Basics Flashcards
Define load, deflection, stiffness, torsional stiffness and compliance
Load - force that acts in a structure
Deflection - response of a structure due to a load (deformation)
Stiffness = load/deflection
Torsional stiffness = torque/angular deflection
Compliance - a structure with low stiffness
Explain poisons effect
As you pull a material it will extend in length and decrease in cross-area
When you compact a material it will decrease in length but increase in cross-area
AoLo = A1L1 (as volume is constant)
How is poisons effect removed?
Use stress and strain instead
Describe shear, torsion and torque
Stress acting in opposite directions can cause shear
Shear modulus = shear stress/shear strain
Torque - force acting away from centre, T = Fr
Torsion - twisting force on material (usually tube)
Draw cantilever bending point loaded and cantilever bending uniformally loaded
Point Loaded - One end is fixed, other has load applied
Uniformly - fixed at both ends, load through out = bending in middle
Define the area moment of inertia
Resistance to deflection from a shape
Rod - Ix = Iy = 0.25πr^4
Tube - Ix = Iy = 0.25π(r2^4-r1^4)
Column - Ix = Iy = (b.h^3)/12
How do asymmetric shapes affects inertia?
Stiffer in wider axis = higher inertia
Describe a tensile test (inc measurements needed)
Measure gauge length, and area of specimen, put on 50mm gauge markers, clamp one end into grip - move machine to grip other end in
Constant load increase - measure load vs extension then plot a stress vs strain graph - will be for engineering stress and strain (assumes area and length constant) as necking occurs
How do you reliably find Ym of a material?
Strain gauge - when conductors are strained their resistance increases (can be used to give strain), only five measurements for localised strain (2mm)
Extensometers - measure accurate extension of the material and general strain
Why is a tensile test not an accurate way of measuring the Young’s modulus?
Machine elastically deforms as well so not reliable values, must use extensometer or strain gauge
What values can be calculated from a tensile test?
UTS - highest stress material can withstand
Yield point - when plastic deformation begins
Young’s modulus - gradient of elastic deformation (only estimate)
Elongation to failure = Δx/x - how long material extends before failure
What’s the difference between 0.2% proof stress and yield stress?
0.2% proof stress is used when yield point is not obvious, and is found using a tangent at 0.2% strain on stress vs strain graph
Define torsional stiffness
Torque per angular displacement
Define poisons ratio and it’s significance
Poisons ratio = -Δεx/Δεy
Where x is strain normal to stress axis, y is strain in stress axis
A poison ratio of 0.5 will conserve volume under load (metals have a ratio of roughly 0.3)
Define the bulk modulus
The compressibility of a material
K = Young’s modulus/3 - 6.poisons ratio
What happens to the modulus of metallic alloys in impacts?
Modulus increases with strain rates - quick strain rates = higher modulus
What are the error sources for a tensile test?
Misshapen specimens Incorrect alignment (shear occurs) Poor surface finish (early onset necking) Poor gripping Internal defects (cause weaknesses)
How do you measure the Young’s modulus of brittle materials?
Can’t use tensile tests as grips damage material
Would use 3/4 point bending tests (more accurate)
Test gives load vs extension which can be converted to stress vs strain graph
Why is hardness testing done?
Most testing is destructive (can’t use material again) but some components need testing before they go into service - hardness testing is used
Describe the 3 types of hardness testing
Brunel test - small spherical indenter is pressed into material, indent size = Brunel hardness
Rockwell scale - pyramid diamond used instead as less damage to material
Vickers teat - same process as Rockwell but Hv = 1.854load/mean length^2
What are common errors and things to avoid in hardness testing?
operator judgment of indent can be wrong, surface should be strain free, at least 5 indents to measure hardness, causing work gardening around indent = indents should be 5.diameters apart, indents should be 3d in from edge (strain field only in material), leaves small work hardened areas in material
Describe the difference between macro and micro hardness tests
Micro hardness tests test coatings/surface layers = reduced force needed = elastic recovery = micro stress appearing > macro stress
Indent load = resistance to penetration . d^metals resistance to strain
What factors need to be considered for material selection?
Material properties (Tm, Ym, strength) Fixed parameters (length/size) Variable parameters (mass, area etc) Then corrosion resistance & processing requirements
Describe the process of developing a performance index
Draw loading situation and material shape
Work out lowest strength deformation mode
Write out fixed & variable parameters and what needs to be max/minimised
Eliminate variable parameters from equations
Simplify and separate material properties - select material
How does the shape of a component affect material selection?
Different shapes have different failure modes and therefore different material properties are needed
Describe the structure and workings of a blast furnace
- iron ore, coke and limestone loaded into top of furnace, CO reacts with FeO to leave Fe
- limestone reacts with impurities and excess oxygen to form slag layer but Fe has highest density so pig iron is tapped off at bottom periodically (bottom of furnace is hottest)
How is pig iron turned into steel?
Pig iron still contains many impurities
- desulphurisation (Mn added to replace S in iron)
- O slowly added to remove C from iron
- P is removed
- iron is degassed
- Alloying elements added to give specific composition
Compare basic oxygen furnace to electric arc furnace for steel making
BOF - 800tonnes/hour, 50% scrap steel & 50% pig iron
EAF - can use 100% scrap but higher energy requirements
Describe aluminium making process
Bayer process:
- Al found in bauxite (also contains iron)
- heated in furnace, iron rich material removed as ‘red mud’
- Refined using electrolysis (@1000°)- carbon anode, graphite cathode, Al and slag layer in between (Al tapped off)
How can metal production process reduce energy requirements
- scrap reduces energy requirements and requires less contaminants to be removed
- insulating furnace
- using more efficient heating methods
Describe the alloying process and the benefits associated
Carried out during liquid phase of metals to get complete diffusion, but solidification can cause segregation
Alloying increase Tm, Ym and UTS due to disrupting regular layers of metal (also decreases ductility)
What are the two different casting routes and detail some methods
Shapes casting (final shape): sand, die, centrifugal, investment casting Semi-finished casting: continuous and ingot (then need processing into final shape)
Describe alloy solidification
When cooling rate < diffusion rate = homogeneous structure
When cooling rate > diffusion = heterogenous structure = worse properties
Higher cooling rate = cheaper production = standard production method
Explain segregation profiles of an alloy that’s been moulded
Solidification is most likely at an interface (as reduced energy requirements) means that will start at mould wall, fast cooling rate = dendritic growth and solute is pushed out from dendrite as solidifies
Means that centre of mould has diff comp to outside = segregation
Describe and explain the microstructure of ingot casted material
Chill zone (nucleation points on mould wall), columnar zone region (dendrites have grown), uneaxial zone (secondary dendrite arms swept into melt pool and act as nucleation points = solidification in centre = stops dendrite growth)
What grain shape is more beneficial and how is this achieved?
Short and random grains = homogenous = good mechanical properties but not ductile (long grains opposite)
Increased equiaxed zone by slower cooling, inoculation (adding ceramic nucleation points), ultrasonic vibration (breaks off dendritic arms) and lower super heat (material less heat to lose before solidifying)
Explain gravity segregation
Denser liquid sinks through casting = higher solute at bottom, or denser dendrites move through liquid = increased grain size
Stopped by insulating to stop reface solidification
Explain ‘A’ and ‘V’ segregation
Dendrites follow flow patterns through material and are re-melted/broken by rich liquid, Liquid then gets trapped and sets up melt pool = solute rich areas when solidify
Stopped by preventing flow patterns from establishing
Explain shrinkage
Upon solidification elements typically shrink by 3-5% (water/ice exception), exaggerated by solid emitting gas that was held in liquid (gas evolution)
Explain the problems with gas evolution in solidifying material
Gas bubbles form at nucleation points, then rise due to low density, fast solidification can form solid around bubbles causing porosity in final material
Stopped by: slow solidification or degassing liquid, pores can be removed from hot working processes
Explain rate of solidification wanted to avoid/limit gas evolution problems
Very slow - so gas has time to rise to surface and escape
Or extremely fast so that larger bubbles don’t have time to form = less porosity
Medium cooling is worst for porosity and thus properties
Describe plastic deformation
Atoms significantly displaced = no recovery as bond broken and new ones made = high energy needed
Defects lower energy needed by decreasing bonds
Plastic deformation occurs when dislocations move = energy above CRSS, lower CRSS = lower properties
Define a slip system and it’s relevance for plastic deformation
Any combination of plane and direction
Slip system with lowest activation energy taken as active slip system, closer packed the system = lower activation energy (CRSS needed in that direction)
5 systems must be active in order to have dislocation movement
Draw and describe an edge dislocation
Extra half plane of atoms which alters local spacing around it, causing a compressive and tension strain field around it = reduced CRSS as less bonds need to be broken
Explain work hardening
Dislocations move = strain fields interact with eachother forming locks and Jogs = more energy required to overcome, significant in FCC as high dislocation density
Only occurs < 0.6Tm, above 0.6Tm annealing occurs which cancels out
Explain solid solution strengthening
To form solid solution atoms must have similar size, structure and valence e-, but minute size difference causes strain fields - interact with dislocation strain fields = increased strength
Explain interstitial atom strengthening
Atoms much smaller that original atoms so can fit in interstitial sites but causes strain field (interacts with dislocation)
However, interstitial can move toward dislocation and cancel our strain field - energy needed to reestablish field for further dislocation movement
Explain precipitate strengthening
More solute added than solubility = excess = precipitates forming during solidification, slow cooling = large precipitate = reduced strength
Precipitate impeded dislocation movement as strain field around it
If coherent = cut by dislocation, if in coherent = dislocation lengthens to move around (both = inc energy)
What is meant by coherent particles?
Slip plane is continuous/same alignment
How does grain size affect material strength?
Slip planes are incoherent across grain boundaries = dislocations stopped and pile up until strain field affects dislocation in next grain = more energy needed to move dislocation through grains = higher strength if fine grains as more grain boundaries present
Describe phase strengthening
Different phases act like large precipitates = incoherent so dislocation lengthens = increased energy needed = increase strength
Going through eutectic point = maximum phase strengthening as max α and β present
