Mechanical Behavior of Materials Engineering Methods for Deformation

Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, Third Edition, by Norman E. Dowling. ISBN 0 -13 -186312 -6. © 2007 Pearson Education, Inc. , Upper Saddle River, NJ. All rights reserved. This material is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education Inc. , Upper Saddle river, NJ 07458.

Suspension bridge

CHAPTER 6 Mechanical Properties of Metals - I 3

Processing of Metals & Alloys 4 • Most metals are first melted in a furnace. • Alloying is done if required. • Large ingots are then cast. • Sheets and plates are then produced from ingots by rolling Wrought alloy products. • Products manufactured by hot and cold working the metal from large ingots are called wrought alloy products • Channels and other shapes are produced by extrusion. • Some small parts can be cast as final product. Example : - Automobile Piston.

Blast Furnace 5

Ingots 6

Classes of Processes Figure 2. 4

Classifying Processes Choice of process is based on design requirements: Material • Shape, dimensions, and precision • Number to be made • Process Families Shaping Primary Shaping Secondary Joining Primary process creates shapes Secondary process Surface Treatment modifies shapes or properties Materials: engineering, science, processing and design, 2 nd

Metal Shaping Process Hot Rolling of Steel (I-beams, rails, sheet & plate) • Passing a piece of metal between two rolls • Hot rolling Greater reduction of thickness in a single pass. • Rolling carried out at above recrystallization temperature. • Ingots preheated to about 12000 C. • Ingots reheated between passes if required. • Usually, series of 4 high rolling mills are used. 9

Metal Shaping Process Cold Rolling of Metal Sheet • • • 10 Cold rolling is rolling performed below recrystallization temperature. Hot rolled slabs have to be annealed before cold rolling. Series of 4 high rolling mills are usually used. Less reduction of thickness. Needs high power.

Metal Shaping Process Extrusion A plastic forming process in which a material under high pressure is reduced in cross section by forcing it through an opening in a die. Hollow tube leaving the extrusion die

Metal Shaping Process Extrusion • • 12 Metal under high pressure is forced through opening in a die. Common Products are cylindrical bar, hollow tubes from copper, aluminum etc. Normally done at high temperature. Indirect extrusion needs less power however has limit on load applied Die Container Metal Direct Extrusion Container Metal indirect Extrusion

Sample metal products made from Extrusion (rods, tubings etc)

Metal Shaping Process Forging • A primary process method for working metals into useful shapes in which the metal is hammered or pressed into shape. Direct • Types: Forging Ø Open die: Dies are flat and simple in shape * Example products: Steel shafts Metal Ø Closed die: Dies have upper and lower impresion * Example products: Automobile Indirect Forging engine connection rod. 14 • Forging increases structural properties, removes porosity and increases homogeneity. Die

Basic Shapes for Open -die Forging A set of closed forging dies used to produce an automobile connecting rod 15

Sample products made from forging (wrenches, crankshafts)

Drawing • Metal Shaping Process Wire drawing : - Starting rod or wire is drawn through several drawing dies to reduce diameter. % cold work = Change in cross-sectional area Original area X 100 Wire or rod Carbide nib • Deep drawing: - Used to shape cup like articles from flats and sheets of metals 17

Wire drawing (rods, wire tubing) Wire is pulled from the die rods, wire, tubing

Sample products made from wire drawing (rods, wire tubing)

Casting Metal Shaping Process A fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold Cast parts 20 Casting Process Casting mold

Sample products made from casting large parts, e. g. , auto engine blocks, turbine blades, jewelry, complex shapes

Process-property Interaction Effects processing can have on material properties: • Soft, stretchy rubber becomes hard and brittle when vulcanized • Annealing a metal increases its ductility • Glass becomes bullet-proof through a specific heat treatment • Electroplating improves corrosion resistance Materials: engineering, science, processing and design, 2 nd

Stress and Strain in Metals 23 • Metals undergo deformation under uniaxial tensile force. • Elastic deformation: Metal returns to its original dimension after tensile force is removed. • Plastic deformation: The metal is deformed to such an extent such that it cannot return to its original dimension

Plastic deformation

v During elastic deformation the metal atoms are displaced from their original positions but not to the extent that they take up new positions. Thus, when the force removed, the metal atoms return to their original positions and the metal takes back its original shape. v The ability of some metals to be extensively plastically deformed without fracture is one of the most useful engineering properties of metals.

Five basic types of stress

Engineering Stress and Strain Engineering stress = ρ = Engineering stress σ = Δl A 0 F (Average uniaxial tensile force) A 0 (Original cross-sectional area) Units of Stress are psi or N/m 2 (Pascals) 1 psi = 6. 89 x 103 Pa Engineering strain = ε = Change in length Original length A Units of strain are in/in or m/m. It is common to convert eng. Strain into percent strain or percent elongation: 27 % engineering strain= eng. Strain *100%=% elongation

Tensile test • Strength of materials can be tested by pulling the metal to failure. Load Cell Specimen Extensometer Force data is obtained from Load cell Strain data is obtained from Extensometer. 28

equipment pilot plant scale

Tensile Test

In most cases engineering strain is determined by using a small length, usually 2 in. , called the gage length, within a much longer, for example, 8 in.

Tensile test samples before and after testing, and the measurements required to calculate the percent elongation and the reduction in area

Sample Problems on Stress Example 1: A 0. 5 in diameter aluminum bar is subjected to a force of 2500 lbf. Calculate the engineering stress in pounds per square inch (psi) on the bar. Example 2: A 1. 25 cm diameter bar is subjected to a load of 2500 kg. Calculate the engineering stress in megapascal (MPa) on the bar.

Stress and Strain in metals Engineering Strain ( ε epsilon): Example 3: A sample of commercially pure aluminum 0. 500 in. wide, 0. 040 in. thick, and 8 in. long that has gage markings 2. 00 in. apart in the middle of the sample is strained so that the gage marking are 2. 65 in. apart (Fig. ). Calculate the engineering strain and the percent engineering strain that the sample undergoes.