Table Of ContentCONFERENCE COMMITTEE
.P NILEUG.SIB Switzerland
W. BOHME Germany
A.J. BRUNNER Switzerland
.A CERVENKA UK
.E CLUTTON UK
D.R. MOORE UK
A. PAVAN Italy
.F RAMSTEINER Germany
P.E. REED The Netherlands
J.G. WILLIAMS (Chairman) UK
INTRODUCTION
It is our pleasure to introduce the proceedings of the Second ESIS TC4 conference which
was held in Les Diablerets, Switzerland on 13 ht to 15 ht September 1999. We were gratified
by the large number of excellent papers which were submitted. The task of choosing those
for presentation and publication was difficult, but the high standard of the presentation and
the written versions justifies the choices made.
The meeting was designed to reflect the activities of the Committee over the last fifteen
years and to look forward to our future activities. We have divided the papers into four
chapters under the headings of Composites, Elastic-Plastic Fracture, Adhesion and Impact
and General Fracture. This is a convenient grouping, but there are many interactions
between the areas, with the common theme of Fracture Mechanics underlying it all.
We hope the book is useful and we thank the many authors for their diligence in preparing
their contribution.
J G Williams
A Pavan
COMPARISON OF FRACTURE TOUGHNESS (GIc) VALUES OF
POLYETHERIMIDE (PEI) AND A CARBON-FIBRE/PEI COMPOSITE:
AN EXPERIMENTAL AND THEORETICAL STUDY
R. Akkerman, P. E. Reed, K. Y. Huang and L. Warnet
Department of Mechanical Engineering, University of Twente. The Netherlands.
ABSTRACT
GIG values are measured for polyetherimide (PEI) under quasi-static and low speed impact
conditions. A novel technique is used to obtain the quasi-static GIG value, based on video-recording of
crack growth under test in a single specimen. Further measurements of Gic for a carbon fibre/PEI
composite are made, which show greatly reduced fracture toughness of the composite compared to
the matrix alone. Causes of the lowered efficiency of translation of the matrix toughness into the
composite are explored, using FEA modelling to examine the effects of matrix/fibre interface
strength and resin-rich layers on the stress fields in the vicinity of the crack tip.
KEY WORDS
Fracture mechanics, polyetherimide, thermoplastic matrix composites,
INTRODUCTION
Thermoplastic matrix, continuous fibre composites have been developed for their toughness and
impact resistance. Impact tests on laminate plates fabricated with a thermoplastic matrix generally
exhibit less delamination after the test than those made with a brittle thermosetting matrix material.
Polyetherimide (PEI) is one of the thermoplastic materials used for thermoplastic matrix composites.
In the present work, the particular grade of PEI used is ULTEM | 1000 (GE Plastics). The same grade
of PEI is also used by Ten Cate Advanced Composites to produce unidirectional carbon fibre/PEI
prepreg material for the manufacture of carbon fibre composite laminates. Carbon fibre reinforcement
provides strength and stiffness to the matrix material, but also affects the fracture toughness. This
paper first studies the fracture toughness of the ULTEM | 1000 PEI material in isolation and then
moves to investigate how the fracture toughness is affected by the introduction of carbon fibre
reinforcement in a CF/PEI unidirectional composite containing 60% fibres. All tests used adopt the
appropriate ESIS protocol developed for measurement of fracture toughness in thermoplastics and
composites.
PEI is recognised as a tough but notch sensitive thermoplastic material. The GE data sheet (1) of
material properties for ULTEM | 1000 gives Izod impact values of 1300J/m and 50J/m for unnotched
and notched PEI specimens respectively, when tested according to the ASTM D256 method (2).
These values show how sensitive PEI is to notches or stress concentrations, although the unnotched
material is very tough. It should also be noted that the ASTM D256 test method does not provide a
fundamental material property value of impact strength, but rather an energy value to break a specific
component under test. The component geometry and test conditions are detailed in the standard.
4 .R AKKERMAN, P.E. REED, K.Y. HUANG, L. WARNET
QUASI-STATIC DETERMINATION OF THE CIG VALUE FOR PEI
The ESIS protocol (3) describes a method for determining the energy release rate under quasi-static
loading conditions. Values of CIG so determined are considered as true material property values. The
method uses either Single Edge Notched Bend (SENB) or Compact Tension (CT) specimens. SENB
specimens have been used in this case. Several beam specimens, each with a different length notch,
are tested to failure in 3 point bend using a universal testing machine. The energy (U) required to
break each specimen is recorded from the force-deflection diagram. A graph is then made of U
against BD~b, the slope of which provides the value of .CIG ,B D and ~bare respectively the specimen
breadth, depth and a calibration factor, values for the latter being given in a Table in the protocol.
In the present work, 6mm thick sheets of PEI were compression moulded in a closed mould using
dried ULTEM | 1000 pellets. Beam specimens were then machined from the plate, with dimensions
conforming to the ESIS protocol, 12mm width, 6mm breadth and length of 53mm, using a 48mm
length between supports. Notches were then cut and sharpened with a razor blade to give nominal
notch depths between 2.5 and 8.0mm in 0.5mm increments. Each specimen was then tested in 3 point
bend to failure, measuring the energy to failure in each case. The central deflection was measured
directly for improved accuracy, using a Heidenhain electronic micrometer to record the movement of
the loading point. After failure, the precise initial crack length was measured from the fracture
surface using a travelling microscope. All results were corrected for indentation at the supports, using
the method described in the ESIS protocol.
041
021
m
001
E
"" 08
~>
~ o6
w
04 g
O2 Z
0
0 5 10 51 20 25 30 5,3
BD~ (mm 2)
Figure 1
Energy / BD~ plot from one set of quasi-static tests on PEI.
The set comprised 24 specimens, each providing one datum point.
Fig.1 shows the energy/BD~ plot obtained from the quasi-static test on PEI which gives a GIG value
of 3900J/m .2
The ESIS protocol (3) requires the testing of a minimum of 20 notched specimens, each with a
different notch length, to obtain the GIG and CIK values for the material. An alternative approach
using only one specimen in conjunction with a CCD camera and timed video recorder, has been
explored with the same PEI material. A single SENB specimen was set up in the universal test
machine in the 3-point bend test rig as was used for the previous series of tests. An initial razor blade
sharpened notch of 2.5mm was made in the single specimen. A CCD camera through a Zeiss
microscope was used to monitor the crack growth during the quasi-static test, which was conducted at
the recommended speed of 10mm/min. The output from the CCD camera was taken to a video
nosirapmoC of Fracture Toughness (Gic) Values... 5
recorder, which had a 25 frames/s recording rate with time display. The load output signal was taken
simultaneously through a data logging system to a computer with its associated time base. Hence the
computer file held the digitised load/deflection record for the test, recorded at 0.1s intervals as the
crack grew. The video recording of the crack growth was analysed using image analysis software, so
preparing a record of crack length against time from the start of the test. These data could then be
included with the load/deflection record and synchronized to the same time base. This complete file
of load/deflection (or energy) against crack length was then processed through the ESIS protocol GIG
analysis to generate the usual U against ~qDB data for each increment of crack length. Fig.2 shows the
results so obtained from a single PEI specimen.
J
8O
6o j
4O
5 10 1E 20 25 30
~DB mm( )2
Figure 2
Energy/BDq~ plot derived from a video recording of a quasi-static test on a single PEI specimen.
The energy/BDq~ data points from the test on the single specimen show very little deviation from the
best-fit line, in contrast to data shown in Fig. 1 for multiple specimen testing. The best-fit line for the
single specimen data also passes through the origin, suggesting no systematic errors. The value for
Gic obtained from Fig. 2 and the single specimen test was 3800J/m ,2 which is comparable with the
3900J/m 2 value obtained from the conventional test which uses a different specimen for each datum
point.
LOW SPEED IMPACT DETERMINATION OF THE CIG VALUE FOR PEI
Further SENB specimens were cut from a cast 6mm thick PEI plate, then notched and prepared in
accordance with the ESIS Standard for the determination of CIK and CIG at high loading rates (4).
This protocol covers the measurement of the fracture mechanics parameters under impact testing
conditions at rates up to l m/s. The high loading rate test method follows the same general analysis as
for the quasi-static testing, but includes corrections for the dynamic oscillations in the load/deflection
curve. Just enough mechanical damping is used at the point of impact loading to bring the oscillations
within limits specified in the protocol. The recorded energy to break under the mechanically
load/displacement curve is also determined after applying a computer-aided curve fitting procedure to
draw a smooth mean load/time curve through the experimental load/time record.
A Dynatup 8250 instrumented falling weight (IFWIM) system was used for the tests. The full series
of 20 notched SENB specimens were prepared, with different initial crack lengths inserted and razor
sharpened. Overall specimen dimensions were identical to those used for the quasi-static tests. Each
specimen was placed in turn in the IFWIM tester and subjected to an impact test at 1.1m/s. It was
found that 2 layers of Parafilm were required at the point of impact to achieve the necessary
mechanical damping without affecting the maximum force. The load/deflection curves recorded from
the IFWIM tests were then processed according to the ESIS protocol, using the prescribed curve
6 .R AKKERMAN, P.E. REED, K.Y. HUANG, .L WARNET
fitting routine and checking that the force fluctuations were within the specified 5% limit of the fitted
curve.
IO0
8o
A .__
40
, - j
o ~ ............................... ~ .................................... ; ...................................... ( ................. " ................. ~ .................................. i: :: ---: .......................... -;--: ........................... ---::-
0 5 10 15 20 25 30 35
~DB mm( )2
Figure 3
Energy/BD~ plot from one set of impact tests of PEI. The set comprised 20 specimens.
The value of GIG determined from the 1.1m/s test was 2800J/m ,2 which is considerably lower than the
value derived by quasi-static testing of the same material. It would therefore appear that the fracture
resistance of ULTEM | 1000 is rate dependent. Frassine and Pavan (5) also observed PEI to be rate
dependent, but observed the fracture energy to increase from 2100J/m 2 to 3200J/m 2 over a crack
speed range of 6-600mm/min. and at 23~ using Double Torsion specimens.
DETERMINATION OF CIG FOR A UNIDIRECTIONAL CARBON FIBRE/PEI COMPOSITE
ULTEM | 1000 PEI is used as a matrix material in the production of CETEX | thermoplastic matrix
composite laminates manufactured by Ten Cate Advanced Composites. Unidirectional T300-PEI
prepreg material is made by drawing the carbon fibres through a bath of PEI dissolved in a solvent to
form a continuous sheet of unidirectionally oriented fibres distributed in a matrix of PEI. The prepreg
sheet so formed is then dried to remove the solvent, leaving a material with a matrix mass content of
32% (60% fibre volume). The prepreg material is then laid up and pressed to form the required
composite laminate.
Gic values for the unidirectional T300-PEI material were determined using the test method described
in the ESIS protocol for the determination of fracture mechanics parameters for continuous fibre
composites (6). Approximately 2mm thick unidirectional T300-PEI laminates were formed from 21
layers of prepreg, pressed in a closed mould at a moulding temperature of 325~ A mt.131 polyimide
starter foil was inserted centrally during lay-up, to provide a pre-crack length of 50mm from the edge
of the plate in the fibre direction. After pressing, 175mm long, 20mm wide specimens were cut for
DCB testing to determine Gic for the unidirectional material. No further preparation of the crack tip
was undertaken, thus all GIc values determined relate to cracks initiated from the starter foil.
Loading blocks were attached to the ends of the specimens. The DCB specimens were tested at 23~
at a rate of 2mm/min.
Two different methods were used to monitor and record the crack growth during the test in relation to
the applied load and displacement of the loading blocks.
(a) The same CCD camera and timed video recording set up was used as previously described for the
PEI specimens. The edges of the DCB specimens were painted with a brittle white paint, which
nosirapmoC of erutcarF ssenhguoT )ciG( ...seulaV 7
was then scratched to provide a suitable rule to identify the precise position of the crack tip (and
hence the crack length) in the analysis of the video recording after test.
(b) The conductive properties of the carbon fibres can also be harnessed to determine the crack length
in a DCB test. The change in resistance of the carbon/PEI specimen, measured across the 2 arms
of the specimen, can be used to measure the increase in length of the crack throughout the test (7).
Crack length determinations using both techniques simultaneously on a specimen are shown in Fig.4.
The two techniques provide very close agreement for the increase in crack length with displacement
of the loading blocks. The resistance measurement technique records a slightly shorter crack length
for a given displacement, leading to a maximum 2.5% variation in the value of GIc determined.
Hence both techniques are suitable for the determination of the crack growth during the test.
Fig. 5 shows the related force-displacement curve from the DCB test on a T300-PEI specimen. Fig. 6
shows the resulting calculated values of Gic against crack length for the DCB test specimen
considered, which were determined using Corrected Beam Theory (CBT) as specified in the ESIS
protocol (6).
O7
65
.~, ss
114
35 ......... ~ v
30
5 10 15 20 25 30
Displacement )mar(
Figure 4. Comparison of crack length determination using the camera method (o) and resistance
measurement (,) technique
5O
O4
--Z
P 3O
o
2O
01
0
0 5 01 51 02 25
Displacement )mm(
Figure 5. Force-displacement graph of a DCB test on unidirectional T300-PEI
8 .R AKKERMAN, P.E. REED, K.Y. HUANG, .L WARNET
1600
1400
1200
..-. 1000
800
600
OO4
OO2
0
50 55 60 65 70 75 80 85 90 95 100
Crack length (mm)
Figure 6. Crack propagation values of GIG against crack length for the DCB test considered,
calculated using Corrected Beam Theory specified in the ESIS Protocol (6)
Fig. 6 shows a value of Gic for the T300-PEI of 1200 J/m .2 This value is much lower than those
previously discussed from measurements of ciG for PEI alone (3900J/m 2 from the quasi-static testing
and 2800 J/m 2 from low speed impact testing). While the fracture toughness of PEI appears to exhibit
a rate effect, the inclusion of the carbon fibres greatly reduces the fracture toughness by a factor of 3
over the quasi-static value for the PEI alone. Frassine and Pavan (5) have observed a similar
reduction in the fracture toughness of the CF/PEI composite over the pure resin. They measured Gic
values for the composite at 23~ which increased with crack speed from 1740J/m 2 at a crack speed of
01 5- m/s to 1900J/m 2 at 10-2m/s at 23~ Crack speeds in the present work were of the order of
3xl 0 2 m/s. While the currently measured GIG value for the T300-PEI is similar to that measured by
Frassine and Pavan, it is noticeably lower. The origins of this difference are not known, although
results are influenced by variations of the processing conditions used to form the laminates.
LOWERED EFFICIENCY OF TRANSLATION OF FRACTURE TOUGHNESS
Several workers have observed similar lowering of efficiency of translation of high resin fracture
toughness into delamination fracture toughness for the composite (8-10). Yee(8) suggested that the
presence of the fibres led to a reduction in the size of the plastic zone at the crack tip. Bradley(9)
attributes the lowered efficiency in the tough matrix composites to constraint imposed by the fibres
on the development of the plastic zone at the crack tip. Dubois and Keunings(10) performed detailed
FE modelling for a unidirectional glass-fibre/poly(arylamide) matrix system DCB specimen, adopting
a square array for the fibre distribution. They found that, on planes perpendicular to the crack passing
through the fibre axes, the plastic zone was large, extending almost to the surface of the specimen.
This contrasted with a small plastic zone in the case of the matrix alone. The overall increase in the
plastic zone for the composite led to a reduction of the plastic energy density within the zone and was
attributed to the reduced fracture toughness for the composite.
In the T300-PEI composite, it is known that the interface bonding of matrix to fibre is relatively poor
and also that resin-rich interlaminar layers occur in the fabricated composite. Due to the method of
manufacture of the prepreg material, one side of the prepreg contains a thicker layer of the PEI resin
than the other. When the multilayer unidirectional laminate is pressed, the resin rich prepreg surfaces
can be brought together in the centre, so creating a resin rich layer in the DCB specimen for the crack
to follow. These two factors were also noted by Bradley(9), but he suggested that interfacial failure
only played a minor role in the reduced fracture toughness of the composite. These two further
factors are now explored, to see if they might also contribute significantly to the reduction in fracture
toughness of the T300-PEI composite over the pure resin PEI.
nosirapmoC of (Gic) Fracture Toughness ...seulaV 9
Fig. 7 shows a SEM photograph of the fracture surface of a DCB T300/PEI specimen. The
photograph reveals general poor adhesion between matrix and fibre, with many nearly 'clean' fibres
and few with adhering matrix. The crack tends to grow more easily along the fibre-matrix interface.
Figure .7 SEM photograph of a carbon-fibre/polyetherimide delamination surface
The mechanisms leading to either interface or matrix fracture have been investigated numerically, by
considering the stress distribution in the matrix around a single fibre. A detailed account of the
analysis is given by Warnet (11) and is only summarised here. The ANSYS finite element package
was used to examine the situation around a single fibre near the crack tip of a DCB specimen at the
moment of crack initiation. The situation is shown in Fig. .8
Figure .8 Location of the region analysed for the stresses around a single fibre near the crack tip of a
DCB specimen.
Fig. 8 shows the cross-section A-A with a single fibre oriented in the x direction, located at distance d
from the crack plane. Square packing of the fibres is assumed. The quarter fibre with matrix is
modelled and the resulting model geometry using 4 noded quadrilateral elements is shown in Fig. .b8
10 .R AKKERMAN, P.E. REED, K.Y. HUANG, .L WARNET
The square cell is of unit edge length and the fibre radius is then related to the fibre volume fraction.
The 60% volume fraction considered requires a dimensionless radius of 0.874 within a unit square, as
indicated in Fig. 8. Material properties of both matrix and fibre used for the calculation are given in
Table .1
~2
/~ /~
T300 230 14 0.2 0.25 0.7xl 0 6- 5.6x10 -6
-
Fibre
E G
GPa GPa /~
ULTEM 56.6x10 6-
1000
Table .1 Thermo-mechanical properties of T300 and Polyetherimide used for the analysis
The analysis is performed in two stages. Initially the residual stresses originating from initial
fabrication are modelled as the unidirectional composite cools from the fabrication temperature of
325~ to room temperature. The Tg of PEI is quoted as 215~ hence a temperature difference of
-193~ is applied in the model for cooling from initial solidification of the PEI to room temperature.
A second stage analysis is subsequently applied, in which a unidirectional displacement field yU is
applied as indicated in Fig. 8, equivalent to that which occurs in the DCB specimen around the crack
tip at the crack initiation condition. This displacement field is extracted from a FEM model of the
complete DCB specimen.
Fig. 9 shows the nodal displacement field yU obtained from the DCB model as a function of the
transverse distance y from the crack tip (from 0 to 1 ply thickness), as well as the longitudinal
distance x from the crack tip (from 0 to 0.5mm). Fig.9 shows that the relative nodal displacement rate
du/dy is highest near the crack tip due to the singularity at this point. It is also noted that the
transverse displacement yu approaches zero at a longitudinal distance of only 0.5mm from the crack
tip plane.
3.0E-06
y,~
2.5E-06 (cid:12)9 (cid:12)9 (cid:12)9 + (cid:12)9 + (cid:12)9
.' t_.~. x (cid:12)9 (cid:12)9 (cid:12)9 + + + (cid:12)9
2.0E-06 4, (cid:12)9 + (cid:12)9 (cid:12)9 + : ,i, + (cid:12)9 + (cid:12)9 + (cid:12)9 x (cid:12)9 x (cid:12)9 x x
4, (cid:12)9 + (cid:12)9 (cid:12)9 x x x x
1.5E-06 (cid:12)9 (cid:12)9 (cid:12)9 + + (cid:12)9 (cid:12)9 + x (cid:12)9 x (cid:12)9 x x x
(cid:12)9 + (cid:12)9 x
(cid:12)9 + (cid:12)9 x
1.0E-06 (cid:12)9 (cid:12)9 + (cid:12)9 + x (cid:12)9 x (cid:12)9 (cid:12)9 (cid:12)9 (cid:12)9 (cid:12)9 (cid:12)9 (cid:12)9
(cid:12)9 + (cid:12)9 x (cid:12)9 (cid:12)9
+ (cid:12)9 x (cid:12)9 (cid:12)9
5.0E-07
(cid:12)9 + Ax x (cid:12)9 (cid:12)9 (cid:12)9
(cid:12)9 (cid:12)9 (cid:12)9 (cid:12)9 (cid:12)9
0.0E+00 (cid:12)9 (cid:12)9 (cid:12)9 - - ~ ~ o _~, ~ o o ,
0.0E+00 5.0E-05 1.0E-04 1.5E-04
y(mm)
(cid:12)9 x=0 + x=6um (cid:12)9 x=13um x x=21um (cid:12)9 x=100um o x=489um
Figure 9. Transverse displacement near the crack tip of a DCB specimen at crack initiation.
x( andy are as defined in Fig. 8b. yU is the transverse displacement at a specific x,y)
The stress situation around a fibre is considered at an arbitrary longitudinal distance x of 0.1 mm. This
leads to a relative nodal dimensionless displacement (uy) of 8xl 0 ~ to be applied to the model shown
in Fig.8 to determine the stress distribution in the matrix around the fibre for 2 extreme conditions (a)
perfect adhesion between fibre and matrix (b) no interfacial adhesion.
