chapter 2 (part 2)

LOGO

Chapter 2 (Part 2): Principle of Material Science and

Engineering

BKG3493 GAS SYSTEM MATERIALS &

COMPONENTS

CHAPTER OUTCOMES

At the end of this part, you should be able to:

i) Explain mechanical properties for metals such as stress, strain, elasticity modulus, hardness, toughness, ductility and etc

ii) Explain mechanical properties for other materials

iii) Aware that engineering design should include the safety factor

METALS & ALLOYS Metals are used in engineering for many reasons, but

they generally serve as structural elements.

Alloy is a metal composed of more than one element. An alloy has primary constituent and primary alloying elements.

e.g. (i) carbon steel (CS) - Fe, C, Mn

(ii) Stainless steel (SS) – Fe, Cr, Ni, C, Mn

The simplest questions that a design engineer can ask about a structural material are “ How strong is

it?” “How much deformation must I expected given a certain load?”

4

ISSUES TO ADDRESS...

• Stress and strain: What are they and why are they used instead of load and deformation?

• Elastic behavior: When loads are small, how much deformation occurs? What materials deform least?

• Plastic behavior: At what point does permanent deformation occur? What materials are most

resistant to permanent deformation?

• Toughness and ductility: What are they and how do we measure them?

5

• Simple tension: cable

Common States of Stress

o

s = F

A s s

Ski lift (photo courtesy

P.M. Anderson)

A o = cross sectional

area (when unloaded)

F F

6

(photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM

o

s = F

A

• Simple compression:

Note: compressive structure member

(s < 0 here). (photo courtesy P.M. Anderson)

OTHER COMMON STRESS STATES (i)

A o

Balanced Rock, Arches National Park

ENGINEERING STRESS

s =Ft

Aooriginal area

before loading

Stress has a pressure unit

Tensile stress, s: Shear stress, t:

ENGINEERING STRAIN

• Tensile strain: • Lateral strain:

• Shear strain:

= tan Strain is always

dimensionless.

/2

/2

/2 -

/2

/2

/2

L/2L/2

Lowo

TENSILE TEST

www.themegallery.com

Basic description of the material obtained (see Figure 6.1)

The load necessary to produce a given elongation is monitored as the specimen is pulled in tension at a constant rate

This test produces a load-versus-elongation curve (see Figure 6.2)

A more general statement about material characteristics is obtained by normalizing the data resulting stress-versus-strain curve

TENSILE TEST

STRESS VS STRAIN-METALS


0

P

As =

Stress? Strain? Units?

0

l

l

D =

Stress σ= Force/Area

Force is also called load (Newton)

So, σ= N/m2= Pa

Usually stress is presented in MPa

Strain Є = extension/ original length

Є=l2-l1/l1 =mm/mm (dimensionless)

1. Modulus of elasticity 2. Yield strength , YS 3. Tensile strength, TS 4. Ductility 5. Toughness Stiffness? Elastic deformation? Plastic deformation?

(1-5) are the key mechanical properties obtained from tensile

test

STRESS VS STRAIN-METALS

ELASTIC DEFORMATION


F

bonds

stretch

return to

initial

1. Initial 2. Small load 3. Unload

Elastic means reversible!

PLASTIC DEFORMATION


1. Initial 2. Small load 3. Unload

Plastic means permanent!

MODULUS OF ELASCITY


Is the slope of the stress-strain curve in the elastic region

Also known as Young’s modulus

The linearity of the stress-strain curve in the elastic region is a graphical statement of Hooke’s law

( σ = EÎ)

This modulus represents the stiffness of the material – its resistance to elastic strain

At the point that the curve is no longer linear and

deviates from the straight-line relationship, Hooke's Law

no longer applies and some permanent deformation

occurs in the specimen.

YIELD STRENGTH (YS)


• Stress at which noticeable plastic deformation has occurred.

when ep = 0.002 tensile stress, s

engineering strain, e

sy

ep = 0.002

The yield strength is defined relative to the intersection of the stress-strain curve with a 0.2% offset

YIELD STRENGTH: COMPARISON


19

Tensile Strength, TS

• Metals: occurs when noticeable necking starts. • Polymers: occurs when polymer backbone chains are aligned and about to break.

sy

strain

Typical response of a metal

F = fracture or

ultimate

strength

Neck – acts as stress

concentrator

en

gin

eeri

ng

TS

str

ess

engineering strain

• Maximum stress on engineering stress-strain curve.

TENSILE STRENGTH (TS)

TENSILE STRENGTH : COMPARISON


DUCTILITY, %EL


Quantified as the percent elongation at failure

%EL =L f Lo

Lo

x100

• Another ductility measure: %AR =

Ao A f

Ao

x100

TOUGHNESS


• Energy to break a unit volume of material • Approximate by the area under the stress-strain curve.

smaller toughness- unreinforced polymers

Engineering tensile strain, e

Engineering

tensile

stress, s

smaller toughness (ceramics)

larger toughness (metals, PMCs)

TOUGHNESS


The toughness of an alloy depends on a combination of strength and ductility

CREEP


Plastic deformation occur at high temperatures, constant load, long time period

HARDNESS


• Resistance to permanently indenting the surface. • Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties.

PROPERTIES OF OTHER MATERIALS


Ceramics and Glasses

PROPERTIES OF OTHER MATERIALS


Polymers

Flexural modulus

* Based on the same specimen geometry for ceramics (MOR)

Stress-strain

curves for

polyester

engineering

polymer

SUMMARY

• Stress and strain: These are size-independent measures of load and displacement, respectively.

• Elastic behavior: This reversible behavior often shows a linear relation between stress and strain. To minimize deformation, select a material with a large elastic modulus (E or G).

• Plastic behavior: This permanent deformation behavior occurs when the tensile (or compressive) uniaxial stress reaches sy.

• Toughness: The energy needed to break a unit volume of material.

• Ductility: The plastic strain at failure.

• Stiffness: resistance to elastic strain – represented by E

“Every engineering design must take into

account the safety factor (time 1.2 to 4)”

chapter 2 (part 2)

Documents