Concept of FATIGUE in welded steel structures

1. Concept of ‘FATIGUE’ in WELDED
STEEL STRUCTURES
Under the guidance of
M H Prashanth
Assistant Professor
NITK
Submitted by
B Harish
16ST05F

2. Overview
• Introduction
• Mechanism and Factors influencing Fatigue
• Effect of fatigue loading on Structural members
and Weld connection
• Fatigue Analysis...S-N approach
...Fracture Mechanics approach
• Indian Standard Practice
• Improvements of Fatigue Strength techniques
• Conclusions and References

3. Introduction
• DEF: A Fatigue failure can be defined as
“the number of cycles (or) the time taken to attain a pre-
defined failure criterion.”
• Fatigue phenomenon is experienced by structures, which are
subjected to moving loads(such as bridges and crane girders)
or structures subjected to cyclic loads .

4. • Fatigue Failure Proceeds in 3 distinct stages:
 crack initiation in the areas of stress concentration (near
stress raisers)
 incremental crack propagation
 final catastrophic failure.
• In most of the welded steel structures the crack initiation
phase does not exist as crack–like weld defects are invariably
present in them.
• In the case of fatigue fracture of engineering structures, the
following are the two main types of fatigue loading.
a) High–cycle fatigue
b) Low–cycle fatigue

5. Mechanism of Fatigue Failure
• At Present 4 mechanisms are proposed for fatigue failure:
1)Orowan’s theory:
Metals contain small,weak regions for
-- orientation of slip (or) areas of stress concentration
results in notch formation
-- if total plastic strain > critical value, crack is formed

6. 2) Mott theory :
cross slip of screw locations  part of metal is extruded from
surface  cavity is left in the interior of crystal  source of
fatigue crack.
3)Wood’s theory :
fine slip movement  static slip bands back & forth of fine
slip bands formation of notches or ridges a fatigue crack
4)Cottrell & Hull theory :
two different slip systems when work with different directions
produce slip at surface  forming intrusions and
extrusions

7. Factors influencing Fatigue Behavior
1. Stress concentration: Points where stress distribution
is different from that adopted ---
a)relative deformations
b)Macroscopic stress concentration`
c)Local geometric stress concentration
2. Frequency of cyclic loading : indicates the number of
fatigue load cycles per second
no influence on fatigue strength if low stress range
and low frequency of loading
3.Size effect :
small members- more fatigue resistance- max size
effect

8. 4.Residual Stress :
compressive residual stress
tensile residual stress
5.Material :
Tests have shown that under ideal conditions, fatigue limit is
approximately 50% of ultimate stress.
However, other factors may considerably alter this fatigue
limit.
The general relation between fatigue limit and ultimate stress
is given below
F1 = 140+0.25 Fu
F1 = fatigue limit for zero to maximum tensile loading in Mpa

9. Effect of fatigue loading
• In structural members:
 Compressive members will not generally produce fatigue failure
The fatigue behavior of tensile members is generally decided
by their connections
• In Weld Connections :
Residual stresses of yield stress level are always present in
the vicinity of welds.
 Fatigue resistance of welded joint is the geometry and
the resulting stress concentration effects.
Fatigue life varies with the type of weld details due to the varying
nature of the defects in the different details

10. • Fatigue failure at a welded joint may occur in
any one or combination of the following
Failure in the line of fusion, due to lack of proper fusion or
microscopic cracks.
Failure in the heat affected zone due to crystalline change in
the base metal.
 Failure at the toe edge of the weld, which is a stress
concentrated point due to the joint design, weld contour
undercut, etc.

11. Fatigue Analysis
• Fatigue analysis involves :
1) determination of nominal stresses in structural members
2)stress concentration factors at critical points
3)safe number of stress reversals before the onset of failure.
• Fatigue life determination
 S-N approach : Constant amplitude
Variable amplitude
 Fracture mechanics approach

12. S-N approach : constant amplitude

14. In order to determine the fatigue strength of a welded
joint configuration, under a given load condition, it is
necessary to test a series of similar specimens.
 Each of the specimens is subjected to constant
amplitude loading and the number of loading cycles
required to produce failure in each specimen is
recorded.
 The relationship between the applied stress , S and the
number of cycles to failure, N, is obtained.
Logarithmic scales are commonly used for both axes,
namely, LogS – LogN

15. • In constant amplitude loading, one cycle equals two reversals.
R = -1 is called the fully reversed condition since Smin= -Smax
R = 0, where Smin = 0, is called pulsating tension.
Example of constant amplitude
loading S-N curve (Log-Log)

16. • For welded joints the relationship between fatigue life
and applied stress range is linear over a wide range of
stress and takes the form
N = number of cycles to failure,
= applied stress range
m, = depending upon the joint type.

17. Variable amplitude loading
Example for Two Constant Amplitude Blocks of
Loading S-N curve(Log-Log)

18. Palmgren-Miner’s rule
• C= part of life decreased due to applied stress ranges
(generally C=1)
C<1 failure occurs if high to low cycle testing
C>1 failure occurs if low to high cycle testing

19. Fracture Mechanics Approach

20. An important parameter K which is defined as stress at the tip of
crack and is known as stress intensity factor(K)
K=Yσ
Y= geometric correction factor(depends on crack
size,shape,loading)
σ = nominal stress
a = crack length

21. Fatigue life assessment by fracture mechanics is based on the
observed relationship between the stress intensity factor range
and rate of growth of fatigue cracks da /dN
= crack extension per cycle
C,m = crack growth constants can be determined by
conducting tests on materials

22. In welded connections, the stress intensity factor and
fatigue strength is given by following equations with initial and final
crack depth ai and af respectively

23. IS code of Practice
• IS: 1024 – 1999 “Code of Practice for Use of Welding in Bridges and
Structures Subjected to Dynamic Loading” covers the use of metal arc
welding in bridges and structures subjected to fatigue loading.
• Welding details-7
Class A
Class B
Class C
Class D
Class E
Class F
Class G

24. • For each of the stress ranges, the maximum allowable number
of cycles N1, N2 ……Nn
should be determined from the tables given in
the IS Code 1024 - 1999.
• Considering the expected number of cycles for each stress
level as n1, n2 ……nn,
the element should be so designed such that
n1 / N1+ n2 / N2+ n3 / N3+-------------------+ nn / Nn >1.0

25. Permissible stress in Welds
Butt Welds
• Butt welds shall be treated as parent metal with a thickness
equal to throat thickness, and the stresses shall not exceed
those in the parent metal.
• In structures subjected to dynamic loading, tensile (or) shear
stresses in butt welds shall not exceed 66 percent of the
permissible stresses.

26. Fillet Welds
• The basic permissible stress in fillet welds based on a
thickness equal to the throat thickness shall be 100 N/mm2.
• Load carrying fillet welds in dynamically loaded structures
shall be designed so that the secondary bending stresses are
not introduced
• The permissible stresses for field welds of structural members
shall be reduced to 80 percent of those specified in Field
welds .

27. • For combined shear and bending stresses, the equivalent
stress fe is given as
(or)
• The equivalent stress should not exceed 0.9Fy where Fy is
the yield strength of the steel.
)

28. Improvements of Fatigue Strength
techniques
• Crack initiation life can be extended by
 Reducing the stress concentration of the weld.
Removing crack-like defects at the weld toe.
Reducing tensile welding residual stress or introducing
compressive stresses.
• The various methods of improvements can be classified
into:
Weld geometry improvement by grinding, weld dressing
Residual stress reduction by peening and thermal stress
relief

29. Conclusions
• various factors affecting fatigue behavior of welded
connections are explained.
• The nature of fatigue in welded connections and its critical
importance are showed.
• Methods of evaluating fatigue lives of welded connections
are described.
• Techniques for improvement in fatigue performance are
presented

30. References
• Fatigue analysis of welded joints:state of development by
Wolfgang Fricke.
• Study of effect of welding joint location on fatigue strength
and fatigue life for steel weldment by Dr. Ali Sadiq Yasir.
• Estimating the fatigue behavior of welded joints by
M.D.Chapetti.
• Owens G.W. and Cheal B.D., ‘Structural Steelwork
Connections’, Butterworths,London, 1989-chapter 6.
• IS:1024-1999, ‘Code of Practice for Use of Welding in Bridges
and Structures Subjected to Dynamic Loading’, Bureau of
Indian Standards.