MEC302 Flashcards
Describe hydrodynamic lubrication
Lubricant is dragged into wedge between components. The lubricant pressure increase supports the applied load
What are the typical applications for hydrodynamic lubrication?
journal bearings, machine slideways, piston ring/liner.
What are the typical film thickness and coefficient of friction for hydrodynamic lubrication?
1-100 µm
0.01-0.03
Describe boundary Lubrication
Surfaces may not be fully separated. Thin chemical layers reduce the tendency of the asperities to adhere.
What are the typical applications for boundary lubrication?
metal cutting, bearing start-up or shutdown
What are the typical film thickness and coefficient of friction for boundary lubrication?
- 001-0.05 µm
0. 1-0.3 S
Describe Hydrostatic Lubrication
Lubricant pumped into the interface to separate surfaces doesn’t require movement
What are the typical applications for hydrostatic lubrication?
machine tool spindles
bearing start up
What are the typical film thickness and coefficient of friction for hydrostatic lubrication?
1-100 µm
0.01-0.03
Describe Elastohydrodynamic
Lubrication (ehl)
As hydrodynamic, but high local pressure causes increase in viscosity and elastic deformation
What are the typical applications for Elastohydrodynamic
Lubrication?
rolling element bearings,
gears, cams and tappets
What are the typical film thickness and coefficient of friction for Elastohydrodynamic lubrication?
- 1-1.0 µm
0. 001-0.01
Describe Solid Lubrication
Low shear strength solid separates surfaces.
Shears more easily than the component materials.
What are the typical applications for solid
Lubrication?
‘dry’ bearings, vacuum,
graphite, PTFE, MoS2
What are the typical film thickness and coefficient of friction for solid lubrication?
-
>0.05-0.3
What assumptions do you make for a thick disc?
axial strain is constant with radius
εA= constant
(dεA/dr = 0)
What assumptions do you make for a thin disc?
axial stress=0
if there is no load or material to push against……..
stress = 0
What does B always equal for a solid shaft?
0
as if not there would be infinite stress at the centre of the shaft which is impossible
What are the lamé equations used for?
thick cylinders subject to internal and external pressure
Where are the maximum stresses for thick cylinders subject to internal and external pressure?
inner surface
what is the axial stress for an open-ended pipe?
0
what is the axial stress for an closed-ends pipe?
EεA+v(σr+σθ)
Describe visual and optical testing techniques
Looking for flaws/ compare to perfect component
Advantages and disadvantages of visual optical testing
Most common
inexpensive
simple equipment
time consuming, not suitable for large areas
only detects flaws on surface
Describe leak testing
For components that are going to be pressurised
leaks detected by electronic listening devices, pressure gauge, liquid and gas penetrant techniques
or simple bubble test with soap
Describe hydro proof testing
pressurising vessel with water to see if it will withstand pressure in service - doesn’t burst then assume largest defect will withstand pressure
water used as less stored energy than gas so catastrophic burst will not occur
Name the methods for visibility enhancements in cracks
dye penetrant
fluorescent penetrant
magnetic particles
Describe liquid or dye penetrant testing
The component is coated in visible or fluorescent dye
Excess dye removed
Dry developer added
The developer acts a blotter drawing dye out of flaws
Surface defects become visible
Describe magnetic particle testing
Induce magnetic field in ferromagnetic material
Dust surface with iron particles
Surface flaws distort field and concentrate particles near them
Need to perform in two orientations as flaws parallel to field won’t be detected
Flaw = disturbance in mag field= flux leakage= particles drawn to disturbance
Describe electromagnetic testing
Electrical currents are generated in a conductive material by an induced alternating magnetic field
Interruptions in the flow of electric currents (eddy currents) cause by flaws in materials conductive properties will cause changes in the mag field
Describe radiography
use of penetrating x-rays ( or gamma) to examine parts and products for imperfections
an x-ray machine is or radioactive isotope is used as a source of radiation
Radiation directed through a part and onto film
Shadowgraph shows internal soundness of the part
SUBSURFACE
volumetric method
expensive only sees flaws perpendicular to x-rays
Describe X-ray testing
X-ray tube: tungsten target hit by electrons–> emits x-rays
copper rod for heat dissipation
dense material needs high energy electrons
Health and safety important and needs skilled people
geometric unsharpness
Describe gamma ray testing
simple, compact, transportable
energy depends on the half-life of radioactive source
intensity depends on the source
Pros and cons of gamma-ray testing
pros: detect subsurface flaws
accuracy depends on radiation direction
tomography (3d maps)
cons: high capital cost
skilled job
Describe ultrasonic testing
Transmission of the high-frequency sound wave into the material to detect flaws
most commonly used is pulse echo
sounds introduce to component and echos are listened to
SUBSURFACE +- 3-5mm accuracy
Advantages and disadvantages of ultrasonic testing
Non hazardous Subsurface Detect depths / location of cracks or flaws Crack length measurement Used for non-metals
Limited sensitivity
Fairly expensive
Needs calibration
Describe acoustic emission testing
Solid material loaded
imperfections in the solid emit acoustic energy called emissions (stress waves)
sensing transducer mounted on solid oscillates at resonant freq when hit by emissions
SUBSURFACE
Pros and cons of acoustic emission testing
can detect continuous cracking and early stage crack formation
works best in high strength materials
under ideal conditions greater sensitivity to crack detection
expensive
difficult to filter out noise
info about energy emissions from metal needed
What is the anagram for detecting errors in ultrasonics?
P rogramme for the
A ssesment of
N on destructive testing in
I ndusrty
Define toughness
the materials ability to resist crack propagation
low toughness - brittle
high toughness - ductile
Define critical load
the smallest load which causes the component to fail
What is the stress concentration factor proportional to?
sqrt(notch length/notch tip radius)
as radius becomes smaller notch becomes more crack like and stresses more concentrated
What are the conditions of LEFM?
crack is large in comparison to grain size (x10 bigger)
applied stress is less than 1/3 of yield stress
What are the crack propagation modes?
Mode I - opening
Mode II - shear
Mode III - antiplane shear
Define Ki
the applied stress intensity factor
this is a mechanics parameter, tells us magnitudes of the crack tip stresses ahead of a loaded crack with a stress profile tending towards infinity at the very crack tip.
Define Kic
Fracture toughness is the material property
tells us how resistant the material is to fracture
if Ki=Kic there will be a fracture
What is the plane strain fracture toughness?
value of fracture toughness for a specimen so thick that fracture the event isn’t influenced by boundaries of the specimen
What fracture surface does plane strain give?
flat
What fracture surface does plane stress give?
slant, or shear
What are the limitations of LEFM theory?
- stress cannot physically be infinite at the crack tip
- plastic deformation will start when σyy reaches the yield strength of the material
σyy stress perpendicular to crack tip
How does a material’s toughness increase at a crack tip?
crack blunting and voids ahead of cracks
crack joins with voids then blunts and stops growing
Name three energy dissipation mechanisms which increase the toughness at the crack tip by consuming work?
Toughness is about consuming work during crack advance
- crack tip plasticity
- ductile fracture mechanics (void formations & growth, blunting, shear localisation)
- crack deflection (grain to grain misorientation, strengthening fibres or particles)
Describe the point of instability in a cracked structure
When the material toughness can not keep up with the increasing crack driving parameter
What is the flow stress?
(σyield +σUTS) /2
What decides whether the material will fail by crack propagation of plastic collapse?
whichever mechanism is the weakest will be the mode of failure
use failure assessment diagram to find out how the material will fail
What is Kr?
the ratio of total elastic stress intensity factor Ki to material toughness Kic
What is Lr?
ratio of primary load to plastic limit load
What is an approximation for the residual stresses in heat-treated welds?
10% σyield
What is an approximation for the residual stresses in not heat-treated welds?
60-100% σyield
When can friction benefit design?
bearing surfaces
brake pads and disk
nuts and bolts
What is friction characterised by?
roughness and waviness
Describe waviness
more widely spaced components on the surface texture includes all irregularities that the spacing is greater than the roughness sampling length
Describe roughness
fine irregularities on the surface texture, this includes irregularities that are a result of inherent action of the production process
What are asperities?
peaks and troughs of the surface at a micro level
What are the three laws of friction?
- the friction force is proportional to the normal force
- the friction force is independent of the apparent area of contact
- the friction force is independent of sliding velocity
Describe what happens to the asperities when a load is applied to a component
At low loads- asperities deform elastically
At higher more realistic loads - the load squashes the tips plastically forming weld joints known as adhesion junctions
the friction force is the force needed to fracture adhesion junctions
Why do polymers make good low friction bearings?
if plastic flow occurs, polymer chains orientate parallel to sliding surface - they shear easily and μ is low
BUT you get heavy wear
Excessive contact stress or deformation can lead to component failure by:
overload - excessive loading
wear - material removed from surfaces
rolling contact fatigue - cyclic contact stresses cause fatigue
seizure - component surface local weld due to high contact stresses
loss of tolerance - excessive deformation of components
In a circular point contact where is the highest stress?
Below the surface
What is the radius of curvature of a flat plate?
infinity
What is the definition of wear?
redistribution of material that adversely alters the surface
Name three mechanical wear processes
adhesive
abrasive
wear caused by fatigue
Name three other wear processes
chemical or corrosive wear
melt wear
erosion
Describe adhesive wear
material sticks together at adhesion junctions and then break apart, softer material most likely to break
Describe abrasive wear
damage to a component surface arising due to motion relative to that surface of harder asperities
Describe wear cause by fatigue
cyclic relative motion between two surfaces
- stress at surface varies with time
- fatigue occurs
- slow growth of cracks
- surface is weakend
- large pieces detach
What are the purposes of lubrication?
- separate surface to reduce friction and wear
- cool medium
- protection from corrosion
What are the five main classes of lubrication?
oils greases emulsions solid lubricants gases
viscosity is most important property of lubricants, lube needs to be able to support load and pressure of components
Crack initiation triggers
- slip in grains
- cracking or debonding of second phase particles
- natural scratches
- machining marks on surface
- corrosion pit or intergranular attack
- brittle surface layer
- porosity from casting
When is an SN curve used?
high cycle fatigue- more than 100000 cycles
when nominal stresses and strains are assumed to be elastic
What is the fatigue limit?
stress below which no fatigue damage seems to occur
What is the thin walled pressure vessel approximation?
hoop stress = pr/t
What is the time of flight related to?
Ultrasonic testing
can be related to a distance below the surface through the speed of sound in the
investigated material and a B-scan or C-scan enables the measurement of crack size in 2D and 3D
respectively.
Describe the variation in microstructure across a typical weld
The variation of microstructure, especially grain size, across the weld is directly related to the thermal profile
experienced by the material during the welding process.
- leads to a variation of strength, –> related
to hardness,
- softer region of heat affected zone favour crack formation (closer to wels zone)
Which mode is most dominant?
Mode I is most dominant because this is generally the mode which opens and grow the crack and is observed
in most cases in practice.
Define the stress concentration parameter
represents the ratio of the finite stress value at the tip of the notch to the nominal stress.
Explain why compound cylinders are used to contain pressurized gas rather than single cylinders
to induce compression at the bore so reducing the hoop stress at the bore, where it is usually a maximum
What are two other ways that hoop stress can be reduced at the bore?
- wire-wound on cylinder under tension to induce compression at the bore.
- Overstrain once with a pressure beyond yield, to leave compressive residual stress distribution near the bore.
Describe the mechanism for the formation of surface striations
striations form on the fracture surface during stage II of fatigue fracture
cracks propagate when they are relatively large compared to the grain size
around the crack tip there is plasticity as it permanently deforms
with every cycle you get more crack growth
Describe circumstances when you would keep using a structure with fatigue cracks
Cracks must be inspectable and measurable
must know or be able to measure the stresses
must have crack growth data
must know the K calibration and therefore the likely
path of the crack
must be able to set a final allowable defect size Remaining life must be long enough to get several, maybe 5, inspections in before the cracks reach a critical
size.
revs/s to rads/s
x 2pi
rpm to rad/s
x 2pi/60
Define fatigue
the process where repeated variations in loading cause failure even when the nominal stresses are below the material yield strength
fatigue is made up of crack initiation and subsequent crack growth as a result of cyclic, plastic deformation
For a concave radius use……
negative radius
