Engineering Materials & Properties v1.1 PDF

Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Introduction to atomic structure of solids 1.3 Basic Structure and bonding of materials 1.4 Properties of materials 1.5 Mechanical properties 1  1.1 Ty...

Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Introduction to atomic structure of solids 1.3 Basic Structure and bonding of materials 1.4 Properties of materials 1.5 Mechanical properties 1  1.1 Types of Engineering Materials Engineering materials: Metals, polymers, ceramics & composites 1.1.1 Metal The earth’s crust is a major source of metals. Minerals are the inorganic materials present in the crust. In certain places, minerals contain a high percentage of a particular metal. 2  1.1 Types of Engineering Materials 1.1.1 Metal  crystalline in structure  opaque and lustrous  change geometrical shape permanently  under external force  good electrical and thermal conductivities 3  high strength, stiffness and ductility  two or more metals are combined together, an alloy is formed Examples: copper, steel, aluminium, gold, brass, bronze, super alloys Applications : car bodies, tin cans, conducting wires 4 1.1.2 Polymers  light, low electrical and thermal conductivities  resistant to atmospheric attack  low strength, not suitable for high temp. applications  Examples: PVC, rubber, nylon  long molecular chains, each chain being formed on the backbone of carbon atoms Notation Applications: carrier bags, plumbing pipes, hose, tyres 5 Polymer Repeat Unit Applications Polyethylene electrical wire insulation, (PE) flexible tubing, squeeze bottles Polypropylene carpet fibers, ropes, (PP) liquid containers (cups, buckets), pipes 6 1.1.3 Ceramics (oxide, nitride, boride, silicide & carbide)  combination of one or more metals with a non- metallic element such as oxygen, nitrogen or carbon hard, porous and brittle but with good thermal & insulating properties, heat resistant  very high compressive strength but poor plasticity Examples: refractory, glasses, concrete, silica Applications: glass wares, electrical insulators, cutting tools 7 1.1.4 Composites  made from two or more materials combined together, each with significantly different physical or chemical properties  within a composite material, one material will serve as a matrix to hold another material, the reinforcement Example : plywood is light and strong, cermets cutting tool is hard and abrasive-resistant 8 Example of Composites + Straw Mud = Brick 9 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Introduction to atomic structure of solids 1.3 Basic Structure and bonding of materials 1.4 Properties of materials 1.5 Mechanical properties 1 1.2.1 Overview of Atomic Structure An atom is the smallest unit of matter that retains all the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids. For example, water is composed of hydrogen and oxygen atoms that have combined to form water molecules. Many biological processes are devoted to breaking down molecules into their component atoms so they can be reassembled into a more useful molecule. 2 1.2.2 Atomic Particles Atoms consist of three basic particles: protons, electrons, and neutrons. The nucleus (center) of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost regions of the atom are called electron shells and contain the electrons (negatively charged). Atoms have different properties based on the arrangement and number of their basic particles. 3 1.2.2 Atomic Particles The hydrogen atom (H) contains only one proton, one electron, and no neutrons. This can be determined using the atomic number and the mass number of the element (see the concept on atomic numbers and mass numbers). Structure of an atom Elements, such as helium, depicted here, are made up of atoms. Atoms are made up of protons and neutrons located within the nucleus, with electrons in orbitals surrounding the nucleus. 4 1.2.3 Atomic Mass Protons and neutrons have approximately the same mass. Although similar in mass, protons are positively charged, while neutrons have no charge. Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its charge. Electrons are much smaller in mass than protons, about 1/1800 of an atomic mass unit. Therefore, they do not contribute much to an element’s overall atomic mass. When considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom’s mass based on the number of protons and neutrons alone. 5 1.2.3 Atomic Mass Electrons contribute greatly to the atom’s charge, as each electron has a negative charge equal to the positive charge of a proton. 6 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Introduction to atomic structure of solids 1.3 Basic Structure and bonding of materials 1.4 Properties of materials 1.5 Mechanical properties 1 1.3.1 What is " valence electrons " ? locate on the outer unfilled shell of an atom, referred to as the valence shell very loosely held when compared with the other electrons within an atomic structure responsible for the chemical behaviour (reaction) of the element with other element eg. Sodium atom displays one valence electron located in its outermost energy level Sodium 11 Na 2 1.3.2 Bonding of Atom Primary atomic bonds (strong bond) can be sub-divided into 3 classes: a) Ionic bonds b) Covalent bonds c) Metallic bonds a) What is “Ionic bonds" ? electron transfer from one atom to another to produce ions which are bonded together by coulombic forces (attraction between positively and negatively charged ions) 3 What is " ionization" ? An atom with few valence electrons is less stable and more likely to loose its valence electrons. This process is called ionization. An atom which loss its electrons acquired a positive electric charge, while an atom which gain electrons acquired a negative electric charge. Sodium chloride (NaCl) has a high degree of ionic bonding Na Cl 4  b) What is “Covalent bonds" ? Covalent bond takes place between atoms that are close to each other in the periodic table. In a single covalent bond, the 2 atoms contributes one electron each to form an electron bond pair. Multiple electron bond pairs can form by the atom with itself or with other atoms. + H2 H H i.e., hydrogen molecule 5 c) What is "metallic bonds" ? Metals normally have one, two or three valence electrons detached valence electrons tend to group together to form a negatively charged electron cloud This electron cloud floats freely among the metal ions and hold the metal atoms together. -ve charged electron cloud +ve charged metal ion 6  Due to the mutual repulsion between the positively charged metal ions, the metal ions assume a regular pattern of arrangement the attraction between the metallic ions and electron cloud causes the metallic ions to remain at certain fixed position this form of repulsion and attraction established a metallic bond 7 The metallic bond accounts for some properties : Example 1 : when a voltage is applied across a Copper crystal, current will flow through electron cloud that is moving constantly around the metal ions. Example 2 : when a external force is applied to a piece of Copper, it deforms without fracturing. This is because its atoms are arranged in a regular way that lets one row of atoms to slide over the other. This phenomenon account for ductility of a metal. Video1 : Ionic and Covalent Bonds (9.54mins) Video2 : Metallic Bond (4.40mins) 8 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Basic Structure and bonding of materials 1.3 Introduction to atomic structure of solids 1.4 Properties of materials 1.5 Mechanical properties 1  1.4 Properties of Materials In selecting a material for a particular application, match the properties of the materials to the required service condition(s) 1. Physical Properties density, melting point, thermal conductivity, thermal coefficient of expansion, electrical resistivity 2. Mechanical Properties strength, hardness, toughness, creep resistance, fatigue resistance 3. Technological Properties machinability, castability, forgeability, weldability, formability, brazeability 2  1.4 Properties of Materials 4. Chemical Properties resistance to oxidation, corrosion and solvents 5. Economical Properties cost of raw materials and manufacturing, time for fabrication, availability of materials 6. Aesthetic Properties appearance, surface texture and ability to accept special finishing How to remember different types of properties 3  1.4 Physical Properties Properties inherent to the nature of material a. Density ◼ Mass per unit volume (kg/m3) ◼ Atoms more closely packed, higher density b. Melting Point ◼ Temperature where solid changes to liquid (oC) ◼ Thermal activity of atoms exceed the strength of inter-atomic bonding (grain boundaries) c. Thermal Conductivity ◼ Ability to conduct heat (W/m K) 4  1.4 Mechanical Properties Mechanical properties refer to the characteristics of a material that are displayed when a force is applied to the material. The force or stress will cause the metal to have either elastic or plastic deformation. 5  Sub-categories of common mechanical properties: Mechanical Properties Strength Formability Rigidity Toughness Durability Modulus of % Elongation Impact Strength Hardness Tensile Strength Elasticity % Reduction in Wear Resistance Yield Point Area Fatigue Strength Compression Shear Creep Strength 6 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Introduction to atomic structure of solids 1.3 Basic Structure and bonding of materials 1.4 Properties of materials 1.5 Mechanical properties 1  1.5 Mechanical Properties 1.5.1 Tensile Properties 1.5.2 Malleability 1.5.3 Hardness 1.5.4 Impact Strength 2 1.5.1 Tensile Properties The tensile properties of a material can be extracted from the stress-strain curve of the material subjected to pulling force. Cast Iron Stress Glass Mild Steel or F Applied Force Aluminium Example of stress- F Plastic strain curves Strain (or Change in Length) 3 1.5.1 Tensile Properties a. Modulus of Elasticity b. Yield Stress / Proof Stress c. Tensile Strength d. Percentage Elongation e. Reduction in area 4 Tension / Compression Tension External force stretches the material Tensile Force Tensile Force Compression External force squeezes the material Gold is one of the most malleable metal and can be hammered into gold leaf. https://www.youtube.com/watc h?v=ONtjLtBBKXk Compressive Compressive Force Force 5 Engineering Stress F = Applied Load (N) Ao = original cross-sectional area (m2) Stress, σ (N/m2 or Pa) Force F Stress, σ = = Cross − Sectional Area A O 6  Engineering Strain Strain, ε (dimensionless ratio) F Change in Length Original Length ∆L Lo Lf At rest Pulled Final Length - Original Length L f - L o Strain, ε = = Original Length Lo 7  Work Example of Engineering Stress / Strain Recall.. Stress, σ = Force = F Cross − Sectional Area A O F Final Length - Original Length L f - L o Strain, ε = = Original Length Lo π d2 π 8.74 2 Io lf Cross section area = = = 60.0 mm2 4 4 7500 Stress, σ = = 125 N/mm2 F 60 Io = 40mm lf = 40.3mm Lf - Lo 40.3 - 40 Unloaded Loaded Strain, ε = = = 0.0075 Lo 40 8  Work Example of Engineering Stress / Strain Recall.. Stress, σ = Force = F Cross − Sectional Area A O F Final Length - Original Length L f - L o Strain, ε = = Original Length Lo π d2 π 8.74 2 Io lf Cross section area = = = 60.0 mm2 4 4 7500 Stress, σ = = 125 N/mm2 F 60 Io = 40mm lf = 40.3mm Lf - Lo 40.3 - 40 Unloaded Loaded Strain, ε = = = 0.0075 Lo 40 9 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Basic Structure and bonding of materials 1.3 Introduction to atomic structure of solids 1.4 Properties of materials 1.5 Mechanical properties 1 Tensile Test Tensile properties are obtained from a tensile test. Subject a test piece, round or flat in the x-section, to an increasing tensile force exerted by a tensile machine. Distance between shoulder Reduce section Gauge length Fillet or radius 2 Tensile Test F Test piece elongates X-section area reduces (localised deformation) Test piece fractures lf lo Tensile machine plots ¤ Strain or Change in Length ¤ Stress or Applied Force 3 A typical stress-strain curve Elastic Plastic C A B D Stress x Strain 4  Elastic Plastic C B Stress A D Proportional or Elastic Limit ( Until Point A) x Strain Linearity between stress & strain Elastic Deformation  Recovers to original shape if stress is removed 5  Elastic Plastic C B Stress A D Plastic Deformation or Yielding ( Beyond Point A) x Strain Permanent (Plastic) deformation occurs ¤ Material will retain its shape and size Point B - proof stress at a specific strain (i.e., x% strain) 6  Elastic Plastic C B Stress A D Maximum Tensile Stress (Point C) x Strain Fracture Stress (Point D) 7 a) Modulus of Elasticity, E (N/mm2 or MPa) Proportional limit (Point A) ratio of applied stress to strain within elastic limit σ N E= ( ) ΔL A ε mm 2 FA N σ= ( ) & ε= recall.. A O mm 2 Lo FA = the applied force at point A Ao = original cross-sectional area ∆LA = change in extension at point A Lo = original gauge length 8 b) Yield Stress (σy) & Proof Stress Some materials, notably mild steel, show a sudden yield with a marked discontinuity on the stress-strain curve. 9 Upper Yield Point (Point E) and material continues to deform to Lower Yield Point (Point F) Point E is normally used and is known as the yield point FE N Yield Stress σy = ( ) AO mm 2 where FE = applied force at point E. 10 Proof Stress In the event where there is no prominent indication of a yield point, Proof stress is used instead of yield stress Proof Stress is a value at a specific strain i.e., ¤ 0.2% strain B or 0.002 strain A Stress 0.2% Proof Stress FB N σ 0.2 = ( 2 ) AO mm 0.2% Strain where FB = applied force at point B. 11  Proof Stress Proof stress Stress or Applied Force x Strain or Change in Length Offset Value 12 c) Tensile Strength Point C Maximum load that the material can sustain Localized deformation (necking) until failure occurs FC N Tensile Strength σ TS = ( ) AO 2 mm where FC = maximum applied force 13 d) Percentage Elongation This parameter is used to measure the ductility of a material. Lf - Lo Percentage Elongation = x 100% Lo F lf 14 e) Percentage Reduction in Area This parameter is another measure of the ductility of a material. A f − Ao Percentage Reduction in Area = ×100% Ao 2 2 = d − D 2 × 100% D Ao Af F 15 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Basic Structure and bonding of materials 1.3 Introduction to atomic structure of solids 1.4 Properties of materials 1.5 Mechanical properties 1  1.5 Mechanical Properties 1.5.1 Tensile Properties 1.5.2 Malleability and Ductility 1.5.3 Hardness 1.5.4 Impact Strength 2 1.5.2 Malleability and Ductility Malleability ability of material to deform permanently without fracture due to compressive deformation i.e. stamping of gold coin. Ductility ability of material to deform permanently without fracture due to tensile deformation i.e. drawing of copper wire. 3 1.5.2 Malleability and Ductility Malleability allows a material to be deformed in all directions by hammering and pressing. Ductility allows a material to be stretched, twisted or bent without breaking. All ductile materials are malleable but not all malleable materials are necessarily ductile. For example, clay can be easily shaped but will break easily when stretched. 4 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Basic Structure and bonding of materials 1.3 Introduction to atomic structure of solids 1.4 Properties of materials 1.5 Mechanical properties 1  1.5 Mechanical Properties 1.5.1 Tensile Properties 1.5.2 Malleability 1.5.3 Hardness 1.5.4 Impact Strength 2 1.5.3 Hardness the ability of a material to resist plastic deformation, scratching or wear by another material. Diamond is the hardest known natural material. http://www.lifegem.com/ 3 1.5.3 Hardness Hardness is the ability of a material to resist scratching or wear. Hardness values: Brinell, Vickers, Rockwell, Knoop Brinell hardness Vickers hardness Rockwell hardness tester tester tester 4 a) Brinell Hardness Scale use a tungsten carbide ball(1 to 10mm) as the indenter with suitable load 1. Ball is pressed into the workpiece 2. Spherical indentation is produced d Where D - diameter of the ball (indenter) d - measured diameter of the indentation 5  Calculation of Brinell Hardness 1. measure the mean diameter of indentation (d) 2. calculate Brinell Hardness Number (BHN) by 2F BHN =   πD D − D 2 − d 2  F is in kg   2F BHN =  2 2  0.102πD  D − D − d  F is in N   BHN can be obtained from table, wrt size of ball, diameter of indentation, load applied 6  accuracy of BH test depend on the applied load load F for different materials: F 2 D Material F/D2 Steels and Cast Irons 30 Copper alloys & aluminium alloys 10 Pure copper & aluminium 5 Lead, Tin & their alloys 1-2 7 Advantages : Applicable to both ferrous and non-ferrous materials Large contact area to provide accurate reading Results are accepted by industry Imperfections of the materials do not affect the readings Limitations : Not suitable for harder materials > 500HB, may damage indenter Large indentation is left after the test Thickness of the test material => dia. of indenter Surface of test materials must be flat and smooth when small dia. indenter is used 8 b) Vickers Hardness Scale use a diamond pyramid with an included tip angle of 136o Where d = diagonal length of the indentation 9  Calculation of Vickers Hardness 1. measure the mean diagonal of the indentation, 2. Vickers hardness number (VHN) can be calculated: HV = 0. 189 F F is in N d 2 HV = 2 F sin 68 o = 0.189 F × 9.8 F is in kg d 2 d 2 10 Advantages : square indentation is geometrically similar hardness value obtained is independent of the magnitude of applied force small indentation mark allows the test to be done on finish or thin components can test on a wide range of materials under varying loads can test on curved surfaces by applying correction factor Limitations : smooth surface is needed in order to focus the edges of the indentation sensitive to imperfections in materials when small load is used 11 c) Rockwell hardness scale measurement is made on the depth of indentation use hardened steel ball: 100 kg load for HRB use diamond or tungsten carbide cone: 150 kg load for HRC HRN & HRT : smaller load for thin workpiece Rockwell hardness = E – e where E is a constant 12  Rockwell hardness 1. Apply minor load of 10kg (eliminate problem of surface roughness) 2. after which the major load is applied 3. release the major load & with the minor load applied, hardness number is read from indicator directly 4. hardness number correspond to the different in depth between the initial and final loading 13 Advantages : quick, simple with direct reading of hardness value reading can be analog or digital types initial minor load avoids errors due to uneven surface different scales allow measurement of variety of material hardness Limitations : difficult to convert into Brinell or Vickers scale require to use appropriate scale to express the correct hardness value heavy indentation may damage the surface of specimen 14 Topic 1 Engineering Materials and Properties 1.1 Types of engineering materials 1.2 Basic Structure and bonding of materials 1.3 Introduction to atomic structure of solids 1.4 Properties of materials 1.5 Mechanical properties 1  1.5 Mechanical Properties 1.5.1 Tensile Properties 1.5.2 Malleability 1.5.3 Hardness 1.5.4 Impact Strength 2 1.5.4 Impact Strength the ability of a material to withstand shock loading and indicates the toughness of a material. Toughness is the resistance of a material to fracture or break. Tough material absorbs more energy when fractured Brittle material absorbs less energy Impact absorbed energy strength = effective area ( Joule/mm2 ) 3  Comparison between Izod and Charpy Impact Tests Izod impact test Charpy impact test stored 135-160J of stored 150-300J of potential energy potential energy specimen is held in a specimen is supported at cantilever manner both ends V notch faces the striker (V or U) notch faces away striker hits the top of the from the striker test piece striker hits the center of the test piece 4  visual examination of fractured surface provide some useful information: Half-portion of the specimen notched section effective area a) Brittle material - sudden break with little evidence of deformation showing granular texture. b) Ductile material - rough and fibrous fractured surfaces. Fracturing may not be completed. c) Ductile-brittle material - mixed mode of fractured surfaces. 5  Appearance of fracture surface Ductile - fracture surface is Brittle - fracture surface is fibrous, plastic flow of bright and sparkling, crystalline structure crystals not plastically deformed 0% 50% 90% 100% Crystalline area 6 Examples of fractured specimens Extracted from - Manchester Materials Science Centre, UMIST and University of Manchester. Low Carbon Steel - A transition from brittle to ductile failure occurs as the test temperature increases. Copper is a tough material and shows no ductile to brittle transition. Austenitic Stainless Steel has very high toughness. The failure is ductile at all temperatures. 7