Experimental Investigation of the Static Behaviour
of a Hole Drilled Steel Plate Reinforced with a Composite Patch
N.G. Tsouvalis1 and L.S. Mirisiotis2
Associate Professor, 2Naval Architect, Researcher
Shipbuilding Technology Laboratory, School of Naval Architecture and Marine Engineering
National Technical University of Athens
Heroon Polytechniou 9, GR-15773 Zografos, Athens, Greece, [email protected]
1
ABSTRACT
This paper presents an experimental investigation of the efficiency of a carbon/epoxy composite patch for reinforcing a steel
plate with a central hole, loaded in static tension. Patches were laminated on one-side of the steel plate using the
conventional hand lay-up method. Common, low cost composite materials were used, in an effort to assess the effectiveness
of a repair or reinforcement that could be executed in situ in the harsh marine environment of a ship by a non expert
personnel. Various methods were investigated for the steel surface preparation before patch lamination and corresponding
roughness measurements were performed. Strains were monitored at three different positions of the specimens. Despite the
low stiffness ratio of the patch reinforcements, experimental measurements indicated an extension of the linear response of
the specimens, a smoother transition to plasticity and an increase in the failure load ranging from 30 to 50%.
Introduction
The various structural parts and details of a steel ship are subjected to a plethora of loading conditions during the vessel’s
operational life. The type and magnitude of these loading conditions vary significantly due to the stochastic nature of sea
waves, which are the major cause for the loading applied to a marine structure. For this reason, it is quite usual to notice
various types of local defects and failures in a ship structure, like plastic deformations and cracks development, not forgetting
corrosion, which increases the susceptibility of the structure to such local defects. Thus, there are many cases in a steel
marine structure when the need arises for local reinforcements or repairs, either permanent or temporary, until the next
programmed inspection and maintenance. This need can also be the consequence of requirements for higher load-carrying
capacity of the structure, which can in turn be the result of either a problematic initial design (i.e. existence of high stress
concentrations which were not initially predicted) or an improper manufacturing procedure or a change in the operational
conditions (i.e. later additional loading). Another very common reason for local repairs is the existence of defects in the steel
structure resulting from accidents or overloading. The basic objective of these local reinforcements and/or repairs is the
increase of the structure’s residual strength in static loading or the elongation of its operational life in cyclic loading.
The most common procedures followed today for the performance of these repair and/or reinforcement work is the use of
either bolted plates or welding. The first solution is quite effective, although it incorporates serious disadvantages like the need
for accessibility to both sides of the plating under repair, the required careful surface preparation (sandblasting is needed most
of the times) and the applicability to only simple and plain geometries. Welding is a more effective solution, however in some
cases the development of high temperatures in a metallic structural part which is under tension can be either dangerous or
prohibitive. Moreover, the execution of hot, sparking works like welding in a highly explosive environment such as the ship
holds and tanks necessitate a very careful and complicated preparation, which can have a substantial effect on the cost of the
repair, since the ship can stay out of operation for a relatively long period. In contradistinction, the use of composite materials
patching is a very attractive alternative to the traditional reinforcement or repair methods, overcoming many of their limitations
and disadvantages and being in many cases the only practically and economically feasible solution.
Composite patch reinforcements in aluminum aircraft structures has successfully applied from the late ΄70s [1]. Nowadays,
composite patching is applied from the air forces of many countries, as well as from many commercial aircraft manufacturers.
It has been reported that in 1999 there were more than 10000 operating composite patches in various aircraft structural parts
[2]. Although there are many literature references dealing with composite patch reinforcements in aluminum aircraft structures
and civil engineering applications, the reported composite patch applications in steel marine structures are very few. The
basic differentiations between the aforementioned two applications, which dictate a separate approach and study of the
problem, include the different stiffness of the base metal (stiffer steel versus the more similar to composites flexible aluminum),
the completely different geometries involved (quite thicker plating), the different loading cases and the different operating and
environmental conditions. The first applications of composite patches in marine structures have been performed in the ΄80s in
secondary structural parts of some navy ships [3 - 6]. Later applications extended to FPSO vessels. The experimental results
were very encouraging in the case of corroded plates and cracked plates which have been weld-repaired before patching.
However, the technique is less proven in the case of cracks which have not been weld-repaired, although experimental results
showed an extension of the remaining life in fatigue loading after patching by a factor varying between 2 and 4 [4]. It has been
reported that a patch repair to a cracked steel plate has survived fatigue loading in the laboratory equivalent to at least 12 ship
years [4,6]. Moreover, nondestructive methods have been developed for monitoring the quality and effectiveness of a
composite patch through its life [4,6]. Some recent efforts are focused towards characterization of the adhesive bonding
between the steel structure and the composite patch, measuring the corresponding critical energy release rate in an extensive
experimental program [7].
The basic aim of this paper is to investigate experimentally the efficiency of a carbon/epoxy composite patch for reinforcing a
steel plate having a hole. The hole was selected as a representative severe defect in the steel plate, resulting in the
development of high stress concentrations in tensile loading. The main parameter investigated was the steel surface
preparation method before laminating the patch. Among the objectives of this work was also to use low cost, easy to find
composite materials and apply a conventional manufacturing method, the hand lay-up technique, that could be easily followed
in the harsh marine environment of a ship by a non expert personnel. The advantages of the hand lay-up technique for this
particular application is the lower level of operator skill needed, the fact that it can be used where vacuum bagging is not
practical, the easiness to apply to vertical surfaces and the low material and consumables cost [6]. Its major disadvantage is
the low stiffness ratio, SR, that can usually be attained, having as a consequence a not very strong reinforcement. A relevant
work involving a steel plate with a hole under static tension is that of Colombi and Poggi [8], who results in a very good
comparison between experimental and numerical results and also in the conclusion that, in all cases examined, failure mode
was interfacial failure at the steel-adhesive interface.
Experimental Program
All tests were performed at the Shipbuilding Technology Laboratory of the School of Naval Architecture and Marine
Engineering of the National Technical University of Athens. Twelve specimens were tested in total, all in static tensile loading.
Steel specimens were provided by Elefsis Shipyards S.A. as common marine grade steel. However, material characterization
tests performed in some steel specimens resulted in an elevated yield stress, σο, of 348 MPa, whereas the measured tensile
modulus, Est, was equal to 183.75 GPa.
The patch material is a common carbon/epoxy system. Carbon reinforcements are twill 2x2 fabric with a weight of 160 g/m2,
manufactured by R&G Faserverbundwerkstoffe. The resin used was the common high viscosity two component epoxy DER
331 from Dow, in combination with the Epamine PC13 hardener from POINTER S.r.l. The mixing ratio of the epoxy was two
parts of resin to one part of hardener by weight. The epoxy system has a nominal pot life of 50 min and was cured at room
temperature. Material characterization tests were performed in standard carbon/epoxy coupons, for the determination of the
composite tensile modulus, Ep and its tensile strength in warp direction, σu, as well as of the fiber ratio, Wf. The relevant ISO
527-1, ISO 527-4 and ASTM D 3171 standards were applied. Results of the tensile specimens are shown in Table 1 for the
six coupons tested (CoV denotes coefficient of variation). Note that the specimens had no tabs and, therefore, in many cases
failure occurred near the hydraulic machine grips. For this reason the measured maximum stresses and strains are rather low,
whereas moduli measurements remain unaffected. Tests for the determination of fiber ratio by weight were performed in two
coupons. The values measured were 48.0% and 46.8%, thus resulting in an average Wf of 47.4%. The manufacturing
procedure followed for the material characterization plate was applied exactly for manufacturing the steel plate composite
patches too.
The geometry of the specimens is schematically shown in Figure 1. The tst = 4 mm thick steel plates have a 14 mm diameter
central drilled hole and are reinforced on one side by the tp = 3.2 mm thick carbon/epoxy composite patches. Note that the net
length of the specimens is 230 mm, since a 50 mm part at each end is inside the hydraulic machine grips during testing. The
composite patch is made of 12 carbon/epoxy layers, laid up with their warp direction parallel to the length of the steel plate.
Table 1. Results of the material characterization tensile tests
Coupon
max stress (MPa)
max strain (%)
Tensile Modulus (GPa)
1
498.7
1.159
39.5
2
490.0
1.069
44.3
3
511.3
1.286
40.0
4
572.7
1.360
41.1
5
263.6
0.660
40.3
6
532.5
1.202
43.9
Average
478.1
1.123
41.5
CoV
22.83
22.10
5.0
Pos. 3
Pos. 2
3.2
50
4
90
Pos. 1
330
Pos. 2
Pos. 3
14
47
70
14
8
Pos. 1
Figure 1. Geometry of the specimens tested
The first four layers adjacent to the steel have a length of 50 mm, the next four are 70 mm long and the final outside four layers
are 90 mm long, covering all the others. Thus the composite patch has tapered ends with 18% slope and an effective length of
50 mm. The patch was manufactured following a conventional hand lay-up procedure directly onto the steel specimens. The
stiffness ratio of the patch over the steel (SR = Ep·tp/Est·tst) for the specific geometry and materials tested has the relatively low
value of 0.18.
The main parameter investigated within this experimental program was the steel surface preparation method before laminating
the patch. Thus, four different methods were studied, namely the use of a simple electric rotating wire brush (WB), the use of
blasting with aluminum oxide grit No. 60-80 (BAO), blasting with 1.2 mm diameter steel shots (BSS) and blasting with Nickel
slag (BNS). Two specimens were tested for each case of surface preparation, plus reference hole drilled unpatched
specimens (REF). Two sets of tests were performed, involving 6 specimens each. The first set was displacement controlled
tests and involved two reference, two WB and two BAO specimens. The constant rate of the imposed displacement was 0.5
mm/min. The second set was force controlled tests and involved two reference, two BSS and two BNS specimens. The
constant rate of the imposed tensile force was 300 N/s. Load, overall specimen elongation and strains were recorded with a
sampling frequency of 4 Hz in the case of displacement controlled tests and 8 Hz in the case of force controlled tests.
Steel specimens were first cut from a plate and the central 14 mm hole was drilled. After that, the specimens went through
one of the aforementioned surface preparation methods and then they were degreased by acetone in order to remove rust,
grease and oil. In the sequence, the (average) surface roughness Ra was measured for each specimen, using a profilometer.
The roughness measurement was performed in four directions, parallel to the specimen longitudinal axis and in 90°, +45° and
-45° angles to this direction. The average of all these measurements was calculated for each specimen and is shown in Table
2. Figure 2 shows a close view of the surface of one of the two BSS specimens, after blasting. In parallel to the above
procedure, the necessary carbon fabric layers were cut in the appropriate dimensions. Steel specimens were degreased once
more and the patch was laminated directly on the steel, following a conventional hand lay-up procedure. The patched
specimens were left in a temperature of 25 °C for a week for proper curing of the resin, in accordance with the resin
manufacturer recommendations.
Strains along the longitudinal axis of the specimen were measured at three positions, shown in Figure 1. More specifically,
strain was measured on the steel at the high stress concentration area just beside the hole (Pos. 1), also on the steel, away
from the patch (Pos. 2) and on the patch, at its centre (Pos. 3). The latter position was selected in order to monitor the
magnitude of the load transferred by the patch, as well as the bond condition between the two adherents, as the strain in this
position suddenly drops to zero after debonding. Strains were not measured at all positions for all specimens due to
restrictions of the data acquisition system. The measurements performed are shown in Table 2. In all cases strains were
measured using 10 mm gage length strain gauges, except at position 3 for specimens BNS-1 and BNS-2 where strains were
measured using an extensometer. Figure 3 presents one of the two BAO specimens just after failure.
Specimen
REF-1
REF-2
WB-1
WB-2
BAO-1
BAO-2
REF-3
REF-4
BSS-1
BSS-2
BNS-1
BNS-2
Table 2. Characteristics of specimens
Measurement of Strain
Roughness
Loading
Ra (μm)
Pos. 1
Pos. 2
Pos. 3
3
3
3
3
2.74
Displacement
control
3
3
3.05
3
3
3.40
3
3
3.60
3
3
3
3
3
3
5.52
Force control
3
3
5.28
3
3
3
6.20
3
3
3
5.88
Results and Discussion
The global response of all specimens tested is shown in Figure 4, which presents the measured overall elongation of the
specimens (distance between the two grips of the hydraulic machine) versus the applied stress at the specimen ends, away
from the patch and the hole. Results for the four reference unpatched specimens REF-1 to REF-4 present a small
discrepancy between each other, with the first two specimens exhibiting yield at 270 MPa applied stress, whereas the two
others exhibiting yield at 250 MPa. Both these values correspond to yield of the specimens around the hole.
Regarding patched specimens, in all cases initial failure appeared as debonding of the patch from the steel plate. Figure 3 is
characteristic of this failure mode, showing the upper half the patch being still bonded onto the steel plate, whereas the lower
part has already debonded and is sliding on the steel surface. All patched specimens exhibit an extension of their linear
response area with respect to the reference unpatched ones, as well as a smoother transition to plasticity. After debonding of
the patch, the specimen response follows exactly the behaviour of the unpatched specimens. This is evident from the results
in Figure 4, especially for the displacement loaded specimens WB and BAO, which present a sudden drop of the applied
loading as the patch debonds and then their response follows a path similar to that of the reference unpatched specimens.
The same happens also for the force controlled specimens BSS and BNS, however in this case a very rapid increase of the
Figure 2. Surface of a BSS specimen after blasting
Figure 3. BAO specimen after failure
400
350
applied stress (MPa)
300
250
200
150
100
50
REF-1
REF-2
WB-1
WB-2
BAO-1
BAO-2
REF-3
REF-4
BSS-1
BSS-2
BNS-1
BNS-2
0
0
1
2
3
displacement (mm)
4
5
6
Figure 4. Variation of overall displacement (specimen elongation) versus applied loading
displacement takes place at the instant of patch debonding, since the loading grip tries to maintain a constant rate of load
increase. Thus the response of BSS and BNS specimens present a jump in displacements just after the patch debonding.
Note that this jump is happening in some tenths of a second, and this is the reason for the temporary load decrease, although
the specimens are force loaded with a constantly increasing rate.
If we define as failure the appearance of plasticity for the unpatched reference specimens and the patch debonding for the
patched specimens, then Figure 4 exhibits a clear increase of the failure loads of the patched specimens. These failure loads
have been extracted from Figure 4 and are presented in Table 3, together with their % increase with respect to the failure load
of the corresponding reference specimens. Table 3 indicates that significant improvement of the failure loads has been
attained, ranging from 30 to 50%. This improvement is lower for the wire brushed WB specimens with low surface roughness,
becoming greater for the blasted specimens with higher surface roughness. Best results are obtained for the BNS specimens,
blasted with the commonly used in the shipyards nickel slag. The failure loads increase can be characterized as quite
significant, especially if we take into account the relatively low stiffness ratio of the reinforcement (only 0.18). Thicker and
stronger patches would result in substantially higher failure loads.
A careful study of the form of the curves shown in Figure 4 indicates that patched specimens enter into some level of plasticity
Specimen
REF-1
REF-2
WB-1
WB-2
BAO-1
BAO-2
REF-3
REF-4
BSS-1
BSS-2
BNS-1
BNS-2
Table 3. Failure load of the various specimens
Failure load
Average
% Difference
Loading
(MPa)
% Difference
270
−
−
270
−
349
29
Displacement
30
control
353
31
377
40
39
373
38
250
−
−
250
−
326
30
Force control
38
363
45
362
45
50
387
55
400
350
applied stress (MPa)
300
250
patch debonding
200
150
100
50
REF-3
REF-4
BSS-1
BSS-2
BNS-1
BNS-2
0
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
strain near the hole, ε1 (με)
Figure 5. Variation of strains at position 1 (near the hole) versus applied loading
before patch debonding. This plasticity takes place at the area under the patch and around the hole, as it is justified by
corresponding strain measurements which will be presented in the following. The level of plasticity attained before patch
debonding do not vary significantly as a function of the specimens’ surface preparation method, with specimens WB (surface
preparation with a wire brush) presenting the lowest level. Specimens BAO, BNS and BSS-2 exhibit approximately the same
level of plasticity when patch debonding happens. Low level of plasticity, as well as a relatively low failure load was measured
for specimen BSS-1, a fact probably relating to special problems of this specific test.
Strain measurements at position 1 near the hole (see Figure 1) as a function of the applied stress are shown in Figure 5. As it
is reported in Table 2, strain measurements at this position were performed only for the force loaded specimens, namely
specimens REF-3 and 4, BSS and BNS. A first conclusion that can be drawn out studying the linear part of the curves in
Figure 5 is that patched specimens exhibit slightly greater strains than the reference unpatched ones. This fact is probably
due to the local bending of the specimens that takes place while they are loaded in tension, a phenomenon which in turn is
caused by the unsymmetrical cross section of the one-side patched specimens. This local bending forces the specimen to
deflect towards the steel part and is becoming more and more intense, as applied load increases. A schematic view of this
local bending can be seen in Figure 6, which presents the displaced shape of the finite element model of the specimens
tested. For reasons of symmetry, only the right half of the specimen was modeled and is seen in Figure 6. Thus, due to this
local bending, strains on the central area of the lower (unpatched side) of steel specimens (position 1) have an additional
tensile bending component in comparison to the uniform tensile strain of the unpatched reference specimens at the same
position. However, differences are quite small, indicating that local bending is not very intense.
Strain measurements shown in Figure 5 verify the previously drawn conclusion that patch debonding occurs when steel area
around the hole has significantly entered into plasticity. Excluding specimen BSS-1 for reasons mentioned earlier, at the time
Pos. 3
Pos. 2
Pos. 1
Figure 6. Displaced shape of a finite element model of half of the specimen
6
5
displacement (mm)
4
REF-1
REF-2
WB-1
WB-2
BAO-1
BAO-2
REF-3
REF-4
BSS-1
BSS-2
BNS-1
BNS-2
3
2
1
0
0
500
1000
1500
strain at position 2, ε2 (με)
2000
2500
3000
Figure 7. Variation of strains at position 2 versus overall displacement (specimen elongation)
of patch debonding, strains on the unpatched surface of the specimens near the hole reach values which vary from 14500 to
17000 microstrains (με), or 1.45 to 1.70%. These values are quite high, implying that strains closer to the hole are even higher
and giving an estimate of the ultimate strain of the adhesive layer of the epoxy resin used.
Figure 7 gives an overview of the strains variation on the steel at position 2, away from the patch and the hole, versus applied
displacement. Applied displacement was chosen instead applied stress since, in this case, strain plots are much more
informative. Measurements at position 2 were performed for all 12 specimens tested. Figure 7 indicates that strains at
position 2 remain in the linear range (< 2000 με) for all specimens, except REF-1 and 2 and the WB ones. The latter four
specimens enter plasticity at position 2, in some instance after patch debonding. Another expected conclusion is that all
specimens exhibit a considerable decrease of the strain increase rate at position 2, when area near the hole enters plasticity,
either before or after patch debonding. For the displacement loaded specimens WB and BAO, strains drop suddenly when the
patch debonds, since the applied load also drops (see Figure 4). For the force controlled specimens BSS and BNS, strains at
position 2 present a jump in displacements just after the patch debonding, similar to, and for the same reasons with the
corresponding jumps shown in Figure 4.
Strain measurements at position 3 at the center of the patch (see Figure 1) as a function of the applied stress are shown in
Figure 8. As it is reported in Table 2, strain measurements at this position were performed only for the displacement loaded
specimens WB and BAO and the force loaded specimens BNS. Strains at this position were not recorded for specimens BSS
due to a malfunction of the data acquisition system. The last point of all curves in Figure 8 indicate patch debonding, after
which strains drop to zero. The patch failure loads resulting from these measurements are in absolute agreement to those
extracted from the results in Figure 4 and reported in Table 3. All specimens except BNS-2 present a similar behaviour, which
can be described by an initial constantly increasing linear part, followed by a temporary reduction of strains as applied load
increases. Regarding the linear part of the response, it can be seen that all patches exhibit an approximately similar stiffness,
with specimens WB presenting a slightly less stiff behaviour than all the others, indicating that patches in these two specimens
carry less load than the other patches, a conclusion which justifies the reduced patch efficiency for these two specimens (see
Figure 4 and Table 3).
As the applied load increases, strains on the patch stop increasing and present a reduction. This phenomenon is caused by
the local bending occurring in the central part of the specimen, due to the unsymmetric nature of the one-side patch repair (see
Figure 6). Therefore, strains on the free surface of the patch consist of two components, one tensile from the overall tension of
the specimen and one compressive caused by local bending. This compressive component is small compared to the tensile
one at the initial loading stages, becoming more important as load increases. This compressive component is the reason for
the reduction of the experimental strains shown in Figure 8, after a certain stage of loading.
As load increases further, the area of steel around the hole enters plasticity, a fact which results in a relief of the local bending,
since strains at the lower part of the specimen can now reach quite high values. Thus, strains on the patch return again to an
400
350
applied stress (MPa)
300
250
200
150
100
50
WB-1
WB-2
BAO-1
BAO-2
BNS-1
BNS-2
0
0
200
400
600
strain at patch, ε3 (με)
800
1000
1200
Figure 8. Variation of strains at position 3 (on the patch) versus applied stress
increasing rate, as shown in Figure 8. Shortly after this new change of the stains rate, patch debonding occurs. It must be
noted at this point that the particularity of specimen BNS-2 can probably be justified by the fact that measurements at position
3 for this specimen were performed using an extensometer, set to have a gage length quite greater than the 10 mm gage
length of the strain gauges and thus not able to monitor the local bending effects.
If we ignore asymmetry and local bending of the patched specimen and apply simple formulae of engineering mechanics for a
body under uniform uniaxial tension, we can use the measurements in the linear part of the curves in Figure 8 to obtain an
estimate of the load that is carried by the patch, before it debonds. Thus, such calculations result in the conclusion that
approximately 10% of the total load only is carried by the patch, in the linear phase of the response. This low value is justified
by the low stiffness ratio of the reinforcement used, since both the patch thickness and the patch modulus of elasticity are quite
low.
Conclusions
The experimental program performed investigated the efficiency of carbon/epoxy composite patches in reinforcing a steel plate
having a hole and loaded in static tension. The composite patches were laminated on one-side of the steel plate, using a
common, relatively low cost, carbon/epoxy system and applying the conventional hand lay-up technique. As a result, the
manufactured reinforcements had a low stiffness ratio of 0.18. The surfaces of the steel specimens were treated with several
methods before lamination, leading to various surface average roughness values. Strains were monitored at three different
positions on the patched specimens. Experimental results showed that initial failure appeared as debonding of the patch from
the steel plate, the consequent behaviour being similar to the standard unpatched reference specimens one, that is it follows
the typical steel response path in the plastic region. The loads where patch debonding occurred are 30 to 50% higher than the
conventional unpatched specimens yield loads. The surface preparation method does not seem to have a significant effect,
with the use of wire brush exhibiting the lowest level of reinforcement and the use of the various types of blasting resulting in
higher failure loads. Strain measurements verified the existence of a moderate local bending, owed to the non-symmetry of
the one-side patch. Monitored strains near the hole showed that steel specimens exhibited substantial plastic deformations
before patch debonding occurs, their magnitude reaching the value of 1.7% just before patch debonding. The gain of using a
patch of such low stiffness, can not in any case be characterized as negligible, since we obtain an extension of the structural
part’s linear response, a smoother transition to plasticity and a significant increase in the failure load.
Acknowledgments
The authors gratefully acknowledge the support of research project PYTHAGORAS II, which is co-funded by the European
Social Fund (75%) and National Resources (25%). Thanks are also expressed to Elefsis Shipyards S.A. for supplying the
steel plates. The authors also acknowledge the support of Messrs A. Markoulis and H. Xanthis for participating in the
experimental activities.
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