About the Course

I. IntroductionMECE 3245 Materials Science Lab

About the Course

Materials Science Lab - MECE3245

1 hour lectures 8- 3 hour Experiments

Wednesdays 2:30-3:30 pmE223-D3

Fridays W249-D3

2- quizzes

Fridays W249-D3

copyright © 2008 by Li Sun

I. IntroductionMECE 3245 Materials Science Lab

Lab Experiments

Basic Phenomena:

Mechanical Behavior • Phase diagrams• Recrystallization of Brass

• Tensile Testing• Charpy toughness testing• Fatigue of metal• Creep

Microstructure / Property Control

• Age Hardening• Heat treatment of Steel

I. IntroductionMECE 3245 Materials Science Lab

Lectures

Basic Phenomena Review: Mechanical behavior:

Crystallography / dislocationsPhase EquilibriaDiffusion

Tensile stress-strainToughnessFatigue

Thermo-mechanical control of microstructure

Work hardeningHeat treatment

I. IntroductionMECE 3245 Materials Science Lab

Lab Schedule

Lecture 01/17 Wednesday, 5-8 lectures depend on progress

First lab starts on Feb, 2nd

02/02 Lab 102/09 Lab 202/16 Lab 302/23 Lab 403/02 Quiz03/09 Open

03/12-03/17 Spring Break03/23 Lab 503/30 Lab 604/06 Lab 704/13 Lab 804/20 Quiz

I. IntroductionMECE 3245 Materials Science Lab

Lab Organization

• Phase diagrams• Recrystallization of Brass• Fatigue of metal• Heat treatment of Steel

• Tensile Testing• Charpy toughness testing• Age Hardening• Creep

Three sessions: 9am-noon, 1-4pm, 4-7pm

Once you sign-up for one session, you can not switch to other two.

I. IntroductionMECE 3245 Materials Science Lab

Sign-up

Three sessions: A, B, C

Each session has four groups: 1, 2, 3, 4

Four student per group

I. IntroductionMECE 3245 Materials Science Lab

What do you need to do?

• Sign-up for the lab section.• Get your Materials Science textbook out (Calister)• Get familiar with the lab manual before each experiment• Read lab report format and requirement

I. IntroductionMECE 3245 Materials Science Lab

Mechanical Properties of Solids

copyright © 2007 by Li Sun

I. IntroductionMECE 3245 Materials Science Lab

Mechanical Properties

Materials response to external mechanical forces.

• Stress and strain: What are they and why are they used instead of load and deformation?• Elastic behavior: When loads are small, how 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?

I. IntroductionMECE 3245 Materials Science Lab

Engineering Stress and StrainF

F

l0

ll0

F

F

lx

y

zO

0

0

0 lll

ll i −=

∆=ε

0AF

=σ (N/m2)

Original cross-section area

l0 : initial length,li : final length

I. IntroductionMECE 3245 Materials Science Lab

Shear and Torsional Stress and Strain

0AF

=τ (N/m2)

θγ tan= no unitF

F

θ

T

φ

T: torqueφ: twist angle

I. IntroductionMECE 3245 Materials Science Lab

Typical Tensile Test Curves

σ

0

ceramic

metal

polymer

ε

I. IntroductionMECE 3245 Materials Science Lab

Stress-Strain Testing• Typical tensile test

machine

specimenextensometer

• Typical tensile specimen

gauge length

Fig 6.2, 6.3

I. IntroductionMECE 3245 Materials Science Lab

Stress-strain Curve and Sample Deformation

0ε

σ

I. IntroductionMECE 3245 Materials Science Lab

Linear Elastic Properties

• Modulus of Elasticity, E:(also known as Young's modulus)

• Hooke's Law:

σ = E ε σ

Linear-elastic

E

ε

F

Fsimple tension test

I. IntroductionMECE 3245 Materials Science Lab

Elastic and Plastic Deformation2. Small load

Fd

Bonds stretch

return to initial

3. Unload1. Initial

1. Initial

elastic + plastic

3. Unload

plasticd

Planes still stretched

2. large load

F

d

Bonds stretch + Planes stretch

Elastic means reversible!

Plastic means permanent!

I. IntroductionMECE 3245 Materials Science Lab

Important Concepts Associated with the σ-ε Curve

Tensile strength: maximum stress on the σ-ε curve.

σTSσy

σf

0 εYield strength: stress when plastic deformation begins to progress.

Fracture strength: stress when fracture happens.

Resilience: strain energy stored per unit volume in material up to yielding point.

Toughness: strain energy stored per unit volume in material up to fracture point.

I. IntroductionMECE 3245 Materials Science Lab

Important Concepts Associated with the σ-ε Curve - continued

Ductility: Percentage of plastic strain at fracturePercentage of area reduction at the fracture surface

I.. %100%0

0 ×⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

lll

EL f

0 ε

σ

l0

lf

εTS

• When ε<εTS, elongation is uniform along the whole test sample.• When ε>εTS, necking happens, and significant part of plastic deformation happens in the neck region. For short gauge the percentage of elongation will be larger than in long specimen.

%EL is sample length sensitive

II.. %100%0

0 ×⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

AAA

RA f

%RA is sample length insensitive

0 ε

σ

l0

lf

I. IntroductionMECE 3245 Materials Science Lab

True Stress and true Strain

Engineering stress and engineering strain

0AF

=σ0

0

0 lll

ll i −=

∆=ε

0 ε

σ

With respect to original cross-section area and length

True stress and true strainWith respect to instantaneous cross-section area and sample length

0ln0 0

1lildlil

l ldT =∫ ∫== ε εε

iT A

F=σ

I. IntroductionMECE 3245 Materials Science Lab

Relation between σ and σT , ε and εTc

iAF

T =σ0

lnlil

T =ε0

0

0 lll

ll i −=

∆=ε

0AF

=σ

Under uniaxial loading and there is not volume change during deformation,

00lAlA ii =

llli ∆+= 0

lllAAi ∆+

=0

00

0ln

lil

T =ε

00ln

lill

T+

=ε

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

01ln

lil

00

0 )(lA

llFAF

iT

∆+==σ

⎟⎟⎠

⎞⎜⎜⎝

⎛ ∆+=

01

llσ

)1( εσσ +=T )1ln( εε +=T

I. IntroductionMECE 3245 Materials Science Lab

Stress-strain Curve

0 ε

σEngineering

True

The true stress increases with increasing strain during the plastic deformation. This is the strain hardening behavior.

I. IntroductionMECE 3245 Materials Science Lab

Strain Hardening

• Curve fit to the stress-strain response:

σT = K εT( )n

“true” stress (F/A) “true” strain: ln(L/Lo)

hardening exponent:n = 0.15 (some steels) to n = 0.5 (some coppers)

• An increase in σy due to plastic deformation.σ

ε

large hardening

small hardeningσy0

σy1