CE 336 Exam 1
What are some properties of low carbon steel?
-Relatively low strength -High ductility and toughness -Least expensive type of steel -Most common type of steel -Machinable and weldable -Alloy contents of generally less than 10% wt
Describe fatigue failure
-Repeated loading results in propagation of cracks, few micrometers at a time -Eventually crack becomes large enough to be unstable -Results in failure of a member or even the entire structure
How is steel recycled?
-Steel scrap is heated and melted by electric arcs striking between electrodes and metal bath -Impurities oxidized and removed (slag)
What are the two approaches to studying failure of materials?
-Strength of materials (SoM) -Fracture mechanics (FM)
Where are low carbon steels used?
-Structural shapes (beams, plates) -Automobile body parts
What are the problems with the SoM approach?
-Unexpected failures well below the ultimate stress -Dictates use of safety factor > 10 to prevent random failure -Ultimate stress is size dependent -Does not consider stress concentration
Explain electroplating
-Zinc applied to steel sheet, strip and small parts by electro-deposition -No alloy layers, thin coating of pure zinc -More expensive than sheet galvanizing
Explain zinc rich paint
-Zinc dust in organic or inorganic binders -Referred to as "cold galvanizing"
Explain metallizing
-Zinc wire or powder melted or sprayed onto the steel surface -Shop or field application -Mechanical bond / no alloy layers
_____ materials exhibit substantial plastic deformation with high energy absorption before fracture
Ductile
What type of material shows extensive elongation and necking?
Ductile materials
Toughness is very important for _____ loading
Dynamic (earthquake, impact)
Group number on the periodic table represents the number of _____
Electrons in the outermost layer
How is epoxy-coated steel made?
Epoxy paint is fusion bonded to the steel surface
What does FEA stand for?
Finite element analysis
Carbon is a _____ agent
Hardening, it prevents dislocation movement
_____ is the most widely used construction material in the world
Concrete
You should try to _____ corrosion, rather than stop it
Control
What are the problems will iron alloys?
Corrosion, fire
In many solids, atoms arrange themselves into a very regular and well-defined structure. They form a _____
Crystal
Define plastic
Deformation is permanent
Define elastic
Deformation will undergo full recovery
_____ materials are very resistant to tension and compression
Metallic
Describe crystalline
Neat and structured
Use of laboratory measured strength for design of large structures can lead to _____
Non-conservative design
Describe amorphous
Out of order
What is a Type III cause of mechanical failure?
Phenomena unknown
_____ is the most widely recycled material in the US
Steel
Why doe we make buildings out of concrete but not out of glass or jelly?
Strength and durability
Flaws (cracks) in materials cause _____ with magnitudes that are far larger than the average stress computed from strength of materials
Stress concentrations
Describe far-field stress
Stress that would exist if crack was not present
Why did the Tacoma Narrows bridge collapse in 1940?
The parapet walls on the side were transferring large wind loads to the structure
What does fracture mechanics describe?
The role of a crack on performance of a material
What is a Type II cause of mechanical failure?
Theory available but ignored (not included in code)
What is Young's Modulus for concrete?
17
What is Young's Modulus for aluminum?
70 GPa
How many electrons are in the 2nd layer?
8
Outer (valence) shell wants to have ___ electrons by any means possible
8
Reinforcing bar diameter is bar size divided by ___
8
How many electrons are in the 3rd layer?
8 primary, 10 secondary
How many electrons are in the 4th layer?
8 primary, 24 secondary
What is Young's Modulus for wood?
8-10 GPa
How many atoms are in the unit cell of a BCC structure?
2
How many electrons are in the 1st layer?
2
What is the atomic number of helium?
2
What is Young's Modulus for steel?
200 GPa
How many atoms are in the unit cell of a FCC structure?
4
What is the atomic weight of helium?
4
Cold forming increases tensile strength by ___-___% and ultimate strength by ___-___%
50-70%, 20-30%
How many atoms are in the unit cell of a HPC structure?
6
What is the coefficient of thermal expansion for steel?
6.5 x 10^-6 in/inF
What are the different types of heat treatments?
-Annealing -Normalizing -Hardening -Tempering
Atomic weight = ?
# of protons + # of neutrons
Atomic numer = ?
# of protons = # of electrons (unless ionized)
What are the steps of steel production?
-Reduce iron ore to pig iron -Refine pig iron into steel -Form the steel into products
What are the different type of protective coatings used to isolate steel from moisture?
-Barrier coatings (standard paint isolates steel from moisture and must be repeated) -Inhabitive primer coatings (pigments that migrate to the steel surface to passivate it) -Sacrificial primers (metal pigments become the anode, give up electrons to the steel, and corrode instead of the steel)
Describe a metallic bond
-Bonding based on positive ion cores in a sea of electrons -Electrons are shared by all ions in the solid -Electrons can flow in the sea of electrons - electrically and thermally conductive -Bonding can be weak (Hg) to very strong (W)
Why does cold formed steel design require special considerations?
-Buckling -Corrosion
What are some properties of high carbon steels?
-Carbon content 0.60 to 1.4% wt -Hardest, strongest, and least ductile carbon steels -Almost always heat treated (hardened, tempered) -Specially wear resistant and capable of holding a sharp cutting edge -Also known as "tool steels"
What are some properties of medium carbon steel?
-Carbon content of 0.25 to 0.60% wt -Can be heat treated to improve mechanical properties -Stronger than low carbon, but lower ductility and toughness
What are some examples of a hexagonal close packed structure?
-Cobalt -Titanium -Zinc
Explain continuous sheet galvanizing
-Continuous in-line dip process for sheet, strip, wire -Coating thickness / alloy layers minimal
Iron alloys are the most common due to:
-Cost -Abundance -Versatility of steel
Why did the Liberty Ships fail?
-Cracks started at corners (stress concentration, fatigue) -All welded structure did not provide barrier to arrest cracks
Where are high carbon steels used?
-Cutting tools (knives, razors, hacksaw blades) -Springs -High strength wires
What are the three spheres of sustainability?
-Environmental -Social -Economic
What are some examples of unexpected failures?
-Failure of Molasses Tank -Liberty Ships -De Havilland Comet
Describe an ionic bond
-Formed when one or more electrons are transferred from one atom to another -Bonding between metallic and non-metallic elements -From extreme sides of the periodic table
Describe a covalent bond
-Forms when electrons are shared between atoms -Both atoms consider the electrons as theirs -Usually strong bonds
What are some examples of a face centered cubic structure?
-Gold -Aluminum -Copper -Lead -Silver -Platinum
Alloy agents are added to steel to improve the following properties:
-Hardenability -Corrosion resistance -Machinability -Ductility -Strength
What are the roles of carbon in steel?
-Hardening agent -Increases strength and elastic modulus of steel -Reduces ductility
The failure of engineering materials are almost always undesirable because:
-Human lives are in jeopardy -Economic losses -Interference with products and services
What are the different types of primary bonds between atoms?
-Ionic bond -Covalent bond -Metallic bond
What are some examples of a body centered cubic structure?
-Iron -Chromium -Tungsten -Molybdenum
Explain zinc plating
-Only used for fasteners and small parts -Tumbled in drum with zinc powder, glass beads, and proprietary chemicals -Mechanical bond
What are the common types of crystal flaws (or defects)?
-Point defects (missing atoms) -Line defects (dislocation, which is a row of missing atoms) -Area or surface defect (grain boundary) -Volume defect (pores, voids)
Where are medium carbon steels used?
-Railway wheels / tracks -Gears -Crankshafts -Other machine parts
What is Young's Modulus for rubber?
0.1 GPa
What is the APF for a BCC structure?
0.68
What is the APF for a HPC structure?
0.74
What is the APF of a FCC structure?
0.74
What are the different types of structural steel?
1) Hot rolled and continuous cast steel for large structural members 2) Cold formed steel used for studs, trusses, and decking 3) Fastening products (nuts, bolts, rivets) 4) Reinforcing steel 5) Miscellaneous (steel forms, deck pans)
The goal of engineering materials is to design structures and materials to:
1) Prevent mechanical failure (stable structures) 2) Resist weathering failure (durable structures) 3) Be environmentally friendly (green structures) 4) Minimize cost (economic structures)
What are the objectives or studying materials science of civil engineering materials?
1) Selection of proper materials for applications (we need to understand the behavior / performance of each material) 2) Design of materials for particular applications
How can the properties of steel be altered?
Applying a variety of heat treatments - can be hardened or softened
_____ is the building block of all matter
Atom
_____ materials exhibit little deformation or energy absorption before fracture
Brittle
What type of material fails without warning (and at small strains)?
Brittle materials
_____ is the alloying element in stainless steel
Chromium
Where is the shear modulus used?
In the design of members subjected to torsion, such as rotating shafts and helical compression springs
_____ bonds are physical bonds, not chemical
Intermolecular (secondary)
Salts are an example of a _____ bond
Ionic
Describe fracture toughness
It is a material property that describes the resistance of a material to crack propagation (analogous to strength)
What is the Charpy V Notch Impact test done to measure?
It is used to measure the toughness of the material or the energy required to fracture a V notched simply supported specimen
How does cold forming increase yield strength, tensile strength, and hardness?
It results in plastic deformation causing strain-hardening that increases yield strength, tensile strength, and hardness
What is the function of limestone in steel production?
It traps impurities into slag
What does LCC stand for and what is it?
Life Cycle Cost - the total cost of a structure over its entire life cycle
For concrete structures, larger objects fail at a (lower/higher) stress
Lower
To build sustainable structures, civil engineers must understand fundamental _____ and _____ of different building materials
Properties, behavior
The nucleus contains...?
Protons and neutrons
The formation of a crack _____ stress
Releases
Large structures observed have _____ tensile/shear strength than measured in lab
Smaller
What is a Type I cause of mechanical failure?
Specification / codes not followed
Name a material that: -Strengthens after slightly damaged -5x as strong in tension as an equal mass of steel -2x as strong as equal mass of kevlar -Remains flexible even in extreme cold -Lightweight: 1lb could form a strand that could stretch around the Earth
Spider web
Why did the De Havilland Comet fail?
Square windows where cracking occurred
_____ steel is highly resistant to corrosion by including at least 11% wt
Stainless steel
What does the ductility of metals depend on?
The ability to break (rupture) and reconnect (reform) interatomic bonds
Describe strain energy
The energy that causes the rod to spring back when the load is removed
Griffith's experiment with glass fibers showed that (thinner/thicker) fibers show lower strength
Thicker
Why are torsion tests performed?
To determine the shear modulus of structural materials
Why are steel tension tests performed?
To determine the yield strength, yield point, ultimate strength, elongation, and reduction
Describe the fracture mechanics approach
To properly design against failure, one must acknowledge the existence of flaws (cracks) inside materials
_____ is an important material property because it indicates how much energy a material can absorb before it fails
Toughness
True or False? Steel manufacturing is a very energy intensive process
True
True or False? You have to fulfill all three spheres of sustainability to be sustainable
True
Number of _____ defines how an atom will bond
Valence electrons
What is the most widely used material in the world?
Water (second is concrete)
Galvanized steel is coated with a thin layer of _____ to add corrosion resistance
Zinc