Mechanical Properties of Metals

The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness. We shall now discuss these properties as follows:
1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called *stress.
2. Stiffness. It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.
3. Elasticity. It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.
4. Plasticity. It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.
5. Ductility. It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminium, nickel, zinc, tin and lead.
Note : The ductility of a material is commonly measured by means of percentage elongation and percentage
reduction in area in a tensile test. (Refer Chapter 4, Art. 4.11).

FIRST MULTICOLOUR EDITION
A TEXTBOOK OF
Machine
Design
(S.I. UNITS)
[A Textbook for the Students of B.E. / B.Tech.,
U.P.S.C. (Engg. Services); Section ‘B’ of A.M.I.E. (I)]
R.S. KHURMI
J.K. GUPTA


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    Sections and sectional views

    A section is used to show the detail of a component, or an assembly, on a particular plane which is known as the cutting plane. A simple bracket is shown in Fig. 8.1 and it is required to draw three sectional views.
    Assume that you had a bracket and cut it with a hacksaw along the line marked B–B. If you looked in the direction of the arrows then the end view B–B in the solution (Fig. 8.2), would face the viewer and the surface
    indicated by the cross hatching would be the actual metal which the saw had cut through. Alternatively had we cut along the line C–C then the plan in the solution would be the result. A rather special case exists along the plane A–A where in fact the thin web at this point has been sliced. Now if we were to cross hatch all the surface we had cut through on this plane we would give a false impression of solidity. To provide a more realistic drawing the web is defined by a full line and the base and perpendicular parts only have been cross hatched. Note, that cross hatching is never undertaken between dotted lines, hence the full line between the web and the remainder of the detail.
    However, the boundary at this point is theoretically a dotted line since the casting is formed in one piece and no join exists here. This standard drawing convention is frequently tested on examination papers.
    Cutting planes are indicated on the drawing by a long chain line 0.35 mm thick and thickened at both ends to 0.7 mm. The cutting plane is lettered and the arrows indicate the direction of viewing. The sectional view or plan must then be stated to be A–A, or other letters appropriate to the cutting plane. The cross hatching should always be at 45° to the centre lines, with continuous lines 0.35 mm thick.
    If the original drawing is to be microfilmed successive lines should not be closer than 4 mm as hatching lines
    tend to merge with much reduced scales. When hatching very small areas the minimum distance between lines
    should not be less than 1 mm.
    In the case of very large areas, cross hatching may be limited to a zone which follows the contour of the hatched area. On some component detail drawings it may be necessary to add dimensions to a sectional
    drawing and the practice is to interrupt the cross hatching so that the letters and numbers are clearly visible.

    Manual of
    Engineering Drawing
    Second edition
    Colin H Simmons
    I.Eng, FIED, Mem ASME.
    Engineering Standards Consultant
    Member of BS. & ISO Committees dealing with
    Technical Product Documentation specifications
    Formerly Standards Engineer, Lucas CAV.
    Dennis E Maguire
    CEng. MIMechE, Mem ASME, R.Eng.Des, MIED
    Design Consultant
    Formerly Senior Lecturer, Mechanical and
    Production Engineering Department, Southall College
    of Technology
    City & Guilds International Chief Examiner in
    Engineering Drawing
    Elsevier Newnes
    Linacre House, Jordan Hill, Oxford OX2 8DP
    200 Wheeler Road, Burlington MA 01803


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    The Blow Molding Process

    THEORETICAL BACKGROUND Blow molding is a fabrication process to convert the raw materials (resin) into finished hollow containers (products). It is a manufacturing process by which hollow plastic parts are formed. Principle of the Most Basic Type of Blow Molding 1. Resin is melted by heaters and plasticized by an extruder. 2. Then it is extruded so that it forms a tube-shaped parison matching the size of the product as it passes through the die. 3. Air is then forced into the parison and press it against the inner walls of the mold. After the parison cools and become solid in the mold, the air is released. 4. Finally, the mold will open and the product is deflashed and ejected or taken out the machine. Higher Institute for Plastics Fabrication WORKBOOK for Extrusion Blow Molding Practical Course Prepared by Extrusion Blow Molding Department 1st Edition 2009
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    Safety factor

    A safety factor (SF) or factor of safety (FS) (also called factor of ignorance) is used with plastics or other materials (metals, aluminum, etc.) to provide for the uncertainties associated with any design, particularly when a new product is involved with no direct historical performance record. There are no hard and fast rules to follow in setting a SF. The most basic consideration is the consequences of failure. In addition to the basic uncertainties of graphic design, a designer may also have to consider additional conditions such as: (1) variations in material property data (data in a table is the average and does not represent the minimum required in a design); (2) variation in material performance; (3) effect of size in stating material strength properties; (4) type of loading (static, dynamic, etc.); (5) effect of process (stress concentrations, residual stress, etc.); and (6) overall concern of human safety.
    The SF usually used based on experience is 1.5 to 2.5, as is commonly used with metals. Improper use of a SF usually results in a needless waste of material or even product failure. Designers unfamiliar with plastic products can use the suggested preliminary safety factor guidelines in Table 7.3 that provide for extreme safety; intended for preliminary dcsign analysis only. Low range values represent applications where failure is not critical. The higher values apply where failure is critical. Any product designed with these guidelines in mind should conduct tests on the products themselves to relate the guidelines to actual performance. With more experience, more-appropriate values will be developed targeting to use 1.5 to 2.5. After field service of the preliminary designed products has been obtained, action should be taken to consider reducing your SF in order to reduce costs.
    Plastics
    Engineered
    Product
    Design
    Dominick Rosato and
    Donald Rosato
    ELSEVIER


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