Engineering Materials' Importance

• Materials are fundamental to technological progress and impact everyone's life.
• Thorough understanding is needed in the properties when using materials for products like bridges, microchips, or vehicles.
• Advancements in computing, transport, and energy are due to improved materials synthesis and processing.
• Automobiles consist of about 20,000 parts made of different materials, each suited to its function.
• Car bodies use steel for strength and formability; bumpers use reinforced plastics.
• Seats use plastics for light weight and mouldability.
• Glass is used for windows because it is transparent, cleanable, hard, and abrasion-resistant.

Engineering Materials' Importance

• Materials drive technology and manufacturing jobs rely on specialized materials.
• Necessary for improved quality of life, security, productivity and economic growth.
• Engineers must consider material properties (hardness, conductivity) for intended applications.
• Behavior during manufacturing and environmental effects must be considered.
• Materials are classified into metals, ceramics, polymers, composites, and semiconductors.
• The structure greatly influences their mechanical, electrical, thermal, optical, and magnetic properties.

Classifications

• Familiar metals are iron, copper, aluminum, silver, and gold.
• Common ceramics are sand, bricks, glass, and graphite.
• Common polymers are nylon, Teflon, and polyethylene.
• Composites are material mixtures like carbon fibres in epoxy matrix.
• Composites are used in tennis rackets.
• Semiconductors like silicon and germanium are used in transistors, and integrated circuits.

Structure and Properties

• Processing affects material structure and therefore its properties.
• Steel cooled slowly from high temperature is soft with low strength.
• Rapidly cooled steel is hard and brittle.

Crystalline Structures of Metals

• Atoms arrange into crystals when metals solidify from a molten state.
• Joining atom centers in a crystal forms a 3-D geometrical shape called a space lattice.
• The smallest space lattice volume, representing atom positions, is a unit cell.
• A unit cell is the building block of a crystal.
• Unit cells of most metals are cubic or hexagonal.
• Three common crystal lattice structures are Face Centred Cubic (FCC), Body Centred Cubic (BCC), and Hexagonal Closely Packed (HCP).

Face Centred Cubic (FCC) Crystals

• Unit cell is a cube: one atom at each corner and one on each face.
• FCC metals are copper, gold, nickel, aluminum, and their alloys.
• Notable features are good ductility and electrical conductivity.

Body Centred Cubic (BCC) Crystals

• Unit cell has atoms at cube corners and one in the cube's center.
• The BCC metals are iron, sodium, vanadium, and molybdenum.
• BCC metals tend to deform plastically.

Hexagonal Closely Packed (HCP) Crystals

• Atoms are at hexagonal prism corners, one in the center of the top/bottom faces, and three in the midplane.
• The HCP metals are zinc, cadmium, cobalt, and titanium.
• HCP metals have good ductility and are easily deformed.
• There are 14 valid 3-D lattices; all crystals belong to one type.
• Lattice types change with temperature.
• Polymorphic materials change lattice type with temperature.
• X-ray diffraction investigates atomic structures, determining crystal structures and atom positions in unit cells.
• Amorphous metals do not have their atoms arranged on a lattice.
• Amorphous material examples: Thermoset plastics, transparent polymers, rubber, and metallic glasses.

Properties of Materials

• Include physical, chemical, mechanical, and electrical properties.
• Material selection depends on these properties.
• Plastics are used for pens due to light weight; copper is used for wiring due to electrical conductivity.
• Physical properties include weight and conductivity.
• Chemical properties involve reactions with acids and other solutions.
• Electrical properties relate to conductivity.
• Adding impurities reduces metal conductivity; but adding phosphorus increases silicon conductivity.

Mechanical Properties

• Describe behavior under mechanical forces.
• Strength, hardness, and ductility are examples of properties.
• Mechanical properties influence manufacturing process selection.
• Cast iron can't be metal-formed due to brittleness and lack of ductility.
• Mechanical properties relate to material resistance to deformation under forces.
• Springs deform under load, then return to original shape.
• Automobile bodies must be permanently deformed during moulding.
• Engineering terms like load types, shear stress, shear strain are defined for comparison.
• Mechanical properties are defined to be independent of the specimen size and geometry.

Types of Stresses

• A solid body can undergo mechanical forces in different ways.

Tensile Load

• A body is under tension and elongates with two equal, opposite pulling forces.

Compressive Load

• A body gets shortened with two equal, opposite pushing forces.

Shear Load

• A body is under shear with two opposing forces acting radially across its cross-section.
• Resisting forces are internal stresses.
• Stress is force per unit area, measured in N/m².
• Stress can be tensile, compressive, or shear based on load type.
• Strain measures material deformation of a material.
• Strain is the ratio of dimension change to original dimension and is dimensionless.

Stress and Strain

• Formulas are used to calculate stress and strain on a cylindrical specimen under axial force.
• Elongation results from external tensile force.
• Contraction occurs perpendicular to tensile force.
• Direct stress causes strain in its direction, with opposite strain at right angles.
• Linear or axial strain is strain in the load application direction.
• Lateral or perpendicular strain is strain perpendicular to load direction.
• Poisson's ratio is the ratio of lateral to linear strain.
• Cylinder returns to its original shape if the applied force is within a limit.
• Stress and strain are related, defining material behavior.

Stress-Strain Relationship

• Cylindrical rod of ductile material undergoes continuously increasing tensile load until it breaks.
• Behavior is like rubber band stretching until it breaks.
• Stress and strain at different loads are plotted on a stress-strain curve.
• Curve has straight line portion from origin to point P.
• Curve then bends, reaching maximum height at point T.
• Curve falls to point R where the bar breaks.
• Bar with stress less than point P returns to its original length when stress is removed.
• Metals and materials stretch/rebound like a stretched rubber band if stress is below the elastic limit.
• Material permanently stretches when applied stress is beyond critical elastic limit value (point Q), never returning to its original length.
• The curve part up to point P is the "elastic region".
• Elastic limit is the max stress where the material returns to original length.
• Proportional limit = elastic limit.
• Curve part to the right of the elastic limit is the "plastic region."
• Stress-strain relation is linear in the elastic region.
• Young's modulus (E) is the ratio of stress to strain in the linear elastic region.
• Young's modulus is a indicative material property.
• The equation E = Stress/Strain is used.
• Material stiffness or rigidity depends on how much it stretches under load.
• Physical significance of Young's modulus (elastic modulus) indicates interatomic force measure.
• Stiff material exhibits small deformation under large load.
• Yield point: stress where strain increases without stress increase.
• Yield strength: stress with specified limiting deviation from stress-strain proportionality.
• Tensile or ultimate tensile strength: the maximum stress a bar can endure before failing (point T).
• Breaking or rupture strength: where the bar breaks (point R).

Non-Linear Stress-Strain Curve

• In ductile materials like mild steel, the stress-strain curve is highly nonlinear after the elastic limit.
• Yield point can be easily identified.
• Some materials like aluminum and bronze don't show a clear deviation point after elastic limit.
• Yield point isn't easily located.
• 0.2% strain is located on X-axis and line is drawn parallel to proportional limit line in order to locate yield point.
• Maximum stress is the breaking strength in some materials.
• Tensile strength and breaking strength are the same.
• Materials have no yield point and are brittle: cast iron and glass.

Toughness

• Ability to withstand suddenly applied load/absorb energy without failure
• Depends on both strength and ductility.
• Alloy steels are used in cutting tools and gears for impact loads
• Measured by total area under stress-strain curve up to fracture point.

Hardness

• Material's resistance to mechanical indentation.
• General indication of strength, wear resistance, and scratch resistance.
• Important in manufacturing; diamond is the hardest known.
• Hard materials are selected for tools and machine structures.

Ductility

• Extent to which a material can sustain plastic deformation before rupture
• Ability to undergo considerable permanent strain/deformation before breaking
• Ductile materials have high ductility; gold is the most ductile metal
• Important materials property shaped by forming/bending (ex: automobile body)
• Materials that fracture with little plastic deformation are brittle.

Brittleness

• Material undergoes very little plastic deformation before rupture
• Exhibits no yielding or necking
• Ruptures suddenly without warning
• Weak and unreliable in tension.
• Ex: Cast iron, glass, and ceramics.

Malleability

• Ability to be flattened into thin sheets without cracking.
• Ductility is tensile quality (deform under tensile stress).
• Malleability is compressive quality (deform under compressive stress).
• Materials can be malleable but not ductile (ex: lead).
• Silver is both malleable and ductile.
• Aluminum, lead, copper, and tin have good malleability.

Resilience

• Material capacity to absorb energy elastically.
• Stored energy is released upon load removal.
• Measured by triangular area under elastic portion of stress-strain curve.
• Material absorbs greater impact energy without plastic deformation.
• Considered when material is subjected to shock/impact loading.
• Important for shock absorbers and springs.

Stiffness

• Ability of a material to resist deformation.
• Material with high Young's modulus is stiffer than a material with lower value.
• Ex: Steel is three times stiffer than aluminum.
• Aluminum rod of same cross-sectional area + stress, exhibits three times more deformation steel rod.

Poisson's Ratio

• Elongation under axial tensile load and transverse dimension decreases.
• Axial compressive load = bar contracts and transverse dimension increases.
• Ratio transverse strain + axial strain= constant (given material) w/in proportionality limit ("Poisson's ratio").

Axial Strain

• If undeformed bar length = L and diameter = b, the deformations length = d and diameter = δb.
• Axial strain εα = δ/L

Transverse Strain

• εₜ = δb/b
• Poisson's ratio v = -εₜ/εα
• Poisson's ratio: distinct material constant.

Modes of Fracture

• It is essential to understand how materials and when they fail.

Material Failure

• There are different modes (or mechanisms) of fracture (or failure)
• The actual mode of fracture is determined by a number of factors and conditions.
• Crystal structure, type of stress, etc.
• Fracture mechanisms are divided into four classes: ductile, brittle, creep, and fatigue.
• If subjected to load above the yield point and the process of deformation continues, fracture occurs.
• Small pores are formed as the metal is pulled away from the weak interface + necking (reduction of cross-section area) proceeds, adjacent pores join up + its precedes

Necking

• Reduction of cross-section area proceeds+ adjacent pores join up + its precedes. When many of these pores have joined form large internal cavity, the rim fails by shear at 45° to applied tensile load axis.
• Ductile fractures require energy to deform material.
• Ductile fractures are important in metal working operations.

Brittle Fracture

• Plastic deformation is necessary for the spread of initial crack in ductile fracutre (plastic deformation is not necessary in ductile fracture).
• brittle fracture may occur, the spread of crack.
• Because of this, brittle metals are weak, since offer resistance to crack propagation
• Brittle metals fracture rather than deform.
• Ductile metals: difficult break/ propagate cracks.
• If load that caused crack to propagate is removed, crack stops

Failure of Brittle Materials

• Failure may occur at stresses less than that of a ductile material
• Impacts and chock loads.
• Presents/leads to serous problems.

Creep Fracture

• In situations like environments, both stress-strain behavior with fracture becomes dependent on the time.
• common situations where loading determines failure possibilities.
• Creep fractures critical in design of steam turbines, aircraft, etc.
• In jet turbine blades=1200°C, creep factor in selecting a suitable blade material.

Fatigue Fracture

• The failures of materials come at random/unexpected times. As such- has catastrophic results.
• Bending metal repeatedly in same spot causes fatigue fracture
• "Progressive fracture."
• Fatigue failures: stresses well below stresses, part can w/stand static conditions.
• Yield point of material NOT need be exceeded by fatigue.
• Fatigue failures CONSIDERED selecting material for withstand alternate loads

Mechanism

• Complex, involving strain hardening with microcracks from crystal structure/surface imperfections that grow.

Factor of Safety

• Takes into consideration if raw material itself defective/contain cracks. In cases material are failures

Overcoming Part Failure

• To prevent material failure
• Essential that design stress kept w/in yield/ultimate stress as case
• Factor of safety aka "safety factor"
• Simple number varies is application

Purpose

• Purpose: avoid a part failure
• Yield strength 1000 N/m2, factor safety is 2.
• Fatigue load + high factor safety: failures +
• Airplane/satellite designs = high factor compared apps: risk lives human beings

Considering Safety Value

• Variation in mechanical properties due to non- homogeneity of raw material
• Uncertainty in method analysis + manufacture
• Environmental
• application

Materials

• Stress for ductile material is yield stress/strength
• Yield strength/designed strength.
• Brittle materials, failure stress is design stress.

Engineering Materials

• Commonly used engineering materials and their properties are alloys of metals.

Cast Iron

• Ferrous metal alloy iron, carbon 2.1-4.5 %, with 3.5 % silicon
• Vibration damping property
• Good compressive strength,weak in tension
• Applications exist with malleable cast iron and spheroidal-graphite cast iron.
• Gray cast iron resist wear + castings
• Malleable cast iron+ part ag/text
• Spheroidal (SG)cast iron+stren ductility steel connecting rods

Steel

• wide range of applications is is an alloy of the elements iron + carbon, w/manganese, silicon, chromium
• Steel -carbon % base 3 groups:
• Low carbon > mild (0.05 to 0.3%C)

Carbon Steels

• Medium Carbon (0.3 to 0.7%C)
• High Carbon 0.7 to 1.5%C gives apps carbon steel dependency

Controlling Manipulation

• End requirements steel- heat processes
• Alloyed on steel for cut material discuessed.

Aluminum

• Nonferrous material
• Excell elecricity + thermal wire very good resistance.
• 1/3 weight steel+ ductility
• Applications: packaging, electricty wires .

Copper

• Excellent electricity + thermal
• Good corrosion
• Flexible, tough, hot/cold conditions
• Excell electricty used

Copper Alloys

• Alloyed-w/ zinc, tin
• Copper+zince=utensil+ household fittings
• Alloy of copper with tin possess corrosion making valves.
• Bronze & Brass: be machined speeds surf. finsih

Lead

• Good density+ workability
• Very corrosion resistance.
• Lead Pipes exist
• Alloys-widely joints metals"

Zinc

• Zinc alloys +Low point
• 4th metal iron (carburetors and fuelpumps automobile parts)

Tin

• +Does not corrode protective coating
• Load,malleability, storing food

Selection of Material

• Consider propeties for particular
• Material selection decision
• Cost 50 Percent

Step 1

• Requirement define by objectivs
• References: cost durabilty

Step 2

• Possible meterial may be met by number meet require

Step 3

• Choice- influence restions raw
• Cost
• • Inhibiti Example Material: coin/razor
• Coin-materials possess hardness
• Coins- ductility materials large copper
• Razor selected hammered, economical