Materials yapa yapa

Question 1

3 points about metals

Answer 1

(metallic bonding)
– Strong, high modulus, ductile
– High thermal and electrical conductivity
– Crystalline, opaque, reflective

Question 2

3 points on polymers (plastics)

Answer 2

(covalent and van der Waals bonding)
– Soft, ductile, low strength, low modulus, low density
– Thermal and electrical insulators
– Optically translucent or transparent

Question 3

Ceramics

Answer 3

(ionic and covalent bonding)
– Metallic/non-metallic element compounds (oxides, carbides, etc.)
– Brittle, crystalline or amorphous, high Temp
– Strong, high modulus
– Electrically and thermally insulating

Question 4

elastic deformation

Answer 4

returns to original shape

Question 5

plastic deformation

Answer 5

– structure retains some
permanent deformation
– many structures involve both
elastic and plastic responses

Question 6

Tensile test

Answer 6

how does it work

Question 7

Draw a stress strain curve for ceramics, metals and polymers

Question 8

draw a tensile test diagram, engineering stress vs engineering strain

Question 9

Name all stages of tensile test

Answer 9

• initial elastic region
• non linear elastic region
• yield stress
• plastic deformation

Question 10

Describe Initial Elastic region

Answer 10

recoverable (O - A)
– Linear Elastic
* Stress is directly proportional to strain
* When the force is removed the specimen returns to it’s original undeformed
shape
– Hooke’s Law

Question 11

Where does yield stress occur

Answer 11

Material Yields at B
– Yield stress σy (yield strength) - The stress at which the onset of
plastic deformation occurs

Question 12

Where does plastic deformation occur (and describe it etc)

Answer 12

Plastic Deformation after B
– Plastic deformation is not recoverable and it is permanent
– When the load is removed the specimen does not return to its
original undeformed shape. The material becomes damaged
– Work hardening – occurs during plastic deformation

Question 13

Define toughness

Answer 13

TOUGHNESS – the amount of
energy a material can absorb
before it undergoes fracture

Question 14

Poisson’s Ratio

Answer 14

as ratio of lateral (contraction) strain to
axial (extension) strain
𝑃𝑜𝑖𝑠𝑠𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 = 𝜈 = − 𝑙𝑎𝑡𝑒𝑟𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛/𝑎𝑥𝑖𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 = − ∆𝑑/𝑑 // ∆𝐿/𝐿

Question 15

Shear Stress

Answer 15

shear stress (τ) acts tangential to the surface of a material element

Question 16

Shear Modulus

Answer 16

shear modulus (G) is described in terms of ratio of shear stress
(τ) to the corresponding shear strain (γ)

Question 17

Elastic isotropic material properties E, υ and G are not
independent and may be related to one another by,

Answer 17

𝐸 = 2(1 + 𝜐) 𝐺

Question 18

Torsion

Answer 18

Angle of twist

Question 19

Stiffness

Answer 19

resistance to elastic deformation

Question 20

Yield strength

Answer 20

ability to resist plastic deformation

Question 21

ductile

Answer 21

large plastic strain before fracture

Question 22

brittle

Answer 22

low plastic strain before fracture

Question 23

hardness

Answer 23

ability to resist localized plastic deformation

Question 24

Formula for true stress related to engineering stress

Answer 24

σ true
= σ eng (1 + ε eng)

ε true
= ln (1 + ε eng)

Question 25

Atomics bonding- what happens to atoms during elastic deformation?

Answer 25

– Atoms are stretched/separated but bonds are not broken – Atoms return to original position on removal of load

Question 26

Name the primary vs secondary bonding types

Answer 26

Primary (Chemical): Strong – Ionic (Ceramics) – Covalent (Polymers) – Metallic * Secondary (Physical): Weak – van der Waals – Hydrogen

Question 27

Ionic Bonding

Answer 27

Transfer of electron(s) takes place between atoms 2Na + Cl2 → 2NaCl – High melting and boiling points

Question 28

Covalent Bonding

Answer 28

Covalent bonding - Outer electrons (valence) are shared between atoms weaker than Ionic Bond – Polymers are primarily bonded together through covalent bonds

Question 29

Metallic Bonding

Answer 29

Metallic bonding – Outer (valence) electron not bound to any particular atom * Drift throughout entire structure – Negative electron cloud – Positive ion cores * Typically high bond strength – Plastic deformation ability (unlike brittle ionic bonds) * Free electrons – High thermal and electrical conductivity

Question 30

Crystalline Materials

Answer 30

A material in which the atoms are situated in a repeating or periodic array over large atomic distances * Upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern * Each atom is bonded to its nearest neighbour atoms * All metals, many ceramics and certain polymers form crystalline structures * Many material properties depend on crystal structure * Structure of crystal minimises energy

Question 31

Unit cell

Answer 31

Smallest repeat structure is called the

Question 32

Three types of metallic bonding for crystal structures

Answer 32

Three common types for metal – Body Centred Cubic – Face Centred Cubic – Hexagonal Close Packed

Question 33

Is lattice structure seen in metals?

Answer 33

Lattice structure typically not seen in metallic materials due to inefficient packing sequence... Polonium is only example

Question 34

Face Centered Cubic (FCC)

Answer 34

atom at each corner + single atom in each cube face Metals include aluminium, copper, gold, lead, nickel and silver * Readily undergo plastic deformation * Configuration allows atoms to slip past each other easily

Question 35

Body Centered Cubic (BCC)

Answer 35

atom at each corner + single atom in each cube Materials include lithium, alpha-iron and tungsten * Tend to be harder and less malleable Atomic Packing Factor = 𝑽 𝒂𝒕𝒐𝒎𝒔 // 𝑽 𝒖𝒏𝒊𝒕 𝒄𝒆𝒍𝒍

Question 36

Face centered Cubic - atoms and coordination number

Answer 36

4 atoms 12 coordination number

Question 37

Body centered Cubic - atoms and coordination number

Answer 37

2 atoms 8 coordination number

Question 38

Simple Cubic - atoms and coordination number

Answer 38

atoms 1 coordination 6

Question 39

Hexagonal Close Packed

Answer 39

Metals include Ti, Mg, Zn * Closed Packed but with much fewer slip planes * Normally brittle / low ductility

Question 40

Anisotropy

Answer 40

Anisotropy — Property value depends on crystallographic direction of measurement If grains textured (e.g., deformed grains have preferential crystallographic orientation): properties anisotropic.

Question 41

Isotropy

Answer 41

properties homogenous if grains randomly oriented: properties isotropic

Question 42

Lattice Parameters

Answer 42

The unit cell geometry is completely defined in terms of six parameters: the three edge lengths a, b, and c, and the three interaxial angles α, β, and γ

Question 43

Three point indices , lattice coordinate position

Answer 43

Coordinate specifications are possible using three point indices: q, r, and s * These indices are fractional multiples of a, b, and c unit cell lengths – q is some fractional length of a along the x axis, – r is some fractional length of b along the y axis, – s is some fractional length of c along the z axis example

Question 44

Crystallographic Directions

Answer 44

A crystallographic direction is defined as a line directed between two points, or a vector – x, y, z coordinate system in unit cell corner – determine coordinates of two points on the direction vector (xi , y i , zi ) – determine length by subtracting coordinates (head – tail) – normalize by dividing by their respective lattice parameters * Convert to smallest integer * They are expressed in square brackets [u v w]

Question 45

Slip

Answer 45

sliding displacement along a plane

Question 46

Slip System

Answer 46

= slip plane and slip direction Slip occurs on densely or close-packed planes, in close-packed directions – Lower shear stress/energy is required for slip to occur in close-packed planes and in close-packed directions

Question 47

Which is more packed, FCC or BCC?

Answer 47

No truly close packed planes like FCC so BCC less packed BCC ductile, but typically less so than FCC

Question 48

HCP are the planes close packed and what are the slip systems like?

Answer 48

Closed-packed but with much fewer slip systems (typically 3 or 6) – HCP metals tend to be quite brittle, with low ductility

Question 49

Schmid's Law

Answer 49

Show that the Resolved Shear Stress (RSS) (t R) on a slip plane in the slip direction is given by, t R = σ cosλ cosf (Schmid’s Law)

Question 50

Critical RSS

Answer 50

Slip will occur The shear stress for initiating slip in different materials – This is a material property – Slip will happen on the plane with highest RSS ( tR ) * For a single crystal – the engineering stress ( σ) required to cause slip depends on angle of slip plane to loading * Engineering yield stress may therefore vary for a single crystal depending on the loading direction

Question 51

Slip Stress for Polycrystalline Materials

Answer 51

Each crystal oriented differently to nominal loading – Slip directions/planes are therefore randomly oriented

Question 52

Crystal Lattice

Answer 52

Periodic arrangement of atoms

Question 53

defect

Answer 53

disrupts the order of the lattice

Question 54

Name the 3 types of lattice imperfections

Answer 54

Point Line planar

Question 55

Name the 3 types of Point defects

Answer 55

vacancy self interstitial substitutional

Question 56

Point defect

Answer 56

* Point Defects – any defect that affects a few neighbouring atoms or lattice points

Question 57

Interstitial atom (Point Defect)

Answer 57

Interstitial atom – A point defect where a smaller atom fits in between other atoms in the crystal lattice – Can be an Impurity or intentional alloying element – Induce minor stress-field on the lattice

Question 58

Point Defect - Substitutional

Answer 58

Substitutional atom – A point defect where an impurity atom substitutes for a host atom – Induce stress-field on the lattice

Question 59

Alloys

Answer 59

Solid solutions where one metal type is dissolved within another to enhance mechanical properties

Question 60

Point defect- vacancy

Answer 60

A location in a crystal lattice where an atom is missing Induce stress-field on the lattice

Question 61

Line Defects- name the three types of dislocation

Answer 61

Edge dislocation – Line defect where an extra half-plane of atoms exists in crystal lattice – Screw Dislocation – Line defect, where path spirals around a dislocation line penetrating otherwise parallel planes – Mixed Dislocations – Edge + Screw defects

Question 62

Edge Dislocation

Answer 62

Edge dislocation – Line defect where an extra half-plane of atoms exists in crystal lattice

Question 63

Screw dislocation

Answer 63

– Screw Dislocation – Line defect, where path spirals around a dislocation line penetrating otherwise parallel planes Screw dislocations occur when the lattice itself is sheared such that there is a misalignment of the atoms * Dislocation line is the edge, or where this misalignment begins

Question 64

Mixed dislocation

Answer 64

– Mixed Dislocations – Edge + Screw defects

Question 65

Dislocation - line defects

Answer 65

linear defect around which some of the atoms are misaligned dislocation density influences strength and increases with applied stress

Question 66

Line defects - Burgers vector

Answer 66

Magnitude of dislocation given by the Burgers Vector, b – Draw loop around defect – Repeat the loop steps in a perfect crystal – b is the vector required to close the loop * b is always perpendicular to dislocation line for edge dislocations – Magnitude of lattice distortion

Question 67

Dislocation Density

Answer 67

Dislocation Density = Total dislocation line length per unit volume – Highly deformed metal – cold worked (rolling, drawing etc.): – Means there has been significant amount of plastic deformation

Question 68

Why are dislocations highest in metals??

Answer 68

The number of dislocations is highest in metals * Atoms easily move to another position with dislocation movement * Motion of dislocations is easiest because metals have non-directional bonding and close-packed directions * Covalent – directional, angular bonding – need to break bond * Ionic – charged ions don’t want to change position

Question 69

Grain Boundaries

Answer 69

Grain Boundaries are planar defects  a 2D interface between adjacent grains (single crystals) in a polycrystalline material * Most common type of “planar” defect * Characteristics can strengthen or weaken a material * Grains randomly oriented * Each grain has it’s defined crystal structure, i.e. FCC, etc