Miriam Cerutti, tech. director at QI Composites Italy
More and more often we hear that a boat had its mast hit by a lightning during a storm. The most of events happen when the boat is in the harbour, and it usually happens to the boat with the tallest composite mast. Why? Not by chance, of course, but due to the physics laws.
A lightning is nothing more than a transition of electric charge from the thundercloud to the ground, happening in a quick and violent way: the electric discharge.
According to the electromagnetism laws, the electric charges distributed on the surface of a conductor concentrate on the surface with a smaller radius of curvature, i.e. the sharpest one, as a boat mast, for instance. While the electric charges move within conducting materials (as metals and carbon fibre), their motion is impeded in non-conducting materials (insulators such as wood). In presence of a bigger charge, the electric field is more intense, thus the potential electrostatic energy is higher too: an electric discharge will get much more likely out of these areas with higher concentration of energy.
The thunder will preferentially run along the way with lower specific electric resistance, until this way is sufficient to transfer the whole discharge.
A composite material mast is a monolithic laminate of carbon fibre impregnated with epoxy resin. Even if carbon is a good conductor, the carbon + epoxy resin composite is a bad conductor compared to copper or steel, thus the mast is not the preferential way for an electric discharge such as the lightning.
It can though happen that the grounding is not carried out correctly, or that it’s undersized for the discharge: thus the lightning will choose at least partially another way.
And what can happen to a composite mast if it’s crossed by a lightning discharge?
First of all it would be good to try to understand which way(s) the electric discharge has followed. The electric discharge goes through the way with the lower resistance, i.e. through the best electrical conductors.
If the electric discharge runs through the mast structure, the composite laminate overheats during and along its path; resin is in effect an insulating material, and the transition of an electric discharge causes resistive heating. The overheat can modify the mast building materials: especially the resin – i.e. the composite matrix, the material keeping together the carbon fibres – modifies its properties with the temperature increase.
- If the maximum temperature reached by the resin is below the glass transition temperature (Tg) – i.e. lower than 100-120°C – the resin remains in its glossy state and its mechanical properties remain as expected. In this case damages on composite caused by the lightning strike are not expected.
- If the maximum temperature reached by the resin exceeds the Tg, but doesn’t reach the combustion temperature – around 200-250°C – the resin state changes and becomes gummy, with a consequent drastic reduction of the mechanical properties. At this state, the laminate inner stresses can cause delaminations, because the resin mechanical properties are no longer sufficient to keep together the carbon fibre layers. After the electric discharge has run through, the material chills and the resin gets back to its original glassy state, but the delaminations are still there and must be repaired.
- If the laminate reaches the resin combustion temperature – i.e. if the temperature is higher than 200-250°C – the resin burns. Combustion is a non-reversible process, and the laminate must be restored where the resin has been damaged.
In summary, the possible damages caused to the mast by a lightning discharge are:
- resin deterioration due to overheating
Which tests can detect these damages?
- Ultrasonic testing detects possible delaminations.
- Chemical tests on the resin – such as DSC and FT-IR – enable to verify its properties and identify the laminate areas where the matrix deteriorated by combustion.
FT-IR test, results
The picture shows two samples of the same laminate: they look the same, but the one highlighted in magenta has been overheated in oven at 350°C for 15 minutes.
FT-IR test enables to identify what the eye cannot see. The following graphs show the FT-IR spectra observed on the same sample at three steps of maximum temperature reached:
Sample heated at 250°C
Sample heated at 350°C
The first graph represents the complete FT-IR spectrum (between 4000 and 650 cm-1) in the three cases; the second graph is a zoom on the most interesting area (between 1800 and 650 cm-1).
The graphs show the characteristic peaks of epoxy resin. It’s clearly visible that overheating destroys molecular bonds: the characteristic peaks get lower until they disappear, while the noise increases until it reaches an indefinite spectrum, except for a wide jagged peak between 1700 and 1600 cm-1 in the third case (see area highlighted in yellow).
But what is FT-IR test, and what else can it investigate?
It’s not magic! The Fourier Transform Infrared Spectroscopy is a chemical non-destructive testing method to identify materials through the analyses of the molecular bonds vibrations. It’s based on the absorption of the infrared radiation by the materials. A component (spectrometer) focuses the infrared radiation on the sample in order to measure both the wavelengths absorbed by the material and the absorption intensity. A spectrum is thus produced through a mathematical operation named Fourier transform. Spectra can provide both qualitative and quantitative information. The wavelengths absorbed by the sample are characteristic of the chemical groups present in the sample. The absorption intensity at a determinate wavelength indicates the concentration of the chemical group responsible for the absorption.
Q.I. Composites lab is equipped with a portable Fourier transform spectrometer designed for in-situ applications and non-destructive analyses. Polymers, liquids, solids, gels, composites, paints, coatings can be tested by this instrument.
In the nautical field, another interesting topic is evaluating the actual cleanliness of the detaching agents before the lamination, or evaluating, after process problems, whether a delamination has been caused by detaching or contamination residue.
For instance, in the following case the release agent corresponds to the blue curve, and the green curves are the spectra of an epoxy-based pre-preg laminate. It’s shown that the surface of that laminate isn’t contaminated, because the absence of the peak at 740cm-1 in the green spectra shows that there’s no trace of contaminant.