The influence of asymmetrical corner reinforcement on the fatigue strength of T-shaped GRP plate connections

SHIP STRUCTURES LABORATORY

LW

\.I

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gi

THE INFLUENCE OF ASYMMETRICAL CORNER REINFORCEMENT

ON THE FATIGUE STRENGTH

OF T-SHAPED GRP PLATE CONNECTIONS

by

ir. H.G. Scholte & J. van Lint

September 1986

Report No.

_{SSL 294}

CONTENTS

Page

Summary. _i

Introduction. 2

Specification of material 4nd specimens. 3

Test procedure. ₄

Test results. ₅

Conclusions and remarks. ₈

Literature. ₉

Table I.

SUMMARY

Connections of orthogonally placed plates of CRP are carried out

by means of a number of reinforcement layers built _{up in a}

symmetrical way. For.some' connections.a number of layers has to be

built up in an upside position, which is time consuming and cost exp ens ive.

'Reducing the number of layers in a bad working position may have consequences with regard to the strength ôf these connectIons. The influence on the f'atigue strength is investigated on asym-metrically built up T-shaped specimens.

An asymmetrical connection results in a considerable reduction

o,f the, fatigue life. However, the enlargement o the throat

thickness with' filler material, seems to have .a positive effect on the fatigue strength.

-2-INTRODUCTION

In manufacturing GRP ship structures many large plate sections have

to be connected more or less perpendicular _{to each other.}

Examples are the connections betweeñ shell _{and bulkhead, déck and}

bülkhead, deck and shell.

These connections, have a T-shaped form and the connections are made

by means of a number of layers of glass _{reinforcement. Normally these}

corner reinforcements are built úp in a symmetrical way, whiéh can be

done without problems in good working conditions _{for the bulkhead}

-shell connection.

However, the deck _{- bulkhead and deck - shell connections are more}

time-consuming and cost expensive due to the fact that only one side

of thecorner connectioncan be laid down in i good workingposition, while the other side has to be built _{up in an upside position.}

Besides it is more difficult to keep _{a good quality of corner}

rein-forcement in the upside position.

So, if the number of layers to be laid in _{an upside position could be} diminished this would lead to _{a reduction of working time and cost of}

labour.

In order to gain more knowledge about the influence _{of an asymmetrical}

corner reinforcement on the strength and capability of the connection a number of 10 different T-shaped specimens has been investigated

under static tensile load first /1/.

The results showed that the number of reinforcement _{layers could be} decreased on one side of the connection from 15 down _{to 6 with a} reduction of static strength of less than 20% (configuEation _G2). Extra layers of reinforcement at the other side did _{not show a}

Sig-nificant effect on the static strength.

Comparing two other configurations (CI and Di) _{a considerable increase}

of static strength (abt. 30%) _{was observed due to an enlargement of}

the throat thickness with filler material at the side of the reduced

number of layers.

So it was decided to carry out fatigue _{tests primarily on the reference}

configuration Al and the modified configurations _{C2 and Dl. However,,} regarding the good result of configuration Dl in _{the static test}

programme,, it was decided to carry out also fatigue tests on other

SPECIFICATION OF MATERIAL AND SPECIMENS

The specimens simulate a deck - shell connection.

The specimens have been fabricated according the specifications _3-204,

3-208,, 3-210 and 3-233 of the Material Handbook _{of van der}

Giessen-de Noord GNM.

Material:

Glass: _{Syncoglas-Zele woven roving 840 g/m2, type 41 RE.}

Glass filament: Bayer roving RV 7804.

Resin: _{Synthese Setarol 3608 T.}

Filler material: Prestocol type TH 75 c

Secimens:

The dirnensiöns of the specimens are shown in figure 1.

The corner connections are built up with different numbers of reinforcement layers. As reference specimen AI has

_asyetrïcal

corner reinforcement existing of 15 layers of reinforcement at both

sides, ergo 30 layers in total. The other specimens _{are all built up}

with anasyetrical corner reinforcement.

A majority of the

specimens has filler material between thereinforcement _{layers in} order to increase the thróat thickness of the corner connection.

TEST PROCEDURE

All specimens were tested on a _{servo steered hydraulic testing machine} with a load capacity of loo kN. The specimens _{were tested under} alter-nating loading with an R-ratio P . /P = R = -1.

min max

The frequency was 1 Hz.

The principle, of loading and supports is _{shown. in figure 2.}

Special supports have been used _{to avoid secondary bending and axial} stresses. With these supports the specimens could be considered as

completely free supported. Figure 3 shows specimen Al5 _{- V12 in the}

test equipment.

Regarding small deviations in flatness of the bottom _{plate and in the}

angle between the bottom plate and the vertical plate,, all specimens have been fixed in the testing machine in, such a way that the vertical

axis of load force was lying in line with the vertical _{plate of the}

specimens. in this. way secondary bending _{stresses could be, avoided.}

The supports of the bottom plate were positioned in vertical direction so that the bottom plate was fully supported by the supports over the

width of the bottom plate in the zero load condition.

Load force and deflection were measured and recorded by digital

recording apparatus. Crack propagation and delamination was measured 'byviva'I tnspection '(figure-4').

TEST RESULTS

The test results are summarized in table I.

For each specimen the configuration of the corner reinforcement, the applied fatigue loading and the number of cycles are given.

Also the amplitude of the total displacement at the centre of the bottom plate is given as well for the undamaged specimen at

the beginning, of the test (N = O) as for the end of the test just before failure (N.= Nmax)..

It seems significant that at the end of the fatigue test the

deflection is about twice the deflection of the undamaged specimen.

At the end of the test some of the specimens are loaded till fracture in order to determine the rest strength after severe

fatigue damage.. The rest strength varies between 65 and 80Z of the strength under static testing /1/..

More particulars about the crack growth are given in the figures 5 - 16. It is remarkable that in a very early stage of loading and fatigue life delamination occurs and crack initiation starts The whole process of crack growth differs completely from the

crack growth in a homogeneous material as steel.

In steel normally one crack will arise at a point of stress cPn-centration and propagate ma direction perpendicular on the

direction-of--the 1'a.rgest working strain-ampl4-tude. Totally

differ-ent, in the nonhomogeneous material of glass reinforced polyester

a complete crack pattern with some dominating cracks can be watched.

These cracks develop not perpéndicular on the direction of the largest working strain amplitude in the structural detail but between and following the layers of reinfórcements. The cracks

arise due to shear forces in the. resin between the layers of

rein-forcement and due to the tensile stresses in the resin in a

direc-tion perpendicular on these layers of reinforcement.

Besides, many specimens had already cracks or small s:pots of delamination before being loaded. It is concluded that these. defects were caused by enclosure of air, by local insufficient' adhesion to, each other of two layers of reinforcement or by large residual stresses due tq shrinkage (figure 3).

However,, as far as could be observed and considering _{the crack} initiation at s:tarting the tests before the full load amplitude

was setted, these defects did no.t have a significant effect upon

the results.

Although in the figures 5 - 16 the location of cracks, the crack length and the crack growth are presented, it must be realised

that the real ¿rack pattern and crack propagation is _{much more}

complicated than shown in these figures.

So, it appeared that many times _{a crack front is not a straight}

line and that at any given moment the crack _{length at the frontside} may differ substantially from the crack length at the back of the

specimen. However, there is not so much difference when comparing

frontside.:with back over the total fatigue.life of a specimen.

The tendencies in crack development _{are equal for both sides of the}

specimens. .

A further study of the figures 5 _{- 16 shows that the first cracks}

are initiated.i.n the vertical direction. During the test the crack growth of. the horizontal cracks starts slowly but dominates at the

end of the test. . . ,

The fatigue life of the specimens is presented in _{figure 17.}

The figure shows. a good. relation between load amplitude _{and fatigue}

life for the reference configuration of the. specimens AI.,

Thetesuits

f configurations C2andH2are significantly lower _than the results of Al and show a fatigue life which is _{about 5% of the}

"fatigue of reference specimen Al.

However, the results ofK2 are almost equal to the results of Al.

The explanation might be that the stress concentration in _{the corner}

of, the, connection is reduced due to enlargement of throat thickness

by the application of filler material at both sides of _{the corner}

connection.

The. results donot give any information _{about the existence of an} eventually lower fatigue limit. Nevertheless it might _{be of interest} to know if there is a lower fatigue limit and what will be the

height of that limit.

This question was also not answered in _{earlier tests on specimens}

of the

in the corners to connect the corner profiles with the bottOm plate.

in figure 18 also th results of these former tests are presented. The higher static strength of these specimens is due to the additional

reinforcement of the glass pins. The test results show that one series had as well a higher static strength as a higher fatigue strength, while a iáwer fatigue iimi.t was-found just below a load amplitude of

15 kN for the tensile part of the load.

However, the other series does. no.t show a lower fatigue limit and despite the glass pins, this, series does not give .a better fatigue resistance at.the lower.load values than the specimens reported here.,

-8-CONCLUSIONS AND REMARKS

In some specimeiis small delaminations have been detected before loading.. These defects.are due to air enclosure of residual stresses in and large shrinkage of the resin..

Delamination occurs and crack initiation starts at very low tensile loads and in a very early stage of fatigue life.

Small defects in the cother have no effect on the static and

fatigue strength of the specimens.

In the first phase of the fatigue life cracks _{are initiated in}

the inner corners very close to the surface of the corner

rein-forcement. This crack initiation may be considered as some kind of stress relieving and does not seem to have, a significant effect on the fatigue strength.

In the second phase of the fatigue life thé crack _{propagation is}

dominated by horizontal cracks between the reinfórcement layers

of the corner profile and especially at the boundary of corner reinforcement and bottom plate.

For the reference specimens of type. AI fatigue tests are carried

out at 4 load levels. The results db not show much deviation and

_1ayQnas:traigt me, which can be extrapolated

to the ultimate

load in the static test (N = 10°)..

Reduction of the number of reinforcement layers at _{one. side of} the connection from 15 down to 6 results in _{a significant} de-crease of the fatigue strength. Compared with reference con-figuration AI the fatigue life of specimen _{types G2 and H2 is}

reduced with a factor 100.

Enlargement of the throat thickness at the reduced _{part of the}

connection (type H2) d.id not result in an improvement of the fatigue strength (comparison of 112 and G2)..

Compensation of the reduction at one side by extra l.ayers of

reinforcement at the other side (type DI) did _{not result in a}

significant increase of fatigue strength (comparison of DI with G2).

IO. Enlargement of the throat thickness at both sides of a specimen

with a reduced number of layers at one side and no compensation

with extra layers at the other side (K2) shows _{a significant}

improvement of the fatigue strength (comparison with HZ) and

reaches almost the original fatigue life of reference specimen

AI.

The results do nôt show a lower fatigue limit.

Further research with regards to the existence and height of a

lower fatigue limit is reconmiended.

Considering the relatively good results with specimen type K2

it looks worthwhile to investigate the influence of the

enlarge-ment of the throat thickness on the stress and strain

distribu-tion at the corner connecdistribu-tion, and on the static and fatigue strength of this structural detail.

LITERATURE

/17

Scholte, H.G. 'The influence of asymetricai corner

reinforce-ment on the static tensile strength of Tshapéd. GRP plate

connections".

Report No.. 290S.S...L.. Deif t,. September.t985.

/2/ 'investigation on the behaviour under fatigue loading of orthogonally placed plates of glass réinforced polyester'. Report No. 211 S'.S.L. Delft, November 1977.

Table I. _{Resúlts of fatigue tests.} j Specimen Composition of layers

Fatiue load

maxmin

Number of Deflection

()

Rest strength (kN)

atN

O

atN

maz. A1.2 -, V15 _{15/15 +15/-15 567} 2.0 4.6 -A1.3 - V20 :15/15 : -'-20/-20 24 2.3 5.6 A1.4 - V24 15/15 +24/-24 1 3.8 8.9 ₂₅ A1.5 V12 _{15/15 +12/-12 4,583 1.8} 4.57 D1.2 - VIS 26/2-v-2 140 2.7 5.16 23.5 D1.3 -. v12

_26/2-2

_+I/12

_{164 2.16 4.25} 22,4 G2.2 V15 15/6

_-i-151-15

_{8 3.9 7.2 18} G2.3, -. V15 15/6 +15/-15 36 3.2 5.0 -112.2 -.V15 15/3-v-3 +15/-15 _{40 3.5 6..7} -H2.3 - V12 15/3-v-3

12/-12

380 2.25 5.15

1(2.2

-V15

5-v-1O/--v-3 +15/-ì5 116 2.64 5.13 -K2 3 - V12 5-v-10/3-v-3 +12/-12 2,550 2 17 4 65

-X1/X2 number of

Layers. Q Q rn X,1 20 150 150 600 X2

Fig.1 Test specimens ; geometry and dimensions.

Q

Q

L400

Fig.2 PrincipLe of Loading and support.

200 G2, 15 6 01 26 2-v-2 íIt2 15 3-v-3 K.2 5-v-10 3-v-3 W V, i

Fig. 3a. Specimen in test equipment.

max. 400 R _min./mav. -1 max. 400 17,3 10.1 io.2 _io.3 Number of cycLes Fig. 5a _{Specimen A.14 -V24: frontside}

10.1 io.2 _io.

Number of cycLes

Fig. 5b

_{Specimen Al4 -V.24: bckside}

Fronfsjde (1) u Indication of Layers (15)-X1 / X2-I1S)

A/A

0/3

Position of cracks horizontal vertical

15

io. io io.6

10t

5 w

_-.

(n j_t CLL, V) w t-DU 3 2 15 R Pmin/Pmaxr -1

Front side (I)

Indication of Layers (15)-X1 / X2-(15) Position ofcracks

A/A

horizontal

0/s

A /A

_vertical H

D IM

0/

10.6 10. 0.

n Indication of Layers (15)-X1 / X2-(15)

A/A

o/u

0/

Position of cracks horizontal vertical

Front side (LI

Frontside II) I n Indication of Layers (5)-X1 / X2-(15)

0/S

Position of cracks horizontal 10.1 10.2 _lo.3 Number of cycles

Fig. 62

_{Specimen Alj -V.20: frontside}

600 R _Pmin/max -1

-

-400 1'max.

R Pin/max

i

10.1 10.2 io

io.'

Number of cycles

Fig. 6b Specimen A.13 -V.20: backside.

10.

15

I

10

loft

4...

w w

J LI -.

¡ . Li '+tfl (u

-2

o 10.6 i

¡A

vertical

_w

Li ru D

IM

I O..

A/A

o/u

0/

Frontside (t) u Indication of layers (15)-X1 / X2-(15)

Ls/A

D/R

0/

Position of cracks horizontal vertical io.1 10.2 _io Number of cycles

Fig.la

_{Specimen A.12 -V15: froutside.}

Frontsjde (1) u Indication of Layers (15)-X1 / X2 -115)

D/R

0/

Position of cracks horizontal vertica L max. 10.1

Fig.7b

_{Specimen Al2 -V.15: backside}

21

15

ion

C w cn

EC

5w w

_LI s L LI '4V) m t.-3 2

i

o 15 10. C W cn

Ec

5W W

LI 'J '4V) (0 3 2 1 o io? lo io.4 10 10.6

Number of

cycles--10.6 io. io

11 Indication of (ayers (15)-X1 / X2-(15)

o/

0/

A /A

o Im

o/.

Position of cracks horizontal vertical Frontside (I)

u

I

ittitil

i___.______i_._t 111111 10.2

_io!'

Number of cycles Specimen Al5 -V.12: frontside

Indication of Layers (15)-X1 / X2-115)

L\/A

o/u

0/

A IA

o Im

0/

Position of cracks horizonaI vertical

Fi.Bb

Frontside (I) io.3 io.4 Number of cycles Specimen A.15-V12; backside

L400

R r Pmin/Pmax r -1 400 Rr D

i/P

_{r -1}

_____'j

p.

n

.0-u

io.5

10ff

5wo.i

LI ru I 0 LI II) (0

cJLj

Front side (1) II Indication of layers 115)-X1 / X2-(6)

A/A

0/w

0/.

A /A

o /a

0/

Position of cracks horizontal vertical Fronfsjde (i) I t _i t

titil

10.1 Indication of layers (15)-X1 / X2-(6)

A/A

o/a

0/.

A /A

D IR

0/

Position of cracks horizontal vertical

---+---i I I

I iii

io.2 Number of cycles 16.9 400 /.00

t

R: Pmin/rnax r -1

R Pjn/Pr -1

max.

15

3 2 1 O 15 10 (J 'f

DU

3 2 o

10.1 _{io.2 io. io. - 10.}

io.6

Number of cycles

Fig. 92 Specimen 6.22 -V.15: frontside

Fig. 9b _{Specimen 6.22 -V.15: backside.} 4

io.6

r' Indication of Layers (15)-X1 / X2-(6)

ti/A

0/rn

0/

Position of cracks horizontal verticaL iO1 102 Number of cycles

Fig. iO _{Specimen 6.23 -V.15: frontside}

Frontside (1)

Number of cycles

Fig.lOb _{Specimen 6.23 -V.15: backside}

400 600 R Pmin/max -1

f

15

10M

5w w

_-.

tu ¡

It,,

Qtj

_tu

t-DL)

3 2 I i i

iii!

i u

iuuuiO

io.4 iø. 10.6 10 Frotside (L) R _{min./max. -1} Indication of layers 115)-X1 / X2-16) Position ofcracks horizontal

0/rn

0/

r'

/A

0/M

vertical

5

0/

-4

U n, Li

-3

-2

-1

I i i

tutti!

i

lu ittuul

i I i I

till

ut uiuil

i i

iiuiiio

10.6

10 10.2 _{io io.4}

Indication of Layers (26)-X1 / X2-(-v-2)

A/A

0/rn

a/s

Position of cracks horizontal vertica' io io.2 _io Number of cycles

Fig.11a Specimen D.12-V.15 :

frontside

Indication of layers Position of (26)-X1 / X2-12-v-2) cracks

A/A

0/rn

0/s

t /A

0/U

0/

Frontside (I) Frontside (1) horizontal verticat i i i i t i 600 R Pmin/Pmax r -1 600 1'rna s. R P0 /rnax r -1

---A--20L o. s s

lo

5

-4

-3

-2

o io.6

II

i i

till

io.2 lo io Number of cycles

Fig.11b _{Specimen Dl2 -V.15: backside}

Indication of layers Position of 126)-X1 / X2-(2-v-2) cracks

Li/A

0/.

Frontsjde (1) horizontal verticaL Frontsjde (1) H Indication of Layers 26)-X1 / X2-2-v-2)

Li/A

o/u

0/

Position of cracks horizontal vertical 400 Pinas. R _Pmin/max -1 A... --LY

-

_/

z,'

R--3

10H

5W w

-(U I L. L) L

DU

2 o 10.6 15 I I i

iiiiiit

I J i I i 11111! i 1111111 i i 11111110 10 10.2 io3 10.6 Number of cycles

Fig. 12b Specimen D.13-V12: backside.

lou

50Jw

L) -(U / O. L) .n m L.

DU

3 2

i

iü.

iol

10.2 _io Number of cycles

4

D

Lì/A

o/u

0/.

Frontside (L) Indication of layers Position of

15)-X1 / X2-(3-V-3} cracks

--horizontal vertical Front side (L) n Indication of Layers (151-X1 / X2-(3-V-3(

Lì/A

o/u

0/

Position of cracks io 10.2 _io3 Number of cycles

Fig.13b Specimen H.22 -V.15: backside

600

R 'min./max. r -1

Number of cycles

Fig. 132 _{Specimen H,22-V.15: frontside.}

R r Pein /Prnaxr -1 iù. 15

lot

5 ro I L 9. _jj D 3 2 o 10.6 15 10 3 2 i i

i tilO

10.6

i

E -4-w E

5W

LP ru / cL -I y) D

E

101 _{10. io.4 10.} horizontal vertical

Locatíos

/

of cracks 15 ---X1 /X2.- 3-V-3 Frontside (I) Isdication of layers (15)X1 / X2-(3-V-3)

A/A

o/u

0/

Position of cracks horizontal vertical n

Lt---I 1111111 I

211111!

0.1 lOE2 10 10.1+ Number of cycles

Fig. 14a Specimen H.23 -V.12: frontside

Frontside U) n Indication of layers 115)-X1 / X2-13V-3)

A/A

D/I

0/.

¿ /A

D lu

0/

Position of cracks horizontal vertical I. i t i i i

iii I

i t i I I

Iii!

iol iO.2 io io.4

Number of cycles

Fig.14b _{Specimen H.23-V.12: backside}

10.6 o 'o ej

i

o₀ ej 15 f 3 ' O

lou

50)0)

ru

-CL tj VIro L____j.__._____L !__i_____1____Ll I i

liii

Fronlside (1) Indication of Layers fS-Vt0)-X1 / X213-V-31

A/A

0/.

Position of cracks horizontal vertical u Frontsjcje IL) u Indication of Layers (5-V-10)X1 / X2-)3-V-3)

A/A

0/

Position of cracks horizontal vert i ca L ax. 600 R _min./rnax. -1

L t I

I tul

i I I._...L....J!....1II I i i i i_i_i_ii i I I 1._J [.1_II t i i u i i ii...O

10.1 10.2 10 1OE 10.6

Number of cycles

Fig. 15a Specimen K.22 -Vis frontside

R _min./'max. -1 G

If

15

15

-3

I t i

itititi

I i

1111111

1 I

uilitli

i i

itiitil

I I

tiiuiO

10.1 iü.2 i0 iO.'+ iO.5 10.6

Number of cycles

-Fig. Specimen K.22 -V15: backside

2

-2

t

A E Li 4-C u I.) ro L. Li 400 L

Frontside fi) Indication of Layers (5V-lOI-X1 / X2-13-V-31 Il Position of cracks horizontal vertical Frontside IL) n Indication of Layers 15-V-101-X1 / X2-13-V-3)

0/

Position of cracks horizontal verticaL max.

L

'.00 R _Pmin/Pmax -1

¡

ie,o

15

15

-4

-3

-2

I

itiitil

I I 111111 1 I 1111111 I

LJJIIII

I I I I II O

io.2 io.3 io. 10.6

Number of cycles

Fig,16b _{Specimen K.23-V12: backside.}

t

j

10

R _Pmin/max -1

.4

C E E w E w

-

'-J ro o.

-4

VI D

-3

/0

_-2

o

-1 L_L_LJL_1 LII I I i

L.Ii

t I _L_.J t I I t LLO

I J I I I i L_LJ_LJ_LJ I

10.6 _iO

10.2 io.3 io.4

Number of cycles

z

-r '-a 20 ru o 4-_o 4-L. ru ci. Fig. 18 io.3

lo'

10 10.6 10 Report. 211 (charge. j) o Report. 211 (charge. II)

i Type A1 specimens (X11X2 15/15)

o Type i specimens (X1/X2 26l2-V-2) A Type 137 specimens (Xi 1X2 c1516)

' Type H2 specimens (X1/X2 15I3-V-3)

D Type K2 specimens (X1 IX2 5-V-10/3-V-3)

max. A1 R _pn

/Prxr -1

600

-

A1 0D FI.2 K2 Ai e...

io. io. io io.6 io.

Number of cycLes

50-40 30 62A

z

c °i o:-.:---.... o 20 4-o L. fu Q-- 10 - 10 V) C a) ci O I I I

io9 io.1 io.2

O I I I 10.0 _{io.1 io?} Number of rycles Fig. 17 50 ck . . /+0 30 62 D

E Type A1 specimens (X1/X2 15/i5)

o Type D1 specimens (X1 IX2 26/2-V-2) A Type G2 specimens (X1IX2 15/6)

U Type H2 soe.cimens (X11X2 15/3-V-3)

D Type K2 specimens (X11 X2 5-V-10/3-V-3)

-. - Al 1