FATIGUE BEHAVIOR OF ADHESIVELY BONDED PULTRUDED GFRP
Ye Zhang, Anastasios P. Vassilopoulos, Thomas Keller
Composite Construction Laboratory (CCLab), School of Architecture Civil and Environmental
Engineering, Swiss Federal Institute of Technology, EPFL, Station 16, CH-1015 Lausanne, Switzerland
[email protected], [email protected], [email protected]
ABSTRACT
In the frame of this work fatigue behavior of adhesively bonded joints composed of pultruded GFRP laminates is thoroughly
investigated. Two types of joints are examined, Double Lap Joints (DLJ) and Stepped Lap Joints (SLP) to point out the
influence of different joint geometries in fatigue life. The entire experimental program comprised of static and fatigue tests
under several different environmental conditions. The aim is to achieve loading patterns that are similar to those that a real
construction would probably face during operational life. Based on this experimental program, a theoretical model for fatigue
life prediction of adhesively bonded joints under different environmental conditions and for different geometrical configurations
is under development. During this first phase of the program, initial fatigue life data for both DLJ and SLJ coupons has been
collected. The failure mode of each coupon configuration has been identified and first steps for the establishment of the
theoretical models have been determined.
Introduction
Engineers design and produce different parts and then assemble them to create an entire structure. The region of connection
between the parts of a structure is named joint. Although the use of joints has solved a lot of problems in constructions it also
induced some others, since it has been proved that joints are weak points. Thus, their behavior should be thoroughly
examined. Joints are usually classified as follows: Bolted/riveted, welded and bonded. Holes that are opened for the
mechanical fastening of structural parts induce stress concentrations that might be detrimental. On the other hand, heating of
the material during the welding process could significantly reduce properties in the joint region. No side effects like the
aforementioned have been reported for bonded joints. No heating is induced and load is transferred quite uniformly. Also,
depending on the adhesive and the geometry of the joint, good inherent damping properties might be achieved. Their major
disadvantage compared to bolted/riveted joints is that bonded joints are permanent.
During the last years, there is a trend to use adhesively bonded joints in the civil infrastructure sector, e.g., [1-3]. Connections
for FRP bridge decks for example [3] include primary and secondary load-carrying joints and non-structural joints that
potentially could be bonded. The objective is to gradually replace the mechanical connections in civil applications by using
simple bonding techniques. Unfortunately, the extended knowledge about joint’s static and fatigue behavior that has been
gained after a lot of years of research in the aerospace industry, e.g., [4], cannot be directly transferred to the civil
infrastructure domain. Reasons for this fact are among others the essential differences in the manufacturing process, the
material architecture, the dimensions of the components and the application environments [5]. The joint type, the bonded
surface, the thickness of the configuration and the type of the adhesive layer are some of the significant design parameters. It
is essential to analyze joint configurations that are applicable in the civil infrastructure domain, understand their behavior under
different loading patterns and environmental conditions and establish mathematical models to assist design processes.
In the present work an investigation of the static and fatigue behavior of adhesively bonded joints composed of pultruded
GFRP laminates and epoxy adhesive was undertaken. Load, displacement, strain of the laminate and local strain at the region
of the overlap length of the joints were continuously recorded during the tests. Part of the specimens was also equipped with
crack gages to measure the crack initiation and propagation. Both Double Lap Joint (DLJ) and Stepped Lap Joint (SLJ)
configurations were used. Having as basis the static tensile strength of each coupon configuration, constant amplitude fatigue
tests were realized under pure tensile loading, at a stress ratio equal to 0.1, (R=0.1). In the current phase of the investigation
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all tests were conducted under normal environmental conditions, i.e., temperature at 23 C and relative humidity at about 50%.
The entire experimental program contains also tests under different environmental conditions to examine the influence of
temperature and humidity on the structural behavior of DLJ and SLJ and determine design allowables based on experimental
data that replicate as much as possible real operating conditions.
Experimental procedure
GFRP laminates were bonded with epoxy resin in order to produce the specimens. The laminates, provided by Fiberline A/S,
Denmark, were manufactured by pultruded technique, using E-glass fibers and an isophtalic polyester resin. In total, they
consist by three types of layers; veil, mat and roving unidirectional glass fibers in a symmetric configuration. However, due to
the method of manufacturing the symmetry cannot be achieved by 100% since fibers from one layer could penetrate to the
other, as shown in Fig. 1. In this photo, captured by an optical microscope, it is clearly shown that interference between two
subsequent layers is present. In the schematic representation, the ideal laminate configuration is also presented. Laminates of
a total thickness of 6mm and 12mm and width of 50mm were bonded with a two component epoxy adhesion system (SikaDur
330, delivered by Sika Schweiz AG) to create the specimens. Their dimensions are presented in Fig. 2 for the Double Lap
Joints (DLJ) and in Fig. 3 for the Stepped Lap Joints (SLJ).
Figure 1. Photo of the pultruded laminates along with the schematic representation of the stacking sequence. Photo captured
by an optical microscope.
Laminates of both 6mm and 12mm thickness were used for the construction of balanced DLJs. The thickness of the adhesive
was constrained to 2mm on a total length of 50 mm on each side of the joint.
Figure 2. Double Lap Joint configuration
Similar geometrical characteristics for the stepped lap joint configuration are presented in Fig. 3. Compared to DLJs, only 6mm
thick pultruded laminates were used for the construction of SLJs, while the total bonded length was 50mm. Another difference
is the existence of adhesive in two small gaps, 2mm each, perpendicular to the main part of the adhesive, as pointed out in
Fig. 3.
Figure 3. Stepped Lap Joint configuration
All specimens were prepared under laboratory ambient conditions and left for at least 5 days before testing. Several
measurements were performed during static and fatigue tests. Along with applied load and grip displacement, strain at
predetermined points on the coupons was recorded using strain gages. Failure of coupons was driven by a crack that was
initiated and propagated in certain area. Thus, crack propagation gages were glued at the side faces of the coupons, to
monitor the crack initiation and propagation during the loading. The crack propagation gages consisted of 20 wires spaced at
1.15 mm intervals perpendicular to the adhesive layer, and covered almost half of the overlap length. The gages covered the
part of the overlap length where stable crack propagation was expected to occur. As the crack propagated, the wires were
broken progressively and thereby the electrical resistance of the gage was increased. The difference in the resistance was
then transformed to crack length. Fig. 4 contains a photo of a DLJ implemented with all measuring devices, mounted on the
test rig, prior testing. On the right hand side of Fig. 4 the instrumentation of DLJ (upper) and SLJ (lower) is schematically
illustrated. Strain gages marked with D are named back face gages. They were placed on all coupons, at the regions where it
was possible to measure bending of the coupons because of crack initiation or propagation. Two more strain gages were
glued outside the area of the joint where the strains were expected to remain uniform and relatively low during the entire test
process.
Figure 4. A double lab joint with all instrumentation mounted on the testing rig. On the right hand side of the figure the
experimental instrumentation for DLJs (upper) and SLJ (lower) is schematically presented
In this first phase of the experimental program, 6 specimens of each type of joints were tested under monotonic loading to
determine the ultimate tensile load of each configuration, detect and observe the corresponding failure modes, and record
crack propagation to calculate the strain energy release rate. Tests at four different load levels were conducted for each type
of joints, DLJ and SLJ, in order to determine the S-N curve. It was planned that each S-N curve would be determined using 12
coupons, three at each one of the four selected load levels, at 45%, 55%, 65%, 80% of ultimate tensile load. Load level
2
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selection was performed in order to have representative experimental data in the region between 10 and 10 cycles. An
exploratory study has been realized to examine if loading frequency is affecting fatigue life. So, two sets of tests were
programmed at two frequencies, 2 Hz which is closer to real conditions in civil engineering applications and 10 Hz that is more
convenient when time is the criterion. In order to exclude influence of temperature rises because of frequency on fatigue life of
joints, thermal gages were introduced to some of the coupons that were tested at the frequency of 10 Hz. Both static and
fatigue tests were realized on a universal Instron 8800 hydraulic test machine of 100 kN capacity.
Results and discussion
The double lap joints seemed to behave linearly up to failure, under application of axial loading of 45.6±2.0 kN, while stepped
lap joints presented a bimodal behavior, as presented by typical load-displacement lines in Fig. 5. Failure of the adhesive layer
in the perpendicular small gaps was the reason for the depicted behavior of SLJs. Following this failure, which usually
happened at about 70%-90% of the ultimate tensile load of the SLJs, a measurable decrease in the joint stiffness was
recorded. Although having a decreased stiffness the joints were capable of carrying additional loading up to the limit of about
10.5±0.5 kN in average.
SLJ
DLJ
40
Load (kN)
30
20
10
0
0.0
0.4
0.8
Displacement (mm)
1.2
Figure 5. Typical load-displacement curves for DLJs and SLJs.
The typical failure modes of the DLJs and SLJs are shown in Fig. 6-7. For both types of joints, the dominant failure mode was
a fiber-tear-off failure in the GFRP laminates. As indicated in Fig. 6, for DLJs failure initiation occurred in the outer mat layers
of the internal thick laminates below the ends of the thinner laminates of the DLJs. Since there are eight potential locations for
crack initiation on each DLJ, it was usually observed that more than one cracks were simultaneously initiated and propagated
in the same laminate layer, leading the joint to a sudden failure. The situation was slightly different for SLJs since in this case
the adhesive in the small perpendicular gaps was failed at the first stage. Consequently, the stepped lap joint was deformed to
an unbalanced single lap configuration and a crack was then initiated and propagated inside the pultruded laminate as
presented in Fig. 7.
The average ultimate load that the DLJ configuration could sustain was more than four times the corresponding ultimate load
of the SLJ configuration. This was anticipated and can be attributed to two main reasons. Firstly, due to fabrication the area
that was assigned to transfer the applied load to the joint was two times more in the DLJs compared to the area in the SLJs. In
addition, DLJs were balanced unlit final failure, while after the initiation of failure in the small gaps of the SLJs bending
moments were induced to the joints increasing the developed stresses.
Figure 6. Failure of a Double Lap Joint under static loading
Figure 7. Failure of a Stepped Lap Joint under static loading. Small vertical gaps failed first and subsequently the crack
propagated in the adherend.
Similar failure modes were observed under constant amplitude fatigue loading. As depicted in Fig. 8, DLJs were more efficient
on carrying higher loads than stepped lap joints and the ratio between them was again, as under static loading, around 4 for all
load levels. There seemed to be an effect of frequency on fatigue life; curves for 10 Hz are steeper, than curves determined at
a frequency of 2 Hz, meaning that for low cycles the increasing of loading frequency was beneficial for the material, but when
the test was continued for more than 103 (DLJ) or 105 (SLJ) cycles the effect was inverted.
2 Hz
10 Hz
40
Load (kN)
30
DLJ
20
SLJ
10
0
102
103
104
105
106
107
N
Figure 8. Initial constant amplitude fatigue test results for both joint configurations at the frequencies of 2 and 10 Hz.
As it was mentioned in a number of publications, loading frequency in general affects fatigue life of composite materials.
Research studies on this subject concluded that dependence of fatigue life of composite materials on loading frequency is due
to heating of the material at higher frequencies, or creep fatigue at lower frequencies, or their interaction, e.g., [6-7]. An
interesting aspect on this issue was reported by Perreux et al in [6]. As temperature rises due to energy dissipation, life of each
specimen depends on its geometrical shape and therefore on its ability to dissipate heat. Thus, reached lifetime may not only
depend on the intrinsic characteristics of the material for higher frequencies, but is also a function of the shape and size of test
specimen. In another work by Crocombe et al [8] it was demonstrated that for the examined adhesive system (a hot cure
rubber toughened epoxy adhesive) and several different joint configurations, frequency was not affecting fatigue life if it was
kept in the range between 2 and 10 Hz. Thermal gages were used, in the present study, to measure deviations of temperature
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with loading cycles. All measurements, showed that temperature in the joints was increased by only 2-3 C during fatigue life.
Fatigue behavior of double lap and stepped lap joints is under development in CCLab, EPFL and new knowledge on the
aforementioned subject but also about the durability of the joints under different environmental conditions would be acquainted
in the near future.
Conclusions
Analysis of static and fatigue behavior of two types of bonded joints was the subject of this paper. Double lap joints and
stepped lap joints, two of the most common used joint configurations in civil infrastructure industry were investigated. Under
static axial loading DLJs exhibited a linear behavior up to failure which was sudden. For SLJs the situation was slightly
different as two discrete phases in the failure process could be identified. At about 70% of the ultimate tensile load the
adhesive in the perpendicular opening was failing and the joint stiffness was decreasing. Consequently the joint was becoming
unbalanced and additional bending moments were inducing on it developing an augmented stress field.
Under fatigue loading similar behavior was observed. Specimens of DLJ configuration were loaded continuously up to a
sudden and brittle failure. On the other hand, SLJ specimens were performed well under fatigue loading until the initial failure
of the adhesive in that perpendicular gap. After this occurrence, fatigue lifetime was reached quite soon. Loading frequency
was proved to affect fatigue life of both joint configurations. It seemed that increased loading frequency was beneficial only for
low cycle fatigue and this effect was inverted for median and high cycle fatigue regimes. Although these first comments are in
accordance with some other published works, safe results could not yet be extracted from the experimental program.
Acknowledgments
The authors would like to thank the Swiss National Science Foundation (Grant No 200021-103866/1), Sika AG, Zurich
(supplier of the adhesives) and Fiberline Composites A/S, Denmark (supplier of the pultruded laminates) for the support of this
research
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