of tensile strength is ceramics

Wide distribution in ceramics (brittle)

what does a failure probability vs. stress plot show

the higher the slope the lower the variability

Factor of safety

accounts for variations in material quality

Hardness

measure of a material's resistance to localized plastic deformation
not a material property

Hardness testing

The small indenter is forced into the material
The indentation shape is converted into a hardness value
need to obtain 3 different readings
variations in microstructure will case changes in hardness

Work hardening

creating plastic deformation within a material to increase yield stress and UTS but lowers ductility

Rockwell hardness test

measures depth of indentation
(cone for hard materials)
(ball for softer materials)

Vickers/Micro vickers hardness test

dimention of indentation is measured

Brinell hardness test

diameter of indentation is measured

Uses of hardness testing

Estimating UTS
quality control
mechanical properties of small samples
changes in microstructures during processing
wear applications
W=(KFS/H)

Thermal expansion and contraction

change in dimension due to temperature

Coefficient of thermal expansion (CTE)

magnitude of expansion

Expansion due to change in termperature

What affects the CTE for a material

As temperature increases bond separation increases
Elastic modulus and bond energies increase

Thermal Stress

Thermal stresses are caused buy a constraint or two materials with different CTE

Constrained system

a metal rod wants to expand as temperature increases

Bimetallic material

The difference in CTE will cause the material to create thermal stress and expand at a different rate
Depending on the properties of each strip the material will deform when heated or cooled

How are CTE and same materials related

Two materials with different CTE values but at the same temperature is equivalent to the same material with different temperatures

Thermal shock resistance

Thermal shocks of ceramics is resistance to cracking at a sudden change in temperature

How can differential thermal expansion be minimized

slow cooling or heating

Prevention of thermal shock

choosing a ceramic with a very low CTE value

Thermal conductivity

rate of heat flow is proportional to the thermal conductivity (k)

Mechanism of heat transportation in solids

by lattice vibrations or waves called phonons
motion of free electrons

Metals heat transport

Free electrons are more significant than phonons

How is thermal conductivity related to electrical conductivity in metals

They are proportionally related

How does atoms affect electrical conductivity

The electron arrangement around the atom and the number of atoms that can move across the energy band

How is thermal energy related to kinetic energy in atoms

The thermal energy increases electron kinetic energy

How is heat transfer related to material purity

the more pure a material the more heat that can be transferred

What is the main factor to heat transfer in ceramics

Due to the lack of free electrons, the vibration/phonon mechanisms transport heat
K_l is inversely proportional to the number of scattering sites
wave scattering sites are primarily 2nd phase particles (air or contaminants)

Ceramics with high conductivity

Diamond, Si, BeO, AlN

How do polymers have heat transfer

mainly due to vibration and rotation of polymer chains
free path is very low due to disordered structure
specialized processing can reduce the disorder of the structure and henhase conductivity

Ohms Law

V=IR

Resistivity

rho=RA/l

Electrical conductivity

rho=1/sigma

What is electrical current flow related to in metals

electron motion

what makes the electrical conductivity of metals high

Free electrons

Conductivity

mu is affected by imparities

How does scattering centers relate to resuestivity

More scattering centers the higher the resistivity

What are scattering centers

-Impurities
-Vacancies
-Interstitial/substitutional atoms
-dislocations

what are superconductivity materials

materials with zero electrical resistance

High Temperature SC

materials that behaves as a superconductor above -200 degrees C

Type of Failues

Catastrophic failure, Fatigue, Creep

What causes materials to fail

-Poor material selection
-Poor design
-Environmental conditions
-Physical damage

What is fracture

-pieces splitting into two
-breaking of bonds between atoms

Brittle fracture

fracture with little plastic deformation
common in ceramics
occurs suddenly
fracture surface is flat on micro-level

Ductile fracture

fracture with large amount of plastic deformation
common in metals and polymers
failure does not occur suddenly
failure surface can be defined as a cup and cone with microvoids

Griffith Theory

An existing crack with propagating when energy is available is equal to the energy required to form two new surfaces

what does a Stress concentrations allow for

the nominal stress to overcome the theoretical bonding strength

Effects of plastic deformation absorbs energy

plastic deformation absorbs energy