MATERIALS SCIENCE&

MANUFACTURING PROCESSES

Fracture, Toughness, Fatigue, and Creep

2

In order to know the reasons behind the occurrence of failure so that we can prevent failure of products by improving design in the light of failure reasons

Why Study Failure

3

ISSUES TO ADDRESS...

• How do flaws in a material initiate failure?• How is fracture resistance quantified; how do different material classes compare?• How do we estimate the stress to fracture?• How do loading rate, loading history, and temperature affect the failure stress?

Ship-cyclic loadingfrom waves.

Computer chip-cyclicthermal loading.

Hip implant-cyclicloading from walking.

Mechanical Failure

What is a Fracture?

Fracture is the separation of a body into two or more pieces in response to an imposed stress that is static and at temperatures that are low relative to the melting temperature of the material.

The applied stress may be tensile, compressive, shear, or torsional

Any fracture process involves two steps—crack formation and propagation—in response to an imposed stress.

5

Fracture Modes

Ductile fracture Occurs with plastic deformation Material absorbs energy before fracture Crack is called stable crack: plastic deformation

occurs with crack growth. Also, increasing stress is required for crack propagation.

• Brittle fracture– Little or no plastic deformation – Material absorb low energy before fracture

– Crack is called unstable crack.– Catastrophic

6

Ductile vs Brittle Failure

Very Ductile

ModeratelyDuctile BrittleFracture

behavior:

Large Moderate

(%EL)=100%

Small

• Ductile fracture is usually desirable!

• Classification:

Ductile: warning before

fracture, as increasing is

required for crack growth

Brittle: No

warning

7

• Ductile failure:

--one/two piece(s) --large deformation

Example: Failure of a Pipe

• Brittle failure:

--many pieces --small deformation

8

• Evolution to failure:

• Resulting fracture surfaces

(steel)

50 mm

particlesserve as voidnucleationsites.

50 mm

100 mm

Moderately Ductile Failure- Cup & Cone Fracture

necking

void nucleation

void growth and linkage

shearing at surface fracture

crack occurs perpendicular to tensile force applied

9

Ductile vs. Brittle Failure

cup-and-cone fracture brittle fracture

Transgranular vs Intergranular Fracture

Transgranular Fracture

Intergranular Fracture

11

• Intergranular

(between grains)• Transgranular

(within grains)

Al Oxide(ceramic)

316 S. Steel (metal)

304 S. Steel (metal)

Polypropylene(polymer)

3 mm

4 mm160 mm

1 mm

Brittle Fracture Surfaces

12

Stress Concentration- Stress Raisers

Suppose an internal flaw (crack) already exits in a material and it is assumed to have a shape like a elliptical hole:

The maximum stress (σm) occurs at crack tip:

where t = radius of curvature

o = applied stress

m = stress at crack tip

Kt = Stress concentration factor

ot

/

tom Ka

21

2t

Theoretical fracture strength is higher

than practical one; Why?

σm › σo

13

Concentration of Stress at Crack Tip

14

Engineering Fracture Design

r/h

sharper fillet radius

increasing w/h

0 0.5 1.01.0

1.5

2.0

2.5

Stress Conc. Factor, K tmo

=

• Avoid sharp corners!

r ,

fillet

radius

w

h

o

max

Kt

15

Crack Propagation

Cracks propagate due to sharpness of crack tip A plastic material deforms at the tip, “blunting” the

crack. deformed

regionbrittle

Effect of stress raiser is more significant in brittle materials than in ductile materials. When σm exceeds σy , plastic deformation of metal in the region of crack occurs thus blunting crack. However, in brittle material, it does not happen.

plasticWhen σm › σy

16

• Crack growth condition:

• Largest, most stressed cracks grow first!

Fracture Toughness: Design Against Crack Growth

Kc = aY c

--Result 1: Max. flaw size dictates design stress (max allowable stress).

maxaY

Kcdesign

amaxno fracture

fracture

--Result 2: Design stress dictates max. allowable flaw size. 2

1

design

cmax Y

Ka

amax

no fracture

fractureσc

σc

Fracture Toughness

Brittle materials do not undergo large plastic deformation, so they posses low KIC than ductile ones.

KIC increases with increase in temp and with reduction in grain size if other elements are held constant

KIC reduces with increase in strain rate

18

• Two designs to consider...Design A --largest flaw is 9 mm --failure stress = 112 MPa

Design B --use same material --largest flaw is 4 mm --failure stress = ?

• Key point: Y and Kc are the same in both designs.

Answer: MPa 168)( B c• Reducing flaw size pays off!

• Material has Kc = 26 MPa-m0.5

Design Example: Aircraft Wing

• Use...aY

Kcc

BA aa cc

9 mm112 MPa 4 mm --Result:

Impact Tests

A material may have a high tensile strength and yet be unsuitable for shock loading conditions

Impact testing is testing an object's ability to resist high-rate loading.

An impact test is a test for determining the energy absorbed in fracturing a test piece at high velocity

Types of Impact Tests -> Izod test and Charpy Impact test

In these tests a load swings from a given height to strike the specimen, and the energy dissipated in the fracture is measured

20

A. Charpy Test

final height initial height

(Charpy)

Impact energy= Kinetic energy + energy absorbed by specimen

Energy absorbed during test is determined from difference of pendulum height

b. Izod Test

Izod test varies from charpy in respect of holding of specimen

22

• Increasing temperature...

--increases %EL and Kc• Ductile-to-Brittle Transition Temperature (DBTT)...

Effect of Temperature on Toughness

Low strength BCC metals (e.g., iron at T < 914°C)

Imp

act

Ene

rgy

Temperature

High strength materials (y > E/150)

polymers

More Ductile Brittle

Ductile-to-brittle transition temperature

Low strength FCC metals (e.g., Cu, Ni)

Fatigue Test

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating loads (e.g. bridges, aircrafts, ships and m/c components)

The term Fatigue is used because this type of failure occurs after a lengthy period of repeated stress of strain cycling.

Failure stress in fatigue is normally lower than yield stress under static loading.

Fatigue failure is brittle in nature even in ductile metals

The failure begins with initiation and propagation of cracks

Types of Cyclic Stresses

Types of Cyclic Stresses

Random Stress Cycle

Terms Related to Cyclic Stresses

Mean stress:

Range of stress:

Stress Amplitude:

Stress Ratio:

4. Creep

Creep is defined as time dependent plastic deformation under constant static load/stress (steam turbines blades under centrifugal force, pipes under steam pressure) at elevated temperatures

At relatively high temperatures creep appears to occur at all stress levels,

But the creep rate increases with increasing stress at a given temperature.

4. Creep Test

A creep test involves a tensile specimen under a Constant Load OR Constant Stress maintained at a constant temperature.

Temperature: Greater than 0.4Tm

Stress & Temp Effects on Creep

1. Time to rupture decreases as imposed stress or temperature increases

2. Steady creep rate increases with increase of stress and temperature

Good Luck