P 5. Solids under stress Revamp Flashcards
Almost there
u right
Experiment to learn?
When the case calls
Define limit of proportionality
- When the proportional behaviour
- of forces and extension stops
Define elastic limit
- Point at which deformation
- ceases to be elastic
Define permanent extension
When it’s permanent, irreversible deformation
Define elastic behaviour
- When extension disappears when force is removed
- returns to its original size and shape
Define plastic behaviour
- When extension decreases
- only slightly when stress is removed
- Doesn’t return to its original size and shape
Define Hooke’s law?
(4-way)
- The tension in a spring or ware
- is proportional to its extension
- from its natural length
- within the limit of proportionality
Hooke’s law formula?
F = kx
Define F
(Hooke’s law)
Force applied/tension
(N)
Define k
(Hooke’s law)
The spring constant
(force per unit extension)
Define x
(Hooke’s law)
The extension from the spring’s natural length
(mm, cm, m)
What do springs look like in parallel?
1 hanging each side
What do springs look like in series?
Hanging on top of each other
If springs are in parallel, what happens to the k?
(2-way… + 1 thing)
- x = doubled
- k = halved
- k1 + k2 = ktotal
If springs are in series, what happens to the k?
(2-way… + 1 thing)
- x = halved
- k = doubled
- 1/k1 + 1/k2 = 1/ktotal
How is the energy stored in stretched spring equation formed?
- Area under line on force-distance graph = wd
- Likewise to Hooke’s law thing
- E = ½ Fx
- Sub in F = kx
- E = ½ kx2
Therefore, what’s the equation to find energy stored in a stretched spring?
(in data booklet)
E = ½ kx2
Define stress
Force per unit cross-sectional area
Formula for tensile stress (σ)?
σ = N/m2
Formula for tensile stress expressed in words?
Tensile stress = Force/Area
Define σ
(Tensile stress formula)
Tensile stress…
(Nm-2 or Pa)
Define N
(Tensile stress formula)
Force
(Newtons)
Define m2
(Tensile stress formula)
Area
(x2)
Define strain
Extension per unit length
Formula for tensile strain (ε)?
ε = m/m
Formula for tensile strain expressed in words?
Tensile strain = extension/length
Define ε
(Tensile strain formula)
… tensile strain
(A ratio, no units)
Define the m’s
(Tensile strain formula)
(2-way)
- Extension above
- Length below
For any material, if we work within the limit of proportionality, what happens to the ratio for stress/strain?
What’s it called?
- Becomes a constant value
- YOUNG MODULUS
Formula for young modulus (E)?
E = σ/ε
What’s an easier equation for young modulus?
E = Fl/Ax
How is the easy young modulus equation developed?
(3 steps)
- E = (F/A)/(x/l)
- = F/A x l/x
- = Fl/Ax
Define elastic?
(2-way)
- Material that regains its shape
- after stresses are removed
Define ductile?
(2-way)
- Material that can be easily stretched
- or drawn into a wire
Define malleable?
(2-way)
- Material able to be hammered or pressed
- into a shape without breaking/cracking
Define brittle?
(2-way + e.g.)
- Material that’ll snap
- without yield
- … without any give
Define stiff?
(1-1-way)
- Small strains for large stresses
- ;Not stretch or bendy
Define plastic?
(2-way)
- Material that undergoes permanent deformation
- under large stress rather than cracking
Define strong?
(in terms of this topic)
Large stress needed to break it
Define hard?
(in terms of this topic)
Resists indentation on impact
Topics left…?
- Stress-strain graph for ductile metals
- Brittle material
- Polymeric materials
Finish in time o7…. it seems we have an alternate path
What are the 3 categories used to classify solid materials?
- Crystalline solids
- Amorphous solids
- Polymeric solids
What type of materials are crystalline solids?
(psh remember 1/4)
- Metals
- Diamond
- Graphite
- Salt
Define crystalline solids
(2 things)
- Atoms arranged into a crystal lattice
- Consists of regularly repeating unit cells
What do crystalline solids exhibit?
Long-range order and symmetry
What’s the jig with poly-crystalline structures?
(3 things)
- Structure split up into many small crystallites/grains
- Randomly arranged
- Forms different grain boundaries
What type of materials are amorphous solids?
(remember 1/3)
- Glass
- Ice
- Ceramics
Define amorphous solids?
(2-way + 2-way)
- Atoms have no long-range order
- … Or symmetry
- Considered to be super cooled liquids:
- atoms randomly arranged, no overall pattern
What do amorphous solids tend to be more?
(2-way)
- Brittle
- with little elasticity
What type of materials are polymeric solids?
(remember 1/4)
- Rubber
- Cellulose
- Polyethylene (ploythene)
- PVC
Define polymeric solids?
(1 thing + 3-way)
- Compromised of long molecular chains
- Consists of basic units:
- monomers repeatedly linked together
- w/ strong covalent bonds
If the polymeric solid is randomly tangled up, what can be said bout’ it?
(2 things)
- Amorphous
- No long-range order
How can a polymeric solid display semi-crystalline properties?
- Stretched rubber
- When chains are untangled
Crystalline solid in molecular terms?
(2-way)
- Long range order
- Lattice arrangement
Amorphous solid in molecular terms?
(2-way)
- Short/no range order
- Random arrangement
Polymeric solid in molecular terms?
Long chain molecule arrangement
Define elastic strain
(1 thing, 2-way + 2 features)
- Force applied to a material
- Atoms stretched apart
- … by a very small distance
- Strain measures = relatively small forces
- Small strains = no bonds broken
Define plastic strain
(2-way-aparts + 1 main)
- Imperfections within a lattice:
- Edge dislocations
- … causing bonds to break
Explain the theory suggesting lattices contain imperfections
(1 thing + 2-way)
- Mistakes within structure
- Incomplete plane of atoms creating
- “an edge dislocation”
What are edge dislocations?
(2-way + 2-way)
- Creates a point of weakness within material…
- where large stresses occur
- These points = material start to fail if…..
- Larger forces applied here
Movement of edge dislocation = ?
Plastic deformation
What’s the symbol representing edge dislocation?
⊥
(wowow)
How the edge dislocation move within a material?
(2-way + 2-way)
- If shear stress applied
- dislocation migrates throughout material
- New bonds made to the left….
- edge dislocation moves to the right
What causes permanent deformation within material?
(2-way + 2-way + 1 thing)
- Not applying force….
- to move the dislocation
- Plastic strain occurred
- has gone beyond elastic limit
- Edge dislocation stays permanently to right
Define work hardening?
(3-way + 2 features)
- When lots of dislocation meet
- Impedes each other
- to stop dislocation propagation
- Material = gets stronger
- Larger stress to cause further strain
Define grain boundaries?
(2 things + 2-way + 2 things)
- Polycrystalline materials have this
- Forms barriers to edge dislocation movement
- Smaller grains = short distance travelled
- … by the dislocation
- Can make the material stronger
- Dislocation can’t jump to next grain
Define necking
(2-way)
- The thinning of the cross-sectional area
- of a material at the weakest point
When does necking occur?
(2-way + 1 thing)
- When material continues to be stretched
- …. after reaching its ultimate tensile stress
- Rapidly leads to materials failing
Define ductile fracture?
(2-way)
- Process of large strains and necking
- leading up to the material failing
What are the 3 ways to strengthen metals?
- Grain size reduction
- Work hardening
- Foreign atoms
Explain grain size reduction?
(Strengthening metals)
(3-way)
- Smaller grains produce
- more barriers
- to dislocation movement
Explain work hardening?
(Strengthening metals)
(3-way)
- Plastically deforming metal
- by creating a greater dislocation density
- makes metal stronger
Explain foreign atoms?
(Strengthening metals)
(2-way + 2-way)
- Making metal alloys by….
- introducing new elements into molten metal
- Has effect of causing changes to lattice
- impedes the movement of dislocation
How is steel made through foreign atoms?
(2-way + 1 things’ definition)
- Since adding similar sizes but different element atoms
- to lattice distorts the lattice slightly
- Interstitial takes place
- (fitting in between the lattice atoms)
Describe through labelled diagrams of the behaviour of ductile materials?
When the time calls…
Show & draw me the stress-strain graph for ductile metals?
(4 stages w/ 2 having 1/3 things)
- Limit of proportionality
- Elastic limit
^^^ Yield strength - Plastic region
^ first half = work hardening
^ second half = necking
^ peak = ultimate strength - Fracture
When the time calls.
What does it mean if brittle materials don’t undergo ductile fracture?
No plastic deformation within material
What do brittle materials undergo?
(2 things + 2-way)
- Elastic deformation
- Usually failing within their elastic limit
- They obey Hooke’s law….
- up to their fracture point
We’re at a considerably good spot
Lock in for A levels first. But it doesn’t hurt to be diligent
Welcome back?
I wish there was a date here but meh it was definitely last year, few months ago
A material(s) that’s brittle and “amorphous”?
- Glass
- Concrete
Why doesn’t glass yield?
(2 things)
- The way fracture occurs within material
- They break suddenly
Why doesn’t glass have any “failures”?
Doesn’t have slippages of atomic planes (ductile materials)
What happens if a bond breaks for a brittle material?
(2-way)
- Crack in atomic planes
- propagates rapidly
In depth of surface imperfections on brittle materials?
(1-2-way)
- Failure of material
- Crack rapidly at right angles
- to the stress forming a flat surface
How the the cracks of a brittle material often propagate?
(2 things)
- By cleavage
- e.g. atomic bonds break along specific crystal planes
Example of how a crack moves through a brittle material?
Like a zip undoing O_o
Major difference between brittle materials w/ bonds and a ductile material?
Bonds don’t reform like the dislocation movement
The 1 way to bend glass then?
Freshly made glass fibre
Explain compressive stress of a brittle material?
(3-way + 1 thing)
- Inside of bend is touched…
- nothing happens
- due to surface under compression
- Surface cracks are closed
Explain tensile stress of a brittle material?
(2-way + 2-way)
- When touched on outside of curve
- … immediately shatters
- Surface cracks are opened
- and rail rapidly
Hence, in what way is brittle materials stronger?
Under compressive forces than tensile forces
What are 2 types of glass you can get for if u increase their breaking stress?
- Fibre glass (thin fibres)
- Toughened glass
How are thin fibres stronger than thick glass rods
(2 things)
- Smaller surface area
- Fewer cracks & surface imperfections
Hence, how are fibre glass created?
(3-way)
- Placing thin fibres into
- resin glue
- Very mouldable
How is toughened glass achieved?
(6-way)
(Understanding problem)
- Rapidly cooling outside of
- molten glass (jets of air)
- Outside cools and contracts
- Inside is soft and able to give
- Once inside hardens and contracts,
- outside cant give and thus placed under compressive stresses
How are toughened glass much harder to form cracks?
(2-way)
- Close up due to
- internal compressive forces
What happens when toughened glass (pre-stressed glass) completely breaks?
(3-way)
- Shatters into tiny pieces
- w/ rounded edges than
- jagged sharp edges
How is pre-stressed concrete achieved?
(3-way)
- Use of steel reinforcing bars/rods
- compensates for concrete’s weakness
- when placed under tension
How is pre-stressed concrete made?
(5-way)
(Understanding problem)
- Steel rods placed under tension + st retched
- Wet concrete placed around steel
- allow to set causing to bond
- Tension removed from steel rods allowing it to contract
- Concrete now under permanent compressive forces
How about how larger structures of pre-stressed concrete is made?
(2-way + 1 thing)
- Fit together sections of
- pre-stressed concrete
- Strong under both compressive & tensile stresses
Explain concrete without metal rods?
(3-way)
- Lower surface under tension
- Force causes cracks in surface
- rapidly opens leading to failure
Explain concrete w/ metal rods?
(2-way + 2-way)
- Lower surface under compression
- due to rods
- Force will cause cracks
- in surface to close
Stress-strain graph for brittle materials?
(Lil situation)
Maybe I’ll find one online and try implement
Define polymeric materials?
(2-way)
- Consists of long chain molecules
- which’re held together by cross bonds
2 examples of polymeric materials?
- Rubber
- Polythene
What does it mean if rubber is an elastic material?
Can go back to original shape after deformation
Stress-strain graph of rubber?
(lil situation)
Well, its in the booklet but…
- Loading and unloading
- Looks like a skewered flame
What is observed during microscopic analysis of a rubber material?
(3 things + 2-way)
- Like monomers together creating polymers
- Bonding = covalent and strong
- Longer chain = weak VDW bonds to form between adjacent lengths of the coils
- Weaker bonds easily broken HENCE
- can stretch exhibiting large strains
In-depth reason how rubber is elastic?
(2 things + 2-way + 2-way)
- After removal of stress and chains curling back up,
- Thermal energy causes atoms & molecules to vibrate
- Thermal agitation makes chain want to become more
- randomly arranged
- Often referred to stretching polymers
- against thermal opposition
In stress-strain graph of rubber, what’s the phenomena called of the contracting curve not coinciding with the expanding curve?
Hysteresis
In stress-strain graph of rubber, what’s the area enclosed known as?
Hysteresis loop
How to gain energy transferred per unit volume from stress-strain graph of rubber?
The area…. the hysteresis loop…
Hence, the 3 behaviours of rubber
(Left - right = loading,
Right - left = unloading)
- Tangled
- Straightening
- Straight
Brother I pray u consistently nail this
Hm.
