Mechanical properties of materials
The proper and efficient use (value engineering) of materials in machine elements requires considerable knowledge of their mechanical properties. The mechanical properties describe the behaviour of the material under use as a machine element in machine.
The most important mechanical properties are creep, ductility, elasticity, hardness, machinability. malleability, resilience, stiffness, strength, and toughness.
Strength
Strength is the ability of the material to resist stress without failure. The measure of the strength is the ultimate stress. Ultimate strength refers to the force needed to fracture the material.
When materials are subjected to a pulling force (tensile force) they stretch as the stress increases. The stress-strain relationship can be graphed, when test specimen is subjected to tensile load. The diagram is a graph between stress and % elongation. With the help of this diagram different-strengths of material can be defined.
When material is subjected to a pulling force the point where the stretch suddenly increases is known as the yield strength. In many design problems when the yield strength of materials is passed it is considered unsafe for further service. When mild steel is subjected to a pulling force it indicates a distinct point where the stretch suddenly increases. This is known as yield point. Some materials like high nickel alloys, monel metal and other similar non-ferrous materials do not show a definite break in the stress strain curve. In this case it is difficult to assign yield point for them. For such materials yield strength of material is defined at the point where 0.5% elongation takes place.
Proportional limit is the maximum stress under which a material will maintain a perfectly uniform rate of strain to stress. However, it is difficult to measure the exact proportional limit.
The maximum stress from which a material can recover is called the elastic limit. It is difficult to specify the elastic limit and so the idea of proof stress has been developed.
Proof stress is the maximum stress a material can withstand without taking more than small amount of set. The amount is usually specified as the smallest that can be measured by an extensometer.
The proportional limit is yield strength at 0.00% offset in the stress-strain relationship graph. Proof stress is yield strength at 0.01% offset and yield strength is yield strength at 0.2% offset on stress elongation curve under tensile load.
Several materials such as structural steel, copper, aluminium, etc. have equal strength in tension or compression, but their strength in shear is about two-thirds of the strength in tension while in grey cast iron the strength in tension and shear is a fraction of the strength in compression.
Shear strength is the force per unit area produced to fracture a specimen when it is impressed along the cross section of material. The material may be subjected to single shear or double shear. The shear strength of steels compared to their ultimate tensile strength ranges from about 50 to 80 per cent, the lower values for the harder materials.
Elasticity
Elasticity is the property of regaining original shape after deformation. All materials used in machine elements and structures (construction) are elastic but the degree of elasticity varies with different materials. This property is exceedingly important in precision tools and machines. Steel is highly elastic material.
Plasticity is the property that enables the formation of permanent deformation in a material. Stiffness is the property by virtue of which a material can resist deformation. Measure of stiffness is the modulus of elasticity. This property is desirable in materials used in machines, columns, beams and machine tools.
Ductility
Ductility is the property of material that enables it to be drawn out or elongated to an appreciable extent (subject to tensile force) before rupture occurs. The percentage elongation and the percentage of reduction of area before rupture of a test specimen are measures of ductility of the material.
Percentage elongation depends on gauge length and so gauge length is required to be stated when percentage elongation is given. Indian Standard Institution recommends gauge length of 5.65 SQRT( A) where A is the cross sectional area of the test specimen.
Brittleness is opposite to ductility. It shows lack of ductility. Brittle materials show little deformation before rupturing.
Materials with more than 15% elongation are usually considered ductile. Those with less than 5% elongation are considered brittle. Those between 5 and 15% elongation are of intermediate ductility. Property of ductility is desirable in machine parts which may be subjected to sudden and severe loads.
Mild steel, wrought iron, copper and aluminium are ductile materials. Cast iron is a brittle material.
For a variety of engineering uses a material requires good combination of strength and ductility. Usually if two materials having the same strength and hardness the one that has the higher ductility is more desirable in engineering practice.
Malleability
Malleability is the property of a material that enables it to undergo great change in shape under compressive stress without rupture.Malleable materials may be hammered or rolled into any desired shape without rupture.
Soft steel, wrought iron, copper and aluminium are malleable metals.
Hardness
Hardness is that property of a material that enables it to resist penetration, indentation, abrasion or plastic deformation.
In selecting a metal to withstand wear or erosion, mainly three properties are considered: ductility, toughness and hardness. However, the most important from wear resistance point is hardness Wear, which may be either due to friction or erosion by steam, oil, and water is resisted by materials having higher hardness. This property is decreased by heating.
Several methods have been developed for hardness testing. Those most often used are Brinell, Rockwell, Vickers, and Scleroscope. The first three are based on indentation tests and the fourth
on the rebound height of a diamond-tipped metallic hammer.
In order to relate one method of testing hardness with another, hardness conversion charts are available.
Tensile strengths are often listed on hardness conversion charts. Although relationships exist between hardness, and tensile strength and yield strength, but there are chances for error. Therefore, use of tensile testing machine is preferred to determine strength.
Resilience
Resilience is that property of a material which enables it to store energy and resist shock and impact. The measure of resilience is the amount of energy that can be stored per unit volume after being stressed to elastic limit. This property is desirable in materials for springs.
Toughness
Toughness is the property which enables a material to be twisted, bent or stretched under a sudden impact or under a high stress before rupture. It is measured by the Izod test or Charpey test. The measure of toughness is the amount of energy that a unit volume of material has absorbed after being stressed up to the point of fracture. This property is decreased by heating.
Creep
Creep is expressed as the plastic behaviour of the metals or plastics under constant load and at constant temperature.Creep is observed as the material deforms slowly but progressively over a period of time under the same load and temperature. There are three stages of creep. In the first stage the material elongates rapidly but at a decreasing rate. In the second stage which is ordinarily of long duration the rate of elongation is constant. In the third stage the rate of elongation increases rapidly until the material fails.
Design engineers are most concerned with second stage of creep, where elongation takes place at a constant specific rate. The percentage of elongation and time required are decided by the requirements of the particular application, viz, 0.1 per cent elongation in 10,000 hours. In rapidly rotating structural members such as rotors and blades of of steam and gas turbines, the clearances are extremely small and critical. The designer will be satisfied with nothing short of experimentally determined stress of 1 creep rate unit (CRU) or 1 per cent in 100,000 hours.
Determination of mechanical properties:
In order to determine the mechanical properties of the material, tests are carried out in mechanical testing laboratories. These tests are carried out according to standard procedures laid down. The simplest test that can be made on most materials is the static tensile test. The procedure to carry out this test is suggested by Indian Standards Institution. The values for the following properties are obtained from the test:
(i) Ultimate tensile strength
(ii) Proportional limit
(iii) Elastic limit or Proof Stress
(iv) Yield point or Yield strength
(v) Percentage elongation
(vi) Percentage reduction in area.
IS 1608 — 1960 is to be adhered to while carrying out the static tensile test.
Other tests commonly employed are compression, torsion, flexure, cold bending, hardness, impact and fatigue. Data of these various tests are usually shown graphically by the stress strain diagrams.
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