fem modelling of elastoplastic stress and strain field in
TRANSCRIPT
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~eries 07 Aerospace Materials 02
FEM Modelling of Elastoplastic Stress and Strain Field in Centre-cracked Plate
I. Kunes
Delft University Press
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FEM Modelling of Elastoplastic Stress and Strain Field in Centre-cracked Plate
li' •
2392 327 3
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Series 07: Aerospace Materials 02
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FEM Modelling of Elastoplastic Stress and Strain Field in Centrecracked Plate
I. Kunes
Delft University Pre ss / 1997
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Published and distributed by:
Delft University Press Mekelweg 4 2628 CD Delft The Netherlands Telephone + 31 (0) 15 278 32 54 Fax +31 (0)152781661 e-mail: [email protected]
by order of:
Faculty of Aerospace Engineering Delft University of Technology Kluyverweg 1 P.O. Box 5058 2600 GB Delft The Netherlands Telephone +31 (0)152781455 Fax +31 (0)152781822 e-mail: [email protected] .NL website: http://www.lr.tudelft .nl
Cover: Aerospace Design Studio, 66.5 x 45.5 cm, by :
··--~I
Fer Hakkaart, Dullenbakkersteeg 3, 2312 HP Leiden, The Netherlands Tel. + 31 (0)71 512 67 25
90-407-1588-2
Copyright © 1997 by Faculty of Aerospace Engineering
All rights reserved. No part of the material protected by th is copyright notice may be reproduced or utilized in any farm or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the publisher: Delft University Press.
Printed in The Netherlands
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Table of contents
Summary ................................. ........................... .............. .... ..... ...................... ..................... 2
1. Experimental results ........................................................... .................. ......... ................. .3
2. Finite element computations ............................... .... .. ...... ..... ............... ....... ... .. ... ............. 5
2.1 . Finite element model of the CCT specimen ....................................................... 5
2.2. Results of computations with non-propagating cmck ........................................ 8
2.2.1. Development of plastic zones and stresses in the CCT specimen. ...... .l 0
2.2.2. Influence of constraining of the loaded edges ...................................... 18
2.2.3. Stmin changes on the edge ofthe CCT specimen ................................ 18
2.3 . Results of computation with propagating cmck .............................................. 20
3. Buckling ofthe CCT specimen ........... .................. ................................. .. ........ .... ........ 22
4. Conclusions .................................................. ......... .. .. ................... ...... ................... ....... 26
References .......................................................................................................................... 27
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Summary
During experiments studying residual strength of a centre-cracked tension (CCT)
specimen from alwninum alloys, unexpected strain changes were measured by strain
gages in the ligament of the specimen [1 J. An elastoplastic FEM research was conducted
to explain this behavior.
The modelling proved that the experimentaIly observed decrease of totaI strain in
the ligament does not occur, when only in-plane deformation takes place, regardless
whether stabie fracture occurs or not. The results of the anaIysis give an interesting
detailed picture of stress and plastic strain changes in the CCT specimen, subjected to
high loads near plastic collapse.
Simple model, explaining the observed strain changes in the ligament, is proposed
for the CCT specimen after its buckling. This model should he verified by a proper FEM
buckling analysis.
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j ... . , IJ .", Wo, . ! ! ibM' ...... ',- ' X' ' ...... .... ~~ ..... _-
1. Experimental results
Experimental work [1] was performed at Faculty of Aerospace Engineering, TU
Delft, to study residual strength of centre-cracked tension specimen made of 2024-T3 and
7075-T6 alwninwn alloys. During tests, studying the influence of buckling on the
residual strength, the suspicion has arisen that the results are influenced by a bending
moment in the plane of the specimen, caused by the tension machine. To measure this
moment, four strain gages were attached to the specimen (two at each side), as shown in
Figure la. The specimen was loaded with controlled displacements ofthe loaded edge.
0.5 mm
y1' t t l' t t t t
I I
thickness ,
0.9 mm
I n- -28.·100 ~
STRAIN GAGES
w-400
o o
x co I
a)
äi a.. ~ .. .. !!! Oi ., ., e 0
250
200 0 ++f
150 000
~
o~
. 0 '
100 "" "" "" tt'!-++ 50
"" . 0"
0 ." 0.00 0.05 0.10 0.15 0.20
Total strain on Ihe edge [%)
b)
Figure I Position of strain gages (a) and measured total strain on the edge of the specimen (b) in experiments [1] .
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II •
The results obtained by the strain gages show no bending moment, but surprising
behavior of tbe measured total strain on tbe edge (see Fig.lb). After surpassing certain
load level, tbe rate of strain increase is reduced. After some further increase of load, tbe
total strain on tbe edge even starts to decrease. The displacements of tbe clamping plates
were increasing during whole tbe test. For the otber three strain gages in Fig. la, tbe plot
of tbe results is essentially the same as tbat one in Fig.} b ..
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2. Finite element computations
2.1. Finite element modelofthe CCT specimen
Elastoplastic fmite element computation of the CCT specimen was perfonned to
find out, whether the experimentally observed behavior of the specimen, described in the
previous paragraph, will occur if only in-plane defonnation without any buckling takes
place.
Program ABAQUS was used for the FEM computations. Finite element mesh was
prepared and the post-processing work was done in PATRAN.
Due to symmetry of the problem, only a quarter of the CCT specimen could be
mode lIed, as shown in Fig.2.
C>
'"
o o
"
C>
'"
I I
,
EXPERIMENT Y'l'
t t t t t t t t t
I ~ ! ~
I -:? g r-r -- x CD 2a,·100 ! ~
\
rigi~PS
mode"ed quartar
~
thicknalS 0.9 mm
+ I ,j, ,j, ,j, ,j, ~ _~~istributad '11
I w-400 loading force
FEM model
Y 200
'''. loaded edga: - straight • no rotation
< ~ - no contractIon
"-0 10
~ l'l
...,
...,
K Ic'
/ 1'\ J x
b:d
Figure 2 Experimental set-up ofthe CCT specimen and its finite element model
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The loaded edge was enforced to remain straight; aIso its rotation and contraction
were constrained. In this way, very stiff clamping of the reaI specimen was modelled. The
finite element mesh for the whole specimen is shown in Fig.3. Only a quarter ofthe mesh
was reaIly used in the FEM caIcuIation. Eight-nodded quadratic plane stress elements
with reduced integration (CPS8R in ABAQUS) were used.
- -X
Ol!
I 2 ao
Figure 3 Finite element mesh of the whole CCT specimen (only a quarter of the mesh was actually used in the computation)
6
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Material properties (elastic modulus E and Poisson's ratio v12) of standard
2024-T3 aluminum alloy were prescribed according to [2], as given in Table 1.
Table 1 Properties of 2024-T3 aluminium alloy used in the FEM computations.
E [MPa] vl2 Tensile curve 72400 0.33 see Table 2
To describe the elastoplastic behaviour ofthe aluminum alloy, the isotropic theory
of plasticity and the von Mises yield criterion was used. The large displacements theory
. was assumed.
The stress-strain curve of the 2024-T3 alloy, necessary for the elastoplastic
computation, was prescribed according to an experiment [3]. The stress-strain curve was
constructed as an average curve from 5 experimental tensile curves. Tests were performed
on specimens from bare 2024-T3 alloy sheet, 0.505 mm thick and 12.65 mm wide. Real
stress and real strain from the average curve were used for the construction of the model
curve. The model stress-strain curve is described in a piece-wise linear form in Table 2.
No experimental data were available in [3] for low values of plastic strain
(cl' < 0.2%). Therefore, the stress value for zero plastic strain (cr = 349.0 MPa) in Table 2
was obtained by linear extrapolation from the two points with the lowest plastic strain
measured (cl' = 0.2% and EP = 0.48%). This gives rather sharp transition from elastic to
elastoplastic part of the stress-strain curve, which is similar to often used bilinear
approximation of the stress-strain curve. For higher plastic strains, however, the curve
from Table 2 describes real material behavior better than any bilinear model. If necessary,
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I '
a more detailed stress-strain measurement or the Ramberg-Osgood equation for standard
2024-T3 aHoy [4] could be used to refine the curve at &1' < 0.2%.
Table 2 Tensile curve of 2024-T3 aluminium alloy used in the FEM computations.
& [%] &1'[%] cr [MPa] 0.0 0.0 0.0 0.48 0.0 349.0 0.7 0.2 361.9 1.0 0.48 380.0 1.98 1.42 407.2 2.96 2.37 429.7 3.92 3.30 450.4 4.88 4.23 467.1 5.83 5.16 483.5 6.77 6.08 498.2 7.7 7.00 510.0 8.62 7.90 521.4 9.53 8.79 532.9 10.44 9.69 542.1 11.33 10.57 550.4 12.22 11.45 558.8 13.10 12.32 563.7 13.98 13.19 568.6
2.2. Results of computations with non-propagating crack
No crack growth was assumed in the frrst phase of elastoplastic computations.
Loading was realised by prescribing displacements of the loaded edge. Maximum
elongation ofthe whole specimen was 8.832 mm (i.e. 4.416 mm for the modelled quarter
of the specimen). This corresponds to the loading force about F= 121.2 kN (or gross
8
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1
stress cr = 336.6 MPa), acting on the whole specimen. The maximum loading force is
almost double ofthe highest load reached in the experiment (Fmax = 65.49 kN).
The computed eIongation of the whole specimen is plotted by dashed Hne in FigA
for gross stress up to cr = 300 MPa. The crosses in Fig.4 mark the experimentally
obtained values.
300
/'
250 FEM - no crack growth /'./ as
FEM - crack I: Experiment D. :::::i!: 200
(Ij growth ++:;. / (Ij
++ -----G) 150 -----~
\ ------(Ij ++ (Ij + \ (Ij 100 • . N 0 • +
\ +# ~ .+ . C!' .1;+ \
+~
50 •• . ,/ DETAIL
.+ "-
0 0 2 3 4
Total elongation [mm]
Figure 4 Total elongation ofthe CCT specimen
At load about cr = 134 MPa, the crack in experiment starts to grow. Therefore, the
compliance of the specimen increases, while the compliance of the model specimen with
non-propagating crack at the same gross stress almost does not change.
Unstable fracture and failure occur prior to a plastic collapse ofthe specimen in this
experiment. In some cases (for a very smal I crack or very small width W of the
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specimen), a plastic collapse may, however, occur before reaching the toughness of the
specimen [5]. Therefore, it may be of some interest to have a look at the hehavior of our
model specimen with non-propagating crack when the loads are higher than the
experimental toughness of the specimen. In that case, large plastic deformations take
place in the specimen, leading finally to a plastic collapse. At the gross stress
approximately cr = 250 MPa, visible effect of plastic deformation on the specimen
compliance may be seen in Fig.4.
2.2.1 . Development of plastic zones and stresses in the CCT specimen
Detailed picture of elastoplastic deformation of the specimen is given in several
following figures. Contour plots of three quantities of interest are given in these figures.
Plastic zone size is shown as the contour plot of effective plastic strain
(1)
Here, dt~ is an increment of plastic strain tensor; the integration takes place along the
whole deformation path.
In a multiaxial case, it is reasonable to take the value of E~fI' =0.2% as the limit
marking the beginning of plastic deformation. The plastic zone boundary may he then
defined in the contour plot of effective plastic strain (see the fust plot in Fig.5) as the
contour corresponding to E~ =0.2% , i.e. the contour N. The contour plot of the
longitudinal stress cry (in the loading direction) is given as the second one in Fig.5. The
last plot in Fig.5 shows distribution of the transverse stress crx (in the direction
perpendicular to the loading one).
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I effective plastic strain [%]
gross stress
235.4 MPa
longitudinal stress [MPa]
transverse stress [MPa]
504 5.0 4.6 4.2 3.8
3.4 3.0 2.6 2.2 ' 1.8 IA 1.0 0.6 0.2
479 446 413 380 347 314 281 248 215 182 149 116 83 50 17
112 88 64 40 16 -8
-32 -56 -80
- 104 -128 -152 -176 -200
-224
A B C 0 E F G H 1
J K L M N
A B C 0 E F G H 1
]
K L M N 0
A B C 0 E F G H I J K L M N 0
Figure 5 Distribution of effective plastic strain, stress in the loading direction and transverse stress in the CCT specimen at gross stress cr = 235.4 MPa.
11
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As apparent from Fig.5, at the gross stress a = 235A MPa the plastic zone is still
relatively smaIl when compared with the ligament of the specimen. Therefore, the
compliance of the model specimen in FigA does not almost differ from the compliance of
a fully elastic specimen.
Stress distributions in Fig.5 correspond weIl with the plots published in [6]. Note
the unloaded area next to the free surf ace of the crack, as seen in the plot of longitudinal
stress ay . In the same part of the specimen, the transverse stress a x reaches high
compressive values, which may cause buckling of the material above and below the
crack. Naturally, the buckling limit and the behavior after its surpassing can not be
specified by this type of analysis.
The areas of higher stress (both ay and a x ) in the corners of the specimen are
caused by the rigid clamping, constraining the contraction of the loaded edges (see
Figure 2).
As the gross stress increases (a = 273 .7 MPa in Fig.6 and a = 285.3 MPa in Fig.7),
the plastics zone size increases, too. The plastic deformation is most intensive in the
direction ofthe highest shear stress (about ±45° relative to the crack axis), giving rise to
the typical butterfly-like shape of the plastic zone. Visible change of the specimen
compliance, caused by high ratio of plastification in the ligament, may be already
observed at these loads in FigA.
The contour plot patterns for both the longitudinal and transverse stresses in Fig.6
and Fig.7 are basically the same as those in the corresponding plots for a lower load
(Fig.5); just the magnitude ofthe stresses increases.
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effective plastic strain [%]
gross stress 273.7MPa
longitudinal stress [MPa]
transverse stress [MPa]
5.4 5.0 4.6 4.2 3.8 3.4 3.0 2.6 2.2 .
1.8 1.4 1.0 0.6 0.2
479 446 413 380 347 314 281 248 215 182 149 116 83 50 17
112 88 64 40 16 -8
-32 -56 -80
-104 -128 -152 -176 -200 -224
A B C 0 E F G H I
J K L M N
A B C 0 E F G H 1 J K L M N 0
A B C 0 E F G H I J K L M N 0
Figure 6 Distribution of effective plastic strain, stress in the loading direction and transverse stress in the CCT specimen at gross stress cr = 273.7 MPa.
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etIective plastic strain [%]
gross stress 285.3 MPa
10ngitudinal stress [MPa]
transverse stress [MPa]
5.4 5.0 4.6 4.2 3.8 3.4 3.0 2.6 2.2· 1.8 1.4 1.0 0.6 0.2
479 446 413 380 347 314 281 248 215 182 149 116 83 50 17
112 88 64 40 16 -8
-32 -56 -80
-104 -128 -152 -176 -200 -224
A B C 0 E F G H I
1 K L M N
A B C 0 E F G H 1
]
K L M N 0
A B C 0 E F G H 1 J K L M N 0
Figure 7 Distribution of effective plastic strain, stress in the loading direction and transverse stress in the CCT specimen at gross stress cr = 285.3 MPa.
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As the load further increases, a new phenomenon appears in the development of the
plastic zone. This is shown in Fig.8, containing contour plots of effective plastic strain for
three subsequent gross stress levels. At stress cr = 286.2 MPa (the first plot in Fig.8), four
small plastic zones have developed on the edges of the specimen approximately in the
±45° directions fiom the two crack tips. During further increase of the load
(cr = 287.0 MPa in the second plot in Fig.8), the edge plastic zones keep growing and
fmally they join the main plastic zones growing fiom the crack tips (the last plot in Fig.8).
The whole described process, from the start of creating of the edge plastic zones till
the moment when they can not be distinguished fiom the crack tip plastic zones, takes
place during a load increment about 2.5 MPa (compare Fig.7 and the last plot in Fig.8) .
. This stress change is less than 1 % of the applied gross stress.
After all the plastic zones have joined, the whole ligament is aIready plastified and
the specimen has lost almost all its bearing capacity. If the loading were performed by
controlled force, the plastic collapse of the specimen would now occur after a very small
increase of the load.
Loading by prescribed displacements allows easier study of the specimen behaviour
after the plastification of the ligament. This state is shown in Fig.9 for gross stress
cr = 293.7 MPa. Here, the plastic zones still develop in the ±45° directions fiom the two
crack tips, reaching as far as the edges. The stress plots in Fig.9, however, are not very
different fiom the state at which no plasticity existed on the edges (Fig.7 for
cr = 285.3 MPa).
15
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gross stress
286.2 MPa
effective plastic strain [%]
gross stress
287.0 MPa
gross stress
287.8 MPa
5.4 5.0 4.6 4.2 3.8 3.4 3.0 2.6 2.2 I.8 1.4 1.0 0.6 0.2
A B C D E F G H I
J K L M N
Figure 8 Distribution of effective plastic strain in tbe CCT specimen at gross stress cr = 286.2 MPa, cr = 287.0 MPa and cr = 287.8 MPa.
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effective plastic strain [%J
gross stress 293 .7 MPa
longitudinal stress [MPaJ
transverse stress [MPaJ
5.4 A 5.0 B 4.6 C 4.2 D
3.8 E 3.4 F 3.0 G 2.6 H 2.2 . 1 1.8 J 1.4 K 1.0 L 0.6 M 0.2 N
479 A 446 B 413 C 380 D 347 E 314 F 281 G 248 H 215 1 182 J 149 K 116 L 83 M 50 N 17 0
112 A 88 B 64 C 40 · D 16 E -8 F
-32 G -56 H -80 1
-104 J -128 K -152 L -176 M -200 N -224 0
Figure 9 Distribution of effective plastic strain, stress in the loading direction and transverse stress in the CCT specimen at gross stress cr = 293.7 MPa.
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2.2.2. Influence of constraining ofthe loaded edges
To find out, whether the creation ofthe edge plastic zones is influenced by the rigid
clamping of the loaded edges (see Fig.2), another computation was performed. All
parameters ofthe computation were the same as in the previous case, only the contraction
of the loaded edge was not constrained.
This time, after the same number of Joading increments (50 increments), only gross
stress cr = 284.2 MPa was reached (corresponding to the tota! elongation ofthe specimen
3.342 mm). This stress is less than the gross stress cr = 286.2 MPa, at which the edge
plastic zones appeared in the previous computation for the fust time (Fig.8).
Nevertheless, four distinct plastic zones on the edges could he observed also in the
second computation. However, to achieve this at least at the highest load applied in this
computation, a little lower plastic limit (E~ff =0.18%) had to be chosen .
According to these results, the phenomenon of the edge plastic zones creation is not
significantly influenced by the rigid c1amping ofthe loaded edges.
2 .2.3. Strain changes on the edge ofthe CCT specimen
The frrst of the two computations without crack growth (the computation with the
constrained contraction ofthe loaded edges) was used for comparison with experimenta!
data from Fig.! b. The strain in the loading direction was taken from the rightmost
element, next to the x-axis in Fig.3. This position was the c10sest approximation of the
strain gages position in Fig.la. The results are plotted by a dashed line in Fig.lO. The rate
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of strain increases with increasing gross stress, as intuitively expected. This is, however,
just the opposite of the experimentally observed trend in Fig.l b.
300 r-------------------------------------~_.
CiS a..
250
:E 200
Cl) Cl)
! 150 -Cl)
Cl)
Cl) 100 o "-Cl
50
----FEM - no crack growth /' /'
FEM - crack growth
\
\?::~~~ \ \
DETAIL:
o ~~~~~~~~~~~~~~~~~_L~~~~ 0 .00 0.10 0.20 0.30 0.40 0.50
Total strain on the edge [%]
Figure 10 Total strain on the edge ofCCr specimen
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iiil6t
2.3. Results of computation with propagating crack.
The results of computations of the CCT specimen with non-propagating crack,
described in the previous paragraph, did not show the experimentally observed behavior
in the ligament.
A more distinct decrease ofthe total strain rate occurs in experiment (Figl.b) only
when the crack starts to grow. Therefore, a crack extension was introduced also into the
FEM model. Another computation was performed with the mesh and material described
in the paragraph 2.1. This time, the loading force was increased in three steps, according
to Table 3.
Table 3 Loading steps in the computation of CCT specimen with growing crack.
Step Crack length a Numberof Totalload Gross stress [mm] nodes released atthe end atthe end
in the step ofthe step ofthe step [kN] [MPa]
I 50.0 - 46.54 129.3 2 54.375 I 62.76 174.3 3 38.75 I 65.58 182.2
The maximum loading force in the frrst step is equaI to the last measured load, at
which the experimental crack did not grow yet. In the second and third model loading
step, the crack extension was prescribed. This was done by simple releasing of one node
at the crack tip at the beginning of each step. Thus, the crack length increment in both
steps was equal to half the edge length of the element immediately before the origina!
crack tip (see Fig.3). Current crack length in each step is also given in Table 3.
Corresponding total load to be applied at given crack length in the second and the
third loading step was found by interpolating from the experimentally measured load-
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,-------- ---
crack length dependence. At the maximwn load applied in the computation, fracture in
the experiment was still stabie (compare the last value in Table 3 and Figure 3). No
attempt was done to model the crack growth during unstable fracture.
The computed load-elongation dependence is plotted by solid line in Fig.4. In the
load range where the model crack propagates, the compliance of the specimen really
increases. That is in an agreement with the experimental data in Fig.4. However, the total
strain on the edge of the specimen, plotted by solid line in Fig.10, has even for the
growing model crack the opposite trend than the experimental data in Fig.l b.
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t f
3. Buclding of the CCT specimen.
Finite element analysis, described in the previous chapter, showed that the strain
changes in the ligament of the eeT specimen shown in Fig. 1 b do not occur when
buckling of the specimen is constrained and only in-plane deformation takes place.
At frrst, this conclusion seemed to contradict the experimental observations, which
showed the same behavior even if buckling of the specimen was prevented by "buckling
guides" [1]. However, more thorough inspection of the experimental set-up showed a
reason for this behavior [1]. Improved clamping of the specimen then eliminated these
.problems, so that the strain changes from Fig. 1 b are now really observed only on
specimens with buckling.
In many discussions about the problem, a simple model was created, which
explains the strain changes in the ligament after buckling of the eeT specimen. The main
idea of the model is shown in Fig.ll. Here, the behavior of the eeT specimen without
buckling (Fig.lIa) and with buckling (Fig.1Ib) is compared.
Under tensile load applied on the eeT specimen, the material above and below the
crack is compressed by high transverse stress (see Fig.6), but still it remains in the plane
of the specimen and does not allow the outer regions of the specimen to approach each
other. When the buckling limit is surpassed, the compressed areas above and below the
crack move out of the specimen's plane; this is shown very approximately by the two
shaded triangles in Fig.II b. In the plane of the specimen, the stiffness of the buckled
material is very low. The buckled triangles carry therefore very little transverse load and
practically do not constrain the transverse movement of the rest of the specimen. The
outer regions of the specimen may thus get much closer to each other. This behavior we
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can more easily understand, when considering the buckled areas to be completely
"omitted" trom the specimen in Fig.ll b.
a) NO BUCKLING
x
Schematic model of half of the specimen
~1'
rigid clamps
'-J/~~~(J
b) BUCKLING
Stress in loading direction along the crack axis x
schematic stress distribution in front of the crack tip
In-plane bending is reduced by the other half of specimen
Buckled areas do not constrain in-plane bending
schematic stress distribution in front of the crack tip
additional stress trom bending
Figure II Simple model comparing behavior of the CCT specimen before and after buckling
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The difference between the state without and with buckling may be also more
obvious, if we consider just one half of the specimen, as shown in the rniddle parts of
Fig.ll a and Fig.ll b. In the forrner figure, the influence of the other half of the specimen
is represented by the prescribed boundary conditions along the axis of symmetry. In the
latter case (with buckling), the buckled areas are really "ornitted", so that the transverse
movement of the rniddle part of the modelled half is not constrained.
Stress distribution in the fust case (Figla) can be obtained by elastic or elastoplastic
analysis of the specimen in the state of plane stress. Stress distribution before the crack
tip is schematically shown for the longitudinal stress component (in the loading direction)
in the last part of Fig.ll a. There is, of course, a singularity or at least a concentration of
the stress at the crack tip.
After surpassing the buckling limit, the rniddle parts of the buckled specimen in
Fig.ll b can move rather easily closer to the vertical axis of symmetry. This additional
in-plane bending induces other stresses, which are superimposed on the original stress
distribution in the specimen, as shown schematically in the last part of Fig.11 b. At the
outer edge of the specimen, the stresses from the additional bending are compressive and
will be therefore subtracted from the tensile stresses existing there before the buckling.
Thus, the rate ofthe stress (and strain) increase at the edge win be reduced during further
loading in the same way, as shown in Figlb.
What will happen after an initiation of stabie fracture?
In the case without buckling (Fig.lla), the crack extension win reduce the ligament
of the specimen, increasing thus the tensile stress in the ligament. Since the in-plane
bending is still almost completely constrained, the rate of the stress on the edge win not
be visibly reduced by the compressive stress from the bending. The total stress (and
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strain) at the edge will therefore increase even faster (the ligament is smaller), as shown
in Fig.l 0 for the specimen with propagating crack.
On the contrary, when some crack extension occurs in the aIready buckled
specimen (Fig.llb), the in-plane bending of each half ofthe specimen wil! he much easier
due to the smaller ligament. Therefore, the compressive stress from the bending wiIl be
increasing at the edge faster then for a shorter crack. During extension of a long crack, the
bending wil! be more and more important. The compressive stress from bending may then
increase at the edge even faster than increases the tensile stress from the tensile load.
Thus, the stress at the edge (and the tota! strain) as a whole wiIl decrease, as shown in
Fig.lb.
Real centre-cracked tension specimen wiIl thus behave in the way described by
Fig.lla, when the load is beIlow the buckling limit or when the buckling guides are used
to prevent buckling. After surpassing the buckling limit, the specimen without buckling
guides wiIl buckle and its behavior wil! be described by Fig.ll b.
The described simple model of specimen behavior after buckling should be verified
by a finite element analysis focused on this problem.
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4. Conclusions
Elastoplastic finite element modelling of eeT specimen was perfonned. The results
describe in detail the development of stress and et'fective plastic strain distribution in the
specimen up to the loads near a plastic COllapse of the specimen. An interesting
phenomenon of edge plastic zone fonnation is observed at high load levels.
The experimentally observed changes oftotal strain in the ligament ofthe specimen
were studied. The fmite element analysis showed that the strain rate decrease does not
occur when buckling of the specimen is constrained and only in-plane defonnation takes
place, regardless whether the crack extends or not.
Simple model explaining the experimentally observed behavior is suggested.
The strain rate decrease at the edge of the eeT specimen is explained by stress changes
induced by buckling of the specimen. Buckling analysis of the eeT specimen by finite
element method shOuld be carried on in future to verify this model.
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References
[1] de VRIES, T.: The Influence of Buckling on the Residual Strength of CCT specimens. [Report] . Faculty of Aerospace Engineering, Technical University of Delft, the Netherlands. To be published in 1993.
[2] Metals Handbook, Vo1.2, Properties and Selection: Non-ferrous Alloys and Pure Metals. 9th ed., American Society for Metals, 1979.
[3] Sinhe, J. ; Faculty of Aerospace Engineering, Delft University of Technology, Delft, [personal communication].
[4] Military Standardisation Handbook - 5E; Metallic Materials and Elements for Aerospace Vehicle Structures. Vol.l, June 1987. Departrnent of Defence, USA.
[5] BROEK, D.: Practical Use of Fracture Mechanics. Kluwer Academic Publishers, Dordrecht, the Netherlands, 1989,522 pp.
[6] FUnMOTO, T. - SUMI, S.: Local Buckling of Thin Tensioned Plate with a Crack. Memoirs of the Faculty of Engineering, Kyushy University, Vo1.42, No.4, December 1982. p.355-368.
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Series 01: Aerodynamics
01 . F. Motallebi, 'Prediction of Mean Flow Data for Adiabatic 2-D Compressible Turbulent Boundary Layers' 1997 / VI + 90 pages / ISBN 90-407-1564-5
02 . P.E. Skare, 'Flow Measurements for an Afterbody in a Vertical Wind Tunnel' 1997 / XIV + 98 pages / ISBN 90-407-1565-3
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01. J.C. Gibson, 'The Definition, Understanding and Design of Aircraft Handling Qualities' 1997 I X + 162 pages / ISBN 90-407-1580-7
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02. I. Kunes, 'FEM Modelling of Elastoplastic Stress and Strain Field in Centrecracked Plate' 1997 / IV + 32 pages / ISBN 90-407-1588-2
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05. A. Vlot / T. Soerjanto / I. Yeri / J.A. Schelling, 'Residual Thermal Stresses around Bonded Fibre Metal laminate Repair Patches on an Aircraft Fuselage' 1998 / IV + 24 pages I ISBN 90-407-1591-2
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rrt "fl'j I j i 11.. I Ir; , 1 • • WEI •• Uil • .. 1 I
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I1
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During experiments studying residual strength of a centre-cracked tension (CCT) specimen from aluminimum alloys, unexpected strain changes were measured by strain gages in the ligament of the specimen (1). An elastoplastic FEM research was conducted to explain this behavior. The modelling proved that the experimentally observed decrease of total strain in the ligament does not occur, when only in-plane deformation takes place, regardless whether stabie fracture occurs or not. The results of the analysis give an interesting detailed picture of stress and plastic strain changes in the CCT specimen, subjected to high loads near plastic collapse. A simple model, explaining the observed strain changes in the ligament, is proposed for the CCT specimen after its buckling. This model should be verified by a proper FEM buckling analysis.
ISBN 90-40 7 - 15 88-2
DELFT AEROSPi\.~~ 9 799040 715883 ENGI N E U l lN G & TE CHNQlOGY