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LESSON 2 Properties of Materials
The properties of a material are those characteristics that help modify and distinguish one material from another. All properties are observable and most can be measured quantitatively.Properties are classified into two main groups,physical and chemical properties.Physical properties involve no change in the composition of the material.Chemical properties are associated with the transformation of one material into another.Physical properties are, in turn, arbitrarily subdivided into many categories. These subdivisions bear names such as mechanical,metallurgical,fabrication,general,magnetic,electrical,thermal,optical,thermonuclear,and electro-optical. Regardless of the name of subdivision,physical properties result from the response of the materials to some environmental variable,such as a mechanical force, a temperature change, or an electro-magnetic field. In the following, the mechanical property of materials will be discussed.
Mechanical Property of Materials
In selecting a material for a product such as a piston in an internal combustion engine, a designer is interested in properties such as strength, ductility, hardness, or fatigue strength. Mechanical properties are defined as a measure of a material’s ability to carry or resist mechanical forces or stresses. When any matter is at rest, the atomic or molecular structure is in equilibrium. The bonding forces in this structure resist any attempt to disrupt this equilibrium. One such attempt may be an external force or load. Stress results from forces such as tension, compression, or shear that pull, push, twist, cut, or in some way deform or change the shape of a piece of material.
Stress and strain Stress is defined as the resistance offered by a material to external forces or loads. It is measured in terms of the force exerted per area. Normal stress is that applied perpendicular to the surface to which it is applied, i.e., tension or compression. Another way of defining it is to say that stress is the amount of force divided by the area over which it acts. An assumption is made that the stress is the same on each particle of area making up the total area. If this is so, the stress in uniformly distributed. The force and the area over which the force acts can be used to calculate the stress produced in the material. With the use of polarized light and models made of photoelastic plastic, it is possible to detect the concentration of stress.
Strain, or unit deformation, is defined as the unit change in the size or shape of material as a result of force on the material. Many times, we assume that a solid body is rigid; that is, when the body is loaded with some force, the body keeps its same size and shape. This is far from correct. Regardless of how small the force ,a body will alter its shape when subjected to a force. The change in a physical dimension is called deformation.
When a piece of material is subjected to a load, it will not only deform in the direction of the load(axial deformation),but it will also deform in a lateral direction. The ratio of the lateral unit deformation or strain to the unit longitudinal deformation or strain is known as Poisson’s ratio.
Stress-strain Diagrams The stress-strain diagram is used to determine how a certain material will react under load. Figure 2.1 is a stress-strain diagram for low-carbon steel. Strain values(mm/mm) and stress values(MPa) are plotted respectively along the horizontal axis and the vertical axis.
The straight-line portion of the diagram up to almost the yield point is known as the elastic region of the diagram. Within this range of stresses, the material will return to its original dimensions once the load, hence, the nominal stress, has been removed.
Beyond the yield point, the material will continue to deform, but with less stress than before, because the material has begun to yield. In this region, known as the plastic region, plastic deformation takes place and when the load is removed, the material will not return to its original dimensions.
The yield point represents the dividing line or transition from the elastic to the plastic region of the curve. When the stress reaches the yield point, a large increase in strain occurs with no increase in stress. The modulus, modulus of elasticity in tension, or coefficient of elasticity is the ratio of the stress to the strain in the elastic region of the stress-strain diagram. The tensile modulus approximately equal to the compressive modulus of elasticity within the proportional limit. This modulus is an indication of the stiffness of the material when subjected to a tensile load. The stiffness of a material is defined as the ratio of the load to the deformation produced.
Not all materials produce stress-strain diagrams on which there is a clear indication of the start of yielding as the load is increased. Cast iron is an example. This situation should not be interpreted to mean that cast iron does not exhibit elastic properties under moderate loads. The modulus of elasticity for these materials is sometimes taken as the slope of a tangent to the stress-strain curve at the origin.
Ultimate Strength of Tensile Strength Ultimate strength or tensile strength is the maximum stress developed in a material during a tensile test. It is a good indicator of the presence of defects in the crystal structure of a metal material, but it is not used too much in design because considerable plastic deformation has occurred in reaching this stress. In many applications the amount of plastic deformation must be limited too much smaller values than that accompanying the maximum stress.
Yield Strength Many materials do not have a yield point. This poses a problem in deciding when plastic deformation begins for such materials. By agreement, a practical approximation of the elastic limit, called the offset yield strength, is used. It is the stress at which a material exhibits a specified plastic strain. For most applications, a plastic strain of 0.002 in./in. can be tolerated, and the stress that produces this strain is the yield strength. This is sometimes expressed as 0.2% strain. The yield strength is determined by drawing a straight line, called the offset line, from the 0.2% strain value on the horizontal axis parallel to the straight-line portion of the stress-strain curve. The stress at which this offset line intersects the stress-strain curve is designated as the yield strength of the material at 0.2% offset.
A second family of stresses is known as shear stress or shearing stress. A shearing force produces a shear stress in a material, which, in turn, results in a shearing deformation. A stress-strain diagram can be plotted using shear stress and shear strain. Such a diagram will show a definite straight-line portion(elastic region)in which the shearing stress is directly proportional to the shearing strain, Like the normal stress-strain ratio, the ratio of the two shear quantities, is known as the modulus rigidity or shear modulus of elasticity.
A material that can undergo large plastic deformation without fracture is called a ductile material. A brittle material, on the other hand, shows an absence of ductility. Consequently, a brittle material shows little evidence of forthcoming fracture by yielding, as a ductile material would do. A ductile material, by yielding slightly, can relieve excess stress that would ultimately cause fracture. This yielding can be accomplished without any degradation of other strength properties.
The ability or capacity of a material to absorb energy during plastic deformation is known as toughness. The modulus of toughness, equal to the total area under the stress-strain curve up to the point of rupture, represents the amount of work per unit volume of a material required to produce fracture under static conditions. Toughness can also be expressed in terms of the ease or difficulty in propagating a crack. It can be measured by the amount of energy absorbed by a material in creating a unit area of crack. A tough material would have no defects in its microstructure. Impact is defined as a sudden application of a load confined to a localized area of a material. Exemplified by the striking of a material with a hammer, this relatively quick application of force, as opposed to a slow or static loading of a material can cause considerable damage to a material that cannot adequately redistribute the stresses caused by the impact. Ductile materials usually survive impact due to their microstructure, which allows slip to take place. Most metals have good toughness and thus have good impact resistance. Due to their inherent nature as compounds of metals and nonmetals, ceramics do not possess the ability to redistribute stresses and plastically deform. Consequently, they have poor toughness, poor impact resistance, and poor fracture toughness.
Malleability, workability, and formability are some terms related to ductility that describe the ability of materials to withstand plastic deformation without the occurrence of negative consequences as a result of undergoing various mechanical processing techniques. Terms such as weldability, brazability , and machinability, although more properly classified under processing properties, are mentioned here as additional examples of terms used to generally describe the reaction of materials to various manufacturing and/or fabricating processes in industry.
Flexural or Bending Strength
Figure 2.2 is a sketch of a simple supported beam. The transverse load or force P bends the beam, thus resulting in normal stresses in compression near the top surface and normal stresses(tensile)at the bottom of the beam. Assuming that the beam material is homogeneous and isotropic, the normal stresses will be at a maximum near the top and bottom surfaces of the beam. These normal stresses are known as flexural or bending stresses. The maximum bending stress developed at failure is known as the flexural strength. For those materials that do not crack, the maximum bending or flexural stress is called the flexural yield strength.
The fatigue or endurance limit is the maximum stress below which a material can presumably endure an infinite number of stress cycles. Fatigue strength is the maximum stresses that can be sustained for a specified number of cycles without fracture. In other words, fatigue strength can be any value on the ordinate of the stress-cycles diagram. The fatigue limit, as determined empirically, is generally below the yield strength. Most design stresses are lower than the fatigue or yield strength of a material primarily because of the adverse effects of surface conditions on the strength of materials. Another term used in describing failures is endurance ratio, which is the quotient of endurance limit to tensile strength. For many ferrous alloys, the endurance limit is about one-half the tensile strength of the metal.
Creep is a slow process of plastic deformation that takes place when a material is subjected to a constant condition of loading(stress) below its normal yield strength. After a certain amount of time has elapsed under constant load, the creep strain(plastic deformation) will increase and some materials will rupture, or fracture, is known as creep rupture. Creep occurs at any temperatures. However, at low temperatures, slip is impeded by impurity atoms and grain boundaries. At high temperatures, the diffusion of atoms and vacancies permits the dislocations to move around impurity atoms and beyond grain boundaries, which results in much higher creep rates. Different types of materials have different creep characteristics, dependent on the structure of materials.
Torsion describes the process of twisting. The torsional stress is the shear stress produced in the material by the applied torque and is calculated using the torsion formula. Torsional yield strength roughly corresponds to the yield strength in shear. The ultimate torsional strength or modulus of rupture expresses a measure of the ability of material to withstand a twisting load. It is roughly equivalent to the ultimate shear strength. The torsional modulus of elasticity is approximately equal to the shear modulus or the modulus of elasticity in shear.
Hardness is measure of a material’s resistance to penetration(local plastic deformation) or scratching. One of the oldest and most common hardness tests, based on measuring the degree of penetration of a material as an indication of hardness, is the Brinell. Brinell hardness numbers(HB) are a measure of the size of penetration made by a 10mm steel or tungsten carbide sphere with different loads, depending on the material under test. The indentation size is measured using a microscope containing an ocular scale. Vickers hardness numbers(HV) employ a diamond pyramid indenter. Rockwell hardness testers, using a variety of indenters and loads with corresponding scales, are direct reading instruments(i.e., the hardness is read directly from a dial).