microscopy Flashcards
what are the three observable levels of material structure
- macro
- micro
- substructure
what is the equipement used in microsscopy
olympus optical microscope with 4x, 10x, 40x, 100x zoom capabilities
what are the four main construction materials
- steel
- concrte
- polymers or FRP
- wood
what is steel made of
iron ore and coal, smelted together in a furncae
why do we use steel
it is strong but ductile
what is ductility and why is it important
ability to deform plastically before failure
gives a warning before failure
how do we make concrete ductile
reinforced concrete
in bending what is happenign at bottom and top
tension at bottom
compression at top
what happens when concrete fails in compression before the steel yields
fails instantly, shear failure
T or F we want the steel inside the concrete to yield
T
why do we use brittle materials
because they are strong, but also concrete is cheap and accessible everywhere
what are the different types of steel and what do they depend on
depend on carbon content:
- eutectic
- hypo-eutectic
-hyper-eutectic
describe ferrite crystals
- extremely low carbon content at room temp
- essentially pure iron arrange in crystal form
- ductility is primarily derived from this phase
describe cementite crystals
- 6.67% carbon content
- strong, hard, brittle
describe pearlite crystals
- alternating thin bands of cementite and ferrite
- has a mixture of the properties of both crystals (depends on %)
- formed at the eutectic point
what is difference between strength and hardness
ability to resist stresses from loads = strength
ability to resist scratching = hardness
why would we need hardness in steel
- if we need to cut steel for bolt holes and threads ex: need to use harder steel to cut through softer steel
what are the components of hypo-eutectic steel
ferrite and pearlite crystals
what are the components of eutectic steel
pearlite crystals
what are the components of hyper-eutectic steel
pearlite and cementite crystals
what is toughness
ability to dissipate energy before failing
caracteristics of high carbon steel, medium and low
high: strongest (highest stress), most brittle
medium: toughest
low: most ductile
how does a coarse grain size affect the mechanical properties of steel?
strength: low
ductility: high
toughness: high
how does a fine grain size affect the mechanical properties of steel
strength: high
ductility: low
toughness: low
what are the three ways the mechanical properties of steel can be improved
1) alloying
2) heat treatment
3) strain/work hardening
what is alloying
mixing one metal with one or more others to improve the properties of the original metal
what are the two types of heat treatment
annealing (slow cooling ) and quenching (fast cooling)
explain what happens during annealing
- crystals have time to form properly
- uniform dispersion of iron and carbon
- reduces localized weak zones
explain what happens during quenching
- crystals dont have time to form
- smaller crystals are formed
- harder/stronger
what happens to steel during strain hardening
- crystals become longer, thus stronger, in one direction
- stronger in tension (steels strong point)
which would fail first? a thick rectangal of steel or a skinny rectangal of steel
the thick one fails first because of stress = F/A so a higher area means a lower stress
which is stronger? a large rectangal of steel or many different pieces of steel stuck together to have the same thickness
- the many pieces because of size effect - less likely to have a critical flaw
- defects initiates early failure: starts a crack that propagates
which is stronger a crack in a large square or a crack in a piece of many that form a large square
- a piece of many
- defects initiates early failure: starts a crack that propagates
what is concrete made of
aggregates and cement mixed together with water
- also aggregates + HCP (hydrated cement crystals at the nano scale)
why dont we just use cement to build things? why do we use aggregates
cement is too expensive
- weaker than aggregates
why dont we just use aggregates to build stuff
- nothing holding them together, no tensile strenght
- too much effort
why dont we use giant rock slabs to build
- too hard to handle/transport
- no steel reinforcement to take tensile stresses
why use concrete?
- best of both worlds: mouldability and handling of cement paste with small aggregates, strength of aggregates
- able to use steel reinforcement to take tensile stresses thanks to hydrated cement paste bonding it
what is a fibre-reinforced polymer?
- fibres (carbon fibre, glass fibre, aramid fibre) load bearing
- polymer (plastic)(matrix) - glues togehter the fibre
- stronger in tension than in compression (rope + support analogy)
- composite material (1 + materials)
what is the use of observing FRP under the microscope
- detecting flaws
- quality control
- meet standard specifications
what is difference between isotropic and anisotropic
- isotropic = strong equally in all directions
- anisotropic = strong more in one direction than the other
is FRP iso or aniso tropic
anisotropic because of two layers of fibres which are strong in one single direction each
what are the parts of a tree
- outer bark
- inner bark
- cambium
- annual growth rings
- rays
- pith
why are latewood cells smaller than earlywood cells within one growth ring
- earlywood cells have more growth requirements, in the spring time (leaves, flowers, nuts, fruits), more nutrients, more sunlight, good weather, thus cells are larger and cell walls are thinner to make up for fast growth
- latewood cells have less growth requirements, in the summe/fall when growth slows down, producing smaller cells with thicker cell walls
difference between hardwoods and softwoods?
hardwood:
- presence of more vessels(tubes that transport water from roots to leaves)
- less cells between growth rings (hardwood gros slower)
- smaller cells and thicker cell walls
- no resin canals
- thicker bark
why does hardwood typically have a higehr mechanical strength than softwood
hardwood has:
- higher density because of a thicker S2 wall
- S2 wall is aligned with length of cell
what is the name given when there is too much carbon (fusion)
cast iron
why would we not use cast iron in construction
- very brittle
- weak in tension
what is the main downside of strengthening steel in connections other than it has reduced ductility?
- reduced weldability (really hard to weld when the strength is increased)
what is the problem with using the alternative connection other than welding? what is the alternative connection?
- bolting
- consumes resources so more expensive
- more time consuming
how do you measure toughness in steel?
- area under the stress-strain curve (most efficient for structural steel)
- why isnt is good for other steels: structural steel are very ductile
- cant take area under the curve for brittle material
- have to do impact tests.. for brittle
