Fracture toughness fatigue and creep
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Fracture, Toughness, Fatigue, and Creep. Materials Science & Manufacturing PROCESSES. Why Study Failure. 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. Mechanical Failure.

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Fracture, Toughness, Fatigue, and Creep

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Fracture toughness fatigue and creep

Fracture, Toughness, Fatigue, and Creep

Materials Science

&

Manufacturing PROCESSES


Why study failure

Why Study Failure

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


Mechanical failure

Mechanical Failure

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?

Computer chip-cyclic

thermal loading.

Hip implant-cyclic

loading from walking.

Ship-cyclic loading

from waves.


What is a fracture

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.


Fracture modes

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


Ductile vs brittle failure

Very

Moderately

Fracture

Brittle

Ductile

Ductile

behavior:

(%EL)=100%

Large

Moderate

Small

Ductile vs Brittle Failure

• Classification:

• Ductile

fracture is usually

desirable!

Ductile:

warning before fracture, as increasing is required for crack growth

Brittle: No warning


Example failure of a pipe

Example: Failure of a Pipe

• Brittle failure:

--many pieces

--small deformation

• Ductile failure:

--one/two piece(s)

--large deformation


Moderately ductile failure cup cone fracture

Moderately Ductile Failure- Cup & Cone Fracture

void growth

shearing

void

necking

fracture

and linkage

at surface

nucleation

s

50 mm

50 mm

• Resulting

fracture

surfaces

(steel)

100 mm

particles

serve as void

nucleation

sites.

• Evolution to failure:

crack occurs perpendicular to tensile force applied


Ductile vs brittle failure1

Ductile vs. Brittle Failure

cup-and-cone fracture

brittle fracture


Transgranular vs intergranular fracture

Transgranular vs Intergranular Fracture

Intergranular Fracture

Transgranular Fracture


Brittle fracture surfaces

Brittle Fracture Surfaces

• Transgranular

(within grains)

• Intergranular

(between grains)

304 S. Steel (metal)

316 S. Steel (metal)

160mm

4mm

Polypropylene

(polymer)

Al Oxide

(ceramic)

3mm

1mm


Stress concentration stress raisers

Stress Concentration- Stress Raisers

σm›σo

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

so = applied stress

sm = stress at crack tip

Kt = Stress concentration factor

t

Theoretical fracture strength is higher than practical one; Why?


Concentration of stress at crack tip

Concentration of Stress at Crack Tip


Engineering fracture design

s

o

m

Stress Conc. Factor, K

t

=

s

o

w

s

2.5

max

h

r ,

w/h

2.0

increasing

fillet

radius

1.5

r/h

1.0

0

0.5

1.0

sharper fillet radius

Engineering Fracture Design

• Avoid sharp corners!

s

Kt


Crack propagation

Crack Propagation

Cracks propagate due to sharpness of crack tip

  • A plastic material deforms at the tip, “blunting” the crack.

    deformed region

    brittle

    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.

plastic

When σm›σy


Fracture toughness design against crack growth

Fracture Toughness: Design Against Crack Growth

Kc=

--Result 2: Design stress

dictates max. allowable flaw size.

--Result 1: Max. flaw size

dictates design stress (max allowable stress).

amax

fracture

fracture

no

no

amax

fracture

fracture

• Crack growth condition:

• Largest, most stressed cracks grow first!

σc

σc


Fracture toughness

Fracture Toughness

  • Brittle materials do not undergo large plastic deformation, so they posses low KICthan 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


Design example aircraft wing

Design Example: Aircraft Wing

• Use...

9 mm

--Result:

112 MPa

4 mm

Answer:

• Material has Kc = 26 MPa-m0.5

• Two designs to consider...

Design B

--use same material

--largest flaw is 4 mm

--failure stress = ?

Design A

--largest flaw is 9 mm

--failure stress = 112 MPa

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

• Reducing flaw size pays off!


Impact tests

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


A charpy test

(Charpy)

final height

initial height

A. Charpy Test

Impact energy= Kinetic energy + energy absorbed by specimen

Energy absorbed during test is determined from difference of pendulum height


B izod test

b. Izod Test

  • Izod test varies from charpy in respect of holding of specimen


Effect of temperature on toughness

Effect of Temperature on Toughness

• Increasing temperature...

--increases %EL and Kc

• Ductile-to-Brittle Transition Temperature (DBTT)...

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

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

polymers

Impact Energy

Brittle

More Ductile

s

High strength materials (

> E/150)

y

Temperature

Ductile-to-brittle

transition temperature


Fatigue test

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


Types of cyclic stresses1

Types of Cyclic Stresses

Random Stress Cycle


Terms related to cyclic stresses

Terms Related to Cyclic Stresses

Mean stress:

Range of stress:

Stress Amplitude:

Stress Ratio:


4 creep

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

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

Stress & Temp Effects on Creep

  • Time to rupture decreases as imposed stress or temperature increases

  • Steady creep rate increases with increase of stress and temperature


Fracture toughness fatigue and creep

Good Luck


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