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# T207 Exam revision

### T207 Notes for 2018 exam

TermDefinition
Abrasive Wear Occurs between two surfaces that are in relative motion. The asperities of each surface gouge material out of the opposing surface as it moves and so over time is worn away. It is normally the softer that is gouged away and the harder remains intact.
Composite Body made of at least two different materials whose constituents have different properties which generates internal constraint (each material interferes locally with the other) when T changes.
Bernoulliâs principle Holds that for fluids in an ideal state, pressure and density are inversely related; in other words, a slow-moving fluid exerts more pressure than a fast-moving fluid.
Assumptions for Bernoulliâs principle 1) points 1 and 2 lie on a streamline, 2) the fluid has a constant density, 3) the flow is steady, 4) there is no friction
Block on an inclined plane tan(Î¸) = sin(Î¸)/cos(Î¸), Where, Î¼=tan(Î¸), Î¸=tan-1(Î¼), Draw a Triangle akin to the diagram to show all the working out. (Block 3, Part 2, page 118)(FullPDF page 582)
Reason for boundary lubrication During sliding contact it is the deformation and shearing of asperities that that give rise to frictional forces. There is contact between the asperities on the two surfaces, the lubricant adheres to the asperities, effectively cushioning them.
How Boundary lubrication works The lubricant adheres to the asperities. The sliding process now combines the deformation and shearing of asperities, with the shearing of the lubricant itself and since the latter requires less force, the overall effect is a reduction in friction.
Cell potential E(cell) = E(metalA)-E(metalB), Positive answer = Metal B corrodes, Negative answer = Metal A corrodes
Corrosion Requires all of the following: Anode - More Active Material, Cathode - Less Active Material, Current - Flow of Ions, Electrolyte - Host for Flow of Ions
Corrosion protection - Electroplating This process deposits a thin layer of metal on the object being protected. The object being protected is the negative terminal, surrounded by a solution of the ions of the metal being deposited on to the object. The negative charge attracts positive ions.
Corrosion protecting - Galvanising This is when the zinc layer stops oxygen/water or salt from attacking the iron. An advantage is that a scratch will still slow corrosion (as Zn is higher in the series).
Corrosion protection - Sacrificial Protection Works by taking a metal higher up the reactivity series and connecting to the iron/steel, it donates its electrons to any iron ions that might have formed, stopping it from corroding.
Creep requirements Requires three conditions to occur; time, heat and stress. Occurs when Homologous Temperature,Th>0.4 Tm Th=T/Tm
Creep definition A constant stress situation that is below yield. It is extremely prevalent in higher temperature conditions. Usually tested with a wire of a given size that has a constant load and the elongation is measured over time.
Creep Curve The rate of deformation is called the creep rate. It is the slope of the line in a Creep Strain vs. Time curve. (Block 5, Part 5, page 81)(FullPDF page 1231)
Elastic Collision In which both conservation of momentum and conservation of kinetic energy are observed. (Block 3, Part 2, page 96/97)(FullPDF page 560/561)
Factors Affecting Fracture in Metals Stress concentration, Speed of loading, Temperature, Thermal shock
Failure assessment diagram (FAD) Gives a 2-parameter approach to assessing a defect. It accounts for the possibility of fracture and plastic collapse separately. (Dinorwig Case Study, page 28) (full PDP page 780)
Failure - Cyclic Loading Stresses can lead to microscopic physical damage to the materials involved. Even at stresses well below the material's ultimate strength, this damage can accumulate until it develops into a crack or other damage that leads to failure of the component.
Failure - Ductile Characterised by tearing of metal and significant plastic deformation. Associated with overload of the structure or large discontinuities. Due to error in design, incorrect selection of materials,improper manufacturing technique and/or handling.
Fatigue Due to cyclic/alternating loading that can be at any stress level. The loading is repeated until the number of cycles imparted causes failure. This is expressed in S-N curves for particular materials.
First law of thermodynamics Energy cannot be created or destroyed in an isolated system.
Forces on an aircraft (Theory of flight) Weight = force of gravity acting down toward earth. Lift = right angle to direction of motion (differences in air pressure). Thrust = propels in the direction of motion. Drag = acts opposite direction of motion(friction and differences in air pressure).
Fracture - Brittle Rapid crack propagation, low energy, without plastic deform. Little or no plastic deform prior. Material imperfection, sharp corner, notches in component, fatigue crack. Cleavage or inter-granular. Assâd wâ nonmetals such as glass, ceramics, hard plas
Friction Force between two surfaces that are sliding, or trying to slide, across each other. As surfaces move, small imperfections move against each other, may form weak bonds-pulls on surfaces and slows them down. (Block 4, part 3, page 143) (fullPDF page 935)
Full film lubrication Completely separated by constant film of lubricant. No asperities in contact and only frictional resistance is that of the fluid's viscosity resisting shear deformation caused by the relative movement of the two surfaces. Example. Skis on snow.
Hammer forces description Mass is mass of hammer, as it REBOUNDS inlet velocity is the same magnitude as outlet velocity but in opp direction - so CHANGE of velocity is 2x inlet velocity (goes in at V1 in one direction, slowed to zero, accel back to the same mag in opp direction)
Hammer forces equations Impulse = change of momentum, Impulse = average force x contact time, change of momentum = momentum out - momentum in = (m V2 - m V1) = m (V2 - V1)
A Homogeneous Body - Non-Uniform Temperature Change Temperature change is different across the body. When the temperature change is rapid the material is said to suffer thermal shock. This is what cracks a glass bottle when boiling water is poured over it. E.g. window in cold climate.Warm inner, cold out.
A Homogeneous Body - Uniform Temperature Change Temperature change is constant across the body. If subject to external constraint and suffers this type of change in temperature can cause issues such as buckling in railway lines and cracking of welds.
Hydrodynamic Lubrication description A self-generating, self-sustaining form of full film lubrication where the two surfaces are separated by a film of lubrication and create a converging oil film in the form of a wedge. (Block 4, part 3, page 179) (fullPDF page 971)
Hydrodynamic lubrication advantages Very low friction (hydrodynamic means that there is a full film of oil between the bearing and race components) Lower wear and longer life than standard bearings. Should run cooler since there is less friction and mainly viscous loss to the oil.
Hydrodynamic lubrication disadvantages Hydrodynamic bearings require forced lubrication to maintain the full film. The correct viscosity of oil is required to avoid contact between metal pieces (temperature and load play into that). More costly than standard bearings
Inelastic collision When colliding objects stick together after the collision.
Innovation by context The creative process going on in the mind of the engineer when solving a problem, and can be anything from a clever change in the design of a computer program that allows it to run faster or use less memory, to something revolutionary like the jet engine.
Innovation by development. Changing the bit that doesn't work, or that could work better, to improve the function of the whole for reasons of cost, performance, ease of manufacture or competitive edge. Example - Black and Decker Workmate.
Motor effect (electric) Means that a simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves.
Petrovâs equation The amount of torque needed to keep a bearing turning at a given speed against viscosity resistance. Power loss, P, associated with this drag torque with the shaft speed, Ï, is given by: P=ÎÏ=(2ÏÎ·r^3 LÏ^2)/C_r, Hence power absorbed would be: P=ÎÏ
Power Absorbed (P) Associated with the drag torque at shaft speed, Ï. Has equation: P=ÎÏ
Pressure profile in a hydrodynamic bearing Differences in pressure around a bearing when static, starting and running. See (Block 4, part 3, page 180-181) (fullPDF page 972-971)
Routine solutions Configuration or reconfiguration of existing devices or components, without innovation, because something is broken or needs to be re-positioned, or there is simply a better way to do it. Example - Adjustment of Hubble Telescope mirror.
Second law of thermodynamics The entropy of any isolated system can never decrease. Remain constant in ideal cases where the system is in a steady state (equilibrium). In all spontaneous processes, the total entropy always increases and the process is irreversible.
Stress corrosion cracking description. This cracking characterised by cracks propagating either transgranularly or intergranularly (along grain boundaries). (AKA SCC). A precursor of this cracking is pitting corrosion, occurring if the stainless steel is not sufficiently resistant to pitting.
Stress Corrosion Cracking (SCC) factors This results from the combined action of three factors: 1- Tensile stresses in the material, 2- A corrosive medium, 3- The use of material susceptible to this.
Stress strain curve Plot of x axis=Strain against y axis=Stress. For different materials with different properties. See (Block 2, part 4, page 65/65) (fullPDF page 355/356)
Temperature Dependent Effects - Accelerating Plot of temperature against viscosity. Modelled in terms of the fraction of the particles that have much higher than average thermal energy. Example: Heat curable glues. (See fig 2.1-b in Block 1, part 2, page 84) (fullPDF page 84)
Temperature Dependent Effects - Gradual Plot of temperature against viscosity. Modelled in terms of the average thermal energy of the particles of the system. Example: Convection in a domestic water system. (See fig 2.1-a in Block 1, part 2, page 84) (fullPDF page 84)
Temperature Dependent Effects - Sudden Plot of temperature against viscosity. Modelled in terms of balance between disorder generated by vibration of atoms & order induced by inter-particle forces. Example: electric kettle auto-off. (See fig 2.1-c in Block 1, part 2, page 84) (fullPDF page 84)
Thermal efficiency The extent to which the energy added by heat is converted to work output.
Thermal stresses When there is a temperature change, but some constraint prevents expansion.
Third law of thermodynamics The entropy of a system approaches a constant value as the temperature approaches absolute zero.
Mechanisms of wear abrasion, adhesion, fatigue and oxidation
Zeroth law of thermodynamics If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.
Entropy The degree of disorder or uncertainty in a system
Torque/Speed graph - Low Power Plot of torque against rotational speed. Closer to stall speed point - low speed, high torque. OR closer to no load speed point - high speed, low torque. (Block 3, part 3, page 39) (fullPDF page 655)
Torque/Speed graph - Max Power Plot of torque against rotational speed. Midway between no load speed point and stall speed point. Medium speed and torque. (Block 3, part 3, page 39) (fullPDF page 655)
Change in thermal energy âQ = mcâT Where, m = Mass, c = Specific heat capacity of the material, âT = Change in temperature
Bernoulliâs equation Ev = p + 0.5Ïv^2 + Ïgh Where, Ev = Energy per unit volume, p = Pressure, Ï = Density, v = speed, g = Gravity, h = Height
Magnetic force equation F = BILsinÎ¸ Where, B = Magnetic flux density, I = Current, L = Length, Î¸ = Angle the current makes with the magnetic field
Frictional Force equation F = Î¼R Where, Î¼ = Coefficient of friction, R = Reaction force
Aerodynamic drag equation FD = (0.5Ïv^2)CD A, Where, FD = Drag force, Ï = Density, v = Speed, CD = Drag coefficient, A = Wing area
Aerodynamic lift equation FL = (0.5Ïv^2)CL A, Where, FL = Lift force, Ï = Density, v = Speed, CL = Lift coefficient, A = Wing area
Moment of inertia for Thick ring or thick-walled hollow cylinder I = 0.5m(Ro^2+ri^2), Where, m = Mass, Ro= Radius (Outer), ri = Radius (Inner)
Archand wear equation k = QH/W, where, k = Wear coefficient, Q = Measured volume of material worn per metre of sliding distance, H = Hardness of the softer surface, W = Normal load
Fracture Mechanics Equation K1= YÏâÏa, Where, Ï = Stress, a = Crack length, Y = E.g. Long edge crack in a finite-width plate etc.
Total Kinetic Energy (Ktotal), Sum of the translational and rotational kinetic energies. Ktotal = 0.5mv^2 + 0.5IÏ^2, Where, m = Mass, v = Speed, I = Moment of inertia, Ï = Angular speed
Mass flow rate equation m = ÏvA, Where, Ï = Density, v = Speed, A = Area
Electrical power Equation P = IV, Where, I = Current, V = Voltage
Power lost to heat (Ptherm) Ptherm = I^2R, Where, I = Current, R = Resistance
Volumetric Flow Rate (Q) Q=vA, Where, v = Speed, A = Area
Created by: Bucks884

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