Tripos Paper Definitons materials Flashcards
Definition of hardness
Hardness is a measure of the resistance to indentation
Usually expressed as the load on the indented divided by projected area of indentation
Yield stress
Yield stress is the NOMINAL stress at the elastic limit when yielding starts ( this is in data book )
At what strain can you approximate en = et and sigma n = sigma t
So far 25 %
Thermoplastic definition + examples
Linear polymer which softens , becomes plastic and melts at a higher temperature
Polyethylene , pvc
Thermoset definition+ example
Cross linked, polymer which becomes less stiff as temperature is raised , but DOES NOT MELT, Rather degrades at high temperatureS
Examples = epoxies , phenolics
Draw the modulus vs TEMPERTAURE graph for an amorphous thermoplastic vs a thermoset , key points
Amorphous = starts high and is GLASSY , drops at TG, , now RUBBERY, drops again, VISCOUS FLOW, now Tm
Thermoset = starts high ( glassy), slightly drops with temperature, at the end DEGRADE
why do higher strength materials have lower KIC or fracture quicker in general
- higher strength is higher sigma y, caused due to hardening so more dislocations
- however higher dislocations reduce potential to palstically deform and hence absorb energy
- they fravture quicker, and so need to use lwoer range for KIC
what is a process zone and
when can you use Kic
how to find process zone
- process is region of inelastic deformation near the crack tip
- locally affects elastic stress field and hence the suitability of using linear fracture mechanics to model the stress at failure
- databook says process zone must be 50 times less than crack size
2) find process using formula for kic and the SIGMA YIELD
Why are natural materials composites not suitable if small dimensions
Small dimensions is likely to be smaller than the micro structural length scales or these materials so not possible
What are 3 hardenings
1) work hardening
2) solid solution hardening
3) PRECIPitate hardening
How does work hardening work
What microstructure properties control the hardening
gliding dislocations on different slip planes interact , and pinning occurs
- dislocations bow out , requiring increase in shear stress until it gives way
2) dislocation densities, higher the harder, and so spacing between pinning points
How does solid solution hardening work
- what angle
Micro structural property
- solute atoms act as weak obstacles to gliding dislocations, ROUHENING THE SLIP PLANW
- dislocations bow out when the line tension pulls the dislocation past the solute atom
- at less than 90
2) concentration, of solute, higher the more pinning, so lower spacing between solute atoms
How does precipitation hardening work
Microstructure
Alloying elements form compounds which act as string obstacles to gliding dislocations ,
- these bow out and leave a residual dislocation around too , and so a lot of resistance is offered
Micro structural = size and volume fraction, so bigger the more impedenet and more the lower the spacing
Why would actual density be different to therotietcal
Because of defects in crystal structure like at grain boundaries, or due to the impurities
Explain why true area of contact statsified load / hardness
- surfaces pressed together at microscopic level thr contact is through surface asperities
- softer steel A will be indented by asperities or steel B ( so determined by A)
- at each contact there will be force local = HA
Sum these up
What are times when’s miners ruke is good but bad(2)
- good because it’s applicable for high cycle fatigue normally
- BUT it ignores sequence in whcih phases of loading occur ! Applying data for any from undamaged specimens to all phases of loading
What is bulk modulus
Hydrostatic loading
How to derrive , including dealing with vik strain
Why 0,5 bug out
1) hydrostatic stress/ volumetric strain
2) hydsfeostaic is same stress everywhere ( compressive tho, take modulus )
3) vol strain is change vol/ orignal, but fir small strain is sum of all strain
- find strain due to passion and done
4) because then negative, meaning compression means increase in volume
What does heat treatment of cabron steel invoice and describe process
Is YM / KIC affected
- quenching snd tempering, is PRECIPITAION HARDENING
- hard precipated are formed during heat treatment which act as strong obstacles which mean dislocations need to bow out COMPLETLEY to pass, increaseing shear stress needed for yielding
2) YM IS NOT ONLY DETERMINED BY BOND STIFFNESS
- KIC is tho, because rest dislocations which increase potential for plastic deformation
Describe log da/ dn vs stress intensity graph
Region one = crack imitation, where crack growth per cycle is 0 below K Th
2 is steady state crack propagation = crack growth per cycle described by Paris law, where ln A is above the axis
3 = kmax = KIC , where fast fracture occurs, crack growth rate increases rapidly
what is a unidirectional composite
what are assumptions derriving the parallel and perpendicular
if alternating layers of unidirectional layers in composite what is the parallel and perp E gonna be now if even amount of layers
1) just where fibres are in same direction one only
2) that fibres are perfectly glued to composite.
- this means parallel same strain
- perpendicular is same stress
3) taking one direction, half E is provided by perp and other half by parallel now, so just multiply by 0.5
1) Describe plot for cyclic stress amplitude vs fatigue life
1) y axis is stress amplitude range / 2, x axis is Nf
- it’s a curve that starts high and ends low
- high region from 0 to 10^4 is low cycle ( thus high range)
- from 10^4 onwards it’s high cycle ( low stress range)
- endurance limit at 10^7 cycles bit it’s a STRESS
2) what is endurance limit
1) endurance limit of a nominally defect free material is the applied STRESS ZAMOLITUDE E ABOUT A ZERO MEAN STRESS at which stresses below cause NO FRACTURE AT ALL, or fracture only at 10[7 cycles
3) how can each mechanism be modelled
- why is plastic strain used for coffin ?
And briefly explain how each high or low still causes failure
- low by coffee , high basquin ( explain each thing, and that plastic strain used because elastic strain is 0 for coffee
2) low cycle is high stress where sigma between yield and uts. - causes plastic deformation and so low fatigue life
3) high cycle is low stress , causes elastic deformation but cracks still develop and cause failure JUST TAKES LONG TIME
4) how ti use laws to predict fatigue life
- if non zero make zero with goodman
- then use miners ruke to determine failure, when ni/ nf summed gives 1
Deacobre true vs nominal stress strain for metal
Nominal - we know it to be elastic, then work hardens, then necking then failure by ductile fracture
True - same YM in tension and then higher stress lower strain, and FAILS AT NECKIJG
- in compression same but NECKING AVOIDED, hence goes for longer strains until crushed
Failure mechanism of ceramic both ways
2) describe ceramic stress strain graph superimposed compression and rtensiom
Tension - failure by weakest flaw
Compression - failure by crushing , crush band propagates steadily until bands of material fai,
2) - has high YM , tension by brittle failure
- compression yield strength goes 10 before , and then digssnolly comes down which is compression
Describe two polymer stress strain curves
1) both are very low modulus, one can have high ductility ( drawing) , other can just fail quickly ( brittle)
What does no volumetric strain which for small strains is sum of strains mean
That incompressible
What is Defintion and difference of K, KIC and fast fracture
K is a geometrical property whcih described the loading at tip of a SHARP CRACK , depends on applied stress and gerometric ( give dimensions )
KIC is a MATERIAL PROPERTY which describes value of K needed at crack to cause fats fracture
Fats fracture is when rate of crack growth increases and happens when K> kic
When do you need zero stress
Miners rule
Basquin , coffee Munson
Paris law DOESNT
Why does failure occur where change of section
SCF amplifies the stress at that point which causes failure l
This dependent on section but radius of curvature
Change to higher radius of curvature ti reduce SCF
Why can polymers EM be changed but like ceramic can’t
Key word
Ceramic p, metal, primary bindign, and so EM determined by atomic packing and bonding , however polymers determined by secondary bindign which can be altered with processing and degree or crystallinity .
Microstructure insensitive = can’t manipulate
ductile failure
you have some inclusions
plasticitiy concentrates stress on inclusions (holes in micrograph), nucleoting these voids causing fracture (I make bigger)
micrograph show this as voids nucleated by inclusions
brittle failure
if carried below ductile to brittle transsition temp, energy absorbed is likely to be low
- micrograph shows no plastic flow
annealed vs drawn/ heat treated
annealed is no dislocation. no dislocation is no strength but a lot of ductile failure , possibility to deform
drawn is work hardening and heat teatetd is precipitaion, so both have high dislocations so stength but not ductility
then compare the wh method, preicipation is better and solid is better
how do composites fail
pulling from sockets, so lines in half, fibres pulled from matrix, so a lot of fricton energy used
weibull why do we use v/v0
the probility of v is the same as n indivudlu test samples v0, hence n v0, hence n is v/vo and thats the exponent
going from stress- life plot to the laws in the plot
paris
stress life plot is stress amplitude vs cycles to failure BOTH LOGGED
hence for low and cycle areas we can idenfity the gradient straight away as the laws
paris comes from the crack growth rate agaisnt stress intensity factor, where you have 3 regions
what is fatigue life influenced by
how to delay initiation
marks machining imperfections etc
- work harden it / surfave hardneing
micrograph showing crack
clamshells is obvious , macroscopic
striations are microscopic , shows the distance of crack every cycle
failure of pressure vessels
- want to avoid catastophic rupture
- hence want it to leak before failure
- this occurs when crack length is greater than thickness
if its less then fast fracture occurs and then it explodes
how to find max load for leak before failuire
- set t to thickness
- set sigma to sigma max becasue we dont want it to plastically fail either
- use KIC so fast fracture occurs here
this gives max load
proof testing steps, how to find therotirecal cycles to failure
1) fill above working pressure
- if it survives then you know the crack size is atleast this high , and this is the maximum initial crack length
2) use this crack length in paris law to predict the numbers of cycle, making final crack either for fast fracture, or if leak before failure just set it to the thickness t (so dependoing on what you testing for)
why does this work
- dont want crack to progoate under fast fracrure conditiosn
amontins laws
f= mew r
f is independent of area of contact
coloimn- friction dynamic is indepndent on sliding v
adhesive vs abrasive
two surfaces similar hardness is adhesive , small bitd are taken off
abrasive, the harder material takes lots of soft out and so rate of wear is higher
what is relationship between shear stress and yield
tow is half yield
wear rate
is volume of material removed/ slidijg distance proprtional to k where slidign ditsnce is disrtsnc e
derrive coeefiecnt of friction
f= mew r
f is shear stress x area/ hardness
convert shear stress and hardness to yield
get w/6, so mew us 1/6
