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UMD / Materials Engineering / ENMA 150 / How long did it take for the buildings to collapse?

How long did it take for the buildings to collapse?

How long did it take for the buildings to collapse?


School: University of Maryland - College Park
Department: Materials Engineering
Course: Materials of Civilization
Professor: Robert briber
Term: Fall 2017
Cost: 50
Name: Updated ENMA Final Review
Description: Final Review
Uploaded: 12/15/2017
14 Pages 139 Views 3 Unlocks

ENMA 150 Materials of  

How long did it take for the buildings to collapse?


- Closed book, Bring a calculator

- Final Exam format  

- Short answer questions

- Essay type questions

- Calculation questions (analogous to the homework problems) - In-class guest lectures and videos are considered part of the class - Important concepts (also review Midterm study guide which has  more details):

∙ Content from the in-class guest lectures on the World Trade Center and  combinatorial materials science, readings and videos shown in class and  in discussion section.

o In-Class Lectures

 World Trade Center

∙ Steel was main material holding the building up

∙ Steel becomes more brittle with higher rate of deformation

What combinations could start a fire?

∙ Exceeding 250 degrees C

∙ The impact of the planes didn’t really cause the collapse

∙ The planes weakened the buildings – they stood for 60 and  

105 minutes

∙ The jet fuel allowed for the heat to rise and spread and  We also discuss several other topics like When do anxiety disorders develop?


∙ Fire was main cause of the collapse

∙ The light, sprayed-on material (fireproofing material) did not  stick well, and most internal components in the impact region

were probably stripped clean

∙ Burning paper and other things kept fire going at a high  

temperature which allowed the metal to melt

∙ Floors started sagging and collapsing

Which fuel has highest energy density?

 Combinatorial Materials Science (Dr. Takeuchi)

∙ Give an explanation of combinatorial materials  


o The idea of combinatorial material discovery is  

based on the idea that they shoot a laser into a  

material that gives off atoms of that material.

They then take the atoms of many materials and  

place it on a chip and test the combinations until  

they find the properties they are looking for. Don't forget about the age old question of When did john locke write “an essay concerning human understanding”?

o Videos

 Making Stuff Cleaner

∙ Electric cars

o Limited by batteries

o Energy density (amount of energy per volume) in  

gasoline is much higher than current batteries

o Tank full of gas will allow car to go much further

o Batteries take many hours to recharge while gas  

takes few minutes to fill

o New batteries are being developed but far from it

o Tesla is building a “Giga-factory” in Nevada to  We also discuss several other topics like What is the tissue type of cardiac muscle?

manufacture lithium ion batteries

∙ Fuel cell

o Hydrogen can be used in a proton exchange  

membrane fuel cell to generate electricity

o In a fuel cell hydrogen gas is combined with  

oxygen in air to form water

o When this occurs, an electron is released when  

the hydrogen is split at the anode in the fuel cell  

and can be used to run an electric motor

o The electron is then combined with a proton (half  

a hydrogen molecule, minus the electron) and  

oxygen at the cathode of the fuel cell to form  


o Hence the only exhaust for a fuel cell car is water

∙ Problems with fuel cells

o Hydrogen is not readily available as a fuel and it  

would take significant energy to make hydrogen  

available as a fuel and a new infrastructure would  We also discuss several other topics like What is the scientific method(optic)?

have to be developed to distribute it to consumers

o It is difficult to store hydrogen in sufficient  

quantities to be useful as fuel in a car with  

reasonable range (300+ miles)  

o Chicken feathers converted to carbon fibers is  

being explored as a possible material for storing  


∙ Aluminum based molten salt batteries

o Aluminum is the most abundant metal in the  Don't forget about the age old question of What are flozin drugs?

earth’s crust

o Molten salts offer high energy density and high  

power density

∙ Solid Oxide Fuel cells (“Bloom Box”)

o Produces electricity directly from oxidizing a fuel o High efficiency

o Long-term stability

o Fuel Flexibility

o Low emissions

o Relatively low cost

o Disadvantage: High operating temperature which  results in longer start-up times

∙ Grid Storage batteries

o Electrical energy is stored during times when  

production exceeds consumption, and returned to  the grid when production falls below consumption o Example: dammed hydroelectricity We also discuss several other topics like What is npp?

∙ Solid Oxide Fuel Cells

o At least 5X more efficient than the ones currently  on the market from Bloom Energy

o Thinner electrolyte layers helped optimize fuel cell design

∙ Artificial Photosynthesis

o Convert sunlight into either electricity (for  

charging batteries)

o Or to split water to form hydrogen for fuel cell cars ∙ Creating energy from sunlight is different from storing  energy in a suitable form of transportation (hydrogen for fuel cells or batteries)

 Making Stuff Smarter

∙ Mimicking structure of shark skin to make a plastic that  is anti-bacterial  

∙ Silicone rubber pads on the “Gecko Robot” that mimics  the tiny hairs on a foot of a gecko to allow the robot to  climb walls

∙ Self-healing materials

o Repair themselves in response to damage

o Example: Seal a bullet hole using a 3 layer  


o Inner and outer layer were a special plastic that  closes up after the bullet passes through

o Center layer is a super-absorbent polymer that  expands dramatically and rapidly when in contact  with fuel

∙ Magnetic rheological fluids

o Contains iron particles and in presence of  

magnetic field increases viscosity

o These can be used as the damping fluids in shock  absorbers

o Example: military Humvee, found in new cars

∙ Meta-materials

o Special class of artificial materials engineered to  

have properties that may not be found in nature

o Metamaterials usually gain their properties from  

structure rather than composition

o Applications: materials that will bend light around  

a material creating an effective invisibility cloak

∙ Size scales (angstroms – nanometers – microns – millimeters –  centimeters – meters) and the associated structures with these size  scales, including the concept of hierarchical structure.

o Angstroms (Å): 10-10 meters

 Missing/extra atoms

o Nanometers: 10-9 meters  

o Microns: 10-6 meters

 Crystals (ordered atoms)

 Second phase particles

o Millimeters: 10-3 meters

 Crystals (ordered atoms)

 Crystal Texturing

o Centimeters: 10-2 meters

 Crystals (ordered atoms)

 Crystal Texturing

o Meters: 1 meter

o Subatomic structure: below the level of the atom (<0.1 nm) o Atomic/Molecular structure: interactions of atoms and molecules  with each other (0.1-2 nm)

o Microscopic structure: groups of atoms/molecules interacting with  each other (10 - 10,000 nm (10 μm))

o Macroscopic structure: large scale structure visible to eye (0.1 mm  and larger)

∙ Historical Ages and approximate dates

o Stone Age - 2my BCE

 Lower paleolithic - 1.5my BCE

 Upper paleolithic - 40,000 BCE

 Neolithic - 9000-8500 BCE

o Modern Era - 8000 BCE

 Chalcolithic (Copper Age)4500 BCE

o Bronze Age - 3150 BCE

o Iron Age - 1200 BCE

 Caesar Augustus rules Rome ~0 BCE

o Modern Age

∙ The Metals of Antiquity; why are many of the metals of antiquity are quite  rare

o Mercury, (Hg) 750BC (native)

 Liquid at room temperature

o Iron, (Fe) 1200BC (native, but rare & in meteorites)

 Smelting is difficult (melting iron 2000 degrees Fahrenheit) o Tin, (Sn) 2000BC (not native)

 Not useful alone

 Tin + Copper = Bronze

o Lead, (Pb) 3500BC (not native)

 Pipe lining  

o Silver,(Ag) 4000BC (native, but rare)  

 Bad for tools

 Found in same ore as lead  

o Copper, (Cu) 4200BC (native)

 Tools

o Gold, (Au) 6000BC (native)

 Electrical connectivity

o Why they are rare?

 Elemental abundance gold is really rare so is silver

 Most common elements in rocks are iron and aluminum and  magnesium

 Earliest metals like gold is one of the scarcest elements yet it  was one of first discovered

 Found in native state – gold, no need to extract

 Some of the earliest metals were some of the most rare  because they can be found in natural state

 Copper and iron are not so tightly packed into their rocks so  easier to extract

∙ Six main properties of materials

o Material Property: Materials trait that determines size and kind of  response when exposed to a stimulus. Desirable that a material  property is independent of material size & shape

o Mechanical: Modulus and Strength, determines response to due  applied load or force.

o Electrical: Conductivity and dielectric constant, determines  response due to electric field

o Thermal (physical): (specific) heat capacity, thermal conductivity, Tm, Tg, density; determines response due to change in  

temperature, application of heat.

o Magnetic: ferromagnetic, paramagnetic, diamagnetic, determines  response due to an applied magnetic field.

o Optical: clarity, index of refraction, reflectivity, determines  response due to light.

o Chemical: chemical reactivity, determines response due to  chemical environment (degradation, oxidation (rust, etc.).

∙ Classification of materials (metals, ceramics, polymers) o Main classes of materials

 Metals

∙ Metallic elements such as aluminum

 Ceramics  

∙ compounds of metallic & non-metallic elements

 Polymers  

∙ plastic and rubber materials

o Secondary types (Advanced materials)

 Semiconductors

∙ Intermediate between electrical conductors and  


 Biomaterials

∙ Materials implanted in the human body

 Composites

∙ Combination of 2 or more materials

 Smart Materials

∙ “Smart” means material is able to sense changes in  

local environment and respond with a known behavior

 Nanoengineered materials

∙ Materials with very small dimensions (on the order of  

nanometers) which have unusual properties

o Silicon Boule – single crystal rod of silicon (pure)

∙ Why the discovery of clay fundamental was in the shift from hunter gatherer to agricultural based societies.

o Used as storage devices

o Instead of going out everyday hunting because there was nowhere  to keep food, the storage allowed to keep food fresh

o This allowed more time for other things such as evolving cities

∙ Mechanical properties of materials including elastic and plastic  deformation, fracture, (Young’s) modulus, stress, strain, yield stress and  strain, fracture stress and strain, creep and fatigue

o 3 types of deformation

 Elastic

∙ Deformation is reversible  

 Plastic

∙ Change in shape of material is not reversible and  

remains after deformation

 Fracture

∙ Material breaks

o Modulus (stiffness)

 Stiffness: measure of the load required to produce a elastic  deformation

o Stress

 Force applied to a certain area of an object

o Strain  

 Response of a system to an applied stress

o Yield stress

 Amount of stress that an object needs to experience for it to  be permanently deformed

o Yield strain

 Amount of strain an object can experience before being  permanently deformed

o Fracture stress

 Amount of stress that an object needs to experience for it to  be fractured or broken

o Fracture strain

 Amount of strain an object can experience before being  permanently fractured or broken

o Creep

 Deformation (often slow) that occurs under a constant stress.  The applied stress is much less than the normal fracture  

stress, but the deformation can be quite large

o Fatigue

 Behavior under cyclic deformation. Cyclic deformation often  results in crack growth and eventual failure. The is called  

failure due to fatigue. The deformation (or applied stress) is  often much less than the normal (single cycle) fracture stress

∙ The parts of a stress-strain curve

∙ Equations to know (with units) for mechanical properties  o Strain

 σ= Eε

∙ σ – Pa

∙ E – Pa

∙ ε – NO UNIT  

o Stress

 σ=F/A

∙ σ – Pa

∙ F – N

∙ A – m2 

o New Length

 ε = lf – lo / lo  

∙ ε – NO UNIT  

∙ l - m

o Elastic stored Energy

 (12¿ σε

∙ σ – Pa

∙ ε – NO UNIT  

∙ FINAL UNIT – J/m3 

∙ The relationship between stress, strain and modulus for elastic  deformation

o Modulous (stiffness): measure of the load required to produce a  particular elastic deformation

 E=σ/ε 

o Stress: force applied to a certain cross-sectional area of an object  σ=F/A 

o Strain: the amount of deformation in the direction of the applied  force divided by the initial length of the material

 ε = (lf – lo) / lo  

 ε=σ/E 

∙ Elastic energy absorbed/volume:  

o Elastic: deformation is reversible

o area under stress-strain curve (joules/m3)

 (12¿ σε or equivalently σ2/2E

∙ Units of force (Newtons); stress and modulus (Pascals), strain  (meters/meters) and how they are related and what the underling basic  unit of length, mass and time (meter, kilogram, seconds) are for Newtons  and Pascals. Understand MPa, GPa (powers of 10 for units)

∙ Smelting of copper and the societal/intellectual implications of converting  a rock (albeit a mineral) to a metal

o Smite Malachite  usable coper

o Allows the separation of the elements to get usable material

∙ Types of Bonding (covalent, ionic, metallic) and relationship to  classification of materials

∙ Crystalline versus amorphous materials, Crystalline grains, simple cubic  crystal structures

o Crystalline

 Dense, regular packing

 Structure of ions, molecules, or atoms that are held together  in an ordered, three-dimensional arrangement

 Highest level of order that can exist in a material

 Ex: Quartz

o Amorphous  

 Non dense, random packing (noncrystalline)

 Irregular and lacks repeating pattern of a crystal lattice  Occurs by adding impurities (interfere with formation of  crystalline structure)

 Ex: Glass

o Crystal structure can be thought of as a small box with atoms,  molecules or ions located in specific areas of the box

o The general arrangement and order of the atoms in the structure  are directly related to the natural properties of the crystal

o Simple Crystal Structures

 Rare due to poor packing (only Polonium has this structure)

o Crystalline Grains

 Take a full form crystal and there are different color Speckles  which are the different crystal grains

 Single grains of crystals

 Polycrystals

∙ Made of many crystals placed together

∙ Point defects, dislocations, role of dislocations in deformation o Point defects

 Vacancies

∙ Empty lattice site

∙ Thermodynamics requires that vacancies are present  

even in perfect crystals

∙ Missing atom in the sheet  

 Self-interstitial

∙ An atom is crowded into a spot between other atoms  

and is not a regular lattice site

o Dislocation

 Linear defect where some atoms are misaligned

 Edge Dislocation

∙ An extra ½ plane of atoms. The extra atoms result in  

strain in the nearby crystal lattice

 Screw dislocation

∙ An atom is crowded into a spot between other atoms  

and is not a regular lattice site

∙ Substitutional versus interstitial atoms, substitutional hardening, strain  hardening

o Substitutional impurity atom

 Occupies the same place in the lattice that the atom it is  “substituting” for occupied

 Generally the substitutional atom must be within ~15% of the size of the original atom

 Many alloys are substitutional alloys where the two  

components can form a solid solution

o Interstitial atoms

 Sit between the regular atoms and (consequently) need to be  significantly smaller than the surrounding atoms

o Substitutional hardening

 Due to the fact that the substitutional atom is generally a  (slightly) different size that the surrounding atoms which gives rise to local strain in the crystal.

 This impedes dislocation motion during plastic deformation  and results in improvement in mechanical properties.  

 This is why bronze has better mechanical properties than  copper

o Strain Hardening

 Due to the creation of dislocations during plastic deformation  These dislocations impede the motion of dislocations which  improves the mechanical properties of the material

 It is also called work hardening

∙ Bronze vs. Copper (why was bronze an improvement over copper?) o Copper

 Used for its excellent electrical and thermal conductivity, good strength, good formability  

 Resistant to corrosion

 Greeks and Romans made tools with it

 Today mostly used in wiring

o Bronze

 Alloy that consists primarily of copper with addition of other  ingredients

∙ Usually adding tin to copper

 Much stronger than copper

 Used in construction of sculptures, musical instruments and  medals, and bushings and bearings

 Resistant to corrosion

∙ What are quasicrystals, what is non-periodic tiling?

o Period tiling

 No gaps same shapes lining up

 Repeats itself

 If you go up 4 feet and over 5 feet you are in center of tile o Non-periodic tiling

 No gaps or spaces, but does not repeat itself

 You might start at fat diamond and go up a certain space and  over a certain space, it might be a different shape

 Not normal pattern

o Quasicrystals

 3D actual material that has kind of structure

 Structural forms that are ordered but not periodic (NON PERIOD TILING)  

 These are 2 concepts that were previous thought to be  contradictory and not allowed in a single material

∙ Constructive versus destructive interference of waves

o Constructive

 Crests line up with crests: waves are “in phase”

 Waves add and keep the wavelength but double the  


o Destructive

 Crests line up with valleys: waves are “out of phase”

 Waves add and cancel out  

∙ Bragg’s Law: λ=2dsinθ, Diffraction problems

o Predicts where the scattering angle 2θ where the diffraction peak  will occur for a given X-ray (or neutron, light or electron) wavelength o





∙ The differences between diffraction and radiography (see diffraction  lecture notes)

o Diffraction

 Gives you atomic structure of the material

 Atomic or Microscopic structure of material

o Radiography

 When you go to dentist or doctor

 Use X-Rays and get picture

 Has same ability of photo, used to see bigger things  

 Macroscopic structure of material

∙ The significance of gold and silver in society (concept of wealth  accumulation, why is gold the logical element for currency); Apple gold. o Gold

 Used for jewelry

 Logical Element for currency

∙ Not gas

∙ Is not reactive

∙ Not radioactive

∙ Rare but not too rare

∙ Can melt easier to shape

∙ Does not tarnish like gold

∙ Not as abundant as silver

o Silver

 Used for jewelry

 Became largely used for many industries since abundance of  element grew

 Highest known electrical and thermal conductivity of all  metals

 Not logical element for currency

∙ Tarnishes

∙ Too abundant to be used as currency

o Apple gold

 Hardened alloy of 18K gold in their high end Apple Watch  Metal matrix composite of gold with ceramic particles (Apple  Gold)

 Ceramic particles are much harder than gold or copper, silver  or palladium generally used in a traditional gold alloy and  provided enhanced scratch and wear resistance

 By definition 1 gram of an 18K gold alloy contains 0.75g of  gold and 0.25g of other materials (combination of copper,  silver and/or palladium)

∙ Approximately 48% less gold present but the gold alloy  is harder and more scratch resistant than traditional  


 Value of the gold In the Apple Watch Edition

∙ $861

∙ Sold for $10,000

∙ The Iron Age, smelting of iron, bloom iron forged to wrought iron, iron  versus steel, the role of carbon, cast iron

o Iron Age

 1200 BCE

∙ Caesar Augustus rules Rome ~0 BCE

 Native  

 Rare

 Found in Meteorites

o Smelting of Iron

 Smelting is difficult (melting iron 2000 degrees Fahrenheit)  Iron is extracted from the iron ore and excess oxygen is  removed  

o Bloom Iron forged to wrought iron

 Bloom Iron  

∙ Iron containing charcoal and slag (very impure)

∙ Very poor mechanical properties

 Forging it into wrought iron (~0.1% C)

∙ Removes slag

∙ Fair mechanical properties (~ bronze)

o Iron vs. steel

 Iron

∙ Chemical element that is found in abundance in Earth’s  crust

∙ Susceptible to erosion from rust

∙ Soft and malleable

∙ Unable to be used for applications until hardened

 Steel

∙ Alloy

∙ Mixing iron with carbon

∙ Extremely strong

∙ Malleable  

 Overall:

∙ Steel is more rust resistant

∙ Steel has Better weldability

∙ Iron is most commonly used to produce steel  

o Role of Carbon

 Transforms soft iron into hard and useful steel  

o Cast Iron

 When too much carbon is added to iron  

 Creating a very brittle material

 Applications: cast-iron pan, piping

∙ Other Math

o Mechanical Properties

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