Application of bonded composites in the repair of ships structures – A 15-year service experience

8/10/2019 Application of bonded composites in the repair of ships structures A 15-year service experience

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Application of bonded composites in the repair of ships structures A

15-year service experience

Ivan Grabovac a,*, David Whittaker b

a Defence Science and Technology Organisation, Department of Defence, 506 Lorimer Street, Fishermans Bend, Melbourne, VIC 3207, Australiab Thales Australia, Garden Island, NSW 2011, Australia

a r t i c l e i n f o

Article history:Received 25 July 2008

Received in revised form 10 October 2008

Accepted 6 November 2008

Available online xxxx

Keywords:

A. Carbon fibre

B. Adhesion

Ship repair

a b s t r a c t

The carbon fibre composite overlays (patches) installed on a Royal Australian Navy frigate to inhibit therecurrence of superstructure fatigue cracking have been in service for 15 years now. The service life these

composite repairs have accumulated is now sufficient for the ship to have gone through several complete

maintenance cycles. The ship has also had a major upgrade/modification programme and been exposed

to a full range of environmental conditions while it has served on its many active deployments. This

paper examines the success of this repair methodology in the light of ship owners expectations. These

expectations have two major aspects which are addressed here.

Thefirstis the effectiveness of the composite repair in restoring the strength and function of the dam-

aged structure, the cost and timeliness in effecting that repair and the disruption incurred prior to and

during the repair.

The secondaspect to determining thesuccess of thecomposite repair is more long term. This comprises

the durability and repairability of the composite repair itself, including the availability of clear, objective,

and documented criteria for inspecting the repair in future years and authorising its continued service.

Furthermore, the ease with which the repair itself can be removed and replaced to facilitate subsequent

maintenance work or modifications to the parent structure, and the ability to survey the parent structure

behind the repair all make up this second facet. Unfortunately, this aspect is less obvious to the repairteam but quickly becomes apparent to the ongoing custodians of the structure. These custodians are also

the ones most likely to be authorising the repair method to be used for similar damage in the future and

so they are important stakeholders to keep on side.

This paper reviews the service history of the carbon fibre overlays since their installation and com-

ments on both the short and long term success of the repair methodology.

Crown Copyright 2008 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Steel and aluminium ships, like any other dynamically loaded

metal structure, will always have the potential to fatigue and crack.

The preferred method for dealing with this is by prevention, with

careful design seeking to keep stresses below the fatigue strengththrough the use of thicker material, expansion joints, etc., and pru-

dent detailing eliminating stress concentrations by rounding cor-

ners, staggering openings, etc. Yet, in spite of these efforts, over

the life of a vessel we can anticipate some cracking, particularly

for high performance vessels such as warships.

The Royal Australian Navy (RAN) has a rigorous programme of

structural inspections for their ships managed by the Centre for

Maritime Engineering (CME) who maintain a centralised database

of all defects, including cracking and rectification work. Effective

monitoring and recording, however, is only part of the strategy

used by prudent ship operators to guard against the potential cat-

astrophic failures associated with fatigue. The rest involves access

to effective permanent and temporary repairs for cracks in vessels.

Permanent repairs for cracking inevitably involve some modifi-

cation of the vessel. The steps involve reanalysing the structure inthe way of the crack to confirm the source of the cracking (typically

excess loading or stress concentration), redesigning the detail to

remove the same (e.g. by moving an opening, adding a bracket,

increasing a radius, etc.) and installation of the approved

modification.

The process can be lengthy and expensive. Installing structural

modifications invariably involves removing equipment, typically

heavy specialised electronic equipment whose size and geometry

prompted the design compromises that led to the cracking in the

first place. Maintenance availabilities long enough to allow com-

pletion of the modification work usually occur only every 5 years

at the docking refits. Also the modification/design approval process

1359-835X/$ - see front matter Crown Copyright 2008 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.compositesa.2008.11.006

* Corresponding author. Tel.: +613 9626 8252; fax: +613 9626 8409.

E-mail address:[email protected](I. Grabovac).

Composites: Part A xxx (2009) xxxxxx

Contents lists available at ScienceDirect

Composites: Part A

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Please cite this article in press as: Grabovac I, Whittaker D, Application of bonded composites in the repair of ships structures A ...,

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has to be rigorous to ensure function, access, and maintainability is

not compromised. In addition, the commercial process of scoping

work, accepting the quotation from the designer, acceptance of

the design work, scoping and putting out for tender the fabrication

work to fit the modification, etc., take time during which the vessel

is in service and the crack is growing.

Temporary repairs that actually arrest the crack growth are

required. Unfortunately, traditional temporary repairs such as dril-

ling a hole at the crack tip or grinding out the crack and re-welding

do not always work. Adding additional material by welding on a

bracket or doubler plate can sometimes work for a while but can

also be as intrusive as a permanent repair and for aluminium,

welding reduces the strength of the parent metal. The only tempo-

rary repair that can be regarded as reliably effective for fatigue

cracks is to crop out some of the surrounding metal along with

the crack and replace with new. Although this resets the number

of load cycles to zero on the fatigue clock it does not remove the

structures predisposition to fatigue in that area and some redesign

and modification will be required at some time. Therefore, a more

effective method of temporary repair is required.

This paper describes such an effective alternate method of

repairing and preventing cracks on ships by bonding a composite

overlay (patch) over the parent metal. The majority of the support-

ing evidence presented is from a trial on an RAN frigate where 2

such overlays have now been in service for in excess of 15 years.

When that trial was started such a method was considered uncon-

ventional for ships, but over the intervening years, quite indepen-

dently, its use has slowly increased.

2. Background

The interest in using the composite materials for an unconven-

tional repair (strengthening) of ship structure was first expressed

by the RAN back in the mid-80s. The FFG-7 Class frigates, due to

the geometry of their continuous aluminium superstructure, dis-

played cracking problems early in their service [1]. Of particular

concern was the neck-down area at the Frame 196 on 02-Deck

which was found to be a region of high stress concentration. To

overcome this problem, structural alterations were made by instal-

lation of two major USN (United States Navy) modifications to the

Class. The first modification strengthened the highly stressed area

by increasing the plate thickness and the second softened major

stress concentrations by staggering the change in plate thickness.

The retro-fit of these modifications to the first of the RAN FFGs

were major tasks that had to be conducted in stages spaced over

several maintenance availabilities.

However, at that time the RAN was also considering an alterna-

tive solution to the problem for which the work by Dr. Alan Baker

at the Defence Science and Technology Organisation (DSTO),

involving the composite patching of the primary and secondary

aircraft structures, was clearly an inspiration[2].In response to RAN interest, the DSTO in close collaboration

with the RAN designed two carbon fibre (CF) composite overlays

for trial aboard HMAS Sydney, Fig. 1a. These were intended for

application by adhesive bonding on the most stressed part of 02-

Deck spanning the area between Frame 188 and 212 on Port and

Starboard, Fig. 1b. Therefore, the main purpose of CF overlays

was to further strengthen the 02-Deck and reduce or eliminate

the recurrence of cracking in the area. The overlays were designed

to reduce the peak stress in the regionof 75.6 MPa [3] byup to20%.

This was achieved through laboratory development work on mate-

rials and processes[46]resulting in installation of both overlays

by DSTO in 1993[7].

This technology was demonstrated in service onboard HMAS

Sydney. During the 15 years of service, vital experience was gained

that included repairs which in turn proved effective in restoring

the strength and durability of composite overlays. During this per-

iod the only crack that propagated directly in the region of the

patches occurred where a portion of a patch de-bonded. Once this

de-bonding was repaired this crack did not reappear. This outcome

is considered as a success that met the RANs objective.

3. Overlay design, major components and application

3.1. Design

A schematic diagram,Fig. 2a, shows the size, construction and

major components of the composite overlay used in the trial. The

CF composite is the structural component designed to withstand

service load. The load is transmitted from the structure to the over-

lay through adhesive layer which forms an interface between the

CF composite and the aluminium deck, Fig. 2b.

The orientation of the load-carrying carbon layer was deter-

mined by the direction of the crack growth and the nature of the

loading. Previous history and analysis has shown [1] there was a

propensity for Mode I cracks (fracture due to tension) to occur

across the width of superstructure and especially around the

knuckle on 02-Deck (Frame 196). Consequently, the carbon fibre

overlays were located on the weather-deck between Frame 188

and 212 on Port and Starboard side and oriented parallel to the

ships longitudinal axis, that is, they were placed on the top of

the superstructure where it began to narrow, Fig. 1b.

The strength of the carbon overlay needed to encounter the

peak fatigue stress (75.6 MPa) in the region of the superstructure

required a unidirectional laminate consisting of 25 plies, each ply

having the weight/area = 300 g/m2 [5]. This strength requirement

was based on the Finite Element (FE) modelling and a series of fa-

tigue test results involving dynamic cycling of large fatigue speci-

mens strengthened with 25-ply carbon composite. A rather

conservative factor of safety of 2 was applied at the time [5]. How-

ever, in more recent work the FE analysis showed that only 15 such

plies are needed to achieve the optimal strength [8]which pro-

duces the required 20% reduction in the superstructure stress level

(i.e. approximately 60 MPa).

3.2. Components

The overlay consists of four distinctive parts i.e. adhesive inter-

face, carbon fibres, GRP layer and edge seal, all designed with a

specific role. The performance of all, however, is conditional upon

a durable adhesion to the aluminium surface which is the key fac-

tor of this technology. This condition is met by preparation of an

aluminium surface as outlined below.

3.2.1. Surface preparation

Surface preparation of aluminium deck consisted of two simpleprocesses (a) grit blasting followed by (b) application of an adhe-

sion promoter.

The grit blasting process was carried out once all the surface

contaminants such as paint, dirt, oil, etc. had been removed. Usu-

ally, a non-metallic, mineral abrasive was used (i.e. Mesh #24) to

achieve white metal finish (Class 3) [9]. Such prepared surface

should attain a uniform appearance resulting in a profile of about

6080 lm. This process activates and greatly increases the surface

area allowing good resin wetting for mechanical interlocking,

Fig. 3a.

The adhesion promoter was an aqueous solution of organo-

functional silane (about 1% in methanol) used in this application

as a coupling agent between the metal substrate and the matrix re-

sin [10]. This solution was applied by brush to a freshly blasted

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surface and allowed to evaporate at ambient or elevated tempera-

ture for about 2 h. Once the solution wets the surface, the silane

molecule reacts with a receptive site on the metal surface. Concur-

rently, the silane also polymerises in reaction with other silane

molecules thus creating a hydrophobic, cross-linked film which

inhibits corrosion of the metal. The final step of silane reaction

with resin matrix was completed once the resin is used to form

adhesive interface (bond-line). This was shown to be a selective

process as not all types of silanes produce strong and durable

bonding with the resin matrices available to composite industry,

Fig. 3b[4,6]. In this instance, a methacrylate silane solution was

found to be the most suitable adhesion promoter when used with

vinyl ester resin. This was determined following a series of Boeing

wedge tests [11]to assess bond durability of specimens exposed to

hot/wet (50 C, 96% RH) and simulated marine (35 C, 5% salt fog)

environments.

After preparation of the adhesive interface and following an

overnight resin cure at ambient temperature, the final product is

shown in Fig. 3c. The surface bonding is not only achieved by silane

coupling, which combines resin and metal, but also the resin that

permeates and wets a large surface area of metal which upon resin

cure provides elaborate mechanical interlocking. When protected

from external elements such a bond should provide good perfor-

mance over an extended period of time.

3.2.2. Adhesive interface

The adhesive interface or bond-line is composed of modified

vinyl ester resin [4,12] and scrimmaterial used to control thickness

uniformity. The interface is usually close to 1 mm in thickness

which is sandwiched between the carbon overlay and aluminium.

Its principal role is to provide durable adhesion, transmit local ser-

vice loads and provide physical barrier preventing direct contact of

carbon fibres with aluminium deck thus avoiding possible galvanic

corrosion. Furthermore, the resin dielectric property was also a

pre-requisite to achieve resin post-cure by low-voltage (100 V),

electrical heating process [12]. The adhesive bond-line thickness

of about 1 mm was found suitable for this purpose since its break-down voltage exceeds 18 kV/mm.

3.2.3. Carbon fibres

The carbon fibres that provide overlay strength and thus rein-

forcement to the structure are good thermal and electrical conduc-

tors. When used on metal structure such as ship, provisions for

avoiding electrical contact must be made to avert galvanic corro-

sion in presence of an electrolyte (i.e. seawater). Otherwise, the

aluminium deck will preferentially corrode since its electro-chem-

ical potential (0.70 to 0.90 V) lies at the anodic or active end of

galvanic series[13].

3.2.4. GRP layer

The glass reinforced plastic (GRP) layer does not contribute tooverlay strength and is intended solely for protection of the under-

lying CF composite against marine environment, impact and abra-

sion. The GRP layer was also bonded to aluminium deck at the

edges which gave a sacrificial area all round, Fig. 2a. This excess

GRP material provides some edge resistance to water ingress but,

as shown in service [7], this interface required additional sealing

for long-term resistance against exposure to the marine

environment.

3.2.5. Edge seal

The seal around overlay perimeter was produced using a two-

part elastomeric sealant. This was found suitable for providing

long-term protection against water ingress at the deck/adhesive/

GRP interface. Early in the trial the edge sealing provided by a coatof paint alone was found to be inadequate. The GRP layer especially

around its periphery loses the bonding strength due to combined

effects of saline conditions and thermal cycling (thermal expansion

different to aluminium).

3.3. Application

In general, a bonded composite overlay may be used as either a

method forstructural reinforcementor alternatively a crack repair.As structural reinforcement, the composite overlay is simply

bonded to a sound structure containing no fractures. Its principal

purpose in such cases is to prevent anticipated degradation of

thestructure that would otherwise develop such defects.

If the structure is already weakened by cracks being present, it

may still be repaired effectively using an overlay. In case such as

this the progress of structural degradation is arrested or signifi-

cantly slowed down. The preferred process is to grind out the crack

and re-weld and then grind the weld flat to affix the patch. This

method is far more effective than simply grinding out the crack

and re-welding alone.

4. Service history (19932008)

A complete service history of bonded composite overlays

aboard HMAS Sydney is summarised inTable 1. The record starts

in April 1993 and covers the period to date. The data consists of

all the activities undertaken by the DSTO Research Team and

Thales Australia performing either the overlay inspection or repair

work.

4.1. Inspections

As indicated in Table 1, there were seven inspections made over

the service period. Initially, the inspections were more frequent

due to a lack of knowledge on service performance; however, at

the later stage with gradual accumulation of experience and

build-up of confidence the inspections were less frequent owingpartly to composite overlays good performance. The methods of

inspections in the beginning involved some kind of instrumental

non-destructive evaluation (NDE) technique as the concerns of

the composite overlays de-bonding from the deck prevailed [14].

Later, the inspections were simple, quick and inexpensive involv-

ing a visual and/or acoustic test such as tap testing to detect the

presence of delamination or de-bonding. This approach was found

to be more suitable and yet adequate and simple enough for the

shipyard maintenance staff to carry out.

4.2. Repairs

In 15 years of service, a total of four repairs were made to the

composite overlays. One repair was a non-structural restorationinvolving only the sacrificial edges of the GRP protective layers

(Table 1, A5). The other three interventions were all structural re-

pairs. Of those, two can be classified as dockyard inflicted damage

that occurred during the ship maintenance or upgrade activities

(Table 1, A7 and A10). The other may be regarded as environment

initiated damage affecting the bonding at the overlay/deck inter-

face which went unnoticed over a length of time (Table 1, A9).

The repairs were always carried out using the original materials

and fabrication steps[15]. A brief summary of all repairs including

the damage contributing factors and other related information is

given below.

4.2.1. Non-structural repair January 1996

Following the RAN feedback relating to some de-bonding ofcomposite overlays from the deck especially around the edges, an


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inspection was carried out in October 1995 (Table 1, A4). The

assessment revealed that parts of the sacrificial area of the GRP

protective layer had failed to adhere to the deck. Evidence was also

found to show that if no repair was carried out, the adhesion of the

composite structural part would be affected. Designing a durable

adhesion at the GRP edge/deck interface was always a concern

throughout the development work, especially for the perimeter

edges which were considered to be the most vulnerable parts to

damage. The main reason for such unease was the composite over-

lays exposure to all elements (solar heating/cooling, water effect,

salt corrosion, mechanical impact and abrasion due to walking

and working on top of it, Fig. 4). The condition of the composite

overlays as found in January 1996 is shown in Fig. 5. Small inci-

dence of bonding failures due to environmental exposure is evi-dent including a part of the edge missing most probably due to

mechanical interference (insets,Fig. 5).

For the most effectiverepair it was decided to removethe sacrifi-

cial GRP edges all around the overlay (Port and Starboard) for their

renewal. This decision was based on the fact that there were too

many small areas of edge disbonds scattered around each of the

12 m perimeters. Certain steps in the course of the repairare shown

in Fig. 6. The steps (ac) relate to the removal of damaged material,

surface preparation and edge reconstruction, respectively.Fig. 6d

showsthesealantmaterial being introduced for thefirst time to seal

the edges at the interface between the GRP sacrificial edge and the

aluminium deck plate. This step was not originally planned during

the overlay installation process in April 1993. However, after about

2 years in service the surface paint coating alone was found inade-

quate to prevent edge degradation, therefore thisadditional protec-

tion was developed and applied in all subsequent repairs. The

sealant material is a commercially available, two-part polysulphide

elastomer.In laboratoryevaluation thisproductwas foundto adhere

well toboth thealuminiumsurfaceandthe compositeoverlay. It also

displayed good resistance to seawater, various hydrocarbon prod-

ucts, solar radiation and could easily be applied, cured and painted

as required. The final material lay-up and configuration is shown

schematically in Fig. 7.

Initially, theGRP edge resistance to cracking andadhesionfailure

was overestimated because unrealistic reliance was placed on sur-

face paint coating. On the 02-weather-deck, which is exposed to all

elements, the coating was found to crack after about 1218 months

allowing water ingress and causing it to flake from the deck surface.

The other contributing factor was a large difference in thermal

expansion between the GRP and the aluminium deck, e.g. the ther-

mal expansion coefficient for E-glass fibre = 5 106 K1 and alu-

minium= 23 106 K1. The magnitude of this problem appears

to be reduced by using the polysulphide sealing applied directly to

aluminium plate and the GRP edge area, Fig. 7. Normally, two coats

are applied and between them a glass tape is inserted for tear resis-

tance. If no damage is made to this most peripheral part of the com-

posite overlay, the edges remain protected for a long time

irrespective of the condition of paint coating. However, that was

not always the case when mechanical interference was involved,

see below, Sections 4.2.24.2.4.

4.2.2. Structural repair January 1998

The first structural repair of damage to the Port composite over-

lay was successfully carried out in January 1998. The damage was

made during the scheduled maintenance work on the 02-Deck

Location of

composite overlay

Fig. 4. Typical maintenance work activity on 02-Deck.

Fig. 5. Examples of degraded edges due to effect of environment and mechanical abrasion.


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when using water jet equipment for removal of the surface paint.

The rotary tool consisting of water jet nozzles operating at high

pressure (approximately 276 MPa) accidentally cut through the

thickness of composite overlay. In the process, the load-carrying

carbon plies were severely affected and required repair. Some of

the repair steps undertaken including the magnitude of the dam-

age are shown in Fig. 8 and a schematic diagram with all the mate-rials and geometry used in the repair of composite overlay is given

inFig. 9. The damage to the overlay was a through-thickness type

therefore the repair procedure required a total restoration from the

metal surface to the GRP protective layer. The carbon fibre plies

were replaced ply-for-ply in a step-wise flush repair mode where-

by each step represented a 4-ply thickness. A total of six steps were

made (only three steps are shown in Fig. 9). The size of replacing

plies was made to fit the area prepared for the repair.

The damage caused to Port overlay by mechanical equipment

was purely accidental. During the ship maintenance work involv-

ing surface recoating, it is a normal practice to employ high effi-

ciency equipment for cleaning a metal surface of remaining

surface paint and other contaminants. When the overlays were ini-

tially installed on the 02-Deck, the perimeter of each was marked

with a 25-mm wide white border frame. On subsequent deck reco-

ating this marker, indicating overlay edge, was omitted therefore

no clear visual guidance existed warning the operator of changing

surface conditions. Regarding the repair shown in Fig. 9, this meth-

od was primarily developed for efficiency and in-field practicality.The modified step-wise technique used 4-ply steps instead of only

1-ply as commonly employed in aerospace repairs involving the

thin skin composite structures. This approach significantly reduced

the time of repair and made it easier especially in this case involv-

ing through-thickness damage to a 25-ply CF laminate. This meth-

od of overlay repair proved fully effective in service and it did not

require any further attention.

4.2.3. Structural repair August 2002

Early in July 2001 the RAN reported water ingress into the com-

partment below on the Port side through the deck plating which

was located near the overlay centre part. This fault was considered

unusual since there was no visual mechanical damage to the com-

posite overlay or the deck plating to allow water passage. On closerinspection of the fault area, a length approximately 200 mm of

composite boundary was found de-bonded from the deck, Fig. 10.

It was establish that this failure was attributed to three major fac-

tors: (i) The boundary seal strip at the fault area was missing, pre-

sumably removed at some stage during the service of the deck

area, and covered with coats of paint. (ii) As shown in Fig. 10 (inset)

this composite region lies next to a depression in the deck topog-

raphy where water tends to accumulate. (iii) Failure to notice the

damage and carry out repair to the sealing strip. It should be men-

tioned, however, that during the earlier repair in January 1996

(Section4.2.1) all edges were sealed and care was taken especially

in this area to prevent water affecting the overlay.

A lack of seal would most probably initiate edge de-bonding

from the deck as experienced during the first 2 years in service.

In this case, it was not known for how long this edge was without

Fig. 6. Non-structural repair January 1996, (a) removal of degraded material, (b) preparations for edge repair, (c) edge reconstruction and (d) sealed and restored edge

configuration.

Edge sealreconstruction

GRP protective layerreconstruction

25 50 100 - 15025

Primer coat

CF Composite

Aluminium Deck Plate

Fig. 7. Overlay edge reconstruction schematic diagram (carbon fibre cross-sectional view).

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the seal and likewise, the conditions of exposure (moisture, salinity

and temperature) which would have had a major influence on

adhesion failure at the composite/deck interface. On this occasion,

only a temporary repair was carried out by the RAN to stop water

ingress into the compartment below.

In May 2002 at ships next maintenance period, preparations forfull repair commenced. The overlay affected part was removed,

Fig. 11a, and ultimately about 2 m length of overlay middle part

prepared for repair in August 2002, Fig. 11b. Prior to overlay repair,

Fig. 11c and d, the cracks in the deck plate were re-welded. The

overlay repair itself was simply a repeat of the work carried out

in January 1998 (Section4.2.2) differing only in size and location

of repair. This restoration was also successfully completed and

no follow-up was required.

It is believed that as a consequence of overlay failure to adhere

to the deck, 1 crack in the deck plate formed. This is shown on

engineering drawing, Fig. 19, designated as S54/S65. There wasalso an additional crack adjoined to overlays outside edge

designated S86. (An engineering account as to the cause of their

formation is presented in Section 4.4.) A picture of the hairline

crack (S54/S65) thought responsible for water entering the space

below is shown inFig. 12.

Fig. 8. Structural repair January 1998, (a) waterjet cleaning tool bottom view, (b)damage made to composite overlay, (c) preparations forrepairand (d)reconstruction of

carbon plies.

CF CompositeScrim Adhesive Metal plate

GRP protectivelayer

Step length=100 mmStep height=4 plies

Ply-for-ply composite

reconstruction

Fig. 9. Schematic diagram of composite overlay repair (half-thick composite shown only side view).


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4.2.4. Structural repair November 2005

During the recent ships upgrade project both overlays (Port

and Starboard) on 02-Deck received various kinds of damage.

The overlays condition following the inspection is briefly illus-

trated in Fig. 13. As a result of various work activities around

and on the surface of both overlays this time the damage was

more extensive. For example, it included not only the edges

but also some delamination and through-thickness cuts, followed

by welding performed in the close proximity to composite over-

lay, Fig. 13(df). The degree and the type of damage varied be-

tween the two overlays, Fig. 14a, therefore the corresponding

repair strategy was developed by rationalising and combining

damaged areas as summarised in Fig. 14b. For example, on thePort side the damaged areas D2 and D3 (Fig. 14a) were com-

bined into a single repair area R2 and R3 ( Fig. 14b) and so on.

This approach has simplified the task and improved the quality

of repair. Again in this instance, emphasis was placed on recon-

struction of overlay termination geometry, i.e. the taper. This is-

sue was addressed successfully right at the start in 1993 and is

considered important to prevent stress concentration in adjacent

areas of the structure and to avoid delamination and peel stres-

ses in the laminate. The taper is simply formed by a successive

ply drop-off method (i.e. decrease in length) when all the plies

are stacked up and is usually aligned with the principal axis of

stress acting on the structure. The angle (h) is typically around

5. The geometry of a repair (i.e. R2 and R3) terminating in a ta-

per is shown in Fig. 15.

Fig. 10. Overlay condition and location of edge failure.

Fig. 11. Structural repair August 2002, (a) initial cut-out, (b) final preparations for overlay repair, (c) reconstruction of carbon plies and (d) vacuum bag consolidation andresin cure.

Fig. 12. Crack in the deck plate found under overlay in 2001.


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Overall, the repairs performed in November 2005 (Fig. 14b)

included:

Port

GRP edge repair including the seal strip (R1).

Repair to CF composite including GRP edges and sealing

strip (R2 and R3).

Starboard

The work involved only the restoration of damage to GRP

edges and sealing material (R4R9).

Through-thickness holes drilled and cuts made were not

repaired only sealed (R10 and R11).

Insummary,the damages inflictedto bothoverlaysduringthe ship

upgrade project areseenso faras some ofthe best examplesof human

interference when no special instructions or warnings are provided.

With this repair the following observations are worth noting:

(a) Care should be taken in correctly diagnosing delamination in

thick laminated overlays. In this instance, the NDE data

obtained on the Port side by using the tap test was correctly

located but incorrectly interpreted as being an adhesive fail-

ure, i.e. de-bonding at the overlay/deck interface. In fact, the

delamination was found at about half-thickness i.e. 1012th

carbon ply (full thickness 25 plies) thus the adhesion to the

deck plate was intact hence a half-thick overlay serviceable.

Fig. 13. A collage of damagetype suffered byoverlayson Port andStarboard side, (a)delamination andabrasion onPort side, (b and c) damageto overlays by various abrasive

tools, (d) location of life raft atop Starboard overlay, (e and f) cuts through overlay thickness to weld life rafts anchor point and feet to deck plate.


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(b) The through-thickness damage caused to the Starboard

overlay by installation of a life raft could not be repaired.

The owner would not consider relocation of a life raft. Con-

sidering a small length of cuts made across the fibres, the

overall loss of local strength was considered acceptable

under the circumstances. In this case, the cavities created

by these cuts were simply filled in with the sealant material.

As mentioned earlier, a conservative factor of safety was

added to the original design which significantly increasedoverlay strength[8].

(c) Close inspection of the overlays physical condition was

made in the vicinity of welding points. The concern was a

very small distance (approximately 25 mm) of these cuts

to the welding spots where high temperatures are gener-

ated. Normally, temperatures in excess of approximately

150 C could seriously affect both the adhesive bond and

the composite overlay. Surprisingly, no such damage as

burning or scorching marks was found on either the bond-

line or CF composite.

4.3. Other repair issues

4.3.1. Time, disruption and cost of repair

The issues considered in this section are specific to processes

and materials used in the repairs that are carried out for the first

time therefore they are not optimised yet. For these reasons the

information provided is indicative only of the time needed to affect

the repair, likely disruption to ships operation schedule and the

approximate cost applicable at the time of repair.

Time and disruption These important issues in conducting a re-

pair were estimated inTable 2. The figures relate to work directly

connected with repair. The repair time was always within the

scheduled ship visiting time to port and at no time was the ship

held back affecting its return to service. The only time of concern

was the initial installation of overlays in 1993. In addition to 14

days of work, there were additional 7 days spent on preparation

involving temporary removal of equipment, erection of rain shel-

ter, installation of diversion barriers for surface rain water, etc. In

general, the time needed for most composite repairs can be esti-

mated provided a prior assessment is possible. For all external

work, these predictions vary subject to weather conditions.

Cost The cost to defence is approximated (where possible) and

it relates to materials and equipment only. These costs are obvi-

ously subject to change with time. For example, the cost of unidi-

rectional (UD) carbon fibre in the early 90s was around AU$55 for a

linear metre (1-m wide), area-weight = 300 g m2. For the samematerial in 2002 the cost was around AU$35. Considering the

labour, this project is regarded as a RAN scientific demonstrator

therefore all the work phases were done by the DSTO scientists

directly involved with the project from start. Exceptions to this

were the last two repairs (August 2002 and November 2005) which

were contracted out by Defence Materiel Organisation (DMO).

4.3.2. Repairability and documentation

Repairability A repair to composite overlays is reasonably sim-

ple although it does require certain technical skills and knowledge,

especially the knowledge in the use of advanced materials includ-

ing surface preparation and bonding to metal is essential. Most

general purpose GRP repairers have insufficient knowledge, train-

ing and equipment to conduct these types of repairs. Therefore, de-fect assessment, NDE, resin infusion, vacuum bagging, grit blasting,

surface preparation, etc. are some of the operations those contrac-

tors are lacking and more often than not short-cuts are done. For a

durable repair such practice is not acceptable especially when the

bonding to a metal surface is made for structural applications.

However, for a competent composite specialist such repairs are

usually not a problem.

Documentation The official RAN documentation describing the

repair methodology is not available yet. The RAN has a specific pro-

cedure to deal with such engineering issues and any changes/addi-

tions to the repair methodology are categorised as configuration

change. Still, there is scientific information on the subject pub-

lished by DSTO. The references include articles that cover all the

development stages, overlays installation, and including the timein service. In addition, there is more detailed DSTO documentation

Fig. 14. Structural repair November 2005, (a) a summary of all damage inflicted

to overlays on Port and Starboard side, i.e. D1D11, (b) repair strategy for both

overlays.


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available on materials, processes and repair for this technology but

only for internal use and licensee information.

4.3.3. Service inspection and maintenance

Inspections The RAN ships are well maintained and serviced

for which a set schedule exists. Regarding the maintenance of over-

lays, the DSTO scientists together with the dockyard contractor

(Thales Australia) used every opportunity for inspection. This oc-

curred on average every 18 months during the ship scheduled visit

to port. From experience collatedso far, the composite overlays can

be left in operation on a ship for years even under most demanding

conditions the marine environment can offer.

Maintenance If no damage is inflicted, the overlays generallyneed little maintenance. Often during the deck recoating work it

is advisable to refresh the surface paint. Questions were raised

regarding the paint cracking and flaking away from the overlay

surface, Fig. 16. As shown, these paint failures have nothing to

do with overlays but are simply a result of many layers being

applied one on top of the other, without old ones being taken

off. However, carefully removing the old paint coat by using a

steel brush and the like is relatively simple task. Care should

be exercised not to cause damage to underlying GRP layer or

the edge seal strip.

4.4. Deck cracking on HMAS Sydney

The engineering records show that all pre-existing faults in deckplate were rectified by welding before overlays installation on 02-

Deck (Port and Starboard). For example, the 600 mm crack found

on the Starboard side,Fig. 17, was re-welded and, following over-

lay installation to strengthen the area, has not reappeared since

Taper angle, ( ) Taper length, ( )aComposite

thickness, ( )b

b

a

l

Ply 'drop-off' length, ( )l

Deck Plating

CFComposite

Fig. 15. Schematic diagram of overlays end taper (side view).

Table 2

Overlay repairs cost and other details (excluding labour).

Overlay installation/repair Disruption to Ship Service Removal equipment and

fixtures

Damages or

injuries

Repair duration,

(persons)

Cost to defence (AU$)

(Materials, consumables,

equip. hire, etc.)

1. Overlays installation

March/April 1993

Work done during ships

scheduled visit to port

Some surface equipment

temp. removed

None 21 Days (5) 3040 K (Est. 1993)

2. Repair January 1996 As above None None 10 Days (2) 34 K

3. Repair January 1998 As above None None 3 Days (2)


13/18

1993. This was confirmed in the most recent review of cracks on

02-Deck that covered the period between 1997 and 2007 for all

the RAN FFG warships still in active service.

A summary of the results of the review for the Starboardarea on

HMAS Sydney, housing the CF overlay, is given in Fig. 18. Evidently,

there were no cracks under the overlay but a few appeared in an

adjoining area, of which S37 was the closest. This crack was en-

tered into the database in 1999 although its presence dates back

to pre-1993. In all these years with CF overlay in its proximity

the crack did not advance.

It is noted that this crack (and others on the Port side, see be-

low) lies in the weld line region related to two USN modifications

known as Ship Alterations (S/A 86 and S/A 146). These were carried

out around 1985 and 1991, respectively, with the purpose of

addressing the cracking problems around Frame 196 by inserting

new thicker plate(s) into the structure.

Crack review of the Portsection produced different results. For

the same period, the database shows 2 cracks (S54/S65 and S86)

registered in May 2001 and located underneath or adjacent to

the overlay,Fig. 19. These are also thought to be associated with

welding for S/A 86 and S/A 146. (Note the two numbers, i.e. S54

and S65, represent two separate surveys of the same crack. The

S54 is the number of the initial survey which identified the crack

prior to the removal of partly de-bonded Port patch. The S65 is

Fig. 17. Deck crack discovered in 1993 before installation of Starboard overlay.

Fig. 18. Starboard section of 02-Deck, with relative locations of Ship Alterations 86 and 146, deck cracks and CF overlay.


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the number of the subsequent survey once access to the metal sub-

strate was made available during the patch repair.)

The crack S86 which is located outside, but close to the overlay,

was reported as static (not growing) for the period 19952005. In

2006 it was rectified by grinding out and re-welding.

More significant is the crack S54/S65 that occurred directly

underneath the de-bonding of the Port overlay. It is noted that

this occurred not only along a weld but at a point where there

are three changes in the thickness of the underlying plate, i.e.

(i) 9.519 mm, (ii) 9.512.7 mm and (iii) 12.719 mm. These

changes of thickness manifest themselves on the upper side of

the plate where the overlay is fitted. The consequent draping

of the carbon plies over such undulation results in the overlay

having to resist longitudinal loads through bending normal to

its plane as well as tension, Fig. 20.

This combined loading case was extensively covered during

the materials evaluation phase[5,6]for which no problems were

identified even when the loading was double that measured in

Fig. 19. Engineering drawing of 02-Deck; Port, showing USN modifications, location of cracks and outline of CF overlay.

T

4xT to 2xT min

T - Difference in thickness between plates

Weld

CF Overlay

Fig. 20. Schematics and moment diagram of combined loading on overlay at the weld region between two plates of different thickness.

Table 3

RAN ships, FFG-7 Class in active service.

Vessel Date commissioned Number of cracks period 19972007 incl.

In superstructure In patch location

1. HMASSydney January 1983 95 1 (S54/S65)

2. HMASDarwin July 1984 39 2 (D5, D32)

3. HMAS Melbourne February 1992 83 1 (M78)

4. HMASNewcastle December 1993 70 0


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this highly stressed part of deck. However, for reasons described

earlier (Section 4.2.3) a portion of overlay de-bonded from the

deck in this critical region. This de-bonding not only reduced

the effective stiffness of the patch in the longitudinal direction,

but may also have had the effect of adding a Mode III loading

(out-of-plane shear, tearing) to any Mode I crack that may have

been there already.

Therefore, combining the individual effects of (i) heat affected

zone in the weld, (ii) the coincidence of cracks appearing in the

plate area with multiple changes in thickness and (iii) a reduction

in patch effective stiffness in the area, the likelihood of cracking as

noted for S54/S65 is high. Perhaps, the easiest method to minimise

the chances of further cracking in this region may be to avoid over-

lay degradation and de-bonding which leads to reduction in its

effective stiffness. In any new applications it would be an added

advantage if overlay bonding is performed to as flat a surface as

practicable. However, this is not a mandatory requirement for this

resin system which was specially developed to account for such in-

stances in which the resin in the adhesive is also exposed to some

tensile loading[5,6].

Finally, the authors suggest caution in the interpretation of sur-

vey results. The discovery of cracks on a plain grey deck is no easy

task and is very dependant on the skill of the surveyor and the acu-

ity of his vision. Cracks on the 02-Deck are typically only hairline in

width when they are first discovered and noted for monitoring.

Consequently, some judgement needs to be exercised in assuming

a crack appeared only shortly before it was discovered.

4.5. Deck cracking on other ships in Class

From this trial alone it is hard to prove the overlays ability to

arrest and prevent cracks in metal structures. For that the reader

is referred to other work such as the FE analysis[8]and early lab-

oratory trials [5]. Apart from only two overlays of limited size

being fitted to one of four vessels, statistically the sample popula-

tion is considered insignificant especially in view that, one ship had

its aluminium structure modified twice and the other three werebuilt to modified designs to address and fix the cracking problems

of this Class. What is an interesting statistic to note however, is

that for the period of 19972007 inclusive, the four vessels experi-

enced an average of 72 reported fractures in areas of their super-

structure unrelated to where the overlays and structural

modifications were fitted,Table 3. As expected, the greatest num-

ber of cracks occurredon the older ships. An exception to this is the

older vessel HMAS Darwin, which was home-ported on the West

Coast of Australia, showing lower number of cracks over that per-

iod. The others were based on the East Coast. Although HMAS Dar-

win had less cracks overall it had more cracks in the critical area

(one Port and one Starboard) compared with a ship of a similar

age, HMASSydney.

With the oldest FFG (HMAS Sydney) entering its 25th year inRAN service, its nominal 15-year design life has well and truly ex-

pired. This vessel recently completed a major weapons upgrade

and it is anticipated to be required by the RAN for another 10 years.

In 2001 the Centre for Maritime Engineering approved a new set

of standard repairs for the myriad of recurring cracks that had be-

come evident up to that point in time. These repairs to date have

been reasonably effective for the areas they apply to. However, in

2001 theRAN trial of carbonoverlays hadnot progressedfor long en-

ough to justify their inclusion in that set of standard repairs.

As the vessels have aged, the cracking has occurred in areas

away from the original hot spots in the narrow section of the

upper deck of the superstructure. Typical locations for the current

wave of cracks are stress concentrations in superstructure longi-

tudinal bulkheads, less stressed portions of the upper superstruc-ture deck (02-Deck) and the narrower portions of the lower

superstructure deck (01-Deck). Certainly, enough time has elapsed

for the standard crack repairs for this Class to be updated on the

basis of the latest survey data.

5. Overlay technology issues

5.1. Service durability

5.1.1. Environment

At sea, the 02-Deck receives significant proportion of tensile

and compressive forces due to hull longitudinal bending especially

in heavy seas which characterise waters south of our continent.

The waters north of Australia are calmer but can be very unpredict-

able including cyclonic winds with heavy rain, and high seas. The

surface temperatures (mid-summer) are generally much higher

and can reach up to 80 C on a ship surface in the northern ports

[16]. As our continent lies under the hole in the ozone layer, the

ship also receives a considerable amount of UV radiation from

the sun.

While in port for maintenance activities the 02-Deck is in con-

stant use either as a walkway or a work area. Often the samearea is

used for cargo offload by overhead crane and erection of scaffold-

ing during the service to main mast. Under those conditions the

overlays are mostly exposed to abrasion and impact.

Meanwhile, both overlays (Port and Starboard) are still in ser-

vice and functioning as originally designed. The time since installa-

tion has exceeded 15 years. The ship has gone through several

complete maintenance cycles and overlays have been repaired sev-

eral times as discussed earlier. Fortunately, no structural damage

occurred to the Starboard overlay therefore, its primary adhesion

to 02-Deck was unaffected in all that time.

5.1.2. Repairs

A total of four repairs to overlays were made. Of those, two were

due to a combined effect of service and environment (Sections

4.2.1 and 4.2.3) and the other two due to a human interference.However, all of those repairs were carried out in a different loca-

tion, which implies that the quality of repair work was satisfactory

to restore overlay function without a need for repair rework. In all

repairs identical materials and procedures were employed. There-

fore, within the scope of repair work done there are indications

that the work and processes applied produce an acceptable and

durable repair to composite overlays.

5.2. Reduction in the cost of ownership

With the increased affordability of carbon fibre suitable for

marine use, applying a carbon overlay is now a far less exotic repair

than it used to be. The maintenance Naval Architect/Engineer can

now (for Mode I and Mode II, in-plane shear fractures) use layersof carbon to reinforce a weak zone and compensate for the anneal-

ing effect in aluminium at a re-welded crack. However the most

graphic demonstration of how the Carbon Patches can be a more

cost effective solution in comparison to modifying the metal struc-

ture underneath was provided by S/A 146. Experience found that

the original modification S/A 86 which thickened the aluminium

structure in the areas of high stress concentration did not stagger

the change in thickness in small enough increments. The second

modification S/A 146 added interim changes in thickness to the

aluminium plate and rounded out some of the sharp corners in

the structure. In contrast, the equivalent detail in the carbon patch,

tapering the fore and aft end of the carbon layers, was achieved

with a pair of scissors prior to lay-up.

Furthermore, a major repair is much more affordable if it can bedelayed and scheduled in with other work. The main cost of a re-


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pair is not the materials or labour required to perform the repair

itself, but the labour involved in shutting down and removing

equipment for access and the reduced utilisation rate of the vessel

spending less time in service. The trial has demonstrated that car-

bon overlay repairs can be carried out effectively with no disrup-

tions and on time allowing those who manage the maintenance

of vessels to obtain this saving without risk. In addition, it was

demonstrated what is required for these overlays to hold on until

the next major maintenance cycle (up to 5 years) or if that is

missed, the next maintenance cycle (another 5 years).

The current generation of older combatants came from a time

when tools such as FE analysis allowed designers to make them

lighter by reducing the plate thickness of their superstructure to

a minimum. For those vessels, new repair options are now

needed as these ships have their length of service extended

and are pressed into roles well beyond those anticipated in the

1970s. The carbon overlays not only provide a lightweight dura-

ble repair, the trial has demonstrated that they can be readily re-

moved and easily replaced when other work is required.

Surprisingly, the overlays showed good damage resistance to ser-

vice loads, impacts and even when hot work (welding) is per-

formed nearby. In general, their presence on a ship does not

push up the cost of other work.

The above points seem to make sense when taking the FFG-7

Class as an example. Of the 51 FFGs built for the USN, 30 still re-

main in active service with that navy, with no anticipated replace-

ment at this stage. Of those no longer in service with the USN,

many are now seeing continued service with other navies, e.g. Bah-

rain (1), Egypt (4), Poland (2), Turkey (8 with 2 more expected this

year), Pakistan (believed to have requested 6). The first of this Class

was theOliver Hazard Perry commissioned by the USN from 1978

until 1997 (20-years service), the oldest still in service with the

USN is theJohn L. Hall commissioned in 1982 (currently 26 years

of service) however, the oldest in service with any navy is the

ORP Gen. T. Kosciuszko commissioned in the USN as Wadsworth

in 1978 (currently 30 years of service). This is an impressive service

history for a vessel designed to strict weight limits and built to aprice. Nor can one describe this Class as now an obsolete weapons

platform with the newest example ROCS Tian Dan built in Taiwan

and commissioned into their navy in 2004 (only 4 years old).

6. Discussion

In the course of service demonstration the composite overlays

were inspected seven times and received a total of four repairs.

Of those, two repairs were necessary due to overlay degradation

caused by a combination of service and environment while the

other two repairs were needed due to human interference occur-

ring in port at ship maintenance time. It is noted, however, that

all repairs were carried out quickly and inexpensively during ship

scheduled call to port and no activity associated with overlays hasever affected operation of the ship.

The defects caused by service/environment factors were cor-

rected in due course; modifications were made, demonstrated

and problems did not reappear. However, for those caused by

humans, the contributing factors remain unresolved. In early ser-

vice days the periphery of both overlays were clearly marked

with white line (seeFigs. 4 and 5), but over time and due to fre-

quent surface recoating that information got lost and both over-

lays blended with their surroundings thus visually became an

integral part of aluminium deck. At maintenance time these

were treated as the rest of deck surface and, as shown before,

received a rather harsh treatment. The lack of instruction for

the maintenance crew plus regular changes of ship officers did

not help the cause.

Considering the conditions the overlays were exposed to over

such a long time in service, the authors are confident that all the

likely problems related to design/materials/processes, expected

to surface-up in such application, were identified and corrected

and modifications demonstrated. It is believed that this technology

has now matured by overcoming one big Milestone the long-term

marine durability. The other important Milestone of achieving clear

and convincing evidence ofcomposite usefulness in repairing andstrengthening the ship structure has partially been achieved for the

reasons explained above.

6.1. Performance indicators

There are essentially twofold expectations from this technology

demonstrator. One is to serve the interest ofVessel Owner and its

maintenance contractorswhich is focused on overlays ability to ar-

rest the existing and prevent the development of new cracking in a

ship structure. The other is that of the Technology Developerwhich,

in addition, is keenly interested in overlays marine exposure and

durability aspect.

6.1.1. Outcomes of interest to Vessel Owner

A summary of relevant indicators include:

TheStandard overlay showed faultless performance no deck

cracking under or in adjacent areas over a 15-year service on

an active RAN ship.

This overlay received two successfully completed non-struc-

tural repairs.

A large crack in butt-weld between plates of changing thickness

(Fig. 17) after re-welding and overlay reinforcement did not

reappear.

A small crack to fore side of starboard overlay (S37,Fig. 18 in

existence before 1993) appears to be arrested since it did not

advance in size.

The overlay showed damage tolerance to cuts made in struc-

tural carbon laminate needed to weld support frame for instal-lation of a life raft.

The welding operations carried out in close proximity caused no

visible damage to overlay. The mass of surrounding aluminium

structure appears to act as an effective heat sink.

ThePortoverlays performance did not match that of its coun-

terpart although it served as a useful means to demonstrate

and qualify a variety of overlay repairs. It received three struc-

tural and one non-structural repairs. Each repair was able to

restore overlays full function and on time.

The deck non-skid coating is fully compatible with the sur-

face of GRP layer. One application seems enough for many

years of service. It will not crack and flake-off with time as

on aluminium surface. On recoating, it needs only one coat

for visual refreshment. If multiple coats of paint accumulateon top of overlay the paint will crack and flaking will start,

see Fig. 16.

6.1.2. Additional outcomes of interest to Technology Developer

Durability and marine exposure indicators include:

Extensive experience of composite durability gained in bonded

structural hybrid configuration.

The resin system displayed outstanding durability, toughness

and tolerance to impact and peel forces especially at overlays

ends.

The marine paint system provided inadequate protection to

water ingress at the composite metal interface.


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The interface without any protection shows susceptibility to

failure due to probable synergistic effects of materials thermal

expansion in the presence of saline conditions.

The use of edge sealant prevents the onset of interface bond

failure the failure mechanism appears to be sensitive primar-

ily to the presence of seawater. On inspection, after years in ser-

vice the bond at the interface protected by a sealant appears in

good order in spite of edges being exposed in service to full

extent of differential thermal expansion between composite

and aluminium.

6.2. Lessons learned

The effect of environmentcan influence bond degradation at the

overlay/deck interface especially around the perimeter after

prolonged exposure to elements. Protection must be provided

by sealing the edges of overlay to prevent failure of adhesion

to the metal structure.

At maintenance time, most of the damage to overlays was

inflicted by human interaction. The problem can easily be over-

come by issuing a relevant set of instructions to the mainte-

nance crew. However, this must be initiated at the CME level

to be fully effective; otherwise, instructions given by scientists

are on a personal level and usually last until next change of

crew.

For surface recoating workof composite overlays a set of instruc-

tions are also needed. This is to avoid unnecessary work and

waste of materials as explained earlier. A most suitable method

of removing old paint coats from the overlay top surface needs

to be explored.

The urgent need toadd composite patches to the existing reper-

toire of approved repairsfor fatigue cracking.

7. Conclusion

The trial confirmed that the carbon fibre patch/overlays meet

the ship owners short and long term expectations for a ship repair

and are a useful addition to the repertoire of the Naval Architect

faced with cracking problems for a ship in service.

In reviewing the trial against the ship owners short term expec-

tations the following points were addressed:

The carbon patch was effective in restoring the strength and

function of the damaged structure. During their 15 years of ser-

vice the only crack that reappeared under the patch occurred

where the bonding of one of the patches had failed. This prob-

lem was addressed and neither the de-bonding nor the crack

has reappeared. In addition the FE and laboratory work done

in support of the trial clearly show the effectiveness of this

method of repair.

Thecost andtimelinessin effecting thecarbonpatches wasdem-

onstrated during their installation and subsequent repairs. The

labour required was at the low end of what could be expected

of a comparablealuminium repair andthe material costs, though

not large, have since dropped considerably. The materials

required are available commercially and the lead times required

for organisingthe repair areno longerthan that requiredto orga-

nise materials and qualified welders for an aluminium repair.

There were no additional disruptions to other activities as a

result of installing or repairing the patches. The grinding out

and re-welding cracks prior to the carbon patch installation

required all of the precautions and permits associated with

hot work onboard a ship. The installation of the patches them-

selves did not require any of the restrictions, safety precautions

or shielding of equipment required for aluminium welding. The

only requirements are ensuring adequate ventilation and

extraction when performed below decks and restricting access

during the patches installation and ambient cure overnight.

The ship owners longer term expectations were met as follows:

Durability The life of the patches to date, 15 years on a

weather-deck, clearly shows that the RANs need for a durable

repair can be met.

Repairability of the composite patch itself During the trial the

carbon patches have been successfully repaired on four

occasions.

The availability of clear, objective and documented criteria for

inspecting the carbon patch repairs Such criteria are well docu-

mented in the technical literature and this was used in the trial

for planning the repairs.

The ease with which the patches can be removed Paradoxically,

the source of the damage experienced by the carbon patches

during their 15 years of service is one of the things that made

the trial a success. The patches can be easily removed with

the abrasive blasting equipment used in preparing the substrate

for painting. Unless attacked by such equipment or an angle

grinder, correctly fitted patches appear able to remain in service

indefinitely.

Theability to surveythe structurebehind therepair Looking at the

cracks that have manifested themselves near or under the

patches during the trial, it can be reasonably presumed that any

cracks under the patches will be unlikely to grow unless there

has been some failure of the edge seal leading to de-bonding.

Traditional management of temporary repairs that included

drilling holes at crack tips, re-welding the same crack repeatedly,

or adding some arbitrary bracket to reduce a stress concentration

is just not adequate. The lead time required for a Class-wide mod-

ification demands that effective interim measures are available

that relieve crews of the disconcerting sight of crack growing in a

bulkhead or deck-head while at sea. The carbon fibre overlay is

now a solution that works if applied correctly.

Ifcarbon fibre patches were endorsed as an approved temporary

repair then the requiredservicehistory would be quickly builtup to

access their suitability as a permanent repair. This could be

achieved by close collaboration between the Vessel Owner, the

Technology Developer and the maintenance contractor which, if

pursued, would lead to higher performance and more cost-effective

patches/overlays than those used in this trial. In the longer term,

the requirement exists for industry to develop an enduring capabil-

ity in thetechnology in order to facilitate its future implementation.

Acknowledgements

Authors wish to acknowledge the following stakeholders,

whom the information presented in this paper is of interest:

DSTO Management, for their continuing interest and support to

the project.

Thales Australia Naval, for long-term dockyard assistance and

overlay inspections.

FFG System Program Office RAN, for the review of the

manuscript.

The Centre for Maritime Engineering, for access to their survey

database and their interest in the outcome of this long-term

trial.


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Likewise, the authors would like to thank the following individuals

for their assistance in providing data and reviewing the paper:

Dr. Richard

Jackson

Thales Australia

Graeme

Emerton

Senior Hull Maintenance Officer, Centre for

Maritime Engineering Department of Defence

David Cox Chief Engineer, Amphibious & Afloat Support

System Program Office Department of DefenceSubrata

Majumder

Platform Systems Manager, FFG Upgrade Project,

FFG System Program Office Department of

Defence

Dr. Stuart

Cannon

Head, Surface Ship Structures, DSTO

Dr. Christine

Scala

Research Leader, AMSS, DSTO

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December, 1984, Bethesda, Maryland, USA.

[4] Grabovac I, Pearce PJ, Baker AA. The use of advanced composites to combat

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Application of bonded composites in the repair of ships structures – A 15-year service experience

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