lecture 11 fatigue 2

8/4/2019 Lecture 11 Fatigue 2

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Outline

at gue - ev ew

Fatigue crack initiation and propagation

Fatigue fracture mechanics

Fatigue fractography

Crack propagation rate

Factors affecting fatigue

-Design factors

- ur ace e ects-Environmental effects

Dr. M. Medraj Mech. Eng. Dept. - Concordia University MECH 321 lecture 11/1

Types of Fatigue

Occurs under dynamic stresses

.

o me a a ures occur n a gue

Occurs in all kinds of materials

Usually breaks .; no, or

very e, o serva e p as c e orma on(some micro-deformation).

low c cle ati ue

high cycle fatigue

- high loads short Nf(104-105 cycles)


low loads long Nf(>105)

Variability in Fatigue Data

Fati ue data isnormally shown as.. values.

evaluate theprobabilities of fatiguefailure at certain stresslevel. This is moreaccurate than averagevalues.


Crack Initiation and Propagation

ree steps:

..

..

na a ure w en area ecreasessufficiently)

Fi ure 6.48 Schematic re . of

Fatigue life:

Nf= Ni + Np Ni is the number of cycles to initiate fracture

a fatigue fracture surface in a

steel shaft. When the crack

length exceeds a .

Np is the number of cycles to propagate tofailure

high cycle fatigue ( stress levels):

value at the applied stress,

catastrophic rupture occurs.The science and Engineering of

th

most of the life is spent in crack initiationand Ni is high

low cycle fatigue (.. stress levels):


. .

Askeland and P.P. Phule. propagation step predominates and Np>Ni


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Fatigue Crack Propagation Rate

Logarithm crack growth rate versuslogarithm stress intensity factor range foraNiMoVsteel.

, -result when log(da/dN)-versus-logKdata are plotted

correspond to the values ofm andlogA, respectively.


Fatigue Crack Propagation Rate

One goal of failure analysis is to predict fatigue life of a

component, given its service conditions.

We are able now to develop analytical expression forFailure by integrating in the linear region:

==

=

f cN a

m

da

KAdN :givesintegratedenwhich wh

)(Limits are initial flaw length, ao which canbe determined byNDT, and critical crack

a mKA0f

0 )(

==cc aa dada 1

N:ivesKforonSubstituti

length ac, which can be determined fromfracture mechanics

a

mmmm

a

m

aYAaYA 00 )()(

this results when assuming thatK and are constant (which often are not)

Also i nores the time needed to initiate the crack.


This analysis is only an estimate.

Example

A re at ve y arge s eet o stee s to e expose to cyc c tens e ancompressive stresses of magnitudes 100 MPa and 50 MPa, respectively. Prior totesting, it has been determined that the length of the largest surface crack is 2.0mm. Estimate the fati ue life of this sheet if its lane strain fracture tou hness is25 MPam and the values ofm andA are 3.0 and 1.0 x 10-12, respectively, for in MPa and a in m. Assume that the parameter Y is independent of cracklength and has a value of 1.0.


Factors Affecting Fatigue Life

Important factors are:

mean stress level

geometrical design

surface condition metallurgical structure

environment

Mean Stress (m)

in stress reversal, m= 0

m > 0, then S-N curve moves to

lower values


fatigue life .


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Effect of Mean Stress


Geometrical Effects

Design Factors

component design is important

notches or stress raisers act as

crack initiation sites for fatigue: grooves, scratches, keyways,

, .

sharp corners and radii, any

discontinuities

all increase the stressconcentration

rounded fillets where gradual

changes of diameter occur inshafts


cra c es an mac n ng mar s re uce e a gue e

Surface polishing . the fatigue properties

Geometrical Effects


Geometrical Effects



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Effect of Surface Conditions


Effect of Welding


Surface Treatments

shot peening localized micro-plastic deformation

us ng sma stee a s s ot) mpact ng on sur ace.

It increases the fatigue properties significantly

(aircraft components etc) also increases yield

strength, hardness and fatigue life.

work hardening occurs in the surface surface hardness


Surface burnishing

introduces a residual ..... stress

Effect of Welding and Shot Peening



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Effect Shot Peening on Mean Stress


Surface Treatments

Case Hardening:

Surface hardenin throu h

carburizing or nitriding increasessurface strength and hardness

form in the surface layer to

~1mm depth or greater

ncrease n ar ness ncreases t e

resistance to fatigue.

compressive stress in case

hardening also generated due todifference in volume of case layer


Effect of Materials Composition


Effect of Grain Size



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Environmental Effects

Thermal Fatigue

created at high temperature by fluctuating thermal stresses (t )

restraint in thermal expansion/contraction during uneven

heating/cooling

= E T

l is the linear thermal expansion coefficient

E is the modulus of elasticity

T is the temperature difference, l T is the thermal strain t

elimination of restraint and temperature gradients (use

expansion gaps)


consideration ofphysical properties, (materials CTE, k)

Environmental Effects

Corrosion Fatigue

s mu aneous e ec o cyc c s ress an c em ca attac

formation ofpits leading to stress concentration on surface and

nucleation of fatigue cracks

corrosion can enhance crack growth rate

protective coatings (painting, galvanizing)

selection of more corrosion resistant material

reducing the corrosive environment


Next time:

Creep


lecture 11 fatigue 2

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