Design & Selection of Materials/ Lecture 03-Engineering materials and Their properties PDF

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Asst.L. Ziadoon.M.Rahi Engineering materials and Their properties Lecture_03 Ministry of Higher Education & Scientific Research University of Kufa-Faculty of Engineering Materials Engineering Department Design & Selection of Materials Lecture 03-Engineering materials and Their properties 4th...

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Ministry of Higher Education & Scientific Research University of Kufa-Faculty of Engineering Materials Engineering Department

Design & Selection of Materials Lecture 03-Engineering materials and Their properties 4th Class

By Assistant Lecturer: Ziadoon.M.Rahi Asst.Prof.Dr.Ali Sabea Hammood 2017/2018

[email protected]

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Contents. 3.1 Introduction. 3.2 The Families of Engineering Materials. 3.3 Materials Information for Design. 3.4

Material Properties and Their Units. i) General properties. ii) Mechanical properties. iii) Thermal properties. iv) Electrical properties. v) Optical properties. vi) Eco-properties.

3.1 Introduction.

Figure 3.1 basic of a good product. Materials, one might say, are the food of design. This lecture presents the menu: the materials shopping list. A successful product—one that performs well, is good value for money, and gives pleasure to the user—uses the best materials for the job, and fully exploits their potential and characteristics. It brings out their flavor, so to speak. 2

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

3.2 The Families of Engineering Materials. It is conventional to classify the materials of engineering into the six broad families shown in Figure 3.2.

Figure 3.2 The menu of engineering materials. The basic families of metals, ceramics, glasses, polymers, and elastomers can be combined in various geometries to create hybrids.

1. Metals:

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

• Stiff. • have relatively high elastic moduli. Most, when pure, are soft and easily deformed. • can be made strong by alloying and by mechanical and heat treatment, but they remain ductile, allowing them to be formed by deformation processes. • Partly because of their ductility, metals are prey to fatigue and of all the classes of material, they are the least resistant to corrosion.

2. Ceramics:

• • • • • • • •

Stiff. Strong in compression, weak in tension. Brittle. electrically and thermally insulating. not impact-resistant medium weight. very temperature tolerant. they have a low tolerance for stress concentrations (like holes or cracks) or for high-contact stresses (at clamping points, for instance). • abrasion-resistant (hence their use for bearings and cutting tools); they retain their strength to high temperatures; and they resist corrosion well.

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

3. Glasses: Soda glass, Borosilicate glass, Silica glass, Glass-ceramics. • • • • • • •

Transparent, or easily coloured. High resistance to corrosion. Easy to shape. Low tensile strength. Low toughness. Costs a lot to make so more economical to recycle. Products : windows, bottles, ovenware, optical fibers.

4. Polymers:

Acrylonitrile– butadiene–styrene

• • • • • • •

Strong, flexible. Electrically and thermally insulating. Not creep-resistant, impact-resistant. Lightweight. Temperature-sensitive. Soft. corrosion-resistant 5

Lecture_03

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Thermoplastic: polymers repeatedly softened by heating and hardened by cooling Thermoset: polymers hardened by curing. Elastomers μ ‘elastic polymers’, can either be thermoplastic or thermoset polymers. 5. Elastomers: the material of rubber bands and running shoes—are polymers with the unique property that their stiffness, measured by E, is extremely low(500– 5000) times less than those of metals) and their ability to be stretched tom many times their starting length yet recover their initial shape when released. Despite their low stiffness they can be strong and tough—think of car tires. 6. Hybrids: are combinations of two (or more) materials in an attempt to get the best of both. Glass and carbon-fiber reinforced polymers (GFRP and CFRP) are hybrids; so, too, are sandwich structures, foams and laminates. And almost all the materials of nature (wood, bone, skin, leaf) are hybrids—bone, for instance, is a mix of collagen (a polymer) with hydroxyapatite (a mineral).

Advantages of composite materials  High specific stiffness (E/ρ) and high specific strength (σult/ρ) ⎯ weight reduction, aerospace and sporting goods.  High corrosion resistance ⎯ Acid, alkali resistance of polymers, chemical and marine applications, infrastructure applications.  High impact resistance ⎯ High internal damping of Kevla fiber/epoxy composites, ballistics protection.  High wear resistance ⎯ Ceramic particle reinforced metal matrix composites, ceramic matrix composites. 6

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

 Resistance to high temperature and extreme mechanical, environmental conditions: ⎯ CCC, resisting a temperature of 3300°C for space shuttle nose cone. ⎯ Graphite/epoxy composite, α=0.02 m/m/°C (steel, 11.7ν aluminum, 23) for satellite trusses (-160° ⎯ 93°) ⎯ Fiber-reinforced ceramic matrix composite (CMC) for applications of high mechanical

properties and high temperature.

 Tailor-able properties ⎯ design both materials and structures.

Choose appropriate combination of reinforcements and matrices. Choose optional fiber orientation and lay-up sequences.

Disadvantages of composite materials • Higher cost • Complexity in mechanical characterization and difficulty in analysis. • Weak in transverse direction and low toughness • Difficulty in attaching (Joining) • Environmental degradation (Polymer matrix absorb moisture).

3.3 Materials Information for Design. The engineer, in selecting materials for a developing design, needs data for the materials’ properties. Engineers are often conservative in their choice, reluctant to consider materials with which they are unfamiliar, and with good reason. Data for the old, well-tried materials are established, reliable, and easily found. If you’re going to design something, what sort of materials information do you need? Figure 3.3 draws relevant distinctions.

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Figure 3.3 Types of material information. We are interested here in the types found in the center of this schematicμ structured data for design “allowables” and the characteristics of a material that relate to its ability to be formed, joined, and finished; records of experience with its use; and design guidelines for its use.

Many attributes that can be structured are common to all materials; all have a density, an elastic modulus, a strength, a thermal conductivity. Structured information can be stored in a database and—since all materials have values—it is the starting point for selecting between them. To design with a material, you need to know: its real character, its strengths, and its weaknesses. How do you shape it? How do you join it? Who has used it before and for what? Did it fail? Why? There is more. Material uses are subject to standards and codes. These rarely refer to a single material but to classes or subclasses. To qualify for best-practice design for the environment, material usage must confirm to ISO 14040 guidelines. Table 3.1 Basic Design-Limiting Material Properties and Their Usual SI Units. Class General Mechanical

Property Density Price Elastic moduli (Young’s, shear, bulk) 8

Symbol and Units ρ (kg/m3 or Mg/m3) Cm ($/kg) E, G, K (GPa)

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Yield strength

σy (MPa)

Tensile (ultimate) strength

σts (MPa)

Compressive strength

σc (MPa)

Failure strength Hardness Elongation

Thermal

Electrical

Optical Eco-properties

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Fatigue endurance limit Fracture toughness Toughness Loss coefficient (damping capacity Wear rate (Archard) constant Melting point Glass temperature Maximum service temperature Minimum service temperature Thermal conductivity Specific heat Thermal expansion coefficient Thermal shock resistance Electrical resistivity Dielectric constant Breakdown potential Power factor Refractive index Embodied energy Carbon footprint

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σf (MPa) H (Vickers) ε (–)

σe (MPa) K1c (MPa.m1/2) G1c (kJ/m2)

η (–) KAMPa−1 Tm (°C or K) Tg (°C or K) Tmax (°C or K) Tmin (°C or K) (W/m.K) Cp (J/kg.K) α (K−1) ΔTs (°C or K) ρe (Ω.m or Ω.cm)

εr (–) Vb (106 V/m) P (–) n (–) Hm (MJ/kg) CO2 (kg/kg)

Asst.L. Ziadoon.M.Rahi

3.4

Engineering materials and Their properties

Lecture_03

Material Properties and Their Units.

The combination that characterizes a given material is its property profile. Property profiles are assembled by systematic testing. In this section we scan the nature of the tests and the definition and units of the properties (see Table 3.1). Property values are listed in Appendix A( page 497 textbook) Ashby.2011. Units are given here in the SI system.

 General properties a) The density, ρ (units: kg/m3), is the mass per unit volume. We measure it today as Archimedes did: by weighing in air and in a fluid of known density .

Figure 3.4 Measuring density by Archimedes’ method. ��

�

� ��=

10

�

�

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Example: What is the density of Samples A & B? Ans: Volume A = 5 × 5 × 4 = 100 cm3 Mass = 200 g Density A = 200 g/100 cm3 = 2g/ cm3

Volume B = 5 × 5 × 2 = 50 cm3 Mass = 200 g Density = 200 g/50 cm3 = 4g/ cm3

Give two possible explanations for why one sample is more dense than the other. The price, Cm (units: $/kg), spans a wide range. Some cost as little as $0.2/kg, others as much as $1,000/kg. Prices, of course, fluctuate, and they depend on the quantity you want and on your status as a “preferred customer” with your chosen vendor. Despite this uncertainty, it is useful to have an approximate price in the early stages of material selection.

 Mechanical properties The elastic modulus, E (units: GPa or GN/m2), is the slope of the initial, linear-

elastic, part of the stress-strain curve (Figure 3.5). Young’s modulus, E, describes response to tensile or compressive loading; the shear modulus, G, describes response to shear loading; and the bulk modulus, K, describes the response to hydrostatic pressure.  Strength: measure of the amount of tensile force per unit area that a material can withstand before it fails. 11

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

 Yield strength: the tensile stress (i.e. force/area) at which material yields.  Ultimate tensile strength: the largest tensile stress a material can sustain.  Shear strength: the largest stress a material can sustain under torsion before it yield or fractures.  Compressive strength: a measure of the amount of compressive force per unit area that a material can withstand before it fails.  Stiffness: the resistance to stretching, bending, or twisting loads.  Ductility: the ability of a material to plastically deform.

Figure 3.5 The stress-strain curve for a metal, showing the modulus, E, the 0.2% yield strength, σy, and the ultimate strength, σts. As explained earlier, the components in both structures shown on the cover are designed to withstand different modes of loading: tension, compression, bending, torsion and internal pressure. Usually one mode dominates, and the component can be idealized as one of the simply loaded cases in Figure 3.6—tie, column, beam, shaft or shell.

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Figure 3.6 Modes of loading and states of stress.

Poisson’s ratio, , is the negative of the ratio of the lateral strain, ε2, to the axial strain, ε 1, in axial loading:

suppose, the system is the isotropic material, the moduli are related in the following ways:

Commonly,

Elastomers are exceptional. For these,

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Example: Young’s modulus E for copper is 124 GPaν its Poisson’s ratio is 0.345. What is its shear modulus, G? By applying the equation :

�=

�

+�

= 46.1 GPa.

The strength, σf (units: MPa or MN/m2), of a solid requires careful definition. For metals, we identify σf with the 0.2% offset yield strength σy (see Figure 3.5), that is, the stress at which the stress-strain curve for axial loading deviates by a strain of 0.2% from the linear-elastic line. It is the same in tension and compression. Strength, for ceramics and glasses, depends strongly on the mode of loading (Figure 3.6). In tension, “strength” means the fracture strength, σt. In compression it means the crushing strength σc, which is much greater; typically.

For polymers, σf is identified as the stress at which the stress-strain curve becomes markedly nonlinear, at a strain typically of 1% (Figure 3.7).

Figure 3.6 Stress-strain curves for a ceramic in tension and in compression. The compressive strength, σc, is 10 to 15 times greater than the tensile strength, σt. 14

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

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Figure 3.7 Stress-strain curves for a polymer below, at, and above its glass transition temperature, Tg.

Strength, then, depends on material class and on mode of loading.

The flexural strength or modulus of rupture, σflex (units: MPa) is the maximum surface stress in a bent beam at the instant of failure (Figure 3.8).

Figure 3.8 The modulus of rupture (MOR) is the surface stress at failure in bending. It is equal to, or slightly larger than, the failure stress in tension.  Toughness: ability of a material to plastically deform before fracturing.  Hardness: ability of a material to resist localized surface indentation or deformation.  Fatigue strength: ability of a material to undergo a number of cyclic loads without fracturing . 15

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

 Creep resistance: ability of a material to resist stretching while under loads over long time periods at elevated temperatures.  Impact strength: ability of a material to absorb sudden dynamic shocks or impacts without fracturing. Cyclic loading can cause a crack to nucleate and grow in a material, culminating in fatigue failure. For many materials there exists a fatigue or endurance limit, σe (units: MPa), illustrated by the Δσ − Nf curve of Figure 3.10. It is the stress amplitude Δσ below which fracture does not occur, or occurs only after a very large number (Nf > 107) of cycles.

Figure 3.10 The endurance limit, σe, is the cyclic stress that causes failure in Nf = 107 cycles.

The hardness, H (SI units: MPa) of a material is measured by pressing a pointed diamond or hardened steel ball into the material’s surface (Figure 3.11). The hardness is defined as the indenter force divided by the projected area of the indent. It is related to the quantity we have defined as σf by:

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

A conversion chart for five hardness scales, relating them to yield strength, appears in Figure 3.12.

Figure 3.11 Hardness is measured as the load, F, divided by the projected area of contact, A, when a diamond-shaped indenter is forced into the surface.

Figure 3.12 Commonly used scales of hardness related to each other and to the yield strength.

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Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

Example: A steel has a hardness of 50 on the Rockwell C scale. Approximately what is its Vickers hardness and yield strength? Ans: The chart of Figure 3.12 shows that the Vickers hardness corresponding to a Rockwell C value of 50 is approximately Hv = 500 and the yield strength is approximately 1,700 MPa.

The toughness, G1c (units: kJ/m2), and the fracture toughness, K1c (units: MPa/m1/2 or MN/m1/2), measure the resistance of a material to the propagation of a crack. The fracture toughness is measured by loading a sample containing a deliberately introduced crack of length 2c (Figure 3.13), recording the tensile stress σ* at which the crack propagates. The quantity K1c is then calculated from:

where Y is a geometric factor, near unity, that depends on details of the sample geometry.

Figure 3.13 The fracture toughness, K1c, measures the resistance to the propagation of a crack. The test specimen containing a crack of length 2c fails at stress σ*. The fracture toughness is 18

Asst.L. Ziadoon.M.Rahi

Engineering materials and Their properties

Lecture_03

where Y is a constant near unity.

Example A glass floor panel contains micro-cracks up to 2 microns in length. Glass has a fracture toughness of K1c = 0.6 MPa.m1/2. When the panel is walked upon, stresses as high as 30 MPa appear in it. Is it safe? Ans: The stress required to make a 2-micron crack (so c = 10−6 m) propagate in glass with a fracture toughness of K1c = 0.6 MPa.m1/2, using Equation with Y = 1, is

The panel is safe.

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Engineering materials and Their properties

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References: 1. M. F. Ashby, Materials Selection in Mechanical Design, Fourth Edition,2011 2. Michael Ashby, Hugh Shercliff and David Cebon" Materials Engineering, Science, Processing and Design, 2007

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