The durability of the composites FRP is one of the main reasons of their use in long-term applications of structural elements in applications from spaceships, wings of airplane, sea ships until civil engineering, many times exposed all them in several environmental conditions. Of the aeronautics and Formula 1, it has tested that the FRP are thermally stable, resistant to the oxide and tolerant to the fatigue.
The reinforced polymers of fibres can be deteriorated because their use, but they can be designed for the worst demanding environmental conditions. During the last 50 years there have been constant improvements in the technology of the resins with an increasing trend to use FRP’s in constructions of high responsibility like bridges or buildings.
Causes of ruin
- Physical ageing of the polymeric matrix
- Heat cycles
- Ultraviolet light
- Chemical products (alkaline solutions, cleaning products)
- Biological attacks (fungi)
Physical ageing of the polymeric matrix
All polymers suffer an extremely slow ageing on their molecular structure, caused basely by the humidity and the temperature; and consequently they can become rigid and fragile. The effect of the ageing is less severe in the thermo stable composites, as the epoxy resin, that in the thermoplastic materials. However, in the majority of cases these effects are not critical because the senior transfer from load is found in the fibres, where the ageing of these is minimum.
When the resin is set forth to the humid air, absorption of water is produced by way of this, which can provoke changes in his properties; the absorption of water then reduces notably the temperature of vitreous transition that is the critical temperature of degradation. However, this doesn’t have to be a motive of worry in the case of applications with temperatures of work that are under the values that the temperature of vitreous transition (120ºC) can attain once altered by the absorption of water (about 80-90ºC). So it is necessary always to have present that the exhibition of the resin at high temperatures, above Tg, could provoke a significant degradation of this and the rigidity of the composite could be seen altered.
Of another side, contemplating the phase of fibres of the composite, the humidity can degrade fibres of glass and aramides, but does not have any known effect about the carbon fibres.
Cycles of heat
The ruin of the material caused for the exhibition to cycles of heat relates the influence of the thermal cycles on the properties of this.
Alterations of the properties of the carbon fiber or the epoxy resins in pure state due to the temperature are not known whenever the temperatures of vitreous transition are respected.
This effect can have importance if the composite contains a high number of gaps interconnected filled in with water, the integrity of the material can then be altered by the effects of the freezing. However, a composite cured correctly should have minor fraction of gaps of 1%.
Prolonged exhibitions to the sunlight can cause that the matrix become hardens and it is discoloured. The effect of the UV is self-screening and only the upper surface of the structure of composite is affected, therefore in wide composites the effect of the degradation is minimum. The carbon fibre and glass fibre are resistant to the UV but the aramides like the Kevlar are degraded faster exposed in the UV. It is advisable that the coatings resistant to the UV like the varnishes are used for protecting the surface of the decolouration and the breaks. The coating acts as protective layer to prevent that the surface of FRP is directly set forth in the UV, because of that the coating will require a good maintenance.
There are quite a lot of lacquers and disposable paintings, which have been developed by sea applications, in environments particularly demanding, that they could be used.
It is probable that there is sporadic contact with chemical products like cleaning products and solvents. The FRP normally have good resistance in the chemical products and are used frequently in the chemical industry. Only the glass fibres with alkaline solutions in touch with these products are degraded in a considerable way. The carbon fibres, however, are inert chemically and the epoxy resins offer a lot of chemical resistance to the unfrozen salts, to detergents and to solvents of cleaning and it has to be avoided to use acetone.
Deformation in constant tension (Creep)
The Aramides and the glass fibres are much more susceptible to break for deformation in constant tension than the carbon fibres because often they are subjected to the degradation induced by alkaline; that in the carbon fibres they have little or zero effect of degradation.
Level of break load to obtain 10% of provability from breaking
after 75 years of tension in normal ambient.
Composites have very good resistance to the fatigue. Because of that they are suitable for applications with many cycles of application like aviation, bridges and sea applications. Glass fibers have worse characteristics of fatigue that those of carbon and aramides. The direction 0º has a very big resistance to fatigue.
Behaviour to fatigue of the carbon fibre unidirectional compared with the aluminium
In the former graph the comparative one to fatigue can be appreciated of a composite by epoxy resin and carbon fibres (graphite) with other composites and with the aluminium, is appreciated clearly the influences of the fatigue in their resistance is less those the aluminium.
The biodegradation due to microorganisms is negligible, because the FRP are late decades to break down in the trash.
The galvanic corrosion takes place in a humid environment when two materials of different galvanic groups feel in touch. The corrosion progresses faster as more separate are the materials in the table of series galvanic that is shown next:
Galvanic series of the materials
The effect of the corrosion can be attenuated isolating the surfaces of contact among the pieces from different material applying protective coatings.
In the cases where metals and carbon composites are found in touch, the measures of prevention to apply are:
- Attempting to minimize the presence of humidity in the nearby zone to the contact
- Keeping the carbon composites isolated electrically of the adjacent metals.
Laminating an intermediate layer of glass fibres between the carbon and the metal. These measures are very advisable in zones of contact with aluminium because the galvanic corrosion is a lot he activates due to the difference of galvanic potential.