Page 321 - AWSAR 2.0

laminate thickness used in wind turbine blade stiffener can be as high as 60 mm. Although carbon fibers in the laminate are good electrical conductors, the matrix in which they are embedded is a poor electrical conductor. So, the overall electrical conductivity of CFRP comes down to approximately one-third of that of aluminium. As a result, during lightning strike, the electric current takes longer to travel through this material. This results in heat generation within the material (resistive heating), leading to ablation/ evaporation of the polymer and breakage of the carbon fiber. Sometimes, the heat generated can burn a significant part of the structure. Even if the surface damage is very insignificant to the naked eye, under continuous loading such as vibration, the former will grow and lead to catastrophic failure in the future.
Researchers had started to study about the consequences of lightning strike on aircrafts in early 1970s. A body called
Society of Automotive Engineers
(SAE) have characterized the
lightning strike for laboratory
studies. According to them,
the maximum electric current
during lightning can go as high
as 200,000 A. This definitely is
a very high amount of current,
and would cause damages
in the airplane/wind turbine
surface.
Generally, the composite
material would be covered with
copper mesh to avoid damages
due to lightning strike. Copper,
being a very good electrical conductor, allows the current to move very fast through the body and prevents heating of the material. This method is used by both aircraft and wind turbine industries. But, adding a new material would always increase the total structural weight, which is a big concern in the aircraft industry.
Ms. Dhanya T. M. || 297
In addition to that, if the electric current is too high, it can even melt and boil copper, which would expose the CFRP surface. In some of the wind turbines, the copper mesh protection was not completely effective. So, it is important to look for alternatives to this method.
One of the commonly accepted methods to decrease the resistive heat generated in a material is to increase its electrical conductivity. This will allow the current to pass through the material very fast and would reduce the heat. Adding carbon nanotubes to the polymer is a very effective method to increase the electrical conductivity of the polymer. But, it has not yet been adopted by the industries as it is a highly expensive method. There are various other chemicals like polyaniline that can be added to the polymer to improve its electrical conductivity. However, adding these chemicals need not improve the mechanical properties of the material. A compromise on
mechanical properties such as strength, hardness, and impact resistance is not at all acceptable. So, a material that has both enhanced electrical and mechanical properties needs to be developed to meet the industrial requirements.
Dr Chandra Sekher Yerramalli and his group (including the author) from IIT Bombay are trying to understand the response of CFRP to lightning strike and are proposing certain measures to mitigate the damage. The study
focuses on the following areas: a) Predicting the temperature distribution in the material during and after the strike, b) Estimating the post-lightning damage area and the strength of the material after the strike. In addition to that, the group is developing new conducting polymers that can be used along with carbon
When struck by lightning, if the whole body of the aircraft is made up of metal (generally aluminium), the electric current enters the body and exits very fast as metal is a good electrical conductor. But, this is not the case if the structural material is not a metal.