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MATERIALS AND PROPERTIES 

The term fibre reinforced plastic covers a wide range of materials of very varied properties and costs. At one extreme there are high performance continuous aligned fibre composites, such as carbon fibre reinforced plastics CFRP. They possess high stiffness and strength but with the disadvantage of high relative cost both for raw materials and possibly fabrication. At the other extreme there are materials such as sheet moulding compounds or glass mat thermoplastics which contain non-aligned short fibres and fillers and have the advantage of lower cost and easier fabrication but much lower properties. Further information is available in the VIRCON tool.

One of the first steps in any design exercise is the consideration of candidate materials. For fibre composites this requires an assessment of each constituent phase and usually focuses on the selection of matrix and reinforcement. In addition to characteristic mechanical. properties, equal attention must be given to compatibility and processability. Clearly these are interrelated as the processes for forming the material and fabricating the structure occur concurrently. Other materials may also feature at this stage depending on the application. These may include fillers, stabilizers, pigments, fire retardants, etc. In almost all cases of structural application the fibre acts as the primary load bearing constituent and the matrix serves as a medium of load transfer on to those fibres. These are the basic mechanics of a composite material. The interface between fibre and matrix is therefore vitally important as this transfer occurs through shear at this connection. A further function of the matrix is to protect the interface and the fibres from the action of any environmental effects. For many components the role of the matrix may not be entirely non-structural, however, as it cannot always be assumed that all loads will act in the direction of the fibres. In the non-fibre directions properties become strongly influenced by matrix characteristics.

During fabrication the materials can be handled in one of two ways: on-line impregnation or use of preimpregnated tows. In the former case the matrix is used in a liquid form, or converted into such by the use of a solvent or by melting, and the fibres are wetted out during component manufacture. Preimpregnated tows (prepregs) on the other hand are fibres that have been combined with matrix in a preliminary processing operation which can then be fabricated into a final component form. This has certain advantages as it eliminates much of the chemistry from the component fabrication, but can limit flexibility and is usually more expensive. 

Whatever choice of matrix and reinforcement is made, the key point is that they are selected as a system. It is only by considering constituents in this way that the full design potential can he realised. Further information is available in the MATDAT tool. 

Matrices and Resins 

In general terms a matrix can take the form of almost any material. However, those that have attracted most interest are those based on polymeric systems. New materials based on metals and ceramics are becoming available, but are still in their formative stages of development. Polymers used as matrices can be one of two types. The first, and most common, are of thermosetting character where solidification from the liquid phase takes place by the action of an irreversible chemical cross-linking reaction. This usually occurs as a result of the addition of other chemicals to initiate and accelerate the reaction and may involve the application of heat and pressure. The second type of polymers are thermoplastic in nature and forming can be carried out as a result of the physical processes of melting and freezing. Generally speaking, these reactions are reversible. 

The type of resin matrix will govern the details of the manufacture technique employed. For example, some thermosets are sufficiently fluid to allow processing without further modification, whilst others require the application of heat or the use of diluents to lower viscosity levels. Prepreg materials, where the resin which is already incorporated and has been allowed to react to an initial stage, can be handled as a solid feedstock which is then consolidated through the action of pressure and temperature. The fabrication of thermoplastics is carried out primarily through the action of heating and cooling. Whatever the application, the selection of matrix cannot be divorced from either design or processing. 

Unsaturated polyester resins 

Polyesters are, certainly in tonnage terms, probably the most commonly used of polymeric resin materials. One of their major advantages is the ability for cure at room temperature. This allows large and complex structures to be fabricated where an oven cure would not be practical. Essentially they consist of a relatively low molecular weight unsaturated polyester dissolved in styrene.

Structure of common thermosetting resins

Curing occurs by the polymerization of the styrene which forms cross-links across unsaturated sites in the polyester. A good degree of chemical resistance gives them wide application. A point to note is that the curing reaction is strongly exothermic (see figure below), and this can affect processing rates as excessive heat can be generated which can damage the final laminate. Shrinkage on cure (approx. 7-8%) can also be a problem.

Typical exotherm for a polyester cure

Because of the popularity of these systems, a family of resins has been developed to offer specific properties. Amongst the most important of these are those tailored for chemical resistance. For example, alkali resistance can be enhanced through the use of the so-called bisphenol modified resins where the number of sites for alkali hydrolysis is reduced to a minimum. A further system related to the polyesters, in that diluents such as styrene are used, is the vinyl ester family. Here the unsaturation occurs at the ends of the polymer chain, giving good chemical resistance and comparatively large strains to failure. In all these cases additives may be used to impart certain characteristics. The most notable are those for fire and flame retardance, and ultraviolet absorbers to improve weathering resistance.

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Epoxy resins

These resins are those most often used for advanced structural applications. They are generally two-part systems consisting of an epoxy resin and a hardener which is either an amine or anhydride. A wide variety of formulations is available to give a broad spectrum of properties after cure and to meet a diverse range of processing conditions. The higher performance epoxies require the application of heat during a controlled curing cycle to achieve the best properties. Prepregs can be made with epoxies where the fibres are impregnated with resin which is then partially cured. The objective of prepreg manufacture is to derive a material that can then be used in a fabrication environment, has an acceptable shelf life and good tack. This last property is important as it permits good adhesion between adjacent layers during assembly operations. Compared with polyesters, epoxies tend to demonstrate better mechanical properties, better performance at elevated temperature (depending on cure cycle) and a lower degree of shrinkage (typically 2 - 3%).

Property	Polyester	Epoxy
Specific gravity Rockwell Hardness Impact Strength (Izod J/m) Thermal conductivity (W/m/K) Thermal expansivity (x105 /°C) Specific Heat (kJ/kg/°C) Volume Resistivity (Ohm cm) Dielectric Constant (@60 Hz) Tensile strength (MPa) Flexural Strength (MPa) Tensile Modulus (GPa)	1.1 - 1.5 M70 - M115 16-32 - - - 1015 3.0 - 4.4 50-75 60-160 3.1-4.6	1.2-1.3 M100 - M110 8-80 0.17-0.21 5-8 1.25-1.80 1017 2.5-4.5 60-85 125 2.6-3.8

 

Phenolic resins

Phenolics are of particular interest in structural applications owing to their inherent fire resistance properties. This is accomplished without the use of fillers which, although effective in inhibiting flame spread, tend to increase smoke generation. The relatively recent development of ambient temperature cure systems has provided a stimulus to their use in a range of applications. They possess two significant disadvantages, however: low toughness and a curing reaction that involves the generation of water. This latter effect can cause problems since if it remains trapped within the composite, steam can be generated during a fire which can then damage the structure of the material.

High temperature thermosetting resins

Because of the increasing level of interest for the use of composites at ever higher operating temperatures, there has been a continuing programme to develop organic matrices with good performance in this respect. There are now a number of options with reported survivable temperature capability in the region of 200°C. Examples include:

• Multifunctional epoxies.

• Polyimides.

• Bismaleimides.

• Polystyryl pyridenes.

• PMR (in situ polymerization with monomeric reactants).

All of these systems have glass transition temperatures in the range 180-400°C.

Most epoxy resin systems in common usage tend to have upper working temperatures around the 100-120°C range. Those rated for higher temperature applications are usually based on an epoxy novolac or a tetrafunctional resin with additions to control viscosity and toughness. Polyimides are a family of polymers with arguably the best upper working temperature attainable in a readily available polymer system ( ~ 300°C). They are, however, expensive, somewhat temperamental to fabricate owing to the chemistry involved and, when cured, prone to microcracking. Bismaleimides are a class of thermosetting resins of somewhat complex chemistry related to polyimides. Generally, these systems have a better upper working temperature than epoxies, though the cured resins are more brittle. Fabrication difficulties can also arise with the material in prepreg form as there is little tack. The polystyryl pyridine polymer was originally developed as an ablative system and hence has excellent high temperature properties with a glass transition temperature in the region of 280°C. Again, fabrication is difficult as, in this case, solvents must be used in a high temperature curing cycle. The last system cited, PMR, is particularly interesting as it involves a different approach in its formulation. As the term PMR suggests, a composite material is formed by impregnating fibrous reinforcement with monomers dissolved in a low boiling point alcohol. The monomers are unreactive at low temperatures, but react in situ at elevated temperatures to form a stable polyimide. The performance of some of the higher temperature matrices is summarized below.

Relative modulus for high temperature matrices

Relative strength for high temperature matrices

The processing of high temperature resin systems is not straightforward as they inevitably involve curing at high temperatures for extended periods of time. The processing cycle can be further complicated by the need to remove the solvent used to ease processing or the requirement to heat the material to reduce viscosity. Also, as a rule of thumb, increased temperature capability results in a loss of toughness which can affect performance or even give rise to cracking because of the residual stresses generated at high temperature curing.

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Thermoplastic matrices

The thermoplastic resin systems are fundamentally different from the thermosets in that they do not undergo irreversible cross-linking reactions, but instead melt and flow on application of heat and pressure. The table below shows details of thermoplastics in current use (or undergoing evaluation).

Thermoplastic matrix systems

They range from common materials, for example nylon and polypropylene, to those that have been specially formulated with particular regard to elevated temperature performance such as those based on phenylene (PEEK, PES, PEK-poly(ether ketone)) and aromatic groups (PPS). Properties of the more commonly considered thermoplastics are given in below.

	Density	Modulus GPa	Tensile Strength MPa	Elongation (%)	Fracture Toughness	Izod Impact
Polypropylene PP Nylon PA Polyphenylene sulphide PPS PEEK PEK Polysulphone PS Polyether imide PEI Polyether sulphone PES Poly amide imide PAI Polyimide PI	0.90 1.15 1.36 1.27-1.32 1.3 1.27 1.29 1.37 1.3-1.4 1.3-1.4	1.1-1.6 1.2-2.9 3.4 3.7 4.0 3.6 3.0 2.6 3.8-4.8 3.5	29-37 61-82 79 92 105 70 103 84 152-193 101-166	200-700 60-300 2-20 50 5 50-100 30 40-80 10-15 9-14	- - - 4.8 - 2.5 - 3 3.2-3.9 -	- - 21 83 - 69 40 84 80 21

 

The main advantages of thermoplastics over thermosets are indefinite shelf life, good toughness and the fact that processing is concerned with physical transformations only. There is no chemistry involved and therefore extended cure cycles are not necessary. As a consequence there is potential for rapid, low cost fabrication with simplified quality control procedures. Post-forming operations which provide scope for added flexibility in component design are also possible. However, there are attendant difficulties as the temperatures required for processing can be high), resulting in expensive tooling requirements where particularly complex shapes are required. Higher temperature performance is also seen as a potential advantage for thermoplastics. The figures below show the effect of temperature on mechanical property retention for a range of systems.

Flexural modulus of thermoplastic composites at elevated temperatures

Flexural strengths of thermoplastic composites at elevated temperatures

Thermoplastic composites are almost always processed in the form of prepreg materials. Impregnation of the fibres to form the prepreg can be difficult owing to high viscosities of the melt thermoplastic or the requirement to use high boiling point, polar solvents. As an alternative post-impregnated forms are available where the basic feedstock does not contain fully wetted fibres, but a system which is only physically mixed (see below).

Forms of thermoplastic matrix material

Full impregnation is achieved on subsequent processing.

Thermoplastics continue to be the subject of much development activity, particularly in the areas of fabrication science and property characterization, since there is some considerable progress required before the potential indicated by basic polymer properties is achieved in a composite. Should these developments be successful, a significant increase in the use of thermoplastics for a variety of applications can be expected.

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Fibres

Fibres are the dominant constituent of most composite systems, and one of the main objectives of any design should be to place the fibres in positions and orientations so that they are able to contribute efficiently to load-carrying capability. The most widely available fibre form for advanced structural applications is continuous tows. These produce highly anisotropic materials of very high stiffness and strength in the direction of the reinforcement. Away from this direction, properties tend to fall away rapidly until at the orientation perpendicular to the fibres they become similar to those of the matrix. It is common for individual plies of unidirectional material to be combined to form a more complex construction. Such a laminate may contain many individual layers, each at different orientations with respect to one another, the sequence of plies being determined by design considerations.

Fibre systems are also available in a number of other forms. Most common are woven cloths. These may have equal numbers of fibres in warp and weft directions and therefore equal properties in those directions, or have a degree of bias giving rise to a level of anisotropy. The type of weave can also be important as this determines aspects of processability, such as cloth drape over a curved surface (see below). Both cloths and tows can be preimpregnated, processed and then cured as a prepreg.

Forms of woven cloth

Discontinuous fibres can also be usefully employed to provide reinforcement to a matrix. Typical of these is chopped strand mat (CSM) which consists of chopped fibres about 30-50 mm long distributed in a random manner in a plane and held together with a resin binder. Such an arrangement provides relatively low enhancement of stiffness and strength, but the resulting laminate is effectively isotropic in the plane of the reinforcement. When considering fibre properties it is important to recognize that these may not be fully achieved within a composite. Fibre damage, misalignment and variations in volume fractions can all be deleterious to property values. Strength levels in particular can be dramatically reduced. In design, composite properties as opposed to fibre properties should be used wherever possible.

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Glass fibres

When glass is drawn into fine fibres its strength increases markedly over that of bulk glass. In the United Kingdom two types of glass were originally used for fibreglass manufacture. Now only one main type is produced commercially for the reinforcement of plastics - E-glass, which is a low alkali-containing borosilicate glass. Glass fibres account for around 90% of the reinforcement used in structural reinforced plastic applications. There are a number of different compositions all based on silicate glasses. The mechanical properties are not strongly dependent on composition, but chemical behaviour which is reflected in terms of durability and strength retention in corrosive environments is influenced by the details of the chemistry. Most continuous fibre for reinforcement purposes is manufactured from E-glass which is of low alkali content. S and R glass fibre compositions are designed to give very high modulus fibre and are used particularly in very high performance laminates associated with the aerospace industry. Cem-FIL AR (alkaline resistant) glass fibres are also produced which are used in the Civil Engineering industry for the reinforcement of cement. The Table below gives typical property values for different glass types.

Glass type	Density	Thermal expansivity (x10-6 °C-1)	Tensile Modulus GPa	Strength GPa                    Undamaged                      Strand from roving
A-glass   E-glass   AR-glass   S or R-glass	2.46   2.54   2.7   2.5	7.8   4.9   7.5   -	72   72   70-75   85	3.5   3.6   3.6   4.5	-   1.7-2.7   1.5-1.9   2.0-3.0

 

In addition to the types shown above, a variant of E-glass-E-CR glass-is becoming increasingly important. This has been specially developed to meet the requirements of acid corrosion. S-glass is noted for its high strength and is used in preference to E-glass where its added cost is deemed acceptable. Typical properties for glass fibres are given in the table above. An essential feature of glass fibre production is the application of size which coats the surface of the fibre. The primary purpose of the size is to protect the fibre from abrasive damage and to enhance the strength of the fibre matrix interfacial bond. The nature of the size can be tailored to ensure compatibility with the proposed matrix.

Typical mechanical properties of 60% unidirectional continuous fibre reinforced composites are shown below

Property	E - Glass	Boron	Kevlar 49	High Modulus Carbon	High Strength Carbon	Natural Fibre (Jute)	Carbon Steel
E11 (GPa)	45-55	200	76	180-300	110-150	45	200
E22 (GPa)	15-25	22	5.5	8	9-10		200
n12	0.3	0.3	0.34	0.3	0.3		0.33
G12 (GPa)	5-9	7	2	5-9	5		76
s1  (MPa)	700-1200	1400	1400	1000-1500	1500-2500	420	1300
s2  (MPa)	20-50	90	30	50	50-70		1300
t12  (MPa)	50-80	140	60	50-80	70
s1C  (MPa)	350-700	2800	280	>1000	1200
s2C  (MPa)	170-220	280	140	>150	>150
e1  (%)	1.8-2.2	0.7	1.8	0.4-0.8	1.1-1.8	1.0	>15
e2  (%)	0.4-0.5	0.45	0.6	0.6	0.5-0.7
r  (g/cc)	2.03	2.05	1.36	1.61-1.67	1.52-1.58	1.20	7.8
Specific Stiffness E11/ r	22-27	98	56	110-185	70-95	35	26
Specific Strength s1 /r	345-590	680	1030	600-910	900-1500	350	170

 

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Macroscopically, glass fibre behaviour at ambient temperature can be described with a linear stress/strain curve to failure. Under continuous loading a comparatively small (< 5% of elastic strain) viscoelastic deformation can also be measured. At higher temperatures creep effects can become more prominent. As would be expected from a brittle material the strength values are sensitive to flaws and stress concentrations and are subject to a significant size effect. This is the reason why fibres taken from rovings which have been mechanically handled have strengths much less than 'undamaged' filaments. Strength effects can be treated using statistical concepts. The probability of failure is strongly dependent on fibre length, i.e. the longer the fibre, the greater the chance of a critical flaw being present and hence the greater the likelihood of failure. A tow of nominally identical fibres will most likely fail below the mean failure stress of individual fibres because the load capacity of the bundle gradually deteriorates as a result of failure of the weaker fibres as the load increases. Catastrophic failure will occur when the load is reached where the breakage of just one more fibre reduces the effective cross-sectional area below that which can just support the current level of load. The figure below shows a plot of the dependence of fibre tow strength on single filament strength values. As can be seen, the strength of a fibre bundle approaches the mean strength of individual fibres with increasing Weibull modulus.

Dependence of fibre tow strength on single filament strength

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Carbon fibres

Carbon fibres are typified by a combination of low density, high strength and high stiffness. The characteristics of an individual fibre are determined by the degree of orientation of graphite planes achieved during manufacture. Currently there is a great range of materials available each with a different combination of properties shown below.

Carbon fibre properties

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Current developments are focusing on the requirement for increasing strength coupled with a capability for high strain to failure. As with glass, carbon fibres are susceptible to damage and require a size for protection and surface treatment to promote matrix adhesion.

One aspect of the behaviour of carbon fibres which is rarely reported is the nonlinear form of the stress/strain curve. All fibre types show a marked rise in stiffness with increasing strain. The figure below shows typical data.

Typical non-linear stress/strain curve for carbon fibre

In some cases the difference between initial and final modulus may be as much as 25%. The. shape of the stress/strain curve can be described by:

E_c = E₀ (1+ fe)

where E₀ is the modulus at zero strain E_c that at strain e, and f is an empirical factor dependent on fibre type.

The figure below shows the variation in modulus with strain for a variety of carbon types.

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Modulus against strain for Torayca carbon fibres

In general, the value of the parameter f increases with fibre modulus. Perhaps the more important issue arising from this effect is not so much the use of this effect in a design - often design strains are low in any case - but in the interpretation of values quoted in the literature. Values based on, say, a secant modulus may be higher than those that should be used. As already indicated current developments in carbon fibre technology are providing ever-higher values of stiffness and strength and, by and large, this can be replicated within a unidirectional lamina. A key point to note, however, is that as properties in the fibre direction improve, those transverse to the fibre tend to deteriorate. As most applications involve transverse loading to some extent, it may be difficult to utilise fibre properties fully as failure may occur in other directions due to secondary loading or even residual stresses generated in the fabrication process.

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Aramid fibres

Aramid fibres are available in two forms: low and high modulus. The difference between the two results from basic structural and orientation variations at the polymer chain level. Low modulus can be converted to high modulus material through an annealing process. The main advantage of aramids is their very low density (lower than glass or carbon), giving high values of specific strength and stiffness combined with excellent toughness. As a result of this latter characteristic the material is frequently used in applications where impact resistance and ballistic energy absorption are important. However, there are some difficulties with the material. Because of the details of the polymer morphology the fibre has low longitudinal shear modulus, poor transverse properties and low axial compression strength. These effects are reflected in resulting composite properties where, in bending and compression the response is nonlinear with relatively low ultimate strengths. As a result they are frequently used in a hybrid construction with other fibres such as carbon with the aramid providing toughness characteristics to the laminate.

Other fibre systems

The vast majority of applications using fibre reinforced polymers employ either glass, carbon or aramid fibres. However, other fibres are of interest:

Boron & Silicon Carbide

Boron and silicon carbide fibres were amongst the first fibres specially developed for advanced composites. They have a density similar to glass, but a tensile modulus up to six times greater. The production route is rather complex, involving chemical vapour deposition (CVD) onto a tungsten wire substrate and the high cost of this process means that the fibres are relatively expensive. Because of their large size and stiffness boron filaments cannot be woven into cloths or handled like other fibres, so they are usually processed in parallel arrays of single thickness preimpregnated sheets or tapes.

Polyethylene fibres

Fibres produced from ultra high molecular weight polyethylene (UHMWPE) are now available and have similar tensile behaviour to that of aramids. A very low density (- 30% lower than aramids) means that specific properties are considerably higher. Polyethylene has good chemical, abrasion resistance and low moisture absorption, but like aramids its composites display poor compression and shear behaviour. A further disadvantage is their low melting point (- 150°C) which limits applications to 130°C or less. Typical property values for aramid, boron and UHMWPE fibres are shown in the table.

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Property measurement

Owing to the complexity of the structure of composites and the unusual effects that can arise as a consequence, it is essential that properly formulated test methods are employed to measure property data. There is enormous potential for generating poor quality information which is misrepresentative of actual material behaviour. Many of the documented procedures available for composites are derived from those developed from monolithic polymers. Examples include:

• ASTM D3410 - compression.
• ASTM D3039; BS 2782 Pt 10 Method 1003 - tension.
• BS 2782 Pt 10 Method 1005 - flexure.
• BS 2782 Pt 3 Method 341; ASTM D2344 - interlaminar shear strength.
• ASTM D3518 -- in plane shear.
• ASTM D3479 - tensile fatigue.

In tensile tests it is common to use tabs bonded to the ends of specimens of either aluminium or GRP (± 45°), the latter being of particular usefulness for testing at elevated temperature to minimize thermal expansion mismatch. Wasting of specimens may be found to be necessary where strength values are to be obtained. Bending tests are also commonly used normally in the form of three-point bending on parallel-sided specimens. The figure below shows details of a typical tensile test specimen.

Test specimen for unidirectional tensile strength and modulus

There is a variety of methods for measurement of in-plane shear properties. One test consists of tensile loading of a ± 45° laminate; the shear modulus being determined by a subsequent analysis procedure. Perhaps the most difficult property values to determine are those in compression. The low modulus of many composite systems means there can be an overriding tendency for specimens to buckle under load. This has dictated the development of a range of devices to support material samples whilst under load.

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Compressive test fixture

Accurate determination of fibre volume is vitally important in the characterization of composites and there are two methods available:

• ASTM D3171 - acid digestion.
• ISO 1172 -combustion.

It should be noted that the latter technique cannot be used for carbon- or aramid-based fibre materials owing to the possible destruction of the fibre phase during test.

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The NPL standards selector allows structured guidance to find the correct standard test method, with supporting information on scope, use, alternative standards, related information.

Standards from ISO TC 61/SC 13 are given below.

Composites and reinforcement fibres ISO 527-4:1997 Plastics -- Determination of tensile properties -- Part 4: Test conditions for isotropic and orthotropic fibre-reinforced plastic composites ISO 527-5:1997 Plastics -- Determination of tensile properties -- Part 5: Test conditions for unidirectional fibre-reinforced plastic composites ISO 1172:1996 Textile-glass-reinforced plastics -- Prepregs, moulding compounds and laminates -- Determination of the textile-glass and mineral-filler content -- Calcination methods ISO 1268:1974 Plastics -- Preparation of glass fibre reinforced, resin bonded, low-pressure laminated plates or panels for test purposes ISO 1268-1:2001 Fibre-reinforced plastics -- Methods of producing test plates -- Part 1: General conditions ISO 1268-2:2001 Fibre-reinforced plastics -- Methods of producing test plates -- Part 2: Contact and spray-up moulding ISO 1268-3:2000 Fibre-reinforced plastics -- Methods of producing test plates -- Part 3: Wet compression moulding ISO 1268-5:2001 Fibre-reinforced plastics -- Methods of producing test plates -- Part 5: Filament winding ISO 1268-7:2001 Fibre-reinforced plastics -- Methods of producing test plates -- Part 7: Resin transfer moulding ISO 1887:1995 Textile glass -- Determination of combustible-matter content ISO 1888:1996 Textile glass -- Staple fibres or filaments -- Determination of average diameter ISO 1889:1997 Reinforcement yarns -- Determination of linear density ISO 1890:1997 Reinforcement yarns -- Determination of twist ISO 2078:1993 Textile glass -- Yarns -- Designation ISO 2113:1996 Reinforcement fibres -- Woven fabrics -- Basis for a specification ISO 2558:1974 Textile glass chopped-strand mats for reinforcement of plastics -- Determination of time of dissolution of the binder in styrene ISO 2559:2000 Textile glass -- Mats (made from chopped or continuous strands) -- Designation and basis for specifications ISO 2797:1986 Textile glass -- Rovings -- Basis for a specification ISO 3341:2000 Textile glass -- Yarns -- Determination of breaking force and breaking elongation ISO 3342:1995 Textile glass -- Mats -- Determination of tensile breaking force ISO 3343:1984 Textile glass -- Yarns -- Determination of twist balance index ISO 3344:1997 Reinforcement products -- Determination of moisture content ISO 3374:2000 Reinforcement products -- Mats and fabrics -- Determination of mass per unit area ISO 3375:1975 Textile glass -- Determination of stiffness of rovings ISO 3597-1:1993 Textile-glass-reinforced plastics -- Determination of mechanical properties on rods made of roving-reinforced resin -- Part 1: General considerations and preparation of rods ISO 3597-2:1993 Textile-glass-reinforced plastics -- Determination of mechanical properties on rods made of roving-reinforced resin -- Part 2: Determination of flexural strength ISO 3597-3:1993 Textile-glass-reinforced plastics -- Determination of mechanical properties on rods made of roving-reinforced resin -- Part 3: Determination of compressive strength ISO 3597-4:1993 Textile-glass-reinforced plastics -- Determination of mechanical properties on rods made of roving-reinforced resin -- Part 4: Determination of apparent interlaminar shear strength ISO 3598:1986 Textile glass -- Yarns -- Basis for a specification ISO 3616:2001 Textile glass -- Chopped-strand and continuous-filament mats -- Determination of average thickness, thickness under load and recovery after compression ISO 4602:1997 Reinforcements -- Woven fabrics -- Determination of number of yarns per unit length of warp and weft ISO 4603:1993 Textile glass -- Woven fabrics -- Determination of thickness ISO 4604:1978 Textile glass -- Woven fabrics -- Determination of conventional flexural stiffness -- Fixed angle flexometer method ISO 4606:1995 Textile glass -- Woven fabric -- Determination of tensile breaking force and elongation at break by the strip method ISO 4899:1993 Textile-glass-reinforced thermosetting plastics -- Properties and test methods ISO 4900:1990 Textile glass -- Mats and fabrics -- Determination of contact mouldability ISO 5025:1997 Reinforcement products -- Woven fabrics -- Determination of width and length ISO 7822:1990 Textile glass reinforced plastics -- Determination of void content -- Loss on ignition, mechanical disintegration and statistical counting methods ISO 8516:1987 Textile glass -- Textured yarns -- Basis for a specification ISO 8605:2001 Textile-glass-reinforced plastics -- Sheet moulding compound (SMC) -- Basis for a specification ISO 8606:1990 Plastics -- Prepregs -- Bulk moulding compound (BMC) and dough moulding compound (DMC) -- Basis for a specification ISO 9163:1996 Textile glass -- Rovings -- Manufacture of test specimens and determination of tensile strength of impregnated rovings ISO 9353:1991 Glass-reinforced plastics -- Preparation of plates with unidirectional reinforcements by bag moulding ISO 9782:1993 Plastics -- Reinforced moulding compounds and prepregs -- Determination of apparent volatile-matter content ISO 10119:1992 Carbon fibre -- Determination of density ISO 10122:1995 Reinforcement materials -- Tubular braided sleeves -- Basis for a specification ISO 10352:1997 Fibre-reinforced plastics -- Moulding compounds and prepregs -- Determination of mass per unit area ISO 10371:1993 Reinforcement materials -- Braided tapes -- Basis for a specification ISO 10548:1994 Carbon fibre -- Determination of size content ISO 10618:1999 Carbon fibre -- Determination of tensile properties of resin-impregnated yarn ISO 11566:1996 Carbon fibre -- Determination of the tensile properties of single-filament specimens ISO 11567:1995 Carbon fibre -- Determination of filament diameter and cross-sectional area ISO 11667:1997 Fibre-reinforced plastics -- Moulding compounds and prepregs -- Determination of resin, reinforced-fibre and mineral-filler content -- Dissolution methods ISO 12114:1997 Fibre-reinforced plastics -- Thermosetting moulding compounds and prepregs -- Determination of cure characteristics ISO 12115:1997 Fibre-reinforced plastics -- Thermosetting moulding compounds and prepregs -- Determination of flowability, maturation and shelf life ISO 12115:1997/Cor 1:1998 ISO 13002:1998 Carbon fibre -- Designation system for filament yarns ISO/TR 13883:1995 Plastics -- Guide to the writing of test methods ISO 14125:1998 Fibre-reinforced plastic composites -- Determination of flexural properties ISO 14125:1998/Cor 1:2001 ISO 14126:1999 Fibre-reinforced plastic composites -- Determination of compressive properties in the in-plane direction ISO 14126:1999/Cor 1:2001 ISO 14129:1997 Fibre-reinforced plastic composites -- Determination of the in-plane shear stress/shear strain response, including the in-plane shear modulus and strength, by the plus or minus 45 degree tension test method ISO 14130:1997 Fibre-reinforced plastic composites -- Determination of apparent interlaminar shear strength by short-beam method ISO 15024:2001 Fibre-reinforced plastic composites -- Determination of mode I interlaminar fracture toughness, GIC, for unidirectionally reinforced materials ISO 15034:1999 Composites -- Prepregs -- Determination of resin flow ISO 15040:1999 Composites -- Prepregs -- Determination of gel time ISO 15100:2000 Plastics -- Reinforcement fibres -- Chopped strands -- Determination of bulk density ISO 15310:1999 Fibre-reinforced plastic composites -- Determination of the in-plane shear modulus by the plate twist method

 

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As examples of the results which can be obtained from a typical test programme (ISO 527 Pt 5), the table below shows elastic and strength data for Fibredux 914/glass reinforced materials.

Lay-up	Strength (MPa)	Modulus (GPa)	Poisson Ratio
[0]	1120 ± 44	52 ± 0.9	0.3
[90]	87 ± 14	21 ± 1	0.1
[0/90]	503 ± 44	33 ± 0.5	0.06
[± 45]	233 ± 4	18 ± 0.1	0.5

 

Further samples were prepared from E-glass rovings and a high strength carbon fibre. The resin used throughout was a standard di-functional liquid epoxide filament winding system. Fibre volume fractions were measured using gravimetric and acid digestion methods. Typically, fibre volume fractions were in the region of 60%. Tensile coupons were cut from plates using a water-cooled diamond wheel and care was exercised over minimizing edge damage. Woven GRP end tabs were bonded to the coupons using a room temperature curing epoxide paste adhesive. Specimen dimensions conformed closely with BS 2782: Part 3: Method 320E: 1976 and recommended CRAG procedures were followed. All tests were carried out under displacement control on a universal testing machine with a calibrated 25 mm ± 2.5 mm extensometer attached to each specimen. In addition a [0/90] strain gauge rosette was used to collect axial and transverse strains and therefore Poisson's ratio as a function of applied load. The stress/strain curves are also shown below.

Stress - strain curve for unidirectional GRP

Stress - strain curve for unidirectional CFRP

Stress - strain curve for transverse GRP

Stress - strain curve for transverse CFRP

 

Stress - strain curve for cross-ply GRP

Stress - strain curve for cross-ply CFRP

The unidirectional T300 samples exhibited a gradual increase in modulus with strain which yields a value of f = 15. This is of similar magnitude to data obtained from single fibre properties. For [0,90]s cross-ply laminates, transverse ply cracking was observed at approximately 0.5%. Failure occurred with little evidence of longitudinal splitting or delamination. The [0,90]s cross-ply GRP failure process was easier to observe than that for the carbon and began with transverse ply cracking at approximately 0.5% strain and this continued until a uniform crack density was established. A noticeable decrease in stiffness was observed after transverse cracking (first ply failure), unlike the carbon fibre reinforced plastic (CFRP) sample. A series of longitudinal splits was also observed to grow from the region of the end tabs until they covered the gauge length. These splits appeared to be less densely spaced than the transverse cracks (approximately 2-5 mm for the axial splits compared to 1-2 mm for the transverse cracks).

Failure appeared to be precipitated from edge delamination growth, usually in the region of the end tabs, which grew rapidly. The ensuing fracture resulted in the characteristic 'brush-like' appearance, typical for this type of material. The [±75]s angle-ply CFRP failed at a very low strain compared with the unidirectional laminate. Typically the failure strain was less than 0.5% with a certain degree of non-linearity. Stress/strain graphs indicated a slowly decreasing modulus. Failure occurred with little evidence of longitudinal splitting or delamination. The [± 75], angle-ply GRP failed at a very similar strain, although at slightly increased stress. Failure strain was again less than 0.5% with a slight degree of non-linearity.

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