CEE 3020 Midterm 1

Accuracy

how closely a computed or measured value agrees with the true value

Inaccuracy

a specific deviation from a given value accepted as known

Bias

a consistent tendency to deviate in a specific direction from a known value

Precision

how closely a group of computed or measured values agrees with each other

Imprecision

a magnitude of scatter in a group of measurements

Normal Distribution (Gaussian distribution)

Describes populations and material properties

Mean

the “center” of the normal distribution (location of the peak)

Variance

the “width” of the distribution

Inherent variability

Composition, manufacture methods, time

Sampling methods

• must ensure that a random and representative sample is taken from the population

• depends on material variability & tolerance level of results

• more variability dictates a larger sample

Lab errors

human error, machine error, environmental factors, combination

Variability

Inherent variability, sampling methods, test procedures, lab errors

Engineering Stress

Engineering Strain

Elastic

An instantaneous response (deformation) to load, and the material must return to its original shape (recoverable) when the load is removed

Plastic

Permanent (unrecoverable) deformation when a material is subjected to loads that exceed its yield strength

Viscous

Deformation at a certain rate

Modulus of Elasticity/ Young’s Modulus

Poisson’s Ratio

Elastic Stress-Strain Curve

Elasto-Perfectly Plastic stress-strain curve

Elasto-Plastic with strain hardening stress-strain curve

Viscoelastic

creep and relaxation

Creep

Relaxation

Factors that influence creep/relaxation

temperation, time, load

Engineering vs True Stress & Strain

Microstructure

composition and arrangement of atoms, formation of “grains",” nature of micro-level imperfections

Chemical Bonds

Serve to ensure that atoms achieve stable electron configurations by adding, removing, or sharing electrons: Primary vs Secondary

Primary Chemical Bonds

Ionic bonds, Covalent bonds, metallic bonds

Ionic Bonds Examples

ceramics, Gypsum, rock salt, calcite

Covalent Bonds Examples

Diamond, Glasses

secondary chemical bonds

hydrogen(water), van der Waals(thermoplastic polymers)

Ionic Bonds Definition

• Certain elements readily lose or gain electrons.

• These elements are referred to as strongly electropositive or strongly electronegative.

• When these elements are brought together, they exchange electrons to form ionic compounds.

• Ionic solids will not conduct electricity unless in molten state or dissolved in water.

• All ionic compounds exist as crystalline solids at room temperature.

Covalent Bonds Definition

• Electrons are shared rather than gained or lost

• Electrons are not shared equally – creates a dipole

• Few compounds have primarily covalent bonds

Metallic Bonds Definition

• Elements are electropositive – all wish to lose electrons.

• Electrons are given up to a common “pool”.

• Atoms become positive with a stable electron configuration

Hydrogen Bonds Definition

• Special case of van der Waals bonding.

• Arises from strong electrostatic interactions between the hydrogen atom and oxygen, nitrogen, or fluorine.

• Without this bond, water would freeze and boil at much lower temperatures – no liquid state at room temperature

van der Waals Bonds

• Weak attraction between molecules with nonpolar bonds – formed through temporary dipoles.

• Attraction can also occur between atoms of a single type.

• Strength of van der Waals bonds between molecules strongly influences the boiling point of compounds.

• Exist in the presence of other bond forces

3-D Lattice Structures

Body center cubic (9 atoms), face center cubic (14 atoms), hexagonal close pack (17 atoms)

Unit Cell Structure of Metals

the smallest group of atoms that reveal the particular ccrystal structure of a unique metal

Number of equivalent atoms

BCC: 2, FCC: 4, HCP: 6

rate of cooling from liquid state:

slow cooling - large grain, rapid cooling - fine grain

Phase Diagram

shows the phases formed in differing mixtures of elements over a range of temperatures (eutectic point, liquidus line, solidus line, alpha, beta)

Alloys

hvae one or more compounds dissolved in a metal

Interstitial atoms

fit between the metal atoms, must have an atomic radius less than 60% of host metal, can dissolve only about 6% into the host

Substitutional atoms

take the place of host atoms in the lattice, if the atoms are similar enough, the compounds can mix easily

Alloy facts

• many alloys are stronger and more ductile than the metals from which they are formed

• the greater the mismatch in atomic size, the greater the strengthening effect

Defects in metals

involves movement of dislocations through the metal crystal structure in response to a shear stress (point, line, planar, volume)

Point defects

vacancies, impurities, interstitials

Line Defects

dislocations

Planar imperfections

grain boundaries

volume or bulk defects

voids, inclusions

Elastic deformation

crystal returns to original shape when force is removed

Plastic deformation

crystal does not return to original shape when force is removed

• slipping of one plane of atoms over an adjacent plane

• twinning- a portion of the crystal forms a mirror image of itself across the plane of twinning

Loading-Unloading-Reloading linear-elastic region

Complete recovery upon unloading

Loading-Unloading-Reloading beyond linear-elastic region

Some irrecoverable or “plastic” deformation occurs - “permanent set”

Uniaxial Tension Test

Most common test performed; most properties are determined using this procedure

Torsion Test

Used to determine shear modulus, shear strength

Charpy V Notch

Measures fracture energy, estimates touchness

Rockwell Hardness Test

Measures metal’s resistance to localized plastic deformation; test uses an indenter that is forced into the surface of the material with a specified force

Stress-strain curve

Ductility

a measure of a material’s ability to undergo sugnificant plastic deformation before rupture or breaking

Toughness

the ability of a material to absorb energy and plastically deform until fracturing (or ability to withstand impact loading).

Hardness

a measure of a material’s resistance to localized plastic deformation, such as a small dent or scratch on the surface of the material

• depends on plastic properties

• Nothing can be inferred about complete stress-strain curve

• several scales: Rockwell, Brinell, Vickers, Knoop

• Conversion between scales is empirical and approximate

Iron Facts

mined from earth as ore, metal extracted by:

• pyrometallurgy: smelting

• Electrometallurgy: uses an electric furnace or electrolytic process to remove metal from ore

• hydrometallurgy: ore is subjected to an aqueous solution from which the metal is dissolved and recovered

Iron Products

• pig iron: obtained from reducing iron ore in a blast furnace (cast into bars)

• cast iron: pig iron that has been melted to be cast in final form

• malleable cast iron: cast iron that has undergone special annealing after casting to make it malleable

• Wrought iron: least amount of carbon, contains blast furnace slag, will harden quickly when cooled rapidly

Classification of steels

steel c< 2% vs. cast iron c>= 2%

Low carbon steels

contain less than 0.3% carbon, low strength, high ductility, good weldability

Medium-carbon steels

contain 0.3%-0.6% carbon

High-carbon steels

contain more than 0.6% carbon, high strength, low ductility, poor weldability

Stainless steel

used for corrosion resistance; in presence of air, stainless steel develops film of chromium oxide (Cr2O3) that protects metal from corrosion (rust)

Stainless steel types

Austenitic, ferritic, martensitic

Austenitic stainless steels

excellent weldability, not stable at room temp; stabilizer: nickel, carbon, manganese, nitrogen

Ferritic stainless steels

low weldability, 12-27% chromium with small amounts of austenite-forming alloys

Martensitic stainless steels

least amount of chromium, high hardenability, and require both pre- and post-heating when welding to prevent cracking in the heat-affected zone (HAZ)

Weldability

the ability of a metal to perform in a satisfactory manner after being joined by welding

Arc welding or “stick welding”

Flux on the electrode (“stick”) shields the molten metal from atmosphere to prevent oxidation.

Gas Welding or “MIG Welding”

“Metal in Gas” uses shielding gas instead of flux.

Electrochemical process

A chemical reaction involving the transfer of electrons between two electrodes

Electrochemical cell (or corrosion cell)

A device which converts chemical energy into electrical energy.

Fundamental components

• Anode - where elctrochemical oxidation takes place

• cathode - where electrochemical reduction occurs

• electrical conductor - the bar

• Aqueous Medium (electrolyte) - water, seawater

steel rusting/corrosion

the transfer of electrons from iron to oxygen

Ductile vs. Brittle failure

ductile: significant strain, yield, before failure

brittle materials: much less/no strain prior to failure

Fracture Mechanics

study of flaws in engineering materials

Fatiguq

continued repetition of these loads can lead to “fatigue failure",” can occur suddenly without prior deformation

Fatigue Life and Fatigue Limit

characterized by S-N curves, life will vary for given material with loading conditions, an “endurance limit” or “fatigue limit” may exist

Miner’s Rule

• accounts for variations in stress amplitude over service life

• account for cumulative damage