Mechanical Engineering Materials and Applications PDF

Lecture 1 Mechanical properties that will be studied: Stiffness, strength, toughness, hardness How materials can gradually degrade: Wear, creep, fatigue, Corrosion Mechanical forces - can cause the material to deform (change shape) and may even cause it to fail (break) Tensile Forces Te...

Lecture 1 Mechanical properties that will be studied: Stiffness, strength, toughness, hardness How materials can gradually degrade: Wear, creep, fatigue, Corrosion Mechanical forces - can cause the material to deform (change shape) and may even cause it to fail (break) Tensile Forces Tensile test (applying tensile stress) Take a sample of material Pull on the ends to stretch it Measure the force (F) Measure the stretch amount (elongation / ΔL) Tensile specimen Can be any shape or size with parallel sides / equal cross section Area of cross section is A0 Make ends bigger so it's easier to grip Tensile testing machine - one head is fixed and the other moves at a constant rate. Specimen undergoing tension - Necking until fracture Info from the tensile test: How much the sample trenches for a given applied force can be plotted: ○ Tensile force Vs Elongation  ○ This relationship is size and shape dependant ○ We need a different relationship so we can ignore the specific geometry and analyse the material better Normalising quantities: *very important quantities as they form the basis for most measurements of mechanical properties Engineering STRESS (σ or S) σ =F/A0 (N/m2 = Pa) Engineering STRAIN (ε or e) ε = ΔL/L0 (no units) Stress strain curve Different shapes in different materials As strain increases, stress can go up or down X = the breaking point of the sample Types of stress Tensile stress - applying a pulling force at right angles to the sample face. Compressive stress - pushing at right angles to the sample face - Shown by a negative value If the force acted not normal to the surface but at an angle to it we can resolve the force into two components ○ One normal to the face (Ft: tensile force) ○ One parallel to the normal of the face ( Fs: shear force) ○ Shear stress/strain Shear stress induces shear strain. If a cube shears sideways by amount ’w’ then shear strain is defined by where ‘𝜃’ is the angle of the shear and ‘l’ is the edge length of the cube. Shear stress is a stress that causes sliding (shear strain)  ○ Shear stress τ = F/A shear strain = γ = W/Lo = tan θ ≈ θ γ = W/Lo = tan θ ≈ θ ○ tan θ ≈ θ at for very small strains, so that's why γ = W/Lo ≈ 90 - θ* ○ (page 6 for shear) https://www.purdue.edu/freeform/me323/wp-content/uploads/sites/2/2022/01/03_shear_stress_strain.pdf Pressure When a solid is subjected to equal compression on all sides P=F/A Conventionally positive when compressive Strain due to pressure is a change in volume, called dilation (dilation) D = ΔV/V0 Any type of stress can be expressed as a mixture if these 3 forms: Tension, Shear and Compression Hydrostatic Pressure: acts on an object floating at a certain depth h in liquid density ρ. p = ρ.g.h + p0 Lecture 2 - Elastic Deformation in tensile, compressive and bending tests Tensile Testing standardisation ASTM 8 - American society for testing materials : ○ Guidelines for standarding tensile testing  ○ ASTM E8 for metals and alloys Strain rate (basic idea) As tensile testing is done at a constant velocity: constant strain rate ○ έ = change in strain (deformation) with respect to time. ○ έ = dε/dt Depending on the deformation speed, the tensile testing results can be different: Actual Tensile test measurement. The machine measures Force Vs Displacement Then it calculates the Stress & the Strain Stress/Strain graph regions Plastic region - permanent deformation until fracture point Elastic region - smaller than 1% of the plastic region, material can return to its original shape Yield point Above a certain stress σy the stress/strain line becomes flatter and curved Elastic region ends at the yield point, which At yield point, stress and strain are called: the yield stress σy and the yield strain εy Linear elastic deformation Straight line portion behaves like a spring, so we can apply Hooke’s Law ○ The Hooke’s Law: displacement is proportional to force. (x ∝ F), Strain is proportional to stress. (ε ∝ σ) ○ If you remove the stress, the strain goes back to zero  E = σ/ε E = Young’s Modulus = the slope of the graph at any given point in the elastic region. Unit of Young's Modulus : M/m2 = Pa Stiffness A material's ability to return to its original shape after an applied force is removed. Stiffness is directly proportional to the elastic modulus. For an element in tension or compression, the axial stiffness: Hooke’s Law ➔ σ=Eε (ε = strain = ΔL / L0) We usually measure E by applying tensile stress to the material. Most materials have the same E value in compression as tension. ➔ τ=Gγ If a material is under shear forces, then the shear strain is proportional to the shear stress. G is the shear modulus ➔ p = -K D (D= dilation = ΔV / V0) If the material is under 3D pressure, then the dilation is proportional to the pressure, and K is called the bulk modulus. The “-” is because of the the shrinkage in volume Poisson’s Ratio When you stretch something it gets longer & thinner ○ Inward strain perpendicular to tensile direction Poisson’s ratio: negative of the transverse (lateral) strain to the tensile (axial) strain. ν = - εtransverse / εtensile If the volume stays consistent, the transverse strain will be exactly half the tensile strain. For most materials 0912°C Fe has the FCC lattice and is called austenite. Carbon can dissolve up to 2.1% Mild steel is at 1000° All the C is in solid solution No precipitates Yield stress is not high Ferrite Fe at room temperature has the BCC lattice and is called ferrite. Carbon can dissolve only to 0.2% Mild steel at 25°  Two phases in the microstructure: Ferrite and cementite particles. Yield stress is higher than austenite Pearlite If the C content is exactly 0.76% (by weight), microstructure will have a striped appearance (lamellar structure). - Thin and flat layers of ferrite (Fe) and cementite (Fe3C) - Layers are less than 1μm thick (depends on speed the steel was cooled from high temperatures) - Yield stress is very high - The thinner the layers = the higher the yield stress Effect of carbon content If the C content of the steel ≅ 0.0%, the microstructure will be 100% ferrite If the C content of the steel = 0.76%, the microstructure will be 100% pearlite If the C content of the steel is < 0.76%, (usually the case) there will be a mix of ferite and pearlite in the microstructure If the C content of the steel is > 0.76% (rarely the case), there will be a little extra cementite together with pearlite in the microstructure. Cooling rate: How fast the material is cooled from a higher temp to a lower temp Furnace cooling: when a sample slowly cools down inside a furnace (many hours) Steel cooled from 1000°C after slow furnace cooling: - Large grains in the microstructure and thicker pearlite layers - Strength not very high Natural cooling = air cooling = Normalising: when steel cools down naturally in the air Steel cooled from 1000°C after Air cooling: - Smaller grains in the microstructure and pearlite layers - Strength is higher Quenching: Very rapid cooling from furnace temperature to lower temperatures ex: oil quenching or water quenching Steel cooled from 1000°C after quenching in water: - The iron lattice had to change from FCC to BCC very quickly = the atomic lattice is distorted. - Martensite = A metastable microstructure that is super strong. Martensite - Using an optical microscope there are no particular features (on this level you could usually see the grains and precipitates) - Using an electron microscope: Lots of needle-shaped / lenticular grains are visible. - These martensitic lenses / needles are very fine, much smaller than the plates in pearlite. Martensite phase has the smallest possible grain size in steel, so it has the highest yield strength and hardness and the lowest ductility and toughness. Tempering martensite. Heated up to a temp between 150°C and 700°C depending on steel type and application) Microstructural features (grains & precipitates) get bigger and rounder. The aim of tempering martensite: to improve toughness and ductility, but it will also reduce strength and hardness. We need to control the tempering temperature and duration to obtain a favourable microstructure and optimum mechanical properties for the specific application we have in mind. Summary - Different classes of steel: Low-carbon (mild steel), medium-carbon, high carbon, low- alloy, high-alloy - Seen that steel can have different microstructures depending on the chemical composition and heat treatment - Saw how pearlite and martensite have high strength due to their respective microstructures. - Covered many other important aspects of steel metallurgy Lecture 10 - Phase diagram of Fe - C Phase diagram Type of chart used for showing which phase(s) of a material exist or coexist at equilibrium under certain conditions (pressure, temp, composition) All information about different microstructures of steel (or other alloys) can be represented in graphical form on a phase diagram Phase diagrams are useful for understanding the effect of heat treatment and chemical composition on the microstructure of steel (and other alloys) Simple Binary Phase Diagram Phase Diagram of copper and nickel Temperature on the vertical-axis, composition on the horizontal-axis. The white between the liquidus and solidus lines is a mixture of both, the mush zone, Solidification of Cu-Ni alloys Micro-segregation: (1) Differences in the chemical composition within a grain.  (2) Non-uniform distribution of elements within a single grain. E.g. High nickel concentration at centre of the grain How to avoid? 1. Slower solidification rate 2. Stirring during solidification 3. Heat treatment after solidification (homogenising) Melting point of pure copper is 1085°C Melting point of pure nickel is 1455°C - For alloys of Cu and Ni, no matter the chemical composition of the alloy, it will be solid below these melting points and liquid above them Pure metals have a fixed solidification/melting point, but alloys solidify or melt over a range of temperature. Fe-C Phase diagram On the right side is Cementite at composition Fe = 6.7% carbon On left side is ferrite 0.022 % Carbon, further up is Austenite , which exists over a wide range of comp & temp Across the top is molten steel. 0.76, 727°C eutectoid point - pearlite structure forms when cooled through this point. eutectoid point: Has one phase above and two phases below. Remember for Phase diagrams 4 phases: Ferrite, Austenite, Cementite and the Liquid phase Anywhere else on the diagram - 2 phases (but no more than 2) ○ Pearlite is not a single phase, it is a mixture of ferrite and cementite  ○ Martensite is a phase, but not on the phase diagram as the diagram shows what happens at equilibrium, ○ Martensite is not at equilibrium -it only happened because we cooled the sample so fast. Fe-C Phase diagram from 0.0 to 2.0 C wt.% FOR EXAM