DM308 Production Techniques 2 Lecture 1 - Structural Properties of Materials PDF

DM308 Production Techniques 2 Lecture 1 – Structural properties of materials Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Introduction Course outline Lecture 1 – Structural pr...

DM308 Production Techniques 2 Lecture 1 – Structural properties of materials Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Introduction Course outline Lecture 1 – Structural properties of materials Lecture 2 – Crystals, defects and interfaces Lecture 3 – Phases and microstructures Lecture 4 – Properties and processing of ceramics Lecture 5 – Properties and processing of glasses Lecture 6 – Properties and processing of light alloys Lecture 7 – Properties and processing of high-temperature alloys Lecture 8 - Properties and processing of composites Useful texts Useful books: 1. “Materials science and engineering” – Callister 2. “Engineering materials” – Ashby and Jones 3. “Materials: Engineering, Science, Processing and Design” – Ashby and Shercliff 4. “Introduction to dislocations” – Hull and Bacon 5. “Phase transformations in metals and alloys” – Porter and Easterling 6. “Light alloys” – Polmear 7. “Superalloys” – Reed 8. “Introduction to composite materials” – Clyne and Hull 9. “Composite Materials and Structures for Engineering Students” - Grove Course assessment • 1 written exam (2 questions set by me) - if still social distancing – exam will be online and open book • 2 pieces of coursework (one of which is set by me) • The coursework format is TBC (likely to be essay or group presentation) • The coursework submission is due by the end of Week 7 • Coursework topic will not be in the exam (= less revision). Please use the MyPlace forum If you have any questions about the lecture content or assessment for my part of DM308: • Do not email the lecturer directly • Post your question to the Discussion Forum under Section 1 • This will help fellow students who have a similar query • Check the forum for existing answers to your questions For general module enquiries, please get in touch with the module registrar – Prof. Yi Qin Materials engineering recap. Materials in an ATM Which materials would you use to build the ATM components? Why would you chose them? FRONT BACK Illuminated sign LCD screen CPU Printer Network adapter Pin entry device Rear service LCD Card reader Cash dispenser Cash cassettes Cabinet What is materials engineering? Materials Science and Engineering is a hybrid discipline specialising in the development of new materials through a holistic understanding of their behaviour: 1. How structure of materials controls their properties 2. How processing determines the material structure 3. How properties determine performance in a given application Central to the discipline is the scientific study of materials using experimental and theoretical techniques, known as characterisation Why is it important? Make the impossible possible Prevent things from going wrong Failed solder joint Material families Metals and Alloys • • • • metallic bonding crystalline conducting mostly break in a ductile manner Ceramics • • • • ionic or strong covalent bonding crystalline insulating or semi-conducting break in a brittle manner Glasses Polymers • • • • covalent bonding non or partly-crystalline mostly insulating can be brittle or plastic • • • • covalent bonding non-crystalline insulating break in a brittle manner Composite materials Metal matrix composites Ceramic matrix composites ` • • • • • MMCs Matrix is ductile Matrix is strong in tension Typically reinforced with ceramics Electrically conducting • • • • Polymer matrix composites • • • • PMCs Matrix is very tough Matrix can have low density Typically reinforced with ceramics CMCs Matrix is hard and wear resistant Matrix has thermal resistance Reinforced with ceramics/metals Natural composites • • • • Nature is smart Typically PMCs Parts of plants and animals The original engineering materials Material properties Engineers have less control over: • Density • Cost • Scarcity • Toxicity Engineers have more control over: • Structural properties (we focus on these for this course) • Functional properties Functional properties • • • • • • • • • Dielectric constant Refractive index Resistivity Piezoelectric constant Magnetic permeability Ferromagnetism Ferroelectricity Piezoelectricity Thermal conductivity N.B. Functional properties will not be covered in this course. Structural properties • • • • • • • • • Stiffness Strength Hardness Ductility Fracture Toughness Wear resistance Environmental Resistance Thermal expansion Thermal shock resistance Structural properties of materials Part 1 – Revisiting Hooke’s law Revisiting Hooke’s law Remember the spring constant experiment from school? The force F is proportional to the extension x, related by a proportionality constant k, which is a property of the spring. It would be more useful to derive a version of Hooke’s law that is independent of the spring shape and only depends on the spring material. See next slide. Stress and strain Stress F = applied force A0 = initial cross-section area Strain l = length l0 = initial length Tension and compression • In tension, • In compression, so the strain and is +ve is –ve • By convention tensile stresses are +ve and compressive stresses are –ve • Thus the modulus E is always positive Stress-strain testing Let us look at a typical uniaxial testing machine. These can either be hydraulically actuated or driven by a servo motor, like the one shown below. Column (houses electrical lead screw drive or hydraulic actuator) Crosshead (moves up and down) Grips (hold specimen in place and transmit force to it) Controller/Computer (controls test and logs data) Load cell (measures applied force) Material specimen (the thing you test) Measuring strain accurately We need to measure the material’s extension accurately to calculate the strain. Using the crosshead position is inaccurate due to slack and elasticity of the system. So the extension must be measured directly from the deforming specimen gauge. strain gauge crosshead Measuring strain electrically A circuit known as a Wheatstone bridge offers a possible solution. Strain gauge www.doitpoms.ac.uk Extensometer Measuring local strain optically Sometimes it is important to know how strain is localised in complex parts. Digital image correlation (DIC) is a contactless method for measuring strain. A computer is used to analyse a speckled pattern painted on the material surface. Achilles tendon Speckle pattern applied Strain mapping Luyckx et al., Journal of Experimental Orthopaedics, 2014, 1:7 Stress vs. strain in brittle materials Ideal brittle materials exhibit purely elastic behaviour before fracture. Elastic behaviour is when all of the strain is recovered when the material is unloaded. In the elastic regime there is a linear dependence between stress and strain. (i.e. Hooke’s Law) The proportionality constant, or elastic gradient, is known as the elastic modulus. (Young’s modulus). The stress at which the material fractures is known as the tensile, compressive or shear strength depending on the deformation mode. Stress vs. strain in ductile materials Consider a ductile material deformed in tension. Up to the elastic limit, the deformation is linear and reversible. (elastic) If stress is increased further, deformation is non-linear and permanent. (plastic) The transition stress between elastic and plastic behaviour is known as the yield strength . The maximum stress sustained by the material is the Ultimate Tensile Strength, . Real stress vs. strain curves The Young’s modulus and yield stress are not easy to determine. Typically one takes the overall gradient up to a certain stress to obtain E. Using this gradient, the intersection with graph at 0.2% strain is usually taken for . Elasticity and bond energy The elastic modulus is actually related to the strength of the bonds between the atoms/ions in a material. Below we see a binding energy curve between two atoms. (spring analogy) repulsion attraction The curve is a superposition of energies arising from the repulsive and attractive forces. These forces are related to the distance between the atoms. U0 is the bond energy and r0 is the equilibrium bond length. Shear In addition to tension and compression there is a third mode of deformation known as shear. Shear strain Shear stress F = applied traction force A = area subjected to traction Shear modulus Different stress scenarios simple tension simple compression biaxial tension pure shear hydrostatic pressure Ashby and Jones What are the stress states? 3D stress and strain We can define matrices to fully describe the stress-strain state in a material: 3D Hooke’s Law i.e. tensor summation notation This means that elasticity is a complex directional property. Finite element analysis The 3D description of Hooke’s law can be used in conjunction with Finite Element Analysis to investigate stresses within complex components and assemblies. Stress concentration in turbocharger impeller (red-yellow areas) Stresses in a pressure vessel (red ≥ yield stress) Structural properties of materials Part 2 – Beyond Hooke’s law Hardness Hardness is the ability of a material to resist scratching or indentation. In ductile materials the hardness correlates with the tensile yield strength. Thus a hardness test is a cheaper, less wasteful method to measure YS. Several tests exist, but the Vickers Hardness Test is the most widely used. Diamond indenter Diamond Pyramid Hardness indent in polycarbonate Specimen Vickers Harndess Number: ebatco.com Toughness Toughness = ability of a material to absorb energy during deformation. The energy per unit volume stored or dissipated by a material is given by the area under the stress vs. strain graph. Stored elastic component and in 3D But toughness is the total energy Measuring fracture toughness The Charpy impact test way is a method of comparing the fracture toughness of different materials. It relies on standardised test specimens with a machined notch, that are broken by the swing of a pendulum hammer. End of Swing Material Specimen Impact Hammer Anvil Specimen Start of Swing Specimen Scale Ductile vs brittle Charpy ductile ductile brittle brittle hevvypumps.com hevvypumps.com` Ductile to brittle transition Some materials may undergo a ductile-to-brittle transition as the temperature is changed. The transition temperature is abbreviated DBTT. It is important to take this into account during design and manufacturing stages, carefully considering all possible end use scenarios. brittle Case study: Titanic The low-grade iron rivets were used in the bow of the ship. The impurities and slags caused them to become brittle in the icy waters of the Atlantic. This led to the catastrophic hull tear during the iceberg collision and ultimately the loss of the ship and death of 1,517 people. Case study: Liberty ships The ships featured a welded (rather than riveted) hull to aid massproduction during WW2. Welds underwent a ductile-brittle transition at low temperatures causing ships to break apart mid-ocean. Cyclic loading In some cases a component may be subjected to cyclic loading. This may place additional demands on the material selected for manufacture. tension-compression tension-tension The material may fail even if cycled within the elastic limit. If this is not taken into account, the consequences can be disastrous. Case study: De Havilland Comet Stress concentration at window corners led to catastrophic crack propagation and loss of three aircraft. Case study: Aloha Airlines 234 1988 1978 Stress concentration at rivet holes exacerbated by high number of flight cycles caused fuselage panel to unzip mid-flight. Stress intensity factors Stress intensity factors K tell us how much the effective stress is increased at the tip of a crack or initiation site. They depend strongly on the following: 1. Crack opening mode (I, II or III) 2. Crack geometry 3. Specimen/material geometry A critical stress intensity factor (e.g. KIC) is the value at which the specimen is fully fractured due to the propagation of the crack. For Mode I, the stress intensity at an internal crack in an infinite plate is given by: (Y = correction factor) The critical strain energy release rate is a good measure of fracture toughness: Fatigue and (S-N curves) Fatigue is the damage done to a material by a cyclic load and can result in catastrophic failure, as we have seen on the previous slide. The S-N curve shows how the number of cycles to failure changes with the stress amplitude. Below a certain amplitude (fatigue limit) the material will have an infinite service life. HCF and LCF High cycle fatigue (HCF) 1. High frequency 2. Low stress amplitude 3. Elastic deformation 4. Large no. cycles to failure We can plot the log of strain amplitude vs. the log of no. of cycles to failure Nf Low cycle fatigue (LCF) 1. Low frequency 2. High stress amplitude 3. Some plasticity 4. Low no. cycles to failure There is no district boundary between HCF and LCF behavior, and the transition occurs depending on the ductility of the material. It is usually in the region of 103 cycles, but can this can vary substantially. Fatigue crack growth rate It is important to predict the rate at which a crack grows during load cycling. A crack that does not lead to fracture is known as a sub-critical. The Paris’ law model predicts that a sub critical crack growth rate exhibits a power law relationship with the stress intensity factor range. The constants C and m depend on the properties of the material, environment and stress ratio. Creep Creep is when a material loaded within its elastic limit exhibits plastic deformation over a long period of time . Creep becomes particularly problematic at higher temperatures. ε Rupture burst steam pipe failed turbine blades in a jet engine = const. = creep rate reep rate minimum c Primary tr atsb.gov.au bearinc.com I εr III II Secondary (Steady State) Tertiary log t Stages of creep Incubation period – no measurable deformation is observed as the defects needed for plastic deformation (called dislocations) to occur must first be generated in sufficient numbers. Not all materials exhibit an incubation stage. Primary creep – defect density continues to increase and an initially high rate of deformation is observed. The deformation rate slowly degreases as the dislocations defects entangle and slow down. Secondary creep – the rate of dislocation entanglement is the same as the rate of dislocation generation and a steady deformation rate is observed for a while. Tertiary creep – when the material reaches a certain level of plastic deformation new types defects (voids) start to form, grow and coalesce. The deformation rate begins to increase progressively. Failure – finally the material is unable to sustain more deformatio and breaks. Plastic deformation and time How much plasticity a material exhibits usually depends on the strain rate. Given enough time and sufficient force even brittle materials can flow. At the same time, very rapid deformation can limits the extent of plasticity. Geological flow Creep Lab test Very high strain rate 10-8-10-6 s-1 10-4-102 s-1 <10-14 s-1 Shock loading 10 -10 s 3 6 -1 >106 s-1 Materials and the environment One important aspect of materials design and selection is the need to consider the operating environment. (How is the material protected?) 1. Will the material react chemically with its environment ? (e.g. corrosion, oxidation, chemical attack, dissolution) 2. Will the temperature of the environment affect the material properties? (e.g. high temperature oxidation and corrosion, DBTT, glass-rubber transition, creep) 3. Will the material be affected by sudden changes in the environment? (e.g. thermal shock) Stress corrosion crack of titanium rocket fuel tank Oxidation of aero engine turbine blades Thermal shock failure of ceramics E.g. electronics are no exception Corrosion is a problem Thermal shock testing of electronics Gold plating to prevent oxidation of contacts Wrapping up Summary • Materials properties are and important aspect of design • Properties are categorised as: Structural or Functional • You should now understand what the different structural properties mean for a material • Structural properties define the operational limits of a material • If the limits are exceeded the materials fail • Fatigue properties are important if there’s cyclic loading • Over time, creep may be an issue in ductile materials • It is important to consider the operating environment when selecting materials (i.e. risk of oxidation, corrosion, thermal degradation)