CHAPTER 1
INTRODUCTION
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CHAPTER 1 INTRODUCTION
1.1 BACKGROUND
In India boats plying in coastal and near shore waters are m ostly made o f steel or wood. Recently, use is also being made o f Glass Reinforced Plastic for building small boats such as fishing trawlers and fast crafts. Steel is heavy and requires the help o f skilled w orkm en such as platers and welders. Steel plates are often beaten to shape and fram es are bent to the curvature o f the hull. W ood suffers from degradation and rot in the m arine environm ent, and is becom ing increasingly expensive and scarce. GRP on the other hand, has the property o f being moulded over a mould and this can be done with the help o f skilled carpenters. The material itse lf is easily transportable, resin in containers and glass fibres in rolls. The fabricated hull, w hen cured is light and strong. All these factors contribute to the ever-increasing dem and for GRP in the construction o f lam inates for boat hulls. However, since the hull laminate is manually built up, the fabrication process lends itself to m any uncertainties.
1.2 UNCERTAINTY IN HULL-LAMINATE BUILD-UP
High quality boat hulls can only be m anufactured in a controlled environm ent.
H ow ever, in India this is not often realized. The resin supplied does not have good quality and does not possess a long shelf life. Again, w orkm anship quality plays an im portant role in the fabrication process, m ostly hand lay-up, and is an im portant contributor to the strength o f the hull laminate. The causes o f uncertainty in the strength o f a lam inate are discussed in Chapter 3. U ncertainty also exists in the prediction o f loads on the hull. This is due to different sea states in existence at different tim es during the year and a lack o f generated data. Chapter 5 deals with the uncertainty o f loads. H ow ever, the study o f uncertainty in this work is m ainly related to the tensile properties o f lam inates m ade from short fibre random ly oriented glass fibre. The com posite behaves as
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an isotropic material. Anisotropy ma\ he introduced owing to the manufacturing process, which orients the fibres in a particular direction.
1.3 R A W M A T E R I A L S
A study ol the uncertainty o! the resin can only he accomplished if the standard physical and chemical properties ol the raw materials are accurately known and their expected strength values compared to those listed in Classification Society Rules. The laminate is built up Irom glass fibres and resin. The fibres mainly ol' glass are laid in various ways in a resin matrix. I he laminate is made up of individual lamina to give the required strength properties.
1.3.1 G la s s fib r e r e in fo r c e m e n t
The major component o f strength of a GRP laminate comes from glass fibres.
Various categories o f glass fibres are used for boat hull manufacture according to their properties and strength requirements. Table 1.1 lists a few o f these properties. Smith (1990) [42]. The table gives the standard physical properties o f glass fibres used in boat building. In the present study the v ariation in strength properties o f glass fibres have not been studied, but instead the standard physical property has been taken from the table.
T a b le 1.1
P h y s ic a l p r o p e r tie s o f G la ss F ib r e
Fibre Specific
Gravity
| Young's i Modulus j (GPa)
, Tensile Strength
! (GPa)
Failure Strain
' (%) !
E-Glass 2.55 ! 72 1 2.4 • 3 [
S2. R-GIass 2.50 1 7S . 3.4 ; 3.5 i
HS Carbon 1.74 1 297 4.1 ( 1.4 !
HM Carbon 2.00 ' 520 2.1 : 0.4
Aramid (Kevlar) 1.45 124 2.8 - 2.5 i
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The most commonly used glass-fibre used in hull manufacture is E-glass, which is characterized by high strength and low cost. Glass fibre is readily available in a variety o f forms such as chopped strand mat (CSM), woven rovings (WR) and unidirectional cloth or tape (UD). CSM used in boat hull fabrication has weights varying between 300 to 900 gm /m 2, Smith (1990) [42], Woven rovings normally have equal distribution o f fibres in the warp and weft directions, or may have half as many fibres in the weft direction as in the warp direction. Woven rovings are normally available in the weight range from 200 to 900 gm/m2. Unidirectional cloth or tape consists o f parallel rovings lightly stitched or bonded together. A very light cloth formed by randomly oriented glass filaments with a w eight 20-50 gm/m2 is used as a surfacing mat to reinforce the gel coat and give a smooth exterior finish to boat hulls. Carbon fibres are o f two types, those giving high strength (HS) and those giving high modulus (HM). Carbon fibres vary in strength from 1.5 to 4.5 GPa. High strength carbon fibres hybridized with glass are often used in fast patrol boats and high performance submersibles. Aramid fibres (Kevlar) have very high specific tensile strength. They are hybridized with carbon or glass fibre to carry high compressive and bending loads. In the present study only CSM laminates have been tested (Chapter 3).However, theoretical strength calculation o f laminates has also been done in (Chapter 6) w ith a combination o f CSM and WR fibres to compute their reliability indices and com pare laminate strength.
1.3.2 Resin types
Resin, the matrix material, used in boat hull construction is usually o f the therm osetting type. A variety o f resins include polyester, vinyl esters, epoxies and phenolics are used. The most commonly used among them are polyester resins. The three different types o f polyester resins are (i) orthophthalic polyester, (ii) isophthalic polyester, and (iii) bisphenol polyester. Orthophthalic polyester formed from maleic and phthalic anhydrides with glycol is mostly used for small boat construction. Isophthalic polyester formed from isophthalic acid is more water-resistant and has superior m echanical properties. This resin has been used in experiments conducted in the present study. The resin is suitable for the hand lay up process. Bisphenol polyester resin has
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m uch im proved w ater and chemical resistance properties. It is more expensive and is selectively used for boat hull manufacture. The polyester resin m ixed with a peroxide catalyst and a cobalt octoate accelerator, 1% by volume gives a hard solid on curing.
Vinyl ester resins are similar to polyester resins, but have superior resistance to w ater and chem ical attack, greater retention o f strength and stiffness at higher temperatures and greater toughness. They have been used in speedboat hulls. Epoxy resins have greater adhesive and water-resistant properties with less shrinkage during curing, but are more expensive. Phenolic resins have good fire resisting properties. The following table, Table 1.2 obtained from published literature summarizes the physical properties o f resins used for boat hull construction.
Table 1.2
Physical Properties of Resin
R esin Type Specific G rav ity T ensile M o d u lu s (G Pa)
O rthophthalic Polyester
Smith Indian manuf.
B.V./D.N.V. Smith Indian manuf.
B.V./D.N.V.
1.23 1.1 - 1.4 Nominal value o f manufacturer
3.2 2.75 3.0
Isophthalic Polyester
1.21 1.1 - 1.4 - 3.6 2.75 3.0
Vinyl ester 1.12 - - 3.4 3.0
Epoxy 1.2 1.1 - 1.4 - 3.0 2.35 3.0
Phenolic 1.15 1.36 with
wood flour
- 3.0 - 3.0
R esin Type
T ensile S tre n g th (M Pa) F a ilu re S tra in (% ) Smith Indian
manuf.
B.V./D.N.V. Smith Indian m anuf.
B.V./D.N.V.
O rthophthalic Polyester
65 49-83 45-50 2 1.3-1.6 1 .5 - 2 .0
Isophthalic Polyester
60 49-83 45-50 2.5 1.3-1.6 1 . 5 - 2 .0
V inyl ester 83 - 45-50 5 1.3-1.6 1.5 - 2.0
Epoxy 85 41-90 45-50 5 3 - 6 1.5 - 2.0
Phenolic 50 49 45-50 2 - 1 .5 - 2 .0
B.V. = B u re a u Veritas D.N.V. = D et N orske V eritas
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From the above table it is observed that the tensile modulus and failure strain o f all the available resin types are less than that specified by Classification Society Rules, w hereas the tensile strength is somewhat less than that put forw ard by Smith (1990) [42], These figures suggest that Indian resins have considerable variability w ith regard to strength and stiffness and in some respect have even less than the stated requirem ents, suggesting inferior quality resin. The subsequent chapters analyze the variation in the strength property o f the lam inates m anufactured from resin processed by the Indian m anufacturers. Since the quality o f resin used by Indian m anufacturers is "Boat G rade", there is not m uch variation in the chemical composition. M oreover, the m anufacturer uses resin that is available in the market, the nom inal values o f Tensile M odulus and Failure Strain o f these resins given in the preceding table are less than the Class values.
1.3.3 Lam inates
A lam inate is m ade up o f individual laminae. A lam ina consists o f a layer o f fibre im pregnated in a layer o f resin. A laminate consists o f a num ber o f layers o f lam ina. M ost lam inates are orthotropic in nature and have planes o f symmetry. The strength o f fibres and th eir orientation in an individual lam ina gives the strength properties o f the laminate.
C areful evaluation o f the elastic properties o f GRP laminates is an im portant part o f the design process. The m ost novel feature o f GRP laminates is the freedom w ith w hich the lam inates can be “tailored” to suit a particular strength and stiffness requirem ent.
Lam inates can be assem bled locally w ithout any high degree o f skill.
B ureau V eritas (1979) [8] gives the physical characteristics o f a few com m on lam inates used in boat building. These characteristics are reproduced in Tables 1.3 and 1.4. The values given in these tables may be taken to be for ideal conditions o f lay-up process in the absence o f any details. The ideal conditions can be taken as clean environm ent, controlled tem perature and hum idity, good w orkm anship and new resin o f requisite strength properties. Lam inates consisting o f four, six and eight layers have only been show n since experim ents have been conducted with these layers o f lam inates only (C hapter 3).
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Table 1.3 All Mat Laminate
No. of layers Total thickness
Mass of glass
Mass of resin
Total Mass
Theoretical Moment
Remarks
(mm) (kg/m2) (kg/m2) (kg/m2) daNmm/mm
4 5.21 1.80 5.4 7.2 55.9 W f = 0.25
6 7.81 2.70 8.10 10.8 125.9 Mat. W eight
8 10.42 3.60 10.80 14.4 223.8 = 450 gm /m 2
Table 1.4
Alternate Mat-Woven Rovings Laminate
No. o f layers Total thickness
Mat re inf.
W.R.
reinf.
Total weight of glass
Weight of resin
Total weight
Theoretical Moment (mm) (kg/m2) (kg/m2) (kg/m2) (kg/m2) (kg/m2) daN mm/mm
4 4.08 0.90 1.20 2.10 3.9 6.00 44.6
6 6.12 1.35 1.80 3.15 5.85 9.00 103.5
8 8.15 1.80 2.40 4.20 7.80 12.00 186.9
Wf (M at) = 0.25 ; W f(W R ) = 0.50 ; CSM =C hopped Strand M at C SM = 450 gm /m 2 ; W R= 600gm /m 2 ; W R =W oven Rovings
1.4 USES IN SHIPS
The following uses o f GRP in boat hull construction and the fabrication processes are com m only employed.
1.4.1 Fishing vessels
W ith the increase in scarcity o f w ood, GRP is being increasingly resorted to as a m aterial for building traw ler hulls o f length less than 25 m. Fyson (1985) [16] com pares
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the m echanical properties o f wood, steel and GRP (Table 1.5). The physical properties o f w ood vary considerably being dependent on wood species. The properties o f w ood also vary according to their m oisture content. W ooden vessels have the added disadvantage o f being held together by frames and fastenings which tend to becom e loose and cause w eakness in the structure. Wood also suffers from rot and degradation. Fyson also states that G RP traw lers are 23 to 28% lighter than their wooden equivalents. The hull w eights o f w ood and steel traw lers are very nearly the same. GRP fishing vessels can have greater fish hold capacity because its hull w eight is less than that o f a w ooden one. For all these reasons GRP is being increasingly used as a boat building m aterial. The study o f uncertainty in the following chapters will lead to better perform ance prediction o f GRP as a hull m aterial.
Table 1.5
Strength and Stiffness of Boat-Building Materials
Strength/Stiffness Properties
Shipbuilding Steel
GRP mat
GRP woven roving
A lum inium W ood (Teak)
Specific gravity 7.8 1.5 1.6 2.65 0.85
T ensile strength, a , M Pa 440 84 213 261 64
M odulus o f elasticity, E G P a
206 7.36 10.3 69 8.1
1.4.2 Warships
W arships are high perform ance vessels. Here speed and strength are at a prem ium . GRP and som etim es FRP (carbon fibre) are used to give lightness and stiffness to the hull. GRP is also used in the hull construction o f minesweepers. This is because o f its low m agnetic signature to avoid activation o f mines.
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1 .4 .3 U n d e r w a t e r v e h ic le s
Mobile underwater vehicles have their hulls made o f fibre-reinforced-plastics.
I his enables them to have considerable weight saving to increase payload and range, as a result o f the high compressive strength o f FRP.
1.5 F A B R I C A T I O N P R O C E S S E S
The nature o f GRP as a material is such that it lends itself to a variety o f fabrication processes. Each fabrication process has its own merits and demerits and is suited for particular hull types. Normally single skin laminates are produced, stiffened by hat stiffeners with foam cores. Flat panels, i.e. decks and bulkheads, are o f sandwich construction. Uncertainty o f laminate strength is also introduced through the following fabrication_processes.
1.5.1 Hand lay up process
The major advantage o f this process is that the boat can be built without much infrastructural facilities or major training o f workmen. This is the reason why in India this method o f laying boat hulls is becoming increasingly popular, with hulls up to 25 m being laid. Preaccelerated resin is mixed with a catalyst and deposited on the gel coat or a ply o f reinforcement by a brush or roller-dispenser. Each ply o f reinforcement, either o f chopped strand mat or woven-rovings is paid out from rolls o f 1 m or 1.5 m width, laid on the previous layer and wetted with resin by rolling or brushing.
1.5.2 Automated lay-up
Where the woven roving is exceptionally heavy, wetting out o f the reinforcement layer becomes extremely difficult and recourse is then made to the automated lay-up process. Here, the reinforcement is paid out from a movable platform mounted on a
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gantry crane. The platform moves vertically and transversely over the hull.
Preaccelerated resin is mixed with catalyst and piped into roller im pregnators through w hich the reinforcement passes. Manual rolling and dragging is then carried out over the m ould by workm en on the platform.
1.5.3 Spray-up process
This method is used where large numbers o f small boat hulls are required. Glass fibre rovings o f lengths less than 50 mm are sprayed together with polyester resin prem ixed with catalyst and accelerator at the spray gun. The glass resin mixture is then consolidated by manual rolling. A weight fraction (percentage o f weight o f fibre in a lam inate) varying between 0.25 to 0.3 is achieved by this method. The method is sem i
autom ated with less control o f thickness, hence producing inferior quality laminates than the m ore com mon hand lay up process.
1.5.4 Pre-preg vacuum bag lay-up
This is a com pression moulding process, involving application o f pressure and som etim es heat to aid curing. The method involves placing over the mould a flexible m em brane, which becom e the bag later. A film o f PVA, release agent, is applied over this m em brane and the laminate laid over. The edges are then sealed and vacuum created under the mem brane. The uncured laminate is thus subjected to a pressure o f 1 bar. The m ould can be heated to ensure faster curing.
1.5.5 Resin Transfer moulding
In this process, the therm osetting plastic is wetted in a separate cham ber and then transferred under pressure through a runner to the m ould im pression for shaping. The
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m ould has vents. The charge (thermosetting plastic) is hot and fluid and thus flows around the corners o f the inserts.
The above two methods o f compression moulding are applicable to small boat hulls up to 4 metres in length.
1.5.6 Vacuum Assisted Resin Infusion Moulding (VARIM)
A nother recent development in fibre reinforced polymer com posite moulding is the V acuum Assisted Resin Infusion M oulding process. In this technique resin infusion takes place by the creation o f vacuum. The three dimensional flow o f resin then takes place through a porous medium, the layers o f fibre reinforcement. The layers o f fibre reinforcem ents are thus wetted out with resin. Heat can then be applied for curing.
1.6 OBJECTIVE OF RESEARCH WORK
Glass fibre reinforced plastics are increasingly being used as a hull material for m anufacture o f small marine craft. The hand lay up process is em ployed to lay successive layers o f laminae to build up the laminate. U ncertainties in material properties and workm anship play an important role in laminate design. The best method to observe the effect o f these uncertainties is a study o f the reliability index obtained from the distributions o f laminate resistance and loading environm ent. The objective o f the present research is to find the changes in the reliability indices as laminate strength properties are varied, incorporating material and w orkmanship uncertainties. Bureau V eritas (1979) [8] recom mends moulding workshop tem perature betw een 15° C and 25°C w ith optimal between 18°C to 22°C, the relative hum idity not exceeding 70%.
H ow ever, the Indian environm ental conditions cannot be m aintained w ithin this range, the usual w orkshop tem perature being 31° C and relative hum idity 78%. M oreover, the m oulding is often carried out in the open and not in a closed and protected w orkshop, thus aggravating tem perature and hum idity fluctuations. The w orkm en have varying
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degrees ol skill and often there is inadequate supervision. The objective o f the present research is to simulate these conditions in an experimental set up and study the laminate strength variation.
1.7 S C O P E
The scope ol the present work lies in the uncertainty analysis o f composite laminates made Irom chopped strand mat and woven rovings. The consequences o f resin uncertainty and weight fraction ( ) uncertainty, which shows workmanship quality, arc studied in the total build-up o f laminates.
1.8 O R G A N I S A T I O N O F T H E S I S
The thesis is organized into seven chapters. Chapter 2 deals with the various levels o f reliability analysis in use. Chapter 3 outlines the test procedures employed and the results obtained from experiments. Chapter 4 obtains theoretically the resisting moment distributions using laminate micromechanics and Monte Carlo simulation technique. The theoretical distributions thus obtained are then compared with the experimental values o f the previous chapter. Chapter 5 discusses the methodology o f calculating load on the bottom laminate o f a fast craft. The load is then given a variation to obtain the frequency distribution. Chapter 6 gives the procedure for calculating the reliability index using the distributions o flo ad and resisting moment obtained in Chapters 4 and 5. The reliability indices are calculated for various laminate configurations norm ally used in boat hull construction.
