ITU Department of Mechanical Engineering. MANUFACTURING PROPERTIES of ENGINEERING MATERIALS Lecture Notes. Prof.Dr.Ahmet Aran - PDF

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ITU Department of Mechanical Engineering MANUFACTURING PROPERTIES of ENGINEERING MATERIALS Lecture Notes Prof.Dr.Ahmet Aran 2007 CONTENTS 1. Engineering Materials and Their Properties 1.1. Classification
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ITU Department of Mechanical Engineering MANUFACTURING PROPERTIES of ENGINEERING MATERIALS Lecture Notes Prof.Dr.Ahmet Aran 2007 CONTENTS 1. Engineering Materials and Their Properties 1.1. Classification of Engineering Materials 1.2. Properties of Engineering Materials Mechanical Properties Thermal Properties Environmental Properties Electrical Properties 2. Manufacturing Processes and Manufacturability 2.1 Casting and Castability Casting Process: Castability: Material Properties Which Affect Castability 2.2. Bulk Forming Rolling Forging Extrusion Material Properties Which Effect Formability 2.3. Sheet Metal Forming 2.4. Machining 2.5. Joining 2.6. Other Processes 1 1. Engineering Materials and Their Properties In this Chapter materials are classified and the most important properties of the engineering materials are listed with short explanations. The properties covered here are especially those properties, which are important in manufacturing processes Classification of Engineering Materials A. Metals and Alloys: Inorganic materials composed of one or more metallic elements. They usually have a crystalline structure and are good thermal and electrical conductors. Many metals have high strength and high elastic module. They maintain their good strength at high and low temperatures. They also have sufficient ductility, which is important for many engineering applications. They can be strengthened by alloying and heat treatment. They are least resistant to corrosion. B. Ceramics and Glasses: Inorganic materials consisting of both metallic and nonmetallic elements bonded together chemically. They can be crystalline (ceramics), non-crystalline (glasses) or mixture of both (glass-ceramics). Generally they have high melting points and high chemical stabilities. They have high hardness, high moduli and high temperature strength. But since they are very brittle they cannot be used as good as metals. Ceramics are usually poor electrical conductors. Ceramics have a high strength on compression C. Polymers: Organic materials which consist of long molecular chains or networks containing carbon. Most polymers are non-crystalline, but some consist of mixtures of both crystalline and non-crystalline regions. They generally have low densities and low rigidity. Their mechanical properties may vary considerably. Most polymers are poor electrical conductors due to the nature of the atomic bonding. Most of them are corrosion resistant, but cannot be used at high temperatures. They generally have a good strength to weight ratio. D. Composites: Materials where two or more of the above materials are brought together on macroscopic level. Usually they consist of a matrix and a reinforcement. They are designed to combine the best properties of each of its components. 2 1.2. Properties of Engineering Materials Each material has a property profile. The properties of engineering materials can be classified into the following main groups: physical and chemical. The physical properties can also be further grouped into categories: mechanical, thermal, electrical, magnetic, optical etc. The chemical properties include: environmental and chemical stability. There are also some general properties which cannot be classified within these groups: Density, ρ (Units: Mg/m 3, g/cm 3 ) The density of a material is defined as its mass (m) per unit volume (V). It is represented in the following equation; ρ = m / V Cost (YTL/kg) Anisotropy Definition: The characteristic of exhibiting different values of a property in different directions. Ex: Rubber reinforced with horizontally placed fibers has a high ultimate tensile strength if pulled parallel to the fibers i.e. horizontally, but a relatively low one if pulled vertically. 3 Mechanical Properties TENSILE TEST 4 Elastic modulus (Young Modulus), E (Unit: GPa) Young's modulus, E, is the slope of the initial, linear-elastic part of the stress-strain curve in tension or compression. But accurate moduli are measured dynamically. It is a measure of the rigidity of the material. Young s Modulus (or Elastic Modulus) is the proportionality constant of solids between elastic stress and elastic strain and describes the inherent (natural) stiffness of a material. It can be expressed in the following equation where, E is Young s Modulus; E = Elastic Stress / Elastic Strain Tangent modulus: (slope of the stress-strain curve at a certain point) Secant modulus: (slope of a line from the origin to a specified point) For isotropic materials it is related to the bulk modulus K and to the shear modulus G by where ν is Poisson's ratio. Commonly v = 1/3, and hence E = K, and E = (8/3)G. 5 Shear Modulus, G (Unit: GPa) The shear modulus is the initial, linear elastic slope of the stress-strain curve in shear. Shear modulus is the ratio of shear stress divided by the shear strain in the elastic region. It can also be referred to as modulus of rigidity or torsion modulus. G = Elastic Shear Stress / Elastic Shear Strain For isotropic materials it is related to Young's modulus E and to the bulk modulus K and Poisson's ratio by When v = 1/3, G = (3/8)E, and G = (3/8)K. 6 Bulk Modulus K, (Unit: GPa) The bulk modulus, K, measures the elastic response to hydrostatic pressure. Ratio of mean normal stress to the change in volume Units: SI: GPa; cgs: dyne/cm 2 ; English: psi where V is the volume. For isotropic solids it is related to Young's modulus E and to the shear modulus G by where v is Poisson's ratio. When v = 1/3, E = K, and K = (8/3)G. Poissons Ratio, v, (Dimensionless) Poisson s Ratio is the negative ratio of the thickness decrease divided by the length increase as a result of a tensile stress applied to a material. Its value for many solids is close to 1/3. For elastomers it is just under 0.5 7 Elastic limit, σ el (Unit: MPa) The elastic limit (proportionality limit) is the stress beyond which there is permanent deformation. Below the elastic limit all the deformation is recovered when the load is removed. The 'elastic limit' of a solid requires careful definition. For metals, the elastic limit is defined as the 0.2% offset yield strength. This represents the stress at which the stress-strain curve for uniaxial (=in one direction) tensile loading deviates by a strain of 0.2% from the linear-elastic line. It is the stress at which dislocations move large distances through the crystals of the metal. It is the same in tension and compression as the dislocations movement is caused by the shear stress, which has its highest value at 45 to the axis of loading. For polymers, the elastic limit is the stress at which the uniaxial stress-strain curve becomes markedly non-linear: typically, a strain of 1%. This may be caused by 'shear yielding' (irreversible slipping of molecular chains) or by 'crazing' (formation of low density, crack-like volumes which scatter light, making the polymer look white). For fine ceramics and glasses, the database entry for the elastic limit is an estimate, based on the tensile strength (which is low due to brittle fracture). When based on direct measurements at high pressures, or on hardness measurements, of the stress required to cause plastic flow, it is very high: higher than the compressive strength, which is lowered by crushing. For composites, the elastic limit is best defined by a set deviation from linear-elastic uniaxial behaviour: 0.5% is taken in the database. 8 Elastic limit depends on the mode of loading. For modes of loading other than uniaxial tension, such as shear and multiaxial loading, the strength is related to that in simple tension by a yield function. For metals, the Von Mises yield function works well. It specifies the relationship between the principal stresses σ 1, σ 2, σ 3 and the yield strength sy (elastic limit): The Tresca function is sometimes more convenient, because it is less complicated: For ceramics, a Coulomb flow law is used: Resilience The maximum amount of energy per unit volume which can be stored elastically. This energy is released upon unloading. This value can be calculated as the area under the elastic part of the stres-strain curve. 9 Yield Strength (Unit: MPa) The stress at which a material exhibits a specified deviation from proportionality of stress and strain (Flow stres). An offset of 0.2% is used for many metals. Only certain metals have a yield point (metals with BCC-Body Centered Cubic crystal structure such as iron). If there is a decrease in stress after yielding, a distinction may be made between upper and lower yield points. 10 Tensile Strength (Unit: MPa) Tensile Strength is the maximum tensile stress a material can withstand before failure. It is a feature of the engineering stress-strain curve and cannot be found in the true stress-true strain curve. For brittle solids: ceramics, glasses and brittle polymers - it is much less than the compressive elastic limit. For metals, ductile polymers and most composites - it is larger than the yield strength by a factor ranging from 1.1 to 3. Ductility The tensile ductility is the permanent increase in length of a tensile specimen before fracture, expressed as a fraction of the original gauge length. Ductility is the ability of a material to undergo large plastic deformation without fracture or failure. It can also be expressed as the reduction of area of the specimen during the tensile test. Units: Dimensionless (strain) 11 Toughness Toughness is the ability of a material to absorb energy without rupturing. It is usually measured by the energy absorbed in a notch impact test, but the area under the tensile stress-strain curve is also a measure. Notch Impact Test 12 Bauschinger Effect: The process where the plastic deformation in one direction causes a reduction in the yield strength when stress is applied in the opposite direction. 13 Uniform (necking) strain It is the value of strain at which the area of the specimen begins having different values at different points, i.e. necking starts. Strain Hardening Exponent An increase in hardness and strength caused by plastic deformation at temperatures below the recrystallisation range. The measure is the exponent n in the equations; σ 1 = Kε t n or lnσ 1 = lnk + nlnε t where a logarithmic scale is used and the curve is true stress, true strain. Strain rate sensitivity It is the measure for how fast strain hardening occurs when a material is deformed plastically. It is defined as m = (lnσ) (ln Ý ε ) Compressive Strength For metals, the compressive strength is the same as the tensile yield strength. Polymers are approximately 20% stronger in compression than in tension. In Ceramics, compressive strength is governed by crushing and is much larger than the tensile strength. Composites that contain fibers (including natural composites like wood) are a little weaker (up to 30%) in compression than tension as the fibers buckle 14 Shear strength The highest value of shear stress a material can withstand before plastic deformation occurs. Impact strength Obtained from the notch-impact test. It is expressed in means of energy. Temper Brittleness A feature of some materials, which causes the material to become more brittle after tempering. It can be obtained from the notch-impact test. Modulus of Rupture When the material is difficult to grip (as is a ceramic), its strength can be measured in bending. The modulus of rupture (MOR) is the maximum surface stress in a bent beam at the instant of failure. One might expect this to be exactly the same as the strength measured in tension, but it is always larger (by a factor of about 1.3) because the volume subjected to this maximum stress is small, and the probability of a large flaw lying in the highly stressed region is also small. (In tension all flaws see the maximum stress.) The MOR strictly only applies to brittle materials. For ductile materials, the MOR entry in the database is the ultimate tensile strength. Units: SI: MPa; cgs: 10 7 dyne/cm 2 ; English:10 3 psi Fracture Toughness The fracture toughness Kc, is a measure of the resistance of a material to the propagation of a crack. It can be measured by loading a sample containing a 15 deliberately introduced crack of length a=2c and then recording the tensile stress σ at which the crack propagates. Fracture toughness is then calculated from where Y is a geometric factor, near unity, which depends on details of the sample geometry. Measured in this way, Kc has well defined values for brittle materials (ceramic, glasses, many polymers and low toughness metals like cast iron). In ductile materials, a plastic zone develops at the crack tip, which introduces new features into the way cracks propagate. This necessitates more complex characterization. Nevertheless, values for Kc are cited and are useful as a way of ranking materials. Hardness Units:(for Brinell, Vickers) SI: MPa; (for Rockwell) Dimensionless Hardness is the resistance of a materials surface to abrasion, scratching and indentation (local plastic deformation). It is often measured by pressing a pointed diamond or hardened steel ball into the surface of the material. The hardness is generally defined as the indentor force divided by the projected area of the indent. Hardness is measured by different hardness techniques: Brinell, Vickers, Rockwell, Shore etc.. 16 Very rough approximations can be made by relating the Brinell hardness to the yield strength σ y of ductile materials by H = 3 σ y. Hardness is a good indicator for 17 controlling or comparison purposes, but has little meaning for scientific purposes or calculations. Creep Strength Time-dependent deformation which occurs when materials are loaded above 1/3T m. Creep Test 18 The creep strain rate at a certain temperature can be given by Creep rate= Ý ε = dε dt The creep strength of a material can be given as a. The maximum stress that will cause less than the specified strain in a given time b. The constant stress that will cause a specified secondary creep strain rate at constant temperature Damping Capacity - Loss-Coefficient, η (Dimensionless) The loss-coefficient measures the degree to which a material dissipates vibrational energy. 19 If a material is loaded elastically to a stress σ max, it stores elastic energy per unit volume. If it is loaded and then unloaded, it dissipates energy equivalent to the area of the stress-strain hysteresis loop: The loss coefficient h is defined as The cycle can be applied in many different ways - some fast, some slow. The value of η usually depends on the time-scale or frequency of cycling. Fatigue strength (Endurance Limit), Units: MPa Endurance limit (or sometimes referred to as the fatigue limit) is the maximum stress amplitude in fatigue below which a material can endure an essentially infinite number of stress cycles and not endure failure. Generally 'infinite' life means more than 10 7 cycles to failure. The endurance limit is usually a band of values, which is caused by small differences in the specimens. Fatigue strength is dependent on temperature. 20 21 Archard wear constant, k A (m/mn) The volume of material lost from one surface, per unit distance slid is called the wear rate. The materials ability to resist wear is given by the Archard wear constant, k A. W = k A.A.p Where A is the area of the surface, p is the pressure) 22 Superplasticity A feature of materials to withstand plastic deformation over their limits at high temperatures (usually half of the melting temperature). This effect is caused by dynamic recrystallisation during deformation of the material. Thermal fatigue When the temperature of a material is repetitively increased and decreased (a cyclic change in temperature) thus generating high temperature gradients continuously, the specimen will fail after a certain number of changes. This is called thermal fatigue and is caused by the inner temperature differences causing stresses inside the material Thermal Properties Melting Point, Tm The melting point is the temperature at which a material turns from solid to liquid. The melting temperature of an alloy is usually less than the melting temperature of the parent metals. Latent Heat of Fusion Lm Latent Heat of Fusion is the heat [energy] required per unit mass to change a materials state to another state i.e. from a solid to liquid or from a liquid to gas, this process is reversible, therefore includes a gas to liquid or from a liquid to solid. Units: SI: kj/kg; cgs: cal/g; Imperial: Btu/lb For pure metals this heat is absorbed at constant temperature (the melting temperature), Tm. Amorphous solids (including many polymers) do not have a sharp 23 melting point. When the change from a solid state to fluid is over a temperature range, it is not appropriate to define a latent heat of melting. Specific Heat C p is the specific heat capacity at constant pressure. It specifies the amount of heat required to raise the temperature of 1 kg of material by 1 C (K). It is measured by the standard technique of calorimetry. Units: SI: J/kg.K; cgs: cal/g.k; English: Btu/lb.F Thermal Conductivity (Unit: W/m.K) Thermal Conductivity is a measure of heat flow through a material. It relates heat flow (the flow of heat energy per unit area per unit time) to the temperature gradient (which describes a temperature difference per unit distance), causing the heat flow. The rate at which heat is conducted through a solid at 'steady state' (meaning that the temperature profile does not change with time, i.e. the surface of the material is always at the temperature T 1 and the inside of the material at distance X is always at T 2 throughout the experiment) is governed by the thermal conductivity λ. It is measured by recording the heat flux J (W/m²) flowing from surface at temperature T 1 to one at T 2 in the material, separated by a distance X: Where ( T T ) 2 1 is the temperature gradient, and λ is to be determined as a material X specific constant. In practice, the measurement is not easy (particularly for materials with low conductivities), but reliable data are generally available. Thermal diffusivity, a (Unit: m 2 /s) When the heat flow is not steady the flux depends on thermal diffusivity a: a = λ / (ρ.c p ) where ρ is the density and C p the specific heat at constant pressure. It shows how quick a heat, which is applied to the material, will be distributed among the material. At high λ and low C p temperature differences will be equalled quickly. Thermal Expansion coefficient, α (1/K) Thermal expansion is the term used to describe the change in dimensions that occurs with most materials as the temperature is increased or decreased. Most materials expand when they are heated. The linear thermal expansion coefficient a is the thermal strain per degree K. If the material is thermally isotropic, the volumetric expansion per degree is 3α. If working with a material, which has a high α value, the 24 cooling differences (ex: the surface cools more quickly) will create high inner stresses, which could for example deform the shape of the material. Units: SI: 10-6 /K; cgs: 10-6 /K; English: 10-6 /F Thermal shock resistance (K) Maximum temperature difference through which a material can be quenched suddenly without damage Maximum Service Temperature (K) Maximum service temperature is the highest temperature at which a material can reasonably be used without the effects of oxidation, chemical change or excessive creep. Minimum Service Temperature (K) Minimum service temperature is the lowest temperature at which a material can reasonably be used without the loss of its original serviceable properties. Glass Transition Temperature, Tg (K) A property of non-crystalline solids which do not have a sharp melting point. It characterises the transition from true solid to viscous liquid in these materials. The Glass Transition Temperature relates to those materials that are non-crystalline solids and is defined by the transition from a true solid to ve
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