Fatigue analysis and test of three dimensional specimens of ship structures

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INSTITUT FÜR SCHIFFBAU DER UNIVERSITÄT HAMBURG Schrift Nr. 2404

Fatigue Analysis and Test

of 3-D Specimens of Ship Structures

Nie Wu

FATIGUE ANALYSIS AND TEST OF 3-D SPECIMENS OF SHIP STRUCTtJRES

*

NIE WTJ

1. Introduction

In a joint research project on fatigue strength analysis of

weld ship structures between Institute of Naval architecture in Hamburg University (IFS) and Harbin Shipbuilding Engineering Ins-titute (HSEI) a fatigue analysis and test of 3-d specimens

modeling a typical ship structure have been performed since

October. 1992. It will be continued after May. 1993. This project

is supported financially by the Federal German Ministry of

Re-search and Development (BMFT).

Design which takes fatigue into account is necessary for all

moving structures, especially for welded ship structures. They

are subjected to various cyclic loads during their lifetime.

Structural fatigue failures of large ships, specially of tankers,

or at hatch coamings of 'open deck' ships were reported in the

past decades. The cracks were found mainly at the rounded edges

of cutouts in transverse members connected to longitudinal

stif-feners. A number of investigations were carred out [1]. [2]. [3]. and useful methods for estimation of fatigue life were proposed. Moreover, a section on fatigue design of welded ship structures

is adopted in the newest issue of ¡ Rules for Classification and Construction, I-ship Technology ' [4]. [5] published by

GermarliS-cher Lloyd in 1992. It is the first Rule on ship structural

design considering fatigue analysis in the world.

Some influence parameters, however, can not be considered by

the detail classification, and large scatter of fatigue strength

exists in previous analyses. The fatigue strength analysis may be carried out by special fatigue tests for lately adopted structu

*

Assoc. Professor, Harbin Shipbuilding Engineering Institute

rai details and unconventional hull shapes, etc. For structural components welded probably by Roboter the fatigue analysis and

tests are necessary to follow the development of welding Roboter in manufacturing ship structures.

This joint research project is divided into four steps:

selection of details to be investigate according to testing

facilities available in Harbin or in Hamburg and definition of

concepts to be followed.

manufacture specimens for tests and select numerical

analy-sis method for stress distribution in the experimental model.

experiment with the first two specimens and compare the

re-suits of measurement and calculation.

a series of testing and investigations of fatigue strength in Harbin Shipbuilding Engineering Institute.

2. Modelling longitudinal/transverse web intersection and numerical calculation

2.1 specimen type

The specimen type would be representive, and be available to

the testing facilities in HSEI and IFS. The intersection between

a longitudinal stiffener and transverse web was chosen for the experimental investigation of a ship structural detail. This

in-tersection is a 3-d model, as shown in Fig.l. The cutout

corres-ponds to shipyard standards and was flame-cut in the same way as

normally in practice. An angle-bar was used as the longitudinal stiffener. It is used in shipbuilding engineering in the bottom

and side structures of ships such as tankers and containers. The

notches in the cutout in web inherent for such weld structures. A

further advantage of this model is that the results of present investigation could be compared with other existing test data.

The material of the specimens is shipbuilding steel GL A, see Table 1.

The dimensions of the specimens are shown in Fig.l, also. The

connection between plate and stiffener was welded by means of the manual arc process. The electrode used is of the type E

according to DIN

o

Fig. i _{Experiment model}

3

(1) web plate 210x600x8 mm

(2) longitudinal stiffener HP 140x7 x700 mm

2.2 Numerical calulation

Present computer programs of finite element offer a large

lib-rary of finite element types. They are helpful to find out the

stresses in structure details. For the purpes of stress distribu-tian analysis, numerical calculation was performed using the fi-nite element method with the computer program ADINA [6] in this

project. The model of specimen for stress calculation is shown in Fig.2. In the figure there are two models chosen for finite ele-ment method. In (a) the load applied to the top of web is presen-ted by prescribed displacement, which is specified mainly

accor-ding to test data, corresponds to results of numerical analysis.

In (b) the load on the top of web was adopted by the actual force

acted by actuator in the loading system. Calculation models are

the half of the specimen as it is symmetric about ay axis.

Each type of finite element approaches the structural beha-viours of the details more or less accuracy. The types of ele-ments should be chosen carefully for fatigue analysis [7]. The

finite element model for transverse web is presented by

two-di-mensional solid elements -- plane stress elements with B nodes [6]. This type of elements is usually most effective for

model-ling plated structures which are loaded in-plane. According to

the possible distortional behaviour of the longitudinal stiffener

and hull plate in the test model they are idealized by plate

element to keep the number of degrees of freedom manageable. As

structures are unsyminetric about ox and oz axes their deformations may be three dimensions. The shell element is f

ormu-lated treating the shell ( or plate ) as a three-dimensional con-tinium with two assumptions used in Timoshenko beam theory, and the Mindlin/Reissner plate theory. The structural behaviour of

this element normally reflects the beam and plate characters for longitudinal stiffener and hull plate under consideration. As an

attention is focused ori stress distribution around the edge area of cutout in transverse web, in which the fatigue cracks usually

are found, much finer mesh is employed at the area near the edge of cutout as displayed in Fig. 3. Meshes of other components in

this test model are rather coarser compared with the mesh of

I, ,/í, //////2//,'Tv' /

\ \ \

z _-i

---Fig.

±nite e1eent

cdeiing c

out area.

Because c

te time cosumpti3fl,

a reasonab-e

-rìe

element mesh for the whole model should be chosen carefully.

Transitional mesh regions were introduced between the coarse mesh

and fine mesh in the middle of longitudinal stiffener and hull

plate to reduce the element number. The finite element analysis

was performed with the computer program ADINA. The CPU time used

is about 25 minutes for each loading level in static stress ana-lysis run in computer VAX-6310. Fig. 4 shows the elastic stress

distribution at whole transverse web. The retults of strains cal-culated by the FE method are in good agreement compared with that measured in the test at the edge of the cutout region as shown in

Fig. 5. It may be noticed that there are no many strain gauges

fixed in this area for measuring strain distribution around the

cutout and no measurements at higher stress level which exceeded

the yield stress of the material. All of these measurements might

be suggested in the further tests of the joint research project

to take the local elastic-plastic strains into account. The

maximum normal stress position obtained by FE calculation and

o

strain measurement are both at trie edge or cutout with 11 to the

minimum section of web, not with 50 600 to the minimum section as shown in other investigations. This may be influenced by the

shape of the cutout. A comparison between different shapes and

size of cutout for the maximum normal stress position is also

suggested in the further investigations, if it is possible.

2.3 Determination of crack initiation life

There are different approaches available for description of

the fatigue life of structures. As the local strain approach has

shown to be useful to estimate the fatigue failure of ship structural details, the so-called Smith parameter

PS

[S]

employed to determinate the crack initiation life N:

= .1 max Ea E (1)

where

0nax local elastic-plastic stress

Ea : local elastic-plastic strain amplitude

E : material modulus of elasticity

i1) (D U) cl 1) (J) cl- U) U) _cl

r

0.0

-50.0

Ç -

/

2000

r

1000

RzO

o\

-[N/mm

200

--

flcme cut edge

F.g.

fig.

0

a log-log bilinear parameter life curve. Fig. 6 is adopted in this project which shows results of strain-controled constant amplitude fatigue tests on small scale speciments havïng

flame-cut edge under the stress ratio R=O and R=-l. Expected crack

initiation lifes N are estimated according to Fig. 6 and Smith parameters. A comparison is listed in Table 2. between the FE

cal-culation and measurements for the first two tests.

3. Experiment and results

For fatigue strength analysis of welded ship structures a

se-ries of experiments is proposed. The size and shape of cutout in

specimens are carefully chosen in order to be available to the

testing facilities in Harbin and Hamburg Shipbuilding Instuitute. Considering comparison of experimental results with the one being

obtained in Harbin the connection components between cylider of

acuator in loading system and the web plate of specimen were de-signed by Institute fur Shipbuilding, Universitat der I-Iarnburg, as shown in Fig.7. An advantage of this design is that to joint the

acuator and specimen and to apply force on the edge of web this

connection system, e.g. the flaps and tongue can be handled easi-ly. In addition, it is suitable for the facilities in the

labora-tories of both sides of the joint research project, expecially

for the test details selected with certain dimension. Moreover,

from the point of economic view it is convenient to assemble and

manufacture with lower cost. The experiment for the first two

speciments were performed in

following steps:

3.1. a specimen test

The aim in this step is to check the connection designed. On-ly few of strain gauges were used in the middle points of web to

control the deformation desired. No strain gauges were arranged

at the high stress distribution area around cutout edge in the

web. According to the experimental records during the test the

first crack initiated at a position lined with about 110 angle to

horizental axis in l.3x10° cycles and failured in 7x1O cylcies.

Compared with the expected initiation life N the result is

h

V

sonable. The measured results in Table 2 demanstrated that the

connection components work well and the design is successful.

3.2. the second specimen test

In this step more strain gauges were used to record the stress

distribution around cutout area of web. Fig. 8 displays positions

at which strain gauges are fixed. DMS Typ 3/120 Lyll strain gauges were adopted for positions No. 26 to No. 32, and DMS Typ

6/120 Lyll used for positions No. 20 to No. 25. The loading range

varies from zero to 55 KN i.e. the stress ratio equals 0. Measurements were recorded step by step during static process of

loading in order to compare with FEM calculation results and that

being tested in 1-ISEI for other specimens. The service cyclic load

is applied to the experimental model with constant amplitudes in

frequency of 15 Hz. In the process of cyclic loading dynamic

mea-surements of strain values from a strain gauge arranged in the

cutout area were roughly displayed with gragh at the screen of oscilloscope in the test system in order to evaluate the crack

initiation life N0 The crack initiation and propagation measure-ments are listed in Table 3. _{It should be noted that a crack had}

been observed with strain values decreasing another crack would

happen in the area at which higher tensen stress exists. In fact, the inspection after the test shows that the second crack

appea-red in the root of connection between web and hull plate as shown in Fig. 8.

It is possible to calculate the crack propagation according to the experimental equation proposed by Paris

d.a/dN = CKm ₍₂₎

where a is the crack lengh, _{N represents the number of loading}

cycles, K denotes the increment of stress intensity factor, C

and m are the constants which depend on the testing environment,

its temptature , loads and their frequencies

¡ as well as the stress ratio. Here C=1.83x103, m=3 are adopted [9]. Fig. 9

dis-plays the experimental results of crack _{propagation and the} re-sults computed using FEM according to equation (2).

/

Fig.

posiL.ion

of crack initiation

t-/

_/

crcç

cra.c-< ,,",, ) 70 s-0 4w 30 20 i0

ï

(a) experimental crack propagation

(2)

(b) calculation crack propagation

A series of investigations

According to the schedule of the joint research project 12

specimens, which are 3-D ship structural detail model consisting

of the same material and manufactured with the same weld manner

as the first two specimens, have been consigned for shipment to

Harbin, China. A series of fatigue tests of these specimens will

be carred out in HSEI. With different stress levels the intial

fatigue uf es can be obtained for each test. It is possible to

observe the crack propagation in some tests with relative high

stress level in the cutout area. All the experimental results

be-ing recorded will offer a basic data in order to compare with

others involving in this joint research project.

Summary and remarks

From the investigation of the first stage in this joint

re-search project the following conclusions can be drawn:

The higher stress ( stress concentration ) normally exists

in the cutout area of transverse web/longitudinal stiffener

in-tersection. The cracks occur near the minimum section of the

cut-out. Positions of cracks initiated will be influenced by the

notches, dimensions of the cutout and other factors.

The crack initiation life can be determined from the

elastic-plastic strain by using a damage parameter life curve and

the predicated life has a good agreement compared with the

expe-rimental results.

At a certain stress level around the cutout area

reasen-able stress distribution can be obtained by using linear FEM in

order to predicate the fatigue life of structures.

For the limited specimen test and numerical analysis there

are further work left to improve the investigation. To compute

the stress distribution in cutout area non-linearity of material due to high stress would be taken into account.

A random loading can be applied on the experimental model to

yield foundimental data to predicate the fatigue life of detailed ship structures during the voyages in their whole lifetime.

Following the development of welding Roboter using in ship

construction the dimension of cutout in web can be different from that for traditional welding. The more stress distribution

analy-sis is suggested for different dimensions and shapes of the cut-out in the web, if there is an opportunity.

Acknowl egement

It should be remarked that the investigations within this

joint research project were financially supported by the Federal German Ministry of Research and Development ( BMFT

) and by the

Structure Labourotary in Institute for Shipbuilding of Hamburg University who provided the test specimens.

The author wish to express his appreciation to Prof. Dr.-Ing H. Petershagen and his colleagues for their helpful advice and

valuable discussions.

Table 1. properties of shipbuilding steel Grad A GL

mechanical properties

16

chemical composition in % ₍ Oxygensteel )

C Si Mn p 5 Al

0.1

0.24

1.11

0.13

0.07

0.04

Table 2. predication of crack initiation life

Table 3. crack initiation and propagation

17

test 1. F=40 KN test 2. F=55 KN

measured computed measured computed

amax, N/mm2 -- 265 349 365 xlo -- 910 948 948 N/mm2 218 223 261 266 N xlO 3 97 4 5 0 l.35 1.1 Number. of cycles xlOO length of crack mm strain observable (show in screen) 100110 --- increase

110120

135136

References

[1] W. Fricke and H. Paetzold

Application of the Cyclic Strain Approach to the Fatigue failure of Ship Structural Details.

Journal of Ship Research, Vol. 31. No. _{3, 1987, pp 177-185} [2] W. Fricke

Lineare und nichtlineare Strukturanalyse von schiffbaulichen

Konstru.kionsdetails am Beispiel einer

Doppelbodenunter-suchung. Institut für Schiffbau der Universität _Hamburg, Report Nr. 454, 1985

[31 H. Paetzold

Beurteilung der Betriebsfestigkeit _von

Längsspantdurchfüh-rungeri auf der Grundlage der örtlichen Dehnung,

Institut für Schiffbau der Universität _Hamburg, Report Nr. 455, 1983

[4] Germanischer Lloyd

Rules for Classification and Construction, _{Part I, Ship} Technology, Sec. 20, 1992

[3] W. Fricke

Guidance and Examples to the Revised _{Fatigue Strength Rules} Germanischer Lloyd, 1992

ADINA

A Finite Element Program for Automatic Dynamic _Incremental Nonlinear Analysis

Report ARD 87-1, ADINA-Engineering, _{Watertown Mass., 1987}

Erkki Niemi

Recommendations Concerning Stress Determination _{for Fatigue} Analysis of Welded Components

11w. Doc. XIII-1458-92

Smith, K. N., Watson, P., and _{Topper, T. H.}

A Stress-Strain Function for the Fatigue _{of Metals} Journal of Materials, Vol. 5 _{1970, No. 4, pp 767-778}

11W Guidance on Assessment of The Fitness for Purpose of

Welded Structures

IIW/IIS-SST-1157-90