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Dr. Alagiriswamy A A, (M.Sc, PhD, PDF)Asst. Professor (Sr. Grade),

Dept. of Physics, SRM-University,Kattankulathur campus,

Chennai

UNIT VLecture 1

MECHANICS OF MATERIALS

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Introduction

Fundamental mechanical properties

Stress-strain relation for different engineering materials

Introduction to Ductile materials /Brittle material

Outline of the presentation

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Types of Materials

Ferrous Metals: iron and steel.

Nonferrous Metals and Alloys: aluminum, magnesium, copper, nickel, titanium, super-alloys, beryllium, zirconium, low-melting alloys, precious metals.

Plastics: thermoplastics, thermosets, elastomers.

Ceramics: glass, graphite, diamond.

Composite materials: reinforced plastics, metal-matrix and ceramic-matrix composites, honeycomb structures.

MaterialsMechanical Properties: strength, toughness, ductility, hardness, elasticity, fatigue, creep, Brittleness, toughness, stifness, resilience, endurance etc.

Behavior Under Loading: tension, compression, bending, torsion, shear.

Physical Properties: density, specific heat, thermal expansion, thermal conductivity, melting point, electrical and magnetic properties.

Chemical Properties: oxidation, corrosion, degradation, toxicity, flammability.

Properties of Materials

Manufacturing Processes for

MetalsCasting: expendable mold and permanent mold.

Forming and Shaping: rolling, forging, extrusion, drawing, sheet forming, powder metallurgy, molding

Machining: turning, boring, drilling, milling, planning, shaping, broaching, grinding, ultrasonic machining, chemical machining, electrical discharge machining (EDM), electrochemical machining, high-energy beam machining

Joining: welding, brazing, soldering, diffusion bonding, adhesive bonding, mechanical joining

Finishing: honing, lapping, polishing, burnishing, deburring, surface treating, coating, plating

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Manufacturing a Product: General

Considerations

Material Selection

Processing Methods

Final Shape and Appearance

Dimensional and Surface Finish

Economics of Tooling

Design Requirements

Safety and Environmental Concerns

Manufacturing;

“The Process of

Converting Raw

Materials Into

Products”

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Bricks and glass do not deform and break easily.

Rubber bands deform a lot but return to their original shape

A paper clip easily deforms but does not easily return to its original shape

The thicker something is, the more force we have to exert to get it to break

What Does our Experience Tell Us?

When do materials deform/break?

Why do they deform/break ?

How do they??

Central Questions

Why Do Materials Differ in Their Mechanical Response ???

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Internal/External stress

relaxation is the key

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ENGINEERS NEED A WAY TO QUANTIFY THESE

DIFFERENCES

Why don’t you think in terms of the chemical bonds and

chemical structures that are present

A deeper approach

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Isotropy ; physical properties – direction independent. Ex: Aluminum, steels/cast irons Anisotropy; direction dependent, Ex: Various composite materials, wood and laminated plasticsElasticity; • able to regain its original shape/size after the deformation

within the elastic limit (Hooke’s law)• Stress is linearly proportional to strainPlasticity; • able to permanently deform, after the stress is removed• Stress and strain no longer linearly relatedYield strength (an important ENGINEERING parameter); • defines the stress at which plastic DEFORMATION begins (Al -

370 Mpa, Steel-1500 Mpa, Cu 490 Mpa)

Some Important Definitions

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Different means of load applied

When a metal stretches, but does not break under a certain load, this point is called the _________ Point.

A: yield B: tensileC: stretch D: ultimate strength

Quiz time

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A close correlation/analogy

Necking begins

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Some terminology

of the term “Strength”

Elastic Strength; The strength value of a material , it s’ behavior changes from elastic to plastic regime

Plastic Strength; plastic to rupture regime

Tensile Strength; Ultimate strength corresponds to maximum load

Compressive Strength; The value of load applied to break-off by crushing.

Shear Strength; The value of load applied (specifically tangential load)

Torsional Strength; The value of load applied (specifically twisting load)

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Resilience property- stores energy and resists shocks or impacts

ToughnessAmount of energy absorbed by a material up to the fracture

Endurance property - withstand varying stresses or repeated application of stress.

Some More terms

Amount of energy absorbed by a material in the ELASTIC

regionToughness

CREEPING; deformation increases even under constant load

E.g..- Rubber stretching, concrete bridge

To be Precise ; A compelling competition

between elastic and plastic deformation

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Stress-Strain Relation for Different Engineering Materials

ferrous metals

non - ferrous metals

Brittle; don’t exhibit yielding before failure

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All dim. in mm

An another example

Structural steel

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Polyamide

Yet another piece of information

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A deeper look on stress-

strain curves

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A close comparison

areationalcrosseousIns

loadeousInsStressTrue T sectantan

tan)(

Original/actual area

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The nominal stress σn = P/A0

where P is the force and A0 the original area of cross section

The nominal strain, εn = (L-L0)/L0

where L is the length of the original gauge length under force P, and L0 is the original gauge length. 0

n0

n L

L1or1

L

L

iT A

P)(

L

dL.

L

L

0t

0L

LIn

)1(0

nL

L

)1(In nt

Engineering stress/strain diagrams - elastic range, while true stress – strain diagrams

plastic range.

0L

Lnt