How Strong is Your Ship

8/4/2019 How Strong is Your Ship

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Underwater archaeology conjures up romantic images of lost civilizations and shipwrecks. The most famous

lost civilization is Atlantis, but my fascination as a boy was with the lost city of Dunwich, in Suffolk, England.

Nowadays it is a small collection of fishermens huts on a windswept beach and some tumble down stone

walls on top of a low sandy cliff. But in the Middle Ages, it was a thriving port, exporting wool and importing

timber and wine. It was a gateway to the fertile lands and the prosperous people of East Anglia. Aggressive

currents and storms caused the silting of the harbour and the erosion of the beaches,

leaving the economy crippled and the people struggling to survive against the sea and

eventually losing.

Surveying shipwrecks is another form of underwater archaeology. Shipwrecks have

been described as time capsules which give insights into the way people lived and

worked. Shipwrecks provide very human stories of tragedy and grief, which are

personalized even to people not directly involved because of the name of the ship.

For example, Vasa, Mary Rose, Titanic and Edmund Fitzgerald all bring to mind specific

events at a point in history.

Underwater archaeology is much more dependent on technology than land based archaeology because of

the remoteness of the sites and the hostile environment. Even in shallow water, divers must use scuba gear

to access the dig. Deep ocean sites such as the wreck of the Titanic need highly sophisticated ocean

technologies to locate and survey the wreck. This includes accurate positioning and charting, side scan sonar

to identify anomalies on the sea bed, and remotely operated vehicles to survey the site and take pictures so

that the findings can be interpreted. It also takes enthusiastic and well prepared people who are determinedto see the job through to the end.

The same underwater technology can also be used to investigate modern accidents. We continue to lose

ships in accidents, and names such as Herald of Free Enterprise, Estonia and Ocean Ranger bring to mind

violent losses in recent years. Enquiries into each accident were held to determine its cause and how similar

accidents might be prevented in the future. In all cases, ocean technology was used to gain insight into the

causes of the flooding or structural failure.

Lost cities and lost ships are examples of how human activity and the oceans are intertwined. In each casewe hoped that we could overcome the powers of the sea but in each case we were wrong. The technical

paper in this issue sheds some light on how ships react to their environment, and hopefully this understanding

will slow the number of ships disappearing and becoming a source of study for future archaeologists.

Whether it is trying to understand an ancient civilization or investigating a recent marine accident,

underwater technology can help us hear the stories of how events came to pass. With more sophisticated

technology we can fill in some important gaps in the narrative, and help to understand our relationship with

the ocean over the centuries past, so that we can learn for the future.

David Molyneux

Technical Editor

From the Technical Editor

Wrecks, Treasure, Antiquity, Vol. 3, No. 4, 2008 47


Copyright Journal of Ocean Technology 2008


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Who should read this paper?

This paper addresses an important issue with regard to how the strength of

structural steel used in marine structures is currently obtained. Results of this

research effort indicate that the interaction between fatigue damage andmechanical properties of structural steel used in a ship hull is important and

should be considered for future design of all such marine structures. Anyone with

an interest in the integrity of the hull of their ship (i.e. all real mariners) will be

intrigued by the work described herein.

Why is it important?

The majority of ships and other such marine structures are built of structural steel.

Current design procedures do not consider the interaction between fatigue damage

and the mechanical properties of the structural steel. This study shows that thestrength of structural steel reduces significantly due to fatigue damage and hence,

the interaction between fatigue damage and mechanical properties of structural

steel is an important consideration for safer design of ships and other marine

structures. To the best of the authors knowledge, this study is the first of its kind.

The results have significant potential for improving the strength and integrity of

future ships and other marine structures. However, additional studies on other

structures and steels will be required before the results may be applied commercially.

About the authors

Sreekanta Das is an Assistant Professor in the Department of Civil and

Environmental Engineering at the University of Windsor. His areas of expertise are

low-cycle-fatigue behaviour of metal and marine structures, and experimental and

finite element analysis of metals and metal structures.

Daniel Grenier is currently a doctoral student at the University of Western Ontario.

His area of expertise is low-cycle-fatigue behaviour of metal.

John Kennedy is an Emeritus and Distinguished Professor of Civil Engineering in

the Department of Civil and Environmental Engineering at the University of

Windsor. His area of expertise is structural engineering.

How strong is your ship really?

Das, Grenier and Kennedy describe the complex

relationship between fatigue and mechanical

strength of ship hull steel.

DAS

48 THE JOURNAL OF OCEAN TECHNOLOGY Reviews & Papers



GRENIER

KENNEDY


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ABSTRACT

A ship hull structure is made of stiffened steel plates. The steel plates are stiffened with steel beams and girders. The

connections between the various structural components are made using a welding process which introduces residual

stresses and strains. The static residual stresses alone may cause material yielding in tension in the plate near the

welded connections. A ship in service experiences continuous fatigue load cycles in addition to static residual and otherlocked-in stresses. Thus, fatigue damage builds up over the service life of a ship. Currently, a ship hull is designed for

fatigue and strength. However, the strength design is undertaken assuming the ship hull material is virgin. Therefore, no

interaction between fatigue damage and strength for material is considered. In reality, a ship in service for a

considerable period of time will have accumulated damage due to fatigue load cycles and this may interact with the

mechanical properties such as strength and ductility of ship hulls steel. Thus, an interaction between fatigue damage

and the mechanical properties of structural steel need to be considered for safer designs of ship hulls. Since the

residual stress alone can cause yielding of steel at the plate-frame welded connections, the cyclic loads during itsservice are expected to produce low-cycle-fatigue load cycles locally at and near these connections. This study was,

therefore, undertaken to understand the effect of low-cycle-fatigue damage on the mechanical properties such as

strength and ductility of structural steel.

KEY WORDS

Ship hull; Residual stress and strain; Low-cycle-fatigue; Fatigue damage; Structural steel; Mechanical properties;

Material strength; Material ductility

EFFECT OF LOW-CYCLE-FATIGUE DAMAGE ON STRENGTH ANDDUCTILITY OF STRUCTURAL STEEL

D. Grenier1, S. Das2 (corresponding author), and J. Kennedy3

1, 2, 3 Department of Civil and Environmental Engineering, University of Windsor, Canada




INTRODUCTION

Several studies were undertaken to evaluate the integrity

of aged ship structures considering fatigue (primarily,

high-cycle-fatigue) as one failure criterion and strength

(yield and buckling strength) as the other failure criterion.

Current design standards and codes usually require that

a mean-minus-two-standard-deviation curve to all S-N

curves be applied to limit the probability of fatigue failure

to 2.4% [for example, DNV, 2002]. These design

standards and codes also require ensuring ship hull

structures strength so that a ship hull does not fail due to

applications of extreme loads caused by slamming or by

collisions or by grounding. However, the strength design


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method recommended is based on the Ultimate Limit

State that relates to the materials yield strength and the

strength design is carried out assuming the ship hull

material is virgin and will remain so during its entire

design life. Thus, no attempts have yet been made toconsider accumulation of fatigue damage on the

materials behaviour. Researchers have expressed

concerns regarding the damage caused by fatigue loads

and its influence on the ultimate load carrying capacity

and ductility for aged ship structures [Dexter and Pilarski,

2002; Gu and Moan, 2002; Huther et al., 2004; Kim et

al., 2005; and Wang et al., 2006].

In reality, a ship hull structure experiences both fatigue

load cycles and extreme loads during its service. A ship

hull structure is made of steel plates stiffened by a steel

frame (beams and girders). The connections between

various structural components (beams, girders, plates,

etc.) are made by using a welding process. The weldingprocess creates localized residual stresses and strains.

The residual stress alone can cause tension yielding of

the steel plate near the welded connections [Osgood,

1954; Kondo and Ostapenko, 1964; Faulkner, 1975;

Wikander et al., 1994; Gao et al., 1998; Hu and Jiang,

1998; Dexter and Mahmoud, 2004; and James et al.,

2006]. A ship in service experiences continuous cyclic

loads due to wave pressures and ship motion in addition

to static residual stresses and other static locked-in

stresses. Thus, fatigue damage accumulates over theservice life of the ship. Since the residual stress alone

can cause yielding of steel in the welded connections,

cyclic loads while in service is expected to impose low-

cycle-fatigue (LCF) load histories in the plate at and near

those welded connections [Wang et al., 2006].

Accumulation of damage is expected to be much higher

and occur at a faster rate when the components of theship hull are subjected to LCF load cycles.

Thus, an interaction between LCF damage and various

mechanical properties of structural steel, such as

remaining strength and ductility, need to be considered

for safer designs of ship hull structure. This is especially

true if the safety of an aged ship hull is to be ensured forextreme load conditions. Therefore, this study was

undertaken to investigate the influence of LCF damage

on the material behaviour, especially the strength and

ductility of structural steel. A series of material tests were

undertaken in this study.




Specimen Test Type Strain Range Fatigue Cycles Specimen Name

1 Pull N/A 0 F000P

2 Fatigue-Pull 0 - 0.003 5,000 F005P

3 Fatigue-Pull 0 - 0.003 20,000 F020P

4 Fatigue-Pull 0 - 0.003 45,000 F045P

5 Fatigue-Pull 0 - 0.003 90,000 F090P

6 Fatigue-Pull 0 - 0.003 135,000 F135P

7 Fatigue-Pull 0 - 0.003 165,000 F165P

8 Fatigue 0 - 0.003 180,000* F180F*Specimen 8 failed after 180,000 fatigue cycles

Table 1: Test matrix.


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TEST PROCEDURE

The following sections discuss the test matrix, material

properties, test setup, and test method used in this study.

Material and Test Matrix

Two different material specimens, as recommended by

ASTM A370 [ASTM, 2006a] and ASTM E606 [ASTM,

2006b], were used in this study. ASTM A370 provides

the specifications and guidelines for determining quasi-

static mechanical properties of materials based on simple

quasi-static tensile tests. ASTM E606 provides thespecifications and guidelines for strain-controlled fatigue

tests for determining LCF life. In this study, the test

procedures followed the recommendations of these two

ASTM standards.

The test matrix used in this study is shown in Table 1.

The specimens are identified by their unique names. Asan example, specimen 4 is named as F045P, where the

first letter (F) indicates that it was a fatigue type specimen

as recommended by ASTM E606. The following number

(045) indicates that the specimen experienced 45,000

strain-controlled LCF load cycles. The last letter indicates

how the specimen was finally loaded to its failure. The

last letter P indicates that the specimen failed due toapplication of quasi-static tensile deformation (Pull test).

An F instead of a P at the end indicates the failure

occurred due to application of fatigue load cycles only

and therefore no quasi-static pull test was undertaken on

this specimen. The first specimen (F000P) was subjected

to a quasi-static tensile deformation only in accordance

with ASTM A370. The objective of this test was todetermine quasi-static material behaviour such as

strength and ductility of virgin steel. The last specimen

(F180F) was subjected to LCF load cycles only in

accordance with ASTM E606 and thus no quasi-static

tensile load or deformation was applied. The objective of

this test was to determine the low-cycle-fatigue (LCF) life

of the material. The remaining specimens (specimens 2through 7) were first subjected to various predetermined

LCF strain cycles followed by a quasi-static tensile




Figure 1: Fatigue and tension test type specimens.


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deformation until they ruptured. Thus, these specimens

(specimens 2 through 7) were first subjected to LCF

strain cycles in accordance with ASTM E606 and then

followed by a quasi-static tensile deformation unit rupturein accordance with ASTM A370. The objective of these

tests was to introduce various levels of LCF damage in

the material before undertaking a quasi-static tensile test.

The load and strain data for all the specimens were

acquired through a data acquisition system.

All specimens were prepared from a 25.4 mm (1 in)diameter carbon steel round bar using a suitable lathe to

ensure no initial strain occurred while preparing the

specimens. The chemical properties of the material are

shown in Table 2. The shape and dimensions of the

fatigue specimen were in accordance with the ASTM

E606 specifications and the specimen is shown in Figure

1. Flat shoulders were used to facilitate the grip by the

flat jaws. The shape of the tensile test specimen is alsoshown in this figure. The dimensions for the tensile test

specimens were in accordance with the ASTM A370

specifications.

Test Setup

The tests were conducted in a fatigue testing machine.

The machine is operated by a hydraulic power unit andcontrolled by an automatic computer control and data

acquisition system. The specimens were aligned and

levelled carefully to minimize eccentricity upon loading.

Suitable hydraulic pressure was applied on all jaws

ensuring that the specimens did not slip over the course

of the tests. Figure 2 shows the laboratory test setup.

Test Method

Quasi-static tension tests were first conducted on both

fatigue and tensile type tests specimens (Figure 1) to

ensure the validity of obtaining quasi-static mechanical

properties of the material using a fatigue type specimen.

Thus, specimens of both types were tested and the test

data for both strain and loads were acquired. Figure 3shows the nominal stress-strain curves obtained from

both tests: (i) using a tensile type test specimen as

specified in ASTM A370 and (ii) using a fatigue type




Extensometer

Specimen

ModularHydraulicGrips

Jaws

Figure 2: Test setup.

Yield

Strength

(MPa)

Ultimate

Strength

(MPa)

Chemical Composition

(%)

C Mn P S546 589 0.191 0.805 0.022 0.038

Table 2: Mechanical and

chemical properties.


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specimen as specified in ASTM E606. The nominal quasi-

static tensile stress-strain behaviours obtained from these

two specimens look very similar and thus it was

concluded that a quasi-static tension test on a fatigue

type specimen with dimensions in accordance with ASTME606 specifications can be used to determine the quasi-

static tensile mechanical properties of the material.

It is often assumed that the steel plate at and near the

welded connection yields in tension due to residual

stresses that develop from the welding process. The

stress-strain behaviour of the steel used in this studyshows that the point of first yield or lower yield point (first

non-linearity in stress-strain curve) occurred at 2500

micro strain (0.25% strain). Sea trial test data measured

from slamming events indicate that the strain value in the

plates at the welded connections can vary between 300

micro strains in tension and 2500 micro strains in

compression in a single load cycle. Thus, a strain range

of 3000 micro strain (0.3% strain) with minimum and

maximum values of strains varying from zero micro strain(0.0% strain) and 3000 micro strain (0.3% strain) were

chosen in this study (Table 1).

Specimen 1 (F000P) was tested in accordance with

ASTM A370 specification to determine the quasi-static

mechanical properties of the virgin material. Specimen 8

(F180F) was then tested in accordance with ASTM E606specification to determine the low-cycle-fatigue (LCF) life

of the material when subjected to a strain range of 0.0%

to 0.3%. At various predetermined fatigue cycle counts

within the fatigue life of the material, six other specimens




0

100

200

300

400

500

600

700

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Stress(MPa)

Strain

ASTM A370 SpecimenASTM E606 Specimen

Figure 3: Nominal stress-strain relationships.


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(specimens 2 through 7 in Table 1) were tested using

ASTM E606 specification before failing (rupturing) the

specimen in quasi-static tension in accordance with

ASTM A370. As an example, Specimen F005P was

subjected to 5,000 fatigue cycles using the same strainrange in accordance with ASTM E606. This specimen

was then subjected to a quasi-static tension test

according to ASTM A370 to determine the effect of LCF

damage on its mechanical properties such as quasi-static

stress-strain behaviour, remaining quasi-static strength,

and remaining quasi-static ductility.

TEST RESULTS

The following sub-sections describe various results

obtained from the tests.

Nominal Stress-Strain Behaviour

Nominal quasi-static tension test data for all specimenswere plotted to evaluate the effect of various levels of

LCF damage on the mechanical properties of the steel

used in this study. The yield strength, ultimate strength,

and ductility of the specimens were compared to identify

the effects of the fatigue damage on various mechanical

properties of the material. Figures 4 and 5 illustrate the

change in the quasi-static nominal tensile stress-strainrelationship obtained from three specimens (F000P,

F005P, and F135P) with various levels of LCF damage.

Stress-strain plots of three specimens are only shown for

clarity. It can be seen from Figure 5 that the accumulation

of fatigue damage introduces an early non-linearity in its

usual linear stress-strain behaviour at a much lower

stress value (approximately at 130-250 MPa). Thisindicates that a decrease in the stiffness of the material

occurs as LCF damage is induced. The initiation of the

early non-linearity occurs sooner as higher fatigue

damage accumulates. This raises a question on how the

modulus of elasticity of structural steel is currently

determined from virgin steel specimens and used fordesign of ship hull structure.

Figure 5 marks the yield and ultimate (tensile) strength on

the nominal quasi-static stress-strain plots for the same

three specimens (F000P, F005P, and F135P). The yield

strength was determined by the conventional 0.2% strain

offset method as shown in this figure. The ultimatestrength was determined by identifying the maximum

stress point in the same stress-strain plot. This figure

shows that the yield strength of the specimens decreased

with the increased fatigue damage accumulation (fatigue

strain cycles). The reduction in yield strength with the

increase in fatigue damage is due to the aforementioned

early non-linearity in stress-strain behaviour. However, thefatigue damage did not appear to have much effect on

the ultimate strength. It is observed that the ultimate

strength occurred at higher strain level as more fatigue

damage was introduced.

Stress Hysteresis

Figure 6 shows the stress hysteresis for specimen F180F.The maximum and minimum stress values at every

10,000 cycles were used to plot the stress hysteresis.

This figure shows that the mean stress relaxation

occurred as the stress hysteresis progressed. It is

observed that the rate of relaxation is exponential and it

is relatively high within the first 20,000 cycles. The mean

stress relaxation continued to occur as the level of fatiguedamage increased. However, the mean stress relaxation





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0

100

200

300

400

500

600

700

0 0.05 0.1 0.15 0.2 0.25

Strain

Stress(MPa)

F000P

F005P

F135P

0

100

200

300

400

500

600

700

0 0.015 0.03 0.045 0.06

Strain

Stres

s(MPa)

F000P

F005P

F135P

E (0.2% Offset)

Yield Stress Ultimate Stress

0

200

400

0 0.002 0.004

Early non-linearity

Figure 4: Effect of fatigue damage on stress-strain relationship.

Figure 5: Influence of fatigue damage on yield and ultimate stresses.


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-200

-100

0

100

200

300

400

500

0 50000 100000 150000 200000

Number of Cycle

Stress(MPa)

Mean Stress

Figure 6: Cycle-dependant response with mean stress curve.

-200

-100

0

100

200

300

400

500

0 0.001 0.002 0.003

Strain

Stre

ss(MPa)

5,000

50,000

180,000

170,000

150,000

1

Figure 7: Stress-strain hysteresis loop.


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and also the stress range decreased significantly and at a

much faster rate towards the last 20,000 to 30,000

cycles; that is, after approximately 85-90% of its fatigue

life was exhausted. At this point, it is expected that

fatigue damage accumulation was much higher and maybe due to formation of larger cracks, thus causing a rapid

decrease in material strength.

Stress-Strain Hysteresis Loop

Figure 7 shows the progression of the stress-strain

hysteresis loops. The number shown beside the loop

represents the actual fatigue cycle count for thatparticular loop. It can be seen that as the fatigue cycles

progressed the material exhibited continuous cyclic stress

softening. The stress softening is more prominent on the

tension side and occurred until failure. It is observed that

the stress limit in tension near the end of its fatigue life

(beyond 150,000 cycles) decreased at an increasing

rate. This may be due to the formation of larger cracks as

discussed in the previous section.

Energy is absorbed by the material as fatigue damage

accumulates. By measuring the area of the load-

deformation hysteresis loops, the absorbed energy for

each cycle was calculated. In this study, the deformation

was measured using an extensometer with a 50.8 mm (2

in) gauge length. The energy absorption for every load

cycle is shown in Figure 8. This figure also shows theinfluence of fatigue damage on the yield and ultimate

(tensile) strength. The figure indicates that the energy

absorption per cycle increased as more fatigue damage

accumulated and thus the strength decreased.

0

100

200

300

400

500

600

700

0 50000 100000 150000 200000

Number of Cycles

Stress

(MPa)

Energy per cycle

Yield Stress

Ultimate Stress

0

1

2

3

4

Ductility(J)

Figure 8: Influence of fatigue damage on strength and energy absorption.


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Fatigue-Strength Interaction

Figure 8 shows the interaction between the accumulation

of fatigue damage and strength of steel. Table 3

summarizes this interaction. Columns 5 and 7 of Table 3

show the remaining strength of the specimens aftervarious levels of LCF damage were induced. It can be

seen that the fatigue damage after only 20,000 cycles

(after about 11% of its fatigue life) caused the yield

strength to decrease by more than 12% and the ultimate

strength to decrease by about 3% (Columns 6 and 8 of

Table 3). Beyond this, the strength appeared to stay

relatively constant until the material was subjected to LCFdamage at close to 85-90% of its fatigue life. The sudden

drop in strength near the end of the fatigue life may be

due to the formation of larger cracks at this stage.

Three distinct segments exist in the strength curves of

Figure 8. The early part of the materials fatigue life

(approximately until 20,000 cycles) lead to a rapiddecrease of about 12-13% in yield strength and about

3% in ultimate strength. This raises a concern on how

the yield strength is currently determined from virgin steel

specimens and incorporated in the ultimate limit state

design method. The next part of the strength curves

between 20,000 cycles and 150,000 cycles, the yield

and ultimate strength stayed relatively constant. The finalpart shows a rapid decrease in the strength occurring

after approximately 150,000 cycles. Therefore, the

fatigue-strength interaction indicates that after about 85-

90% of the materials fatigue life has been exhausted,

rupture may occur quickly without much warning. It

should be noted that current design standards and codes

do not recognize the interaction between the LCFdamage and the strength even though researchers

expressed concerns about damage that can occur at the

welded connections due to LCF loading. Thus, it may be

desirable to consider the effect of LCF damage on the

strength of ship hull material.

Fatigue-Ductility InteractionFigure 9 shows the interaction between the accumulation

of LCF damage and the remaining ductility. The ductility

Specimen Fatigue Parameters Pull

Test

Strength Parameters

Fatigue

cycles

Strain

Range

Yield Level Ultimate Level

Strength(MPa)

StrengthReduction

(%)

Strength(MPa)

StrengthReduction

(%)

F000P 0 N/A Y 546 0 595 0

F005P 5,000 0 0.003 Y 521 4.6 584 1.8

F020P 20,000 0 0.003 Y 480 12.1 577 3.0

F045P 45,000 0 0.003 Y 478 12.5 576 3.2

F090P 90,000 0 0.003 Y 469 14.1 569 4.4

F135P 135,000 0 0.003 Y 478 12.5 578 2.9F165P 165,000 0 0.003 Y 361 33.8 468 21.3

F180F 180,000 0 0.003 N 0 100 0 100

Table 3: Influence of LCF damage on the strength.


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0

200

400

600

800

0 50000 100000 150000 200000

Number of Cycles

Ductility(J)

Figure 9: Influence of fatigue damage on material ductility.

was determined by calculating the area under the load-

deformation curve obtained from quasi-static tension

tests and expressed in Joules. It can be observed that the

effect of LCF damage after 5,000 cycles (after about 3%

of LCF) caused the ductility to decrease slightly (by about

5%). Beyond this point, the ductility appeared to remain

relatively constant. The sudden drop in ductility near the

end of the fatigue life seems to be due to formation of

larger cracks at that stage. Therefore, the fatigue-ductility

interaction behaviour indicates that the ductility of

structural steel used in this study decreases rapidly when

the fatigue life is approached (beyond 85-90% of LCF

life). Hence, the drop in ductility of structural steel used in

this study may not be a matter of concern until about 85-

90% of the LCF life is exhausted.

CONCLUSIONS

This study was undertaken to understand the effect of

accumulation of LCF damage on quasi-static material

behaviours of structural steel plate near the welded

connections. Based on this study, the following

conclusions are made and the conclusions are limited to

the type of structural steel and strain values used in this

study.

1. LCF damage introduces an early non-linearity in

the materials behaviour at a much lower stress value

(approximately 130-250 MPa). Therefore, the stiffness

of material reduces as more fatigue damage is

introduced.


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2. This study shows that a rapid decrease of about 12-

13% in yield strength and about 3% in ultimate strength

occurs at a very early stage of fatigue damage

accumulation. This raises a concern on how the yield

strength is currently determined from virgin steelspecimens and incorporated in the current design

method. The interaction between the damage caused by

LCF load cycles and the material strength is important

and needs to be considered in the design practices.

3. The drop in ductility of structural steel due to

accumulation of fatigue damage may not be a matter ofconcern until about 85-90% of LCF life is exhausted.

However, soon after a rupture may occur in the steel

without much warning if it is subjected to further LCF

load cycles.

ACKNOWLEDGEMENT

The authors acknowledge the financial support received

from the Natural Sciences and Engineering Research

Council of Canada (NSERC).

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How Strong is Your Ship

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