Concept of ‘FATIGUE’ in WELDED STEEL STRUCTURES

Under the guidance of M H Prashanth Assistant Professor NITK

Submitted by B Harish

16ST05F

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

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 .

• 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 fatigueb) Low–cycle fatigue

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

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

Factors influencing Fatigue Behavior1. 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

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

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

• 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.

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

S-N approach : constant amplitude

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

• 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)

• 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.

Variable amplitude loading

Example for Two Constant Amplitude Blocks of Loading S-N curve(Log-Log)

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

Fracture Mechanics Approach

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

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

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

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

• 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

Permissible stress in WeldsButt 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.

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 .

• 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.

)

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

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

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.