MATSCI 201 Lecture 8 Mechanical Properties of Metals Fall 2024 PDF

University of Science and Technology MATSCI 201 Fundamentals of Materials Science and Engineering LECTURE 8:Mechanical Properties of Metals By: Dr. Worood A. El-Mehalmey, PhD. 18/11/2024 Materials Science Program...

University of Science and Technology MATSCI 201 Fundamentals of Materials Science and Engineering LECTURE 8:Mechanical Properties of Metals By: Dr. Worood A. El-Mehalmey, PhD. 18/11/2024 Materials Science Program 1  MATSCI 201 Lecture 8: Mechanical Properties of Metals q Introduction q Concepts of Stress and Strain o Tension Tests o Stress-Strain Behavior ü Elastic Deformation Anelasticity ü Plastic Deformation q Tensile Properties o Yielding and Yield Strength o Tensile Strength o Ductility o Resilience o Toughness 2 Chapter 6: Mechanical Properties of Metals Introduction: Many materials are subjected to forces and loads when in service. àIn such situations it is NECESSARY to know the characteristics of the material and to design the member from which it is made such that any resulting deformation will not be excessive and fracture will not occur. The MECHANICAL BEHAVIOR of a material à reflects its response or deformation in relation to an applied load or force. à Structural Engineers is to determine stresses and stress distributions within members that are subjected to well-defined loads. à Materials And Metallurgical Engineers, however, are concerned with producing and fabricating materials to meet service requirements as predicted by these stress analyses. 3 Chapter 6: Mechanical Properties of Metals Introduction: The mechanical properties of materials are ascertained by performing carefully designed Laboratory Experiments that REPLICATE as nearly as possible the service conditions. à Factors to be considered include: ü The nature of the applied load. o It is possible for the load to be tensile, compressive, or shear, and its magnitude may be constant with time, or it may fluctuate continuously. ü The duration of the applied load. o Application time may be only a fraction of a second, or it may extend over a period of many years. ü The environmental conditions. o Service temperature may be an important factor. 4 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: If a load is static or changes relatively slowly with time and is applied uniformly over a cross section or surface of a member, the mechanical behavior may be ascertained by a simple STRESS–STRAIN TEST; these are most commonly conducted for metals at room temperature. Ø Tension Tests: A specimen is deformed, usually to fracture, with a gradually increasing tensile load that is applied uniaxially along the long axis of a specimen. The Tensile Testing Machine is designed to elongate the specimen at a constant rate, and to continuously and simultaneously measure the instantaneous applied load (with a load cell) and the resulting elongations (using an extensometer). 5 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Tension Tests: The output of such a tensile test is recorded (usually on a computer) as load or force versus elongation. à These load–deformation characteristics depend on the specimen size. For example, it requires twice the load to produce the same elongation if the cross-sectional area of the specimen is doubled. To minimize these geometrical factors, load and elongation are normalized to the respective parameters of ENGINEERING STRESS and ENGINEERING STRAIN. 6 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Tension Tests: A. Engineering Stresses is defined by the relationship: F is the instantaneous load applied perpendicular to the specimen cross section, in units of newtons (N) or pounds force (lbf) A0 is the original cross-sectional area before any load is applied (m2 or in.2). The units of engineering stress are mega pascals, MPa (SI) (where 1 Mpa=106 N/m2), and pounds force per square inch, psi B. Engineering Strain ! is defined according to: L0 is the original length before any load is applied and li is the instantaneous length. Sometimes the quantity li - l0 is denoted as "l and is the deformation elongation or change in length at some instant, as referenced to the original length. 7 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Stress-Strain Behavior: The degree to which a structure deforms or strains depends on the magnitude of an imposed stress. For most metals that are stressed in tension and at relatively low levels, stress and strain are proportional to each other through the relationship. à Hooke’s Law relationship between engineering stress # and engineering strain for ! = E" à E = $ Elastic Deformation the constant of proportionality E (GPa or psi) is the modulus of elasticity, 8 or Young’s modulus. Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Stress-Strain Behavior: a) Elastic Deformation: is a change of the shape of the body as a reaction to applied stress. This deformation is only temporary and once the stress is released, the un-deformed shape of the body is restored. à This modulus may be thought of as stiffness, or a material’s resistance to elastic deformation. The greater the modulus, the stiffer the material, or the smaller the elastic strain that results from the application of a given stress. 9 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Stress-Strain Behavior: There are some materials (i.e., many polymers) for which this elastic portion of the stress–strain curve is not linear. à The tangent modulus is taken as the slope of the stress– strain curve at some specified level of stress, à The secant modulus represents the slope of a secant drawn from the origin to some given point of the !-" curve. 10 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Stress-Strain Behavior: The magnitude of the modulus of elasticity is a measure of the resistance to separation of adjacent atoms, that is, the interatomic bonding forces. With increasing temperature à the modulus of elasticity decreases. 11 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Stress-Strain Behavior: Elastic deformation is time independent. à It has also been assumed that upon release of the load, the strain is totally recovered— that is, that the strain immediately returns to zero. In most engineering materials, however, there will also exist a time-dependent elastic strain component. à Elastic deformation will continue after the stress application, and upon load release, some finite time is required for complete recovery. This time-dependent elastic behavior is known as ANELASTICITY. ü For metals, the anelastic component is normally small and is often neglected. ü However, for some polymeric materials, its magnitude is significant; in this case it is termed VISCOELASTIC BEHAVIOR 12 Chapter 6: Mechanical Properties of Metals Exercise: A piece of copper originally 305 mm (12 in.) long is pulled in tension with a stress of 276 Mpa (40,000 psi). If the deformation is entirely elastic, What will be the resultant elongation? 13 Chapter 6: Mechanical Properties of Metals Concepts of Stress and Strain: Ø Stress-Strain Behavior: b) Plastic Deformation: For most metallic materials, elastic deformation persists only to strains of about 0.005. à As the material is deformed beyond this point, the stress is no longer proportional to strain (Hooke’s law ceases to be valid), and permanent, non- recoverable, or Plastic Deformation occurs. The transition from elastic to plastic is a gradual one for most metals; some curvature results at the onset of plastic deformation, which increases more rapidly with rising stress 14 Chapter 6: Mechanical Properties of Metals Tensile Properties: Ø Yielding and Yield Strength: Most structures are designed to ensure that only elastic deformation will result when a stress is applied. à It is therefore desirable to know the stress level at which plastic deformation begins, or where the phenomenon of YIELDING occurs. For metals that experience this gradual elastic–plastic transition, the point of yielding may be determined as the initial departure from linearity of the stress–strain curve; this is sometimes called the proportional limit, as indicated by point P (Yield Point). Proportional limit à followed by Yield Point (Elastic Limit) 15 Chapter 6: Mechanical Properties of Metals Tensile Properties: Ø Yielding and Yield Strength: The stress corresponding to the inter-section of this line and the stress– strain curve as it bends over in the plastic region is defined as the yield strength !y (MPa or psi). Yield Strength à it measures the resistance to plastic deformation. Determination of Yield Point (initial departure from linearity) and Yield Strength (stress at Yield Point); 1. Straight Line à parallel at 0.002 2. Nonlinear à the stress at some amount of strain (ex: ∈ = 0.005) 16 Chapter 6: Mechanical Properties of Metals Tensile Properties: Till Yield Point Elastic Plastic (YP) /Yield Deformation Deformation. Strength (YS) After yielding, the stress necessary to continue plastic deformation in metals increases to a maximum, point M à Tensile Strength (TS): the maximum stress that can be sustained by a structure in tension. (if this stress is applied and maintained, fracture will result) à Necking. à Fracture Strength (FS): the stress at fracture. 17 Chapter 6: Mechanical Properties of Metals Exercise: From the tensile stress–strain behavior for the brass specimen shown in the Figure, determine the following: (a)The modulus of elasticity (b)The yield strength at a strain offset of 0.002 (c)The change in length of a specimen originally 250 mm (10 in.) long that is subjected to a tensile stress of 345 MPa (50,000 psi) 18 Chapter 6: Mechanical Properties of Metals Tensile Properties: Ø Ductility: It is a measure of the degree of plastic deformation that has been sustained at fracture. à A metal that experiences very little or no plastic deformation upon fracture is termed brittle. Ductility may be expressed quantitatively as either percent elongation or percent reduction in area. Percent elongation (%EL) is the Percent reduction in area (%RA) is percentage of plastic strain at fracture defined as 19 Chapter 6: Mechanical Properties of Metals Tensile Properties: Ø Ductility: à As with modulus of elasticity, the magnitudes of both yield and tensile strengths decline with increasing temperature; Just the reverse holds for ductility—it usually increases with temperature. ↑ Temperature à ↓ Tensile and Yield Strengths ↑ Temperature à ↑ Plastic Deformation à ↑ Ductility 20 Chapter 6: Mechanical Properties of Metals Tensile Properties: Ø Resilience: It is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. à The associated property is the modulus of resilience, Ur, which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding. The resilient materials are those having high yield strengths and low moduli of elasticity; such alloys are used in spring applications. 21 Chapter 6: Mechanical Properties of Metals Tensile Properties: Ø Toughness: It is a mechanical term that may be used in several contexts. For one, toughness(or more specifically, fracture toughness) is a property that is indicative of a material’s resistance to fracture when a crack (or other stress- concentrating defect) is present. à It is the area under the ! − # curve up to the point of fracture. Hence, even though the brittle metal has higher yield and tensile strengths, it has a lower toughness than the ductile one, as can be seen by comparing the areas ABC and AB’C’ in the Figure 22  Chapter 6: Mechanical Properties of Metals Exercise: Of those metals listed in Table 6.3, (a)Which will experience the greatest percentage reduction in area? Why? (b)Which is the strongest? Why? (c)Which is the stiffest? Why? Bà Most Ductile; highest strain at fracture Dà Strongest; highest Yield and Tensile Strengths Eà Stiffest; highest elastic modulus 23  Summary: Mechanical Behavior VS Crystal Structure What do you expect? RECALL à In general, dislocations are Harder and Stronger à Higher Stiffness à more mobile at higher temperatures, Less Ductile à Brittle Fracture (a) enabling plastic deformation and ductile Less Stiffness à More Ductile à Ductile fracture. Fracture (c) 24 Summary: à Decreasing Brittleness à Decreasing Ductility and Malleability HCP BCC FCC FCC BCC HCP HCP APF = 0.74 BCC APF = 0.68 FCC 3 slip Systems APF = 0.74 No. Atoms = 6 48 slip Systems No. Atoms = 2 12 slip Systems CN = 12 No. Atoms = 4 CN = 8 CN = 12 25 26 27