Freshly drawn fibres have greater tensile strength than fibres that have been exposed to the atmosphere because surface cracks develop gradually on exposure to the atmosphere or to any other gas or liquid above the surface. So the fibres need something to protect them from cracks forming and propagating.
Resins are exactly the component needed to do that. The three most commonly used resins for Fibreglass are Polyester, Vinyl ester and Epoxy. There are of course big differences between the properties of different resins.
The most commonly used resin is polyester, a thermosetting plastic, meaning that it sets solid when heated and will not melt when subjected to heat again (that type of plastic is thermoplastic). It is when they are solid they have the properties we are looking for. With fracture- energy of approximately 100 Joules per m2 they are 10 to 50 times tougher than glass. But they've only got a tensile strength of about 5-8MPa and are therefore 20 times weaker than glass.
As said, Fibre glass is the combination of glass fibres embedded in a resin to combine the properties of it’s constitute. The job of the resin (matrix phase) is to bind the glass fibres (disperse phase) together and share stress amongst the fibres. Also the resin protects the fibres from crack formation.
The structure of fibre glass is designed such that if one fibre breaks within the resin the stress is taken up by the other fibres.
Fibreglass materials are said to be anisotropic, that is, their strength, stiffness and other mechanical properties depend on the orientation of the fibres. Therefore, it is difficult to specify figures for a composite material because unlike a metal there is more than one element present, the resin and fibres can be present in varying amounts and the fibres can be deliberately aligned to give strength is one direction, so the tensile or compressive strength measured in another direction would give an inaccurate measure of the material’s properties.
While researching I noticed on many websites and books different figures had been specified. For example for tensile strength the Advancing Physics CD-Rom, has a figure of 240 MPa for Strength, whereas the table from an Internet source gives a figure of 340MPa.
High strength: Fibre glass has superior properties than to those of it’s constitute materials. It provides superior tensile, flexural and impact strength behaviour.
Light weight: A fibre glass component weighs roughly half its steel counterpart for bearing the same load. In the case of unidirectional reinforced sections, the weight reduces to 20% of a similar steel section.
High strength to weight ratio: It is amongst the strongest commercial materials available. For the same weight Fibre glass is stronger than concrete, steel and aluminium. Stiffness can be obtained without substantial increase in weight
Design flexibility: Fibre glass can be designed in many shapes - corrugated, ribbed and contoured, in variety of shapes
Dimension stability: Fibre glass components hold form and shape even under severe mechanical and environmental stress
Excellent Weather ability and Chemical Resistance: Fibre glass has excellent resistance to moisture and chemicals over large temperature range. It does not rust, rot, corrode or swell and is, maintenance free
Light Transmission: Fibre glass panels can be made translucent, something unique for structural materials. This property ensures transmission of natural light thereby saving power cost.
I have compared Fibreglass (listed as GFRP) to Aluminium, Mild Steel, Oak, Pine and carbon fibre which are all common materials used in a wide range of products world wide.
The graphs give a brief indication of these material’s properties, an important conclusion that when compared to carbon-fibre, fibreglass is a practical alternative for wood and metal, as its cost is more comparative to these materials, whereas its mechanical properties, particularly its relatively low density and high strength, which are unusual to see in materials other than composites.
Fibreglass is now the most popular material for building boats of almost every type. One of the main advantages of this material is their high strength to weight ratio.
The resin used is unsaturated being a plastic which mean it will not corrode. Boats can be made from one continuous piece of fibreglass with no joints or gaps to allow water into the boat.
Fibre glass also has many advantages to the making of car bodies. It has a strength to weight ration greater then steel that is traditionally used for car bodies. Other benefits for its use in car bodies is that it can be moulded easily into any shape, it can be painted on easily and it has excellent weather ability.
Car bodies can be made lighter then when using traditional steel with the same strength. This brings benefits like improved acceleration, braking and even fuel economy.
