Module 2 Selection of Materials and Shapes. IIT, Bombay

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1 Module 2 Selection of Materials and Shapes

2 Lecture 1 Physical and Mechanical Properties of Engineering Materials

3 Instructional objectives At the of this lecture, the student should be able to appreciate (a) general classification of engineering materials, and (b) physical and mechanical properties of engineering materials Engineering Materials Materials play an important role in the construction and manufacturing of various parts and components. An appropriate selection of a material for a given application adds to economy, working and life of the final part and component. Classification of Engineering Materials Engineering materials can be broadly classified as metals such as iron, copper, aluminum and their alloys, and non-metals such as ceramics (e.g. alumina and silica carbide), polymers (e.g. polyvinyle chloride or PVC), natural materials (e.g. wood, cotton, flax, etc.), composites (e.g. carbon fibre reinforced polymer or CFRP, glass fibre reinforced polymer or GFRP, metal matrix composites or MMC, Concrete, Ceramic matrix composites, Engineering wood such as plywood, oriented strand board, wood plastic composite etc.) and foams. Properties of Engineering Materials Material property is the identity of material, which describes its state (physical, chemical) and behavior under different conditions. The material properties can be broadly categorized as physical, chemical, mechanical and thermal. The physical properties define the physical state of material and are independent of its chemical nature. The physical properties of engineering materials include appearance, texture, mass, density, Melting point, boiling point, viscosity, etc. The chemical properties describe the reactivity of a material and are always mentioned in terms of the rate at which the material changes its chemical identity, e.g. corrosion rate, oxidation rate, etc. The mechanical properties describe the resistance against deformation, in particular, under static and dynamic mechanical

4 loading condition. The mechanical properties include elastic modulus, Poisson s ratio, yield strength and ultimate tensile strength, hardness and toughness, etc. The thermal properties describe material behavior under thermal loading and include thermal conductivity, specific heat, thermal diffusivity, coefficient of thermal expansion, etc. For a given application or service, an engineering material is selected based on a set of appropriate material properties, often referred to as attributes, that would be requisite to sustain various expected loads. Figure depicts a schematic representation of material family, which is utilized in selection of materials for a target application. Figure Organized classification of materials and properties [1] Physical Properties Physical properties describe the state of material, which is observable or measurable. Color, texture, density, melting point, boiling point, etc. are some of the commonly known physical properties. Color: Represents reflective properties of substance Density: Amount of mass contained by unit volume of material. The higher the density the heavier is the substance. (SI unit: kg/m 3 )

5 Melting point: Melting point is the temperature at which material changes its state from solid to liquid. (SI units: K) Boiling point: Boiling point is the temperature at which material changes its state from liquid to gaseous. (SI units: K) Chemical Properties Chemical properties are the measure of reactivity of a material in the presence of another substance or environment which imposes change in the material composition. These properties are always mentioned in term of the rate of change in its composition. Corrosion rate, oxidation rate, etc. are some of the chemical properties of material. Corrosion rate: Corrosion rate is measured in terms of corrosion penetration for given period of time at specific surrounding condition. Corrosion rate is given by length of penetration per unit time. (Units: mm/year) Oxidation rate: Oxidation rate is measured in terms of amount of material consumed forming oxide or amount of oxide scale formed for given period of time at specific surrounding temperature. Oxidation rate is given by amount of mass of material lost or thickness of scale formed during oxidation per unit time. (Units: gms/min or μm/min). Mechanical Properties Mechanical properties describe the behavior of material in terms of deformation and resistance to deformation under specific mechanical loading condition. These properties are significant as they describe the load bearing capacity of structure. Elastic modulus, strength, hardness, toughness, ductility, malleability are some of the common mechanical properties of engineering materials. Every material shows a unique behavior when it is subjected to loading. Figure shows a typical stress-strain curve of C-steel under uniaxial tensile loading. Point A indicates the proportional limit. Stress strain behavior is linear only up to this point. Point B represents the point at which the material starts yielding. Between point A and B, the stress strain behavior is not linear, though it is in elastic region. Point C is referred to the upper yield point. The material behavior after point D is highly nonlinear in nature. Point E is the maximum stress that the material can withstand and the point F schematically indicates the point of rupture.

6 Figure Stress-strain curve for carbon-steel [3] Stresses computed on the basis of the original area of the specimen are often referred to as the conventional or nominal stresses. Alternately, the stresses computed on the basis of the actual area of the specimen provide the so called true stress. Within the elastic limit, the material returns to its original dimension on removal of the load. The elastic modulus is referred to the slope of the stress-strain behavior in the elastic region and its SI unit is conceived as N.m -2. The elastic modulus is also referred to as the constant of proportionality between stress and strain according to Hooke s Law. Beyond the elastic limit, the materials retains a permanent, irreversible strain (or deformation) even after the load is removed. The modulus of rigidity of a material is defined as the ratio of shear stress to shear strain within the elastic limit. The bulk modulus is referred to the ratio of pressure and volumetric strain within the elastic limit. Figure 2.1.3(a) to (c) schematically shows the uniaxial tensile, shear and hydrostatic compression on a typical block of material. When a sample of material is stretched in one direction it tends to get thinner in the other two directions. The Poisson's ratio becomes important to highlight this characteristic of engineering material and is defined as the ratio between the transverse strain (normal to the applied load) and the relative extension strain, or the axial strain (in the direction of the applied load). For an engineering material, the elastic modulus (E), bulk modulus (K), and the shear modulus (G) are related as: G = E/2(1+ν) and K = E/3(1-2ν), where ν refers to the Poisson s ratio.

7 (a) (b) (c) Figure Schematic presentation of (a) tensile, (b) shear and (c) hydrostatic compression [4] The strength (SI units: Pa or N/m 2 ) is the property that enables an engineering material to resist deformation under load. It is also defined as the ability of material to withstand an applied load without failure. Based on the typical stress-strain behavior of an engineering material, a few reference points are considered as important characteristics of the material. For example, the proportional limit is referred to the stress just beyond the point where the stress / strain behavior of a material first becomes non-linear. The yield strength refers to the stress required to cause permanent plastic deformation. The ultimate tensile strength refers to the maximum stress value on the engineering stress-strain curve and is often considered as the maximum load-bearing strength of a material. The rupture strength refers to the stress at which a material ruptures typically under bending. Different material behaves differently when subjected to load. Figure indicates the different in stress strain behavior of typical cast iron, low carbon steel, and aluminum alloy. Cast iron, being a brittle material generates steeper curve than low carbon steel or aluminum alloy. There is no sign of yielding prior to failure, so the yield point has to be found out graphically. The yield point strength in the case of low carbon steel and aluminum alloys can be identified easily. The hardness is another important mechanical property of engineering material and refers to the resistance of a material against abrasion / scratching / indentation. The hardness of a material is always specified in terms of the particular test that is used to measure the same. For a measure of resistance against indentation, Vickers, Brinell, Rockwell, Knoop hardness tests are common. Alternately, for a measure of resistance against scratch, Mohr s hardness test is followed. The basic principle used in these testing involves the pressing of a hard material against the candidate material, whose hardness is to be measured. The Brinell hardness (figure 2.1.5) test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball

8 subjected to a load of 3000 kg. For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation. The typical Brinell hardness values of some of the commonly used engineering materials are as follows: aluminum 15, copper 35, mild steel 120, austenitic stainless steel 250, hardened tool steel 650, and so on. Figure Comparison of behavior of different material Another important mechanical property of engineering materials is the toughness that provides a measure of a material to withstand shock and the extent of plastic deformation in the event of rupture. Toughness may be considered as a combination of strength and plasticity. One way to measure toughness is by calculating the area under the stress strain curve from a tensile test. The toughness is expressed in Joule to indicate the amount of energy absorbed in the event of failure or rupture. Figure shows the schematic set-ups of Izod impact test and Charpy impact test. In both the cases, impact loading is applied in notched specimen of predefined dimension. Energy absorbed during the breakage of the specimen is the measure of the toughness. In a

9 similar line, resilience of a material refers to the energy absorbed during elastic deformation and is measured by the area under the elastic portion of the stress strain curve. Izod and charpy tests are two important methods for evaluating toughness of a material. Figure Brinell Hardness Test [5] Figure Schematic set-up of (a) Izod Test and (b) Charpy Test [6] Thermal Properties The thermal properties of an engineering material primarily refer to the characteristic behaviors of the material under thermal load. For example, thermal conductivity is a measure of the ability

10 of material to conduct heat and is expressed as W.K -1.m -1 in SI unit. The specific heat refers to the measure of energy that is required to change the temperature for a unit mass and is expressed as J.kg -1.K -1. The product of density and specific heat is often referred to the heat capacity of a unit mass of material. The thermal diffusivity refers to the ratio of thermal conductivity and heat capacity of a material and provides a measure the rate of heat conduction. The thermal diffusivity is expressed in terms of m 2.s -1. When a material is subjected to both thermal and mechanical loading, two more characteristics of materials - coefficient of thermal expansion and thermal shock resistance - become significant. The coefficient of thermal expansion provides a measure of unit change in strain of a material for unit change in temperature and is expressed in terms of K -1 in SI unit. The thermal strain in material is considered to be isotropic in nature. The thermal shock resistance provides a measure to which a material can withstand an impact load which is either thermal or thermo-mechanical in nature. The thermal shock resistance is expressed as Kσ T (1- ν ) αe, where K is the thermal conductivity, σ T maximal tension the material can resist, α the thermal expansion coefficient, E the Young s modulus and ν the Poisson s ratio. Getting Familiar with Different Materials Metals Metals have free valance electrons which are responsible for their good thermal and electrical conductivity. Metals readily loose their electrons to form positive ions. The metallic bond is held by electrostatic force between delocalized electrons and positive ions. Metals are primarily used in the form of alloys which depict a combination of two or more materials, in which at least one is metal. The iron based alloys are characterized as ferrous alloys. For example, steel is an alloy of iron, carbon and other alloying elements, brass is an alloy of copper and zinc, bronze is an alloy of copper and tin, and so on. Metals and alloys are typically characterized by an excellent blend of mechanical and thermal properties. Table indicates the typical material properties and common applications of some of the widely used metallic materials.

11 Material Properties Table Common material properties of metallic materials [7] Iron Copper Aluminum C-Steel AA6061 Ti-6Al-4V Type Pure Pure Pure Fe- Alloy Al-alloy Ti-Alloy Density (kg.m -3 ) Melting Solidus = 855 Solidus = Temperature (K) Liquidus = 924 Liquidus = 1933 Boiling Temperature (K) Young s Modulus(GPa) Shear Modulus(GPa) Bulk Modulus(GPa) Poisson s Ratio Yield Strength (MPa) Ultimate Tensile Strength (MPa) Coefficient of Thermal Expansion X (K -1 ) Thermal Conductivity (W.mm -1.K -1 ) Specific Heat (J.kg -1.K -1 ) Application Heat Exchanger Aerospace, Construction, Electrical conductors Utensils, Naval Construction, Chemical transport, Aircraft fittings, Pistons, Bike frames Aerospace, Marine, Power generation, Offshore Industries

12 Ceramics and Glasses Ceramics are non-metallic in nature and refer to the carbide, boride, nitride and oxides of Aluminum, silica, zirconium, etc. However, the ceramics possess excellent resistance to thermal and chemical corrosion and wear resistant. Ceramics are also good thermal and electrical insulators. Table indicates the typical material properties and common applications of some of the widely used ceramics. Table Material properties and applications of commonly used Ceramics [7] Material Glass Alumina Silicon Carbide Silicon Nitride (Soda-lime glass) Properties Density (kg.m -3 ) Melting Temperature (K) Young s Modulus(GPa) Shear Modulus(GPa) Bulk Modulus(GPa) Poisson s Ratio Ultimate Tensile Strength (MPa) Coefficient of Thermal Expansion X 10-6 (K -1 ) Thermal Conductivity (W.mm -1.K -1 ) Specific Heat (J.kg -.K -1 ) Application Cutting wheels, polishing clothes High temperature furnace, Heat shield Balls and roller of bearing, Cutting tools, Engine valves, Turbine blades Windows, food Preparation

13 Polymer Polymer is a chain of molecules connected by covalent (sharing of electrons) chemical bond. Three types of polymers are most common: (a) thermoplastics which can be reworked on heating, (b) thermosets which cannot be worked with after curing is over, and (c) elastomers, which typically provide very high elastic deformation. The polymers cannot withstand high temperature due to their low transition temperature Table indicates the typical material properties and common applications of some of the widely used polymers. Table Properties Material Material properties and applications of commonly used Polymers Polyvineyl chloride (PVC) Bakelite Silicone Type thermoplastic elastomer Elastomer Density (kg.m -3 ) High density silicone-2800 Melting Temperature (K) Young s Modulus(MPa) 1-5 Yield Strength (MPa) Ultimate Tensile Strength (MPa) (Flexible-rigid) Coefficient of Thermal Expansion X 10-6 (K -1 ) 52 Thermal Conductivity (W.mm -1.K -1 ) Specific Heat (J.kg -1.K -1 ) Application Plumbing Electrical Insulators Electrical appliances, Structural application (below 200 C)

14 Natural Materials The most common examples of natural materials are wood, cotton, flax, wools, bamboo, jute which primarily come from the plants or animals. Most of the natural materials are recyclable and require considerable processing operations before use. Table indicates the typical material properties and common applications of some of the widely used natural materials. Table Properties Type Material properties and applications of commonly used natural materials Material Oak Wood Wool Flax European Oak Density (kg.m -3 ) Ignition Temperature (K) Heat of Combution (Kcal/g) Young s Modulus (GPa) 9-13 Shear Modulus(GPa) Bulk Modulus(GPa) Poisson s Ratio Yield Strength (MPa) Ultimate Tensile Strength (MPa) Coefficient of Thermal Expansion (K -1 ) Thermal Conductivity (W.mm -1.K -1 ) Specific Heat (J.kg -1.K -1 ) 0.17 Application 4.9 Longitudinal: 3.5 Transverse: Furniture, Packaging Fabric, Thermal insulator Fabrication of twine

15 Composite Composite material is formed by combining one or more different materials. Unlike alloy system each constituent is distinguishable and retain their properties. Composite materials consist of matrix material with reinforcement to enhance its strength. Few common examples of composites are FRP (Glass/carbon fiber reinforced polymers), Metal matrix composites. Using composites one can combine attractive qualities of other materials and engineer properties to demand. On the other side they are expensive and difficult to fabricate and join. Table indicates common properties and applications of composites. Table Material properties and applications of commonly used composites Material Carbon fiber reinforced Aluminum Titanium Alumina Properties polymer matrix matrix matrix Cermet matrix Density (kg.m -3 ) Young s Modulus(GPa) Poisson s Ratio Yield Strength (MPa) Ultimate Tensile Strength (N.mm -2 ) Application Mechanical components, Protection screen, Sporting equipments Aerospace, Sporting equipments, Electronic packaging Aerospace Turbines High temperature Mechanical Application Cutting tools, Polishing materials Foams Foam is a substance formed by trapping many gaseous bubbles in liquid or solid. Solid foams are very important class of structure due to its light weight. The foams can be metallic (eg. Titanium foam), ceramic (alumina foam) or based on polymer (polyurethane foam). The metallic foams are commonly used for medical implants. The ceramic foams are used typically as insulators while the polymer based foams are primarily used for packaging and acoustic insulators.

16 Exercise Choose the correct answer. 1. Hydrostatic stress results in (a) linear strain (b) shear strain (c) both linear and shear strain (d) None 2. Toughness of a material is equal to the area under part of the stress-strain curve. (a) Elastic (b) Plastic (c) Both elastic and plastic (d) None 3. During a tensile loading, the length of a steel rod is changed by 2 mm. If the original length of the rod has been 20 mm, what is the amount of strain induced (a) 0.1 (b) 2 (c) 0.9 (d) is an example of a chemical property. (a) Density (b) Mass (c) Acidity (d) Diffusivity Answers: 1. (d) 2. (c) 3. (a) 4. (c) References 1. M F Ashby, Material Selection in Mechanical Design, Butterworth-Heinemann, G E Dieter, Mechanical Metallurgy, McGraw-Hill, ROORKEE/strength%20of%20materials/homepage.html, ( ) ( ) ( ) ( ) ( ).