E-Glass is of lower strength and stiffness than the other fibres being considered, but is also considerably lower in cost. The fibre is denser than the alternatives and resulting structures reinforced with E-Glass will therefore be considerably heavier than those reinforced with higher performance fibres. Typically an E-Glass structure may be over twice the weight of a carbon or aramid reinforced composite structure, although this will still be dramatically lighter than a conventional structure in steel, concrete or even aluminium.

The fibre is electrically non-conductive and offers good corrosion resistance. It should be noted however, that the fibre can be attacked by strong alkaline solutions and needs to be protected with suitable resins &/or surface tissues in such situations.

Aramid fibres such as Kevlar offer very high tensile strengths and relatively high elongation to failure. This results in them being very good for absorbing large amounts of energy, for example in structures subject to impact forces. The fibre has very low density and therefore results in extremely lightweight structures. Most FRP materials exhibit slightly lower strengths in compression than tension, but this is particularly true for aramid reinforced FRP and any areas subject to significant compressive stress will require very careful consideration if reinforced with aramid fibres. Aramid fibres have a negative coefficient of thermal expansion.

Carbon

Carbon fibres are available in a wide range of grades offering different mechanical properties. All carbon fibres offer relatively high strengths and stiffness, but are brittle and fail at relatively low strain levels. They have a negative coefficient of thermal expansion.

Carbon fibres are the only fibres considered here that are electrically conductive. In many applications this may not be relevant but may be critical in others, for example near railway power lines or for producing radomes. It may also be necessary to consider the possibility of electrolytic corrosion in conjunction with other materials. Carbon exhibits a very high electrolytic potential and behaves as a noble material in the galvanic series. While it will not corrode itself, it may cause galvanic corrosion in other materials particularly aluminium.

Polyethylene

Polyethylene (PE) fibres exhibit high tensile strengths, but lower modulus than other high performance fibres such as aramid or carbon. They also exhibit very low compressive strengths. They can provide very high levels of impact strength and are being used in applications such as lightweight armour plating. PE fibres have a highly negative coefficient of thermal expansion.

The mechanical properties of a FRP laminate are highly dependent on manufacturing techniques, quality, fibre volume fraction, control of fibre direction and straightness etc. The style of reinforcement may also affect the resulting laminate properties. For example a heavy woven fabric may result in significant crimp in the fibres lowering both strength and modulus values.

Mechanical properties are highly dependent on the quantity of reinforcing fibre contained within the laminate, which is generally expressed as either a volume or mass fraction, or a resin:fibre ratio by weight, which are related as follows;

The possible range of fibre contents within a laminate will be dependent on the manufacturing process. Higher fibre contents generally result in higher mechanical properties, but thinner laminates, resulting in a similar load carrying capability for a given quantity of reinforcement.

It should also be noted that hybrid reinforcements can be produced by mixing different types of fibre within a laminate.

The majority of FRP composites fail in a brittle manner in tension and may possess significantly different properties in compression than they do in tension. The brittle nature also results in greater sensitivity to stress concentrations, notches, holes etc than may be the case with ductile materials such as structural steel. Despite the brittle failure mode, FRP composites will often give signs of being overstressed before catastrophic failure, in the form of acoustic emissions, crazing, cracking, whitening and delamination.

Typical E-Glass Laminate Properties

E-glass is the most commonly used reinforcement for FRP composite structures in the construction industry and a wide variety of laminate properties may be produced. Typical properties are shown in Table 2 compared to steel and concrete.

Unidirectional (UD) Laminate Properties

Typical laminate properties are shown in Table 3, for a variety unidirectional reinforcement materials with a fibre volume fraction of 40%.

Variation of UD Properties with Fibre Direction

The properties presented in Table 3 are for unidirectional (UD) laminates in the fibre direction. It must be understood that these properties can reduce dramatically in other directions and as most realistic structures will experience loads in several directions it is important to be able to predict the laminate properties in other directions.

Laminate theory has been developed for predicting properties in other directions and for analysing multidirectional laminates. Laminate theory is well covered in several textbooks and many laminate analysis computer programs are available.

The prediction of laminate strength is more complex than the prediction of elastic properties, but several failure criteria have been developed and incorporated into laminate analysis programs. As an example of the increased complexity of analysing FRP composite materials compared to conventional isotropic materials such as metals, the Tsai-Wu failure criterion may be compared to the well-known von-Mises failure criterion commonly used for predicting the failure of metallic structures;

Von-Mises Failure Criterion

σ_O² = σ_x² - σ_x σ_y + σ_y² + 3 σ_xy²

σ_x (1/X_t + 1/X_c) + σ_y (1/Y_t + 1/Y_c) + σ_x² / (X_t X_c) + σ_y² / (Y_t Y_c) + τ_xy² / S² + F_xy σ_x σ_y = 1

Full definitions of these failure criteria are contained in the references at the end of this section.

Fortunately, the analysis of FRP composite laminates is easily automated and several laminate analysis software programs are available.

Interlaminar Properties

One of the most significant differences between FRP composites and metals is that the through-thickness or interlaminar properties of FRP composites can be significantly lower than their in-plane properties. This can be of particular importance around joints or in highly curved areas where significant through-thickness stresses may be generated.

Interlaminar properties are highly resin dependent, but interlaminar strengths can often be lower than the strengths of the resin alone, due to stress concentrations around the fibres. They are also dependent on laminate quality, reinforcement type, volume fraction and style.

Interlaminar shear strengths are easily tested with a short beam shear test and values are often quoted by resin suppliers for typical laminates. Interlaminar shear strengths may typically vary from 10 MPa for polyester laminates to 80 MPa for high quality epoxy laminates.

Interlaminar tension strengths can also vary considerably, with typical values around 10 MPa for polyester laminates.

Fatigue Strength

Fatigue strength is highly dependent on resin properties and laminate quality. Fatigue failure often occurs progressively, starting with fibre debonding and resin cracking. It should be noted that FRP laminates can be prone to fatigue failure in compression as well as in tension, in contrast to metals which will generally only develop fatigue cracks in tension.

Creep and Stress Rupture

FRP composite structures subject to long-term loading can suffer from creep and stress rupture and allowance for this must be made during design. The creep properties are highly dependent on resin properties, fibre types and environmental conditions such as temperature and humidity.

Conclusions

There is a vast amount of published data available regarding the engineering properties of FRP composites, which may be very useful during initial design. Once basic laminae data has been established propriety software can be used to predict the properties for a variety of laminate configurations, enabling highly optimised structures to be produced.

However, due to the almost limitless possible combinations of fibre and resin types and laminate configurations it will often be necessary to confirm laminate properties with coupon testing.

Special care will be required during the design and analysis of FRP composite structures due to the low through-thickness properties and the brittle nature of tensile failure.

Further Reading

DSM, Dyneema Fibre Properties.

Du Pont de Numours International SA, Switzerland, Kevlar Fibre Properties.

Toray Industries, Japan, Carbon Fibre Properties.

Hollaway L (1994), BPF Handbook of Polymer Composites for Engineers.

J A Quinn - Composites Design Manual, 2^nd Edition, James Quinn Associates Ltd, Liverpool, 1998.

Link to NPL site for details of CoDA program www.npl.co.uk/cog/

The CoDA program enables preliminary design analysis of components or sub-components with plate or beam geometries to be quickly and easily undertaken. CoDA can synthesise the properties of composite materials, laminates and sandwich structures from fibre and matrix, or material databases.

Outline

Glossary

Definition

Manufacture

Properties

Design

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