Composites
in Industrial Plants An Introductory Introductory Guide
Preace The Queensland Government’s Fibre Composites Action Plan – New Technology Taking Shape launched in April 2006 sets out over 50 initiatives under six theme areas, ranging rom innovation to skills and training. The Fibre Composites Action Plan identied the potential or signicant benets rom increased use o composites in target sectors such as mining, minerals processing and associated inrastructure. Deborah Wilson Consulting Services (DWCS) and GHD were engaged to undertake a study to assess this opportunity and develop approaches that make the choice o composites in mining applications easier, and more relevant to delivering cost savings and other benets to industry. The Queensland Government, through the Department o Employment, Economic Development and Innovation (DEEDI), unded this study as part o a larger initiative to help one o the State’s most promising new industries grow and compete on a global level. The aim o the study was to deliver: •
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case studies on successul use o composites in the mining industry and the benets composites deliver business case inormation on the use o composites in dierent applications in mining, minerals processing and associated inrastructure
Disclaimer This publication was unded by the Queensland Government (through the Department o Employment, Employment, Economic Development and Innovation). It is distributed by the Queensland Government as an inormation source only. The State o Queensland makes no statements, representations, representations, or warranties about the accuracy or completeness o, and you should not rely on , any inormation contained in this publication. Readers should not act or rely upon any inormation contained in this publication without taking appropriate proessional advice relating to their particular circumstances. The Queensland Government disclaims all responsibility and all liability (including without limitation, liability in negligence) or all expe nses, los ses, damages an d
inormation covering availability, technical guides and benets o using composites in common applications in the mining industry
costs you might incur as a result o the
improved links between composites suppliers, manuacturers and the mining industry to better respond to mining industry needs
in any way, and or any reason.
inormation kits, presentations and technical seminars on the ndings and applications where composites deliver value to the mining industry a model or the composites industry to use in proling valuable applications or composites in other industries.
This introductor y guide addresses a number number o these aims. It has been prepared ollowing a review o relevant technical literature and discussions with the composites industry.
inormation being inaccurate or incomplete
Composites in Industrial Plants An Introductory Guide
Table o contents
List o abbreviations
1.
Introduction ___________________________________ 3
2.
Overv iew o materials and products _______________ 4 2.1 Qualitative comparison o materials ___________ 4 2.2 Benets o composites ______________________ 5 2.3 Product applications ________________________ 6 2.3.1 Current applications _____________________ 6 2.3.2 Future applications ______________________ 7 2.3.3 Pipes and ducts _________________________ 7 2.3.4 Tanks and process vessels ________________ 8 2.3.5 Launders _______________________________ 9 2.3.6 Joints and ttings________________________ 9 2.3.7 Coatings and linings ____________________ 10
3.
Composite product manuacturing _______________ 11 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8
Components______________________________ Fibre reinorcement ________________________ Resins ___________________________________ Additives ________________________________ Cores____________________________________ Example o a composite laminate ____________ Manuacturing processes ___________________ Manuacturers ____________________________
11 11 13 14 14 15 15 15
4.
Australian case stories _________________________ 16
5.
Technical peror mance _________________________ 18 5.1 Design___________________________________ 5.2 Standards________________________________ 5.3 Guides __________________________________ 5.4 Relative peror mance o materials ___________ 5.5 Service lie _______________________________ 5.6 Mechanical properties _____________________ 5.6.1 General _______________________________ 5.6.2 Strength ______________________________ 5.6.3 Fatigue________________________________ 5.6.4 Creep _________________________________ 5.6.5 Abrasion resistance _____________________ 5.7 Thermal properties ________________________ 5.8 Chemical properties _______________________ 5.9 Electrical properties _______________________ 5.10 Perormance o composites in re____________ 5.11 UV resistance _____________________________ 5.12 Working with composites on site_____________ 5.13 Inspection and testing _____________________
18 18 19 19 20 20 20 20 21 22 22 22 23 26 26 27 28 28
6.
Economic comparison__________________________ 30
7.
Environmental comparison _____________________ 31
8.
Reerences ___________________________________ 32
9.
Australian manuacturers o composite industrial products ____________________________ 34
10. Australian composites design and engineering service providers ___________________ 41 11. Acknowledgements____________________________ 43
ACI
American Concrete Institute
AS
Australian Standard
BS
British Standard
CFRP
Carbon Fibre Reinorced Plastic
CTE
Coecient o Thermal Expansion
F RP
F ibre Reinorced Plastic
GRP
Glass Reinorced Plastic
HDT
Heat Distortion Temperature
ISO
International Standards Organisation
PTFE
Polytetrafuorethylene
PVC
Polyvinyl Chloride
PVDF
Polyvinylidene Fluoride
UV
Ultraviolet (sunlight)
A composite is a material
1
Introduction
A composite is a material made up o two or more components so the benecial properties o each component are utilised. In this guide, composite reers to a material composed o a thermosetting resin and bre reinorcement. Composites are also reerred to as breglass, glass reinorced plastic (GRP), bre reinorced plastic (FRP) and carbon bre reinorced plastic (CFRP). As there are many dierent resins, reinorcements and methods o putting the two together, there are a multitude o materials which can be described as composites.
made up o two or more components so the benecial properties o each component are utilised.
Composites oer unique products in many o Queensland’s most important industry sectors, including advanced manuacturing, aerospace, building and construction, deence, inrastructure, marine, mining and transport. As composites are light-weight and corrosion-resistant, the materials have the potential to reduce costs, save time and provide a saer work environment. At a time o fuctuating steel prices and long delivery times, composites oer a real alternative to reduce capital and operational costs, and downtime. Composites’ light-weight nature provides operational savings or trucks and mobile equipment, and their corrosion-resistance prevents the hazards o rusting steel structures. Composites have been used in many Australian industries since the 1940s. For example, in the minerals processing and chemical industries, the materials are used in a variety o applications including tanks, pipes, process vessels and foor grating. In the mining industry, the materials are used in applications including ducts, truck bodies and rock bolts. It seems the Bronze Age and Iron Age have passed, and the composites age is now upon us. The Queensland Government is capitalising on Queensland’s strengths as a world leader in the research, development and commercialisation o bre composites technologies through the implementation o its Fibre Composites Action Plan, and signicant investment under the Smart Futures Fund.
Carbon bre-epoxy drill rod prototype with embedded strain gauges and carbon nanotube-epoxy threads Image courtesy of Teakle Composites
For more inormation on Queensland’s Fibre Composites industry please visit: www.composites.industry.qld.gov.au Lucy Cranitch, GHD, produced this guide. It aims to provide an introduction to composites in the mining, mineral processing and chemical industries, and to assist in the decision to purchase a composite component. It does not provide design details o composite components. For more inormation on GHD please visit www.ghd.com.au
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2
Overview o materials and products
2.1
Qualitativecomparisonofmaterials
The table below provides a quick comparison o materials. Table 1
Qualitative comparison o materials
Material
Advantages
Disadvantages
Mild steel
High strength
Susceptibility to corrosion
High stiness
Susceptibility to atigue
High ductility
High weight High energy required or production
Stainless steel
Corrosion resistance
High cost
Aluminium
Low weight
Susceptibility to corrosion in strong acids and alkalis
High ductility
High energy required or production
Ease o recycling Plastic (polyethylene, polypropylene, polyvinyl chloride (PVC), etc)
Composite
Corrosion resistance
Susceptibility to creep
Low cost
Low stiness
Low coecient o riction
Non-conductive properties can be a disadvantage
Ease o recycling
Limited temperature resistance above 200°C
Corrosion resistance
Limited temperature resistance above 250°C
Low weight
Sensitivity to impact damage
High strength Conductivity or non-conductivity Low coecient o riction
Wagners Composite Fibre 100 x 100 mm pultruded sections Image courtesy of Wagners CFT Manufacturing Pty Ltd
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2.2
Benets of composites
Corrosion resistant With the selection o correct materials, composites will not deteriorate in acids, alkalis, solvents and salt water, and can be used rom pH 0 to 14. Composites are thereore used widely in tanks, pipes and process vessels in chemical extraction o base and precious metals. Plant operating time can thereore be maximised. Both minerals processing and chemical plants use this durable material or plant inrastructure, such as gratings and hand rails, where rusting o steel structures can place the saety o plant personnel at risk. Since composites do not require painting, there are also reduced maintenance costs. Durable Composite materials are durable due to their high strength and high resistance to atigue, abrasion and creep. Agitated tanks made rom composites have been ound to operate successully or many years despite the cyclic loads experienced. In pipelines, resistance to abrasion combined with a low coecient o riction aids process fow and reduces downtime. This overall durability o composites reduces the need or maintenance and repair, which maximises plant running time. Light in weight Composites are relatively light in weight compared to steel, iron and concrete. For example, typical composite pipes are approximately 25 per cent o the weight o ductile iron and 2 per cent o concrete equivalent pipe mass per metre. The reduced weight o composite pipes, tanks and process vessels has led to lower transportation and installation costs or the mining industry, and reduced plant downtime through enabling installation at sites where access is restricted. Where electrical guarding and hatches need to be lited by plant operators, the composite option at less than 10 kg per sheet is certainly preerable to the steel option at more than 20 kg. This also applies to hatches and all components that must be lited to ensure the saety o all personnel.
All FRP (handrails, stair treads, landing and support structure) stair platorm Image courtesy of Exel Composites
Electrically insulating or conductive For saety reasons, the electrical insulation o process equipment is critical where high electric currents or voltages are used. Composites that are electrically insulating are used in high electric currents or voltage environments, such as pot rooms in aluminium processing and in electrowinning. The radio and magnetic transparency o composites is useul in a number o applications. In applications where static charge can build-up, static dissipation and grounding o equipment is critical to keep plants operating and to prevent res where fammable solvents are used. Conductive properties can also be built into the composite equipment or applications such as solvent extraction. Thermally insulating Where high temperature fuids are stored in vessels or pipes, thermal insulation is critical or saety. The use o composites in these applications can reduce or eliminate the need or insulation with external temperatures typically being less than 60°C or fuids and liquors up to 100°C. Furthermore, being an insulator, the transer o heat rom composite materials to any body part is very much less than that rom a conductive material such as stainless steel. Flexible in design and manufacture Composite materials oer solutions to many manuacturing problems due to the vast array o resins, reinorcements and unique manuacturing methods used to produce them. Such fexibility in design and manuacture can result in cost and time savings. For example, it is relatively simple or composite materials to create compound curves in metallic materials. Also, while large covers usually require large support structures, the light weight nature o composites means it is possible to design covers that are supported on the edge o a vessel without the requirement or intermediate supports. Composites manuacturing processes, such as hand lay up, also enable unique designs to be manuactured at relatively 5
The ability o composites to conorm to any shape and bond with steel and concrete enables rehabilitation and retrot.
low cost. The ability o composites to conorm to any shape and bond with steel and concrete enables rehabilitation and retrot. For example, composite materials are well used in the lining o process vessels. Composite materials’ fexibility in design and manuacture also means on-site manuacture o very large vessels, such as lament winding o large tanks, is possible
2.3
Product applications
2.3.1
Current applicat ions
Composites can be used in many applications in the mining and process industries, including: Mining •
ducts or ventilation, chilling and cooling in underground operations
•
cuttable rock bolts (used in reinorcement), rib bolts and brackets
•
mobile and stationery containers or water, diesel and other liquid storage on site
•
bore casings and insulation in underground structures
•
theodolites and legs or survey equipment.
Mineral and chemical processing •
•
•
Chemical resistant FRP piping system with coupling or use in highly corrosive environments Image courtesy of A.C.Whalan Composites
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tanks or storage o corrosive and non-corrosive materials process vessels including gas cooler condensers, electrostatic mist precipitators, leach tanks, reactor tanks, thickeners, electrolytic cells, cell bearers, mixer settlers, spent tanks, cr ystallisers, solvent ext raction and electrowinning cells, and pulse columns mineral sands separation equipment including spirals, cone concentrators and hydrocyclones
•
cooling towers
•
linings or concrete and steel tanks and equipment
•
claustra walls and panels
•
ans, blades, bafes, agitators, bottom scrapers and mixing tools
•
pipes, ttings and launders including products or abrasive (e.g. slurry) and non-abrasive materials
•
nozzles, fanges, elbows, reducers, branches, tees and joints
•
ducts or transporting process gases and ume extraction
•
scrubbers and waste gas towers, quench towers and demisters
•
dampeners/valves
•
gratings, ladders, walkways, handrails, steps and platorms
•
inspection hatches, hoods and covers
•
structural applications such as support beams, channels and angles
•
roth crowders or fotation tanks
•
protective guards on machines
•
consoles
•
telescopic handles or sampling and testing
•
stacks, fues and other large structures
•
use o composites to repair ailed plant components.
Mine site inrastructure •
guards, grating, walkways, platorms, kick rails, stairs and ladders
•
rebar and stay-in-place ormwork or concrete
•
polymer concrete
•
concrete foor and bund coatings and lining
•
cable supports, trays and ladders
•
pumps
•
power poles including cross-arms
•
wall and roo sheeting as well as purlins in site buildings
•
window and door rames
•
water treatment and supply
•
bridges
•
trusses
21 mm solid FRP rods supplied to customer as concrete rebar to eliminate any electrostatic intererence with its equipment
•
manhole covers
Image courtesy of Exel Composites
•
railway sleepers
•
drains and sumps
•
poles to remove high voltage lines.
Port inrastructure •
guards and inspection hatches
•
gratings, ladders, walkways, handrails, steps and platorms
•
structural panelling, sheet piling and other applications in marine environments.
2.3.2
Future applications
The advantages o composites described above have led to investigations into new applications or composites, including: •
•
•
•
truck bodies, cabs, panels and engine casings (ully breglass cabs have been used by Leader trucks and Mack trucks since the 1970s) access ladders, hand rails and steps attached to major mining and earth moving equipment wear blocks long and short conveyors including supports, covers and hoods, guards and rollers
•
wash plant pipes and air receivers
•
port loading inrastructure
•
gag ducts or re suppression in underground mines.
2.3.3
Pipes and ducts
From pipes carrying sulphuric acid in leaching o copper bearing ore, to waste water, composite pipes have widespread use in the chemical and minerals processing industries in Australia. Key benets include resistance to corrosion in chemical environments, increased hydraulic fow and reduced operating costs through comparatively low riction compared to steel. Conductive composite pipes are much saer than plastic pipes in solvent extraction plants, and have been ound to be more cost eective and durable than the alternative SAF2507 stainless steel.
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In underground mining, composite ducts are used or ventilation as its light weight nature enables much easier installation and lighter supports than other products. In the chemical and minerals processing industries, composite ducts are used or applications like transporting sulphur dioxide in plants manuacturing sulphuric acid, and in minerals processing plants to extract umes. There are a range o standards and guidelines available or the design and manuacture o composite ducts and pipes. Those most widely used in Australia include: AS 3571
Plastics piping systems—Glass-reinorced thermoplastics (GRP) systems based on unsaturated polyester (UP) resin—pressure and non-pressure drainage and sewerage; and pressure and nonpressure water supply
AS 2634 (obsolescent)
Chemical plant equipment made rom glass-bre reinorced plastic (GRP), based on thermosetting resins
AS/NZS 2566
Buried fexible pipelines
BS 7159
Code o practice or design and construction o glass-reinorced plastics (GRP) piping systems or individual plants or sites
BS 6464
Specication or reinorced plastic pipes, ttings and joints or process plants
BS EN ISO 14692
Petroleum and natural gas industries—glass-reinorced plastics (GRP) piping
ISO 10467
Plastics piping systems or pressure and non-pressure drainage and sewerage—glass-reino rced thermosetting plastics (GRP) systems based on unsaturated polyester (UP) resin
ISO 10639
Plastics piping systems or pressure and non-pressure water supply—glass-reinorced thermosetting plastics (GRP) systems based on unsaturated polyester (UP) resin
ANSI/AWWA C950
Standard or berglass pressure pipe
ISO 10639
Plastics piping systems or pressure and non-pressure water supply using GRP systems based on unsaturated polyester (UP) resin. Composite pipes can be used at low and high pressures. For example, the API 15 HR specication or high pressure breglass line pipe covers pipes rated or 3.45 MPa to 34.5 MPa. For above ground pipes and ducts, BS 6464 contains inormation on installation including supports, guides and anchors. Pipe support spacing is important and the ratio o the vertical defection o a pipe to the horizontal span between supports is oten limited to 1:300. For pipe supports, a minimum contact arc o 120° under the pipe is typical and rubber packers between the support and the pipe can help reduce point loads. For buried pipes, AWWA C950 contains inormation on design whilst AS 2566 and BS 6464 can be used or installation. Inormation on trench preparation, backlling material and installation procedures are given in these standards. It is possible to make continuous radius bends, including elbows and long radius bends, as a single unit with no longitudinal joints in composites. 2.3.4
Tanks and process vessels
In the chemical and minerals processing industries, composite tanks and process vessels have a long history o successul use in chemical environments which readily corrode steel and attack concrete.
FRP Fuel tanks
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Sulphuric and hydrochloric acids are widely used in processing copper, lead, nickel and zinc. In these manuacturing plants, composites are used to construct leach tanks, thickeners, electrolytic cells mixer settlers, spent tanks and pulse columns. In sulphuric acid manuacturing plants, composites are widely used in radial fow scrubbers, gas cooler condensers and electrostatic mist precipitators.
While the corrosion resistance o composites is a key benet, the relatively low cost o composites compared to alternative materials such as stainless steel, duplex and other alloys has also accelerated their acceptance. The ollowing standards and guides are applicable to composite tanks and vessels: AS 2634 (obsolescent)
Chemical plant equipment made rom glass-bre reinorced plastic (GRP), based on thermosetting resins
BS 4994 (superseded)
Specication or design and construction o vessels and tanks in reinorced plastics
BS EN 13121
GRP tanks and vessels or use above ground. Design and workmanship
BS EN 13923
Filament-wound FRP pressure vessels. Materials, design, manuacturing and testing
ASME RTP-1
Reinorced thermoset plastic corrosion resistant equipment
ASTM D3299
Standard specication or lament-wound glass-ber-reinorce d thermoset resin corrosion-resistant tanks.
As well as storage tanks and process vessels, composites can also make internal components such as bafes and weirs. For example, composite fanges, manways and other xtures can be built into the composite tank or vessel. It is important to reinorce areas o composite tanks and vessels subject to higher loads. Shells should be reinorced with external circumerential reinorcing ribs to provide rigidity, particularly where agitators are not independently supported. Floors should be reinorced where intermediate supports are needed or tank roos. Roos should be reinorced where personnel and/or other equipment need to be supported. Inormation on supports or tanks and process vessels is given in the standards. It is standard practice to use concrete slabs as supports, however, concrete ring beams lled with compacted sand nished with a layer o sand and oil mixture can also be used. 2.3.5
Launders
There is no design standard specically or composite launders, although BS 6464 contains some applicable inormation. The stiness o the launder should be sucient to prevent sag, twist, camber or spreading without ull length supports or restraints while the launder is operating. It is advisable to reinorce o-take areas o launders. 2.3.6
Joints and fttings
The type o joints aects the durability and cost o pipelines. Common methods o joining composite pipes are butt and strap; rubber ring type and fanged joins. Restrained joints eliminate the need or and thus cost o thrust blocks etc. Butt and strap joints used with composite pipes are restrained, have similar chemical resistance to the parent pipe material and are less susceptible to leaks. However, in terms o installation butt and strap joints are slow and costly and do not tolerate misalignment or movement well. Whilst rubber ring type joints are not restrained, they are quick to install and tolerate some degree o misalignment and movement. Thus rubber ring type joints are particularly useul or buried pipelines. There are a number o requirements or durable butt and strap joints. The strength o the joint must be at least equivalent to that o the parent material. The required widths o pipe joints are given in the standards, and where accessible, the internal surace o the joint should be laminated. Since joints are hand laid, their thickness must be that o a hand laid pipe, even or joints in a lament wound pipe. To prevent ingress o fuids into the laminate, all cut ends must be sealed with resin. Tees, branches and other similar joints can be prepared using similar techniques to those employed or standard composite butt and strap joints.
FRP fange installed at a ertilizer (phosphates) manuacturing acility in Australia Image courtesy of Lucy Cranitch, GHD
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Flanged joints are also widely used and fanges can be made rom composite materials. The thicknesses o composite fanges depend on the design, but are generally greater than that o metal fanges. ANSI dimensions are commonly used or bolt patterns, and composite fanges can be manuactured to be compatible with most existing fanges made o PVC, steel and ductile iron. It is important to ensure composite fanges are fat to provide a good seal, so ull fat-aced fanges with steel backing rings are oten used. It is important to never mix ull ace composite fanges and raised ace fanges as this readily results in leaks and ailures. To avoid point loads caused by nuts directly in contact with the composite fange ace, washers should be used under nuts, relies can be cut into the ace o the fange and care must be taken with bolt torque. All cut outs or bolt holes must be sealed with resin to enhance durability. A number o standards are applicable to fanges. AS 4087
Metallic fanges or waterworks purposes
AS 2129
Flanges or pipes, valves and ttings
AS 4331.1 (ISO 7005)
Metallic fanges (steel fanges)
2.3.7
Coatings and linings
Composites can be used in conjunction with concrete or steel to provide a corrosion-resistant lining or coating. This may be in the orm o an internal corrosion protection to steel or concrete tanks, or as a protective layer on concrete foors or bunds. The ollowing standards and guides are applicable to composite coatings and linings: BS 6374-4
Lining o equipment with polymeric materials or the process industries. Part 4: Specication or lining with cold curing thermosetting resins
ACI 515.1R
Guide to the use o waterproong, damp-proong, protection and decorative barrier systems or concrete. The ollowing steps are typical in applying a bonded composite layer to concrete: 1.
The concrete should be let 28 days to cure prior to application o any coating or lining.
2.
Surace preparation o the substrate is important. Abrasive grit blasting (high pressure water or grit blasting) o the surace is required to improve bonding o the coating or lining.
3.
Remove dust or grit by vacuuming and/or sweeping.
4.
Wash the surace to remove oils, greases and other contaminants.
5.
Dry the substrate.
6.
Test or suitability o the coating or lining. Various tests are required depending on the substrate, or example pH, moisture and surace pull-o tests are required or concrete.
7 .
Fill voids with a resin-based ller.
8.
Prime.
9.
Apply the basecoat, consisting o resin reinorced with bre mats or with llers.
10. Apply the top coat, and i required spread silica aggregate to provide slip resistance. Quality control during the coating or lining process is important. This should include wet lm thickness tests, adhesion tests, coating sensitivity tests and resin gel time tests. I an additional conductive primer coat is applied, spark testing can be conducted once the basecoat is applied.
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Composite products consist
3
Composite product manuacturing
3.1
Components
o a combination o bres, resins, additives, and in some cases, cores.
Each component contributes to the overall properties, perormance and appearance o the composite product. The precise type o materials and manuacturing process used are determined by the specic properties required or the nal product. The ollowing principles are essential or the production o good-quality composite products: •
•
•
•
quality o materials—resins, glass bres, additives and cores quality o design—quantity, orientation and suitability o bres, suitability and volume o resins, suitability and volume o additives, and suitability o cores quality o manuacturing—consistency and control o the manuacturing and curing processes. Full curing o the product is essential to attain optimum mechanical properties, prevent heat sotening, limit creep and reduce fuid diusion quality o transport and installation practices.
As the composite material itsel is made at the same time as the part, quality assurance and inspection throughout these processes are essential.
3.2
Fibrereinforcement
The role o the reinorcement in a composite part is to carry the applied load. The actors which aect the contribution o the reinorcement to the composite properties are: •
the type o reinorcement
•
the orm o reinorcement
•
the quantity o reinorcement (resin-to-reinorcement ratio)
Fibreglass borehole liner
•
the orientation o the reinorcement.
Image courtesy of Teakle Composites
Type: Many dierent types o reinorcement are available, including E glass, ECR glass, C glass, carbon, aramid (Kevlar) and many other less common bres. Carbon bre is used in the mining industry primarily to provide conductivity. The bulk o the reinorcements are made o glass. E glass is the most widely used bre type due to its high strength and relatively low cost. C glass is used where excellent chemical resistance is required, usually in the orm o a tissue as described in the table below. ECR glass is sometimes used to provide better resistance to chemicals. The ollowing table, taken rom the Eurocomp Design Code, compares typical glass bre properties. Compared to steel, glass bres have approximately 2.5 times the strength with only one third o the density, and higher dimensional stability. Table 2 Comparison o properties o glass fbre t ypes and steel
Property
E glass
C glass
Steel
Specic gravity
2.54
2.50
7.8
Tensile strength (MPa)
3400
3000
1350
Tensile modulus (GPa)
72
69
200
Elongation (%)
4.8
4.8
10–32
Coecient o thermal expansion (10–6/°C)
5.0
7.2
11.5
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Form: Fibres are available in many orms, as described in the ollowing table. Table 3 Forms o reinorcement
Reinforcementform
Description
F ilament
Individual bres as initially drawn rom the raw materials. F ilament s are processed ur ther beore use.
Continuous strand
Filaments gathered in continuous bundle. Continuous strands are processed urther beore use.
Milled bre
Continuous strands hammer-milled into lengths o 0.8 to 3 mm. Milled bres are used as llers and additives to control heat distortion and improve surace quality in compounding and casting.
Chopped strand
Strands chopped to 5 to 60 mm leng ths.
Roving
Strands bundled together without twist. Rovings are used in various manuacturing processes including lament winding and pultrusion to give high strength in the direction o the bres.
Yarn
Twisted strands. Yarns are processed urt her beore use such as in the manuacture o clot hs.
Chopped strand mat
Non-woven mat o chopped strands in random orientations. This reinorcement is widely used to give strength in all directions and good inter-laminar adhesion.
Continuous strand mat
Non-woven mat o continuous strands in random orientations.
Tissue/veil
Fine non-woven mat o continuous laments that are uniormly distributed over the surace in random orientations. Tissues have relatively low strength. Their purpose is to support a resin-rich layer which protects the composite rom moisture and chemicals, through preventing these fuids entering the laminate along the bres.
Unidirectional abric
Rovings in one direction held together by a small amount o bres woven or stitched at 90°. Unidirectional abrics give strength in one direction.
Woven roving
Rovings woven into a abric in a particular pattern, usually a plain weave. Woven rovings give strength in t wo directions.
Cloth
Fabric made rom yarns woven in a par ticular pat tern. Cloths give streng th predominantly in two directions.
Stitched abric
Layers o bres held together by stitching. Stitched abrics give strength predominantly in two directions and have higher interlaminar strength than cloths.
Multi axial abrics
Fabric made rom yarns or rovings in more than two directions. Multi axial abrics give strength in three or more directions.
Needle punched and combi-mats
Quantity: The manuacturing process has a large eect on the quantity o reinorcement in composites. Fabrics with closely packed bres will give a higher volume raction o reinorcement than those abrics with large gaps between bre bundles. The weight per unit area o reinorcement varies greatly rom as low as 20 g/m2 or tissues, to 300 or 450 g/m2 or chopped strand mat, to 800 g/m2 or woven rovings, and to well over 1600 g/m2 or lament wound rovings. As a general rule, the strength and stiness o a composite are proportionate to the quantity o reinorcement present. However, the laminate strength peaks at an optimum bre volume o about 70 per cent, above which the strength declines due to a lack o resin to hold the bres together. Orientation: The tensile strength o bres is greatest in longitudinal direction rather than width. Fibres must thereore be oriented in the direction o the load, and orientation can be designed to suit the particular loading requirements o the Fibreglass cloth composed in a swirl pattern 12
It is helpul to distinguish between two broad groups part. Unidirectional bres run in one direction only, whereas abrics have bres in predominantly two directions, and chopped strands are oriented in all directions. The combination o reinorcements results in an anisotropic material, where its properties vary with direction.
3.3
o polymers—thermoplastic and thermosetting.
Resins
While the bres are the principal load-carrying members, the surrounding matrix o resin maintains them in the desired orientation and location. It also allows the applied load to be transerred between the reinorcing bres. Another very important unction o the resin is to provide a barrier to the environment, which protects the composite rom the elements, such as water and chemicals. Resins are also reerred to as ‘polymers’ as they are made up o many (poly) long-chain molecules (mers). It is helpul to distinguish between two broad groups o polymers—thermoplastic and thermosetting. Thermoplastic polymers melt when heat is applied. This is because their long chains are not chemically bound together (i.e. they are not cross-linked). Thermosetting polymers, on the other hand, do not melt when heated because their long chains are chemically bound together (i.e. they are cross-linked). The resins used in composites (and those described here) are all thermosetting polymers. There are a great variety o resins. The most common groups are polyester, vinyl ester and epoxy. Whilst re retardant versions o these resins are available, phenolic resins are also used in situations where re retardant properties are required. Resins are supplied to composite manuacturers in a liquid state, and during the manuacture o the composite part the resin is cured to orm a solid. This process o curing the resin is a chemical reaction in which the cross-links are ormed between the polymer chains. Beore curing, the resin is in a liquid state as the polymer chains can fow easily. Once the polymer chains are linked together, the polymer chains can no longer fow and the resin becomes a hard solid.
Spent Electrolyte Tank installed at Cause Nickel, Kalgoorlie Image courtesy of Marky Industries Pty Ltd
Polyester and vinyl ester resins supplied to the composite industry are dissolved in styrene monomer. This reduces the viscosity, so that the resin fows more readily to allow ease o spreading and ensures ull bre-wetting, complete impregnation and minimal voids. The styrene monomer is also a key component in the curing process o polyester and vinyl ester resins, orming the cross-links between the polymer chains.
Polyesterresins provide good strength at a relatively low cost and are used widely in the marine industry, and in pools, spas, transport, casting, inrastructure and automotive applications. Various types o polyester resins provide a wide variety o properties relating to water and chemical resistance, weathering and shrinkage during curing. Vinylesterresins are used primarily where improved water and chemical resistance, heat resistance or improved fexibility is required. Standard and high perormance vinyl ester resins are widely used in the mining and chemical industries due to their high resistance to acids, alkalis and solvents. Epoxyresins have a dierent structure to polyester and vinyl ester resins. They are usually sold as a two-pack system—Part A and Part B and these two parts must be mixed strictly in the ratios given by the supplier. The part A is the resin and the part B is the hardener and there are a number o dierent types o each. Epoxy resins are not dissolved in styrene monomer and do not shrink as much as polyester or vinyl ester resins when they cure. Epoxy Resins provide particularly good mechanical strength and adhesion and have good stiness, toughness, heat resistance and water resistance. Epoxy resins tend to be more expensive than polyester resins. Epoxy resins are widely used in piping and inrastructure.
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3.4
Additives
The ollowing additives can be incorporated into the resin:
Fillers are powders used to add bulk to the resin, which reduces costs and enhances the compressive strength o the composite material. Fillers can also reduce the exotherm (heat build-up) and shrinkage during curing. Fillers may be added to the resin at up to 50 per cent by weight (or dense llers) or 35 per cent by volume. Addition o ller over these amounts should be avoided as it reduces the fexural and tensile strengths, as well as the chemical resistance o the composite. Thixotropes are powders added to the resin to allow it to hold up onto a vertical surace. The addition o thixotropes is required when the resin must not run or sag when it is applied to steep moulds or to vertical walls (such a lining o a tank). Thixotropes allow the resin to fow when a shear orce is applied (i.e. when resin is orced through a spray gun), and prevent the resin rom fowing when the orce is removed. Pigments can be incorporated into the resin to produce a specic colour and to provide UV resistance. Fibreglass drill rod joint assembly in Instron testing machine Image courtesy of Teakle Composites
UVinhibitorsandabsorbers can be added to the resin to improve its UV resistance. Flameretardants can be added to the resin to improve its resistance to re. Inhibitors are chemicals added to the resin to slow down the curing reaction, so more time is available to work with the resin during manuacture beore it cures. As resins can cure in storage, inhibitors help to extend the resin’s storage lie. Promotersandaccelerators are chemicals added to the resin to speed up the curing reaction to enable manuacture in a reasonable timerame. While additives improve many properties o composites, they can also impair other properties at the same time. For example, some re retardants can reduce the composite’s resistance to weathering and chemicals. Additives should thereore be careully selected.
3.5
Cores
Some composite parts incorporate core materials, primarily to impart stiness without increasing weight. Cores may also be used to increase the impact strength, atigue resistance, thermal insulation and sound deadening eect. For a panel, the fexural stiness is proportional to its thickness cubed, which means as thickness increases, stiness increases dramatically. Cores can be used in specic areas o a structure where extra stiness is required (e.g. stiening ribs) or throughout the area o a laminate as a sandwich panel. A sandwich panel consists o a core with reinorcement and resin on either side (skin). In a sandwich panel, the adhesive layers between the skins and the core must be able to transer the loads and thereore be at least as strong as the core material. Without a good bond, the three components work as separate beams and the stiness is lost. Figure 1 shows a sandwich panel under a bending load. As a result o the bending, the upper section is placed under compression, the lower section in tension and the core in shear. Shear strength and stiness o a core material are important.
Skin
Compression
Shear
Core
Skin
Tension
Figure 1. Bending a sandwich panel
14
C = Tissue M = Chopped Strand Mat W = Woven Roving
3.6
Exampleofacompositelaminate
Figure 2 shows the wall o a composite tank or pipe to illustrate the layers that make up the composite material.
The reinorcement sequence is oten given on drawings in the ormat below, in order rom the internal surace to the outer surace: C/2M/4(MW)/M/C* Reinorcements: C = 40 g/m2 C glass or synthetic tissue such as Nexus tissue. M = 450 g/m2 E glass powder bound chopped strand mat. W = 800 g/m2 E glass woven roving. C* = 40 g/m2 C glass or synthetic tissue such as Nexus tissue with resin containing wax and UV inhibitors or pigment.
3.7
Primary corrosion barrier C
M
M
Alternating chopped mat & woven roving to desired thickness W
M
M
Vinyl Ester Resin
M
M
M
C resin/wax topcoat
Figure 2. An example o the makeup o a composite wall
Manufacturingprocesses
Formation o a composite product involves combining layers o reinorcement with resin. A chemical reaction o the resin then converts it rom a liquid to a solid to bind everything together as a whole. This chemical reaction is called curing, and is activated by catalysts or polyester and vinyl ester resin and a hardener or epoxy resins. The catalyst or hardener must be added to the resin prior to combining the resin with the reinorcement. It is important to achieve good cure o resins in a timely manner. This can be achieved through adjusting the chemicals involved in curing, including the inhibitors, accelerators and catalyst or hardener, and through taking account o the temperature during manuacture. There are a number o dierent manuacturing processes.
Handlayupinvolves the manuacture o a part in a mould. Resin is rst applied to the mould surace, then layers o glass which are wet by the resin and consolidated with rollers. VacuumInfusionPro cessing(VIP) involves the lay up o dry glass on a mould. A fexible lm (‘bag’) is then laid over the glass and sealed to airtight and then the resin is pulled through the glass under the orce o a vacuum. ResinTransferMoulding(RTM) uses two matched moulds – a bottom mould and a top mould. This process thereore produces parts with two nished suraces. Filamentwinding is perormed on a machine that winds glass bres onto a cylindrical mandrel in a prescribed pattern to orm the desired nished shape (e.g. a pipe). Fibres in the orm o continuous rovings are routed through a bath o resin beore reaching the mandrel. Ater curing, the tube is removed rom the mandrel. Pultrusion is used or the manuacture o products o a constant cross-section. The glass bres are pulled through a die (as compared to ‘extrusion’ where the material is ‘orced’ through a die) in a continuous process, injected with resin, shaped by the die and then cured.
3.8
Manufacturers
Australia’s composites industry is represented by Composites Australia Inc. Composites Australia is a membership-based, not-or-proft association dedicated to increasing the awareness and general usage o composites in Austr alia. Composites Australia has access to an extensive database o organisations in the Australian composites industry including raw material suppliers, manuacturers, designers and engineers, research and development agencies and training and education providers. See section 9 o this guide or contact details or a number o Australian composite product manuacturers, or contact Composites Australia at:
Level 15, 10 Queens Road, Melbourne Victoria 3004 Telephone: + 61 3 9866 5586 or 1300 654 254 Facsimile + 61 3 9866 6434 in
[email protected] www.compositesaustralia.com.au 15
4
Australian case stories
The ollowing tables provide examples o where composites have been used in Australia. Table 4 Current composite components in Australian mining and minerals processing plants
End user
Industry
Location
Components
Rio Tinto
Aluminium
Gladstone, QLD
Hoods or ume tanks, pipes, claustra walls in pot rooms
Adelaide Chemical Company
Copper
Burra, WA
Acid leach tanks (agitated), tank, slurr y pipe, grating, gas cooling tower
Xstrata Copper Reneries
Copper
Townsville, QLD
Electrolyte pipework, polymer concrete Electrolytic cells, galvanizing tank, acid storage tank, grating, wall cladding, roong
BHP Billiton, Olympic Dam
Copper, uranium, gold, silver
Roxby Downs, SA
Mixer settlers, Jameson cells, pipes in solvent extraction and electrowinning, bund linings, ducts, electrolytic cells, stack, tanks, electrostatic mist precipitators
Kanowna Belle Gold
Gold
WA
Roaster stack, an to stack ducting
Posgold Ltd
Gold
WA
Tanks
Nystar
Lead
Port Pirie, SA
Roo and wall sheeting, cable ladder to support cabling
Heraeus Ltd
Metals
VIC
Fume extraction ducting or precious metals recovery plant
Rennison Mine
Mining
Burraga, NSW
Pump
Centaur Mining — Minproc/Davy JV Cawse Nickel
Nickel
WA
Settler tank and lids
Kombalda Nickel Smelter
Nickel
WA
Process equipment in the sulphuric acid plant
Kalgoorlie Nickel Smelter
Nickel
Kalgoorlie, WA
Electrostatic mist precipitators, scrubber
BHP Billiton, QNI
Nickel
Yabulu, QLD
Leach tanks, linings in the stage 2 organic running tank and the cobalt sulphate discharge storage tank, lining o gas cooler condensers
Sunmetals
Zinc
Townsville, QLD
Cooling towers, grating
Xstrata
Zinc and lead Mt Isa, QLD
Froth crowders or fotation tanks
Nyrstar
Zinc
Leach reactor tanks and wash down tanks, electrolytic cells, spent tanks, launders, cooling towers, tank covers, cell bearer, bafes or tank, copper sulphate reactor tanks, mercury removal towers, oreshore stacks, pipework, precipitators, concrete tank linings, tanks, agitator blades, segmented clarier covers, tank, dampeners, butterfy valve, gas cooling towers and internals.
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Hobart, TAS
Table 5
Current composite components in Australian chemical processing plants
End User
Industry
Component
Ferro Corporation
Ammonium and sulphur products
Sieve tray scrubbing tower
Nuarm Chloralkali Plant
Chloralkali plants
Sodium hypochlorite storage tanks, chlorine headers, chlorine scrubber, anolyte tank
Incitec Pivot
Sulphuric acid and ertilizer
Settlers, pipes, radial fow scrubber, 2 gas cooling towers, ducts, drying tower inlet maniold, 8 electrostatic mist precipitators
Alcoa
General chemical
Tank
Australian Chemical Company
General chemical
Mist eliminator vessel or copper roaster
NSW Brickworks
General chemical
Freestanding insulated ume stack
Chemplex efuent treatment plant
General chemical
Pipework
Feld Proctor Gamble
General chemical
Tank
ICI Operations
General chemical
Tank
Koka Chrome Ind. Co Ltd
General chemical
Fume extraction ducting or plating plant
Metalok (S) Pte Ltd
General chemical
Plating line ume exhaust ducting
Pritcorp Sdn Bhd atty alcohol plant
General chemical
HCl vapour scrubber, glycerine reactor/settler, acidulated soap storage surge tank, tank
SCM Milenium Chemicals
General chemical
Titanium dioxide stack, chlorine scrubber
Tiwest
General chemical
Titanium dioxide stack, plant pipework
Toxide Group Services
General chemical
Ducting (ume extraction), stack (steel supported)
Unizon Singapore
General chemical
3600 cm vertical scrubber
Delta (BHP) EMD Plant
Manganese dioxide
Electrolytic cells, storage tanks or resh and spent electrolyte
Cold Rolling Sdn Bhd
Steel
Pipe (or pickle line), lining o steel prefux tank, lining o steel acid pickling tank
Tubemakers
Steel
Acid pickling tank
BHP Pellet Plant
Steel
Waste gas tower, ne scrubber, quench tower, ne scrubber demister, pre-quench scrubber
Minnehasa
Sulphuric acid
Mercury removal tower.
50 m Composite Fibre Conveyor. Modulus design or easy transport, assembly and dismantling. Capacity: 400 tone per hour Belt speed: 2 m/s Conveyor span: 24 m Number o spans: 2 Incline angle: 20 degrees Image courtesy of Wagners CFT Manufacturing Pty Ltd
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5
Technical perormance
This section is particularly aimed at people who are relatively unamiliar with composites, and or those who would not normally have considered them or applications. This section aims to raise awareness o the properties o composites and the actors to be considered in their use. Properties o composites and their raw materials given in this document are typical or average gures. It is important to use the actual product data rom the suppliers when designing products with composites.
5.1 Finite element buckling analysis o a large breglass nozzle under external pressure Image courtesy of Teakle Composites
Design
Composites are less successul when they are used to replace another material without considering its specic design. For example, composite pipes are less sti than metallic pipes, and thereore the supports need to the placed more closely together when installing composite pipes. Such actors have been considered in the various design standards or composites. It is important to engage composite designers and also have 3rd party verication where appropriate. Specialist designers can be contacted directly or through the composite manuacturer. Consideration o the various loads must be perormed diligently and by those who have the background and knowledge o the materials and structures. Section 10 o this guide contains details or a number o Australian composites design and engineering service providers.
5.2
Standards
Standards can be accessed at www.sai-global.com and other online stores. AS 3571
Plastics piping systems—glass-reinorced thermoplastics (GRP) systems based on unsaturated polyester (UP) resin—pressure and non-pressure drainage and sewerage; and pressure and non-pressure water supply
AS 2634 (obsolescent)
Chemical plant equipment made rom glass-bre reinorced plastic (GRP), based on thermosetting resins
AS/NZS 2566
Buried fexible pipelines
AS 2376.2 (superseded)
Plastics building sheets—glass bre reinorced polyester (GRP)
AS 2424 (superseded)
Plastics building sheets—general installation requirements and design o roong systems
AS/NZS 4256.3
Plastic roo and wall cladding materials—glass bre reinorced polyester (GRP)
AS/NZ 2924
High-pressure decorative laminates—sheets made rom thermoset ting resins—classication and specications
AS/NZS 3572
Plastics—glass lament reinorced plastics (GRP)—Methods o Test
BS 4994 (superseded)
Specication or design and construction o vessels and tanks in reinorced plastics
BS 6464
Specication or reinorced plastic pipes, ttings and joints or process plants
BS 6374-4
Lining o equipment with polymeric materials or the process industries, Part 4: specication or lining with cold curing thermosetting resins
BS EN 13121
GRP tanks and vessels or use above ground. Design and workmanship
BS EN ISO 14692
Petroleum and natural gas industries—glass-reinorced plastics (GRP) piping.
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5.3
Guides
ACI 440.1R-01
Guide or the design and construction o concrete reinorced with FRP bars, Committee 440, American Concrete Institute, Farmington Hills, MI. (May 2001), www.aci-int.org
ACI 515.1R
A guide to the use o waterproong, damp-proong, protection and decorative barrier systems or concrete. A guide or Flowtite GRP pressure and non-pressure pipe, engineering design guidelines, Iplex Pipelines Australia, www.iplex.com.au
5.4
Relativeperformanceofmaterials
Table 6 Composite properties* compared to other materials * The properties in this table are indicative only
Random glass composite
Bidirectional glass composite
Unidirectional glass composite
Aramid composite
Carbon composite
Aluminium
Mildsteel
Stainless steel
Fibre content by weight (%)
25–50
45–70
50–90
40–55
40–59
0
0
0
Density (g/cm3)
1.4–1.9
1.5–1.9
1.6–2.2
1.4
1.5
2.6–2.8
7.8
7.92
Tensile strength (MPa )
48–170
190–440
410–1730
345–2067
410–2700
80–480
200–800
190–552
Tensile modulus (GPa )
6–18
12–25
21–62
19–80
30–180
70
190–210
193–200
Compressive strength (MPa)
115–170
98–280
210–480
102–172
360
84–338
410–480
220–552
Compressive modulus (GPa)
6–9
8–17
-
16–19
-
-
-
-
Flexural strength (MPa)
90–340
200–450
690–1860
301
378
310
413
551
Flexural modulus (GPa)
5–17
9–23
27–48
15
28
69
207
193
In-plane shear strength (MPa)
62–96
55–83
110–140
-
-
276
-
-
In-plane shear modulus (GPa)
2.8–3.0
3.0–4.0
4.1–5.2
-
-
26–30
75–80
-
Tensile elongation (%)
1.6–2.1
3–4.5
2.4
2–2.6
1–1.5
2.5–23
22–37
40
0.15–0.52
0.19–0.35
0.3 (in bre 1.7 (in bre direction) direction)
34 (in bre direction)
140–200
43–50
110
1.4 (90° to bres)
0.8 (90° to bres) 23
11–14
16–18
Material
Thermal conductivity (W/m°C)
Coecient o linear thermal expansion (10–6/mm/°C)
18–33
9–16
9 (in bre direction)
–4 (in bre –0.5 (in bre direction) direction)
14 (90° to bres)
57 (90° to bres)
25 (90° to bres)
19
Figure 3. Stress strain curves o various materials A. Composites
s s e r t S
Strain
B. Common metals
Yeild and ultimate strength can be considered the same. Design is to ultimate using saety actor.
s s e r t S
5.5
Strain
C. Non-reinforced plastics
Yeild strength lower than ultimate. Design is to yeild using saety actor.
s s e r t S
Strain
Non-linear curves depending on polymer.
Servicelife
It is typical to speciy a minimum design lie o 20 years o continuous operation or composite process equipments in the mining industry. In other industries, such as underground water pipelines, a design lie o 100 years is more typical.
5.6
Mechanicalproperties
5.6.1
General
The mechanical properties o composites depend on a number o actors: •
resin-to-glass ratio
•
orientation o bres
•
method o abrication.
Composites are anisotropic, which means their properties vary with direction. For the mechanical properties discussed below, it is important to remember the values will be dierent in the direction o the bres to that normal to the bres. In terms o strength, composites have the greatest strength in the direction o the bres. In the direction normal to the bres, the resin and the bre-resin interace determine the strength, which may be one or two orders o magnitude lower than in the direction o the bres. Designers must thereore avoid stress systems that result in signicant loads normal to the bres. Detailed design literature and programs are available to estimate the eect o combinations o bres in dierent directions on the overall capacity o the composite. Calculations o the anisotropic properties o composites require the application o the theory o anisotropic elasticity or use o simpler means to obtain reasonable estimates. For this type o work, the reader is reerred to the various standards, guides and sotware programs available. 5.6.2
Strength
The rule o mixtures is used to calculate the strength o composites. This rule takes into account the relative ractions o the strength o both the bres and resin. Tensile strength The bres in composites are the principal contributor to the tensile strength o the component. The resin has signicantly lower strength and acts to bind the bres together and transmit the loads between them.
Anti-static cable tray supplied for the Blacktip Offshore Gas Production Platform Image courtesy of Exel Composites
20
Compressive streng th The strength o the resin has a much greater infuence on the compressive strength o composites than it does on the tensile strength. This is because the resin must have sucient compressive strength to prevent the bres rom undergoing local buckling or kinking under compression. The resin also helps to prevent ailure through longitudinal splitting. The resistance to buckling under compression can be improved at the design stage by incorporating edge fanges, double curvature and troughs.
Shear strength When subject to shear stress, the load-bearing abilities o the bres and matrix, and the extent to which stresses are transerred between them, aects the stiness and strength o composites. Most composites contain planes o weakness between the layers which can result in interlaminar ailure in shear. The property o interlaminar shear strength describes this behaviour. Composites made rom abrics which have some bres in the z direction (through-wall thickness), such as stitched cloths or chopped strand mat, are more resistant to interlaminar ailure than composites made rom abrics without bres in the z direction. Flexural strength Flexure/bending involves a combination o tensile, compressive and shear orces. At a simple level, the tensile, compressive and shear properties o the materials can be used in the design or fexure. However, fexural strength is seldom the limiting criterion in composites, as stiness more oten dominates the design. Stiffness The stiness o composites is low compared to steel, although carbon brereinorced composites are an exception. Since the tensile strength-to-weight ratio o composites is high and stiness low compared to steel, stiness tends to be the key determinant in structural design with composites.
Flowtite™ GRP Pipe (Continuous Filament Wound) installed in South-East Queensland’s western corridor recycled water pipeline Image courtesy of Iplex Pipelines Pty Ltd and Fibrelogic™ Pipe Systems
The stiness o composite parts can be increased by: •
•
•
•
•
selecting bres with a higher elastic modulus (e.g. carbon bres) sandwich construction. Since stiness is a unction o thickness, cores can be incorporated into a composite to provide rigidity, while keeping the weight low localised increase in thickness, or example, progressive thickening along a local edge or fanging along the edge o a panel ribs can be incorporated into the reverse side o the part compound curves or local corrugations. A olded plate construction can be used to achieve the required stiness rom the overall geometry o the structure.
For most composites with more than about 50 per cent volume o bres, the stiness in tension is dominated by the bres, and the resin contribution is insignicant. 5.6.3
Fatigue
Fatigue is the progressive damage that occurs when a material is subject to cyclic loading and when the stress values o each cycle are less than the ultimate stress limit. For example, in the mining and chemical industries, tanks and process vessels with internal agitators can be subject to constantly imposed stress cycles and are thereore susceptible to atigue. The atigue behavior o steel tends to involve intermittent propagation o a single crack, while the material close to the crack is virtually unchanged. In contrast to this, cyclic loading o composites results in the ormation o many micro-sized cracks. Since the small cracks in composites are spread uniormly in the material rather than concentrated in a single area, a greater area o material is involved in resisting atigue ailure. Furthermore, as the ormation o each small crack absorbs energy, composites tend to have good atigue resistance compared to most metals. However, as damage accumulates, a critical point is eventually reached at which the material can no longer sustain the applied load and ailure occurs. To improve the atigue resistance o composites, resins which are tougher and have greater resistance to micro-cracking should be used, and the amount o voids and other deects in the laminate should be minimised. It is also important to ensure the load normal to the direction o the bres is minimised.
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