Name: Dhanya T M
Department: Aerospace Engineering
Program: Ph D. (5th year)
Name of supervisor: Prof. Chandra Sekher Yerramalli
1. Introduction: Lightning
The electrically charged cumulonimbus thunderclouds are considered as the primary source of lightning. The clouds typically have positive charges accumulated at its top and negative charges collected at its bottom. At some stage in the electrification of the cloud, a collection of negative charges called "step leader" move towards the ground. Meanwhile, the positive charges accumulated at the ground underneath called "streamer" would advance towards the "step leader." Once they meet, conductive path forms, and electricity flow happens from the "step leader" to the "streamer." This is called "cloud to ground lightning (Fig. 1 (a))." When the "step leader" advances, if an object such as aircraft or a wind turbine happen to be in its neighbourhood, the "streamer" can originate from that particular object (Fig. 1(b)).
The lightning-related aircraft accidents were reported in the early twentieth century itself. The probability of occurrence of lightning strike damage to a structure depends on its ability to conduct electricity. If the structural material is a good conductor like metal, the electric current can enter and exit it without much resistance. If the structural material is a poor electrical conductor, the electric current experiences resistance and as a result, the electric energy gets converted to heat energy and the material's temperature increases. This phenomenon is termed as resistive heating and can even lead to ablation of the material.
In the recent past, aircraft and wind turbine industries started using polymer composite materials extensively due to its high strength to weight ratio, corrosion resistance, etc. Fiber reinforced polymers (FRP) are the most commonly used polymer composite materials in these industries. The FRP sheets (laminate), are manufactured by embedding fibers, typically carbon/ glass fibers inside the polymer matrix (eg. Epoxy, Vinyl ester). The FRPs are generally poor electrical conductors as compared to metals. Due to the higher resistance of these materials, during lightning strikes, the electric current takes longer time to travel through it. This results in resistive heating in the material, leading to ablation/ evaporation of the polymer matrix and breakage of the fiber. Even if the surface damage area is insignificant to the naked eye, under continuous loading such as vibration, the former will grow and lead to catastrophic failure in the future.
Another serious concern is the residual strength of the material after the strike. It is well known that the compressive strength of FRPs is one of the deciding factors in the design of structural components as it is weak in compression. The uneven heat distribution in the FRPs after the lightning strike would cause a change in material properties, which in turn lead to a reduction in the strength of the material.
3. The Finite element analysis
The laboratory experiments were the only source for understanding the nature of material damage due to lightning strikes until recently. The technological advances helped researchers explore numerical modeling of lightning strike material damage. Prof. Chandra Sekher Yerramalli and his group (including the author) from IIT Bombay has been conducting numerical studies on predicting the temperature distribution in the material after the strike and estimating the post-lightning material damage volume. Further, the effectiveness of the commonly used lightning protection system (copper mesh) was studied. Furthermore, the residual strength of the material after the strike was evaluated. The commercial software ABAQUS was used for the analysis.
1. Finding the temperature distribution and the damage volume
The time-varying temperature distribution during and after the lightning strike on Carbon Fiber Reinforced Polymers (CFRP) was obtained by the coupled electrical-thermal analysis. The total volume of the material exceeding ablation temperature (300 0C) would give the CFRP damage volume.
3.2. Effectiveness of copper mesh protection
Covering the surface, especially the lightning attachment points with metals, is a commonly used lightning protection method in aircraft and wind turbines. Generally, copper mesh protection is provided if polymer composites are used as structural components. Copper, being a very good electrical conductor, allows the current to move very fast through the body and prevents heating of the material.
The carbon fiber reinforced polymer (CFRP) laminate with copper mesh mounted on its top was modelled for the analysis. The CFRP damage volume was calculated as mentioned in the previous subsection. The study showed that the CFRP damage decreased with increase in copper mesh thickness. Further, the CFRP damage volume decreased with decrease in the copper mesh hole area percentage. These results were published in the journal "Materials Today Communications" (doi:10.1016/j.mtcomm.2018.05.009). But, when copper mesh is used there is an inevitable trade-off between the additional structural weight and damage reduction. So, it is necessary to explore sustainable alternatives to copper mesh protection.
3.3 The residual compressive strength of CFRP without lightning protection
The lightning current with amplitudes 10 kA, 50kA, 100 kA and 200 kA were considered for the study. A micro-mechanical model of the CFRP was subjected to electric current strike. After the strike, a compression analysis was conducted on the model to get the residual compressive strength. The preliminary study shows that the compressive strength of CFRP decreased with an increase in the lightning strike's peak current amplitude. A significant reduction of compressi
ve strength (almost 70%) was observed with a peak current of 200 kA. This study was published in the proceedings of the "American Society for Composites – Thirty-fifth technical conference on composite materials".
As mentioned above, a sustainable alternative to copper mesh protection is needed, especially when the additional structural weight is a serious concern. The scientific community is still exploring methods like providing a conductive top layer (protective layer) on FRPs or even making conductive polymers. The ongoing research in our laboratory is currently focussed on obtaining the critical conductivity required of a protective layer, for minimal degradation in the compressive strength of the fiber reinforced composite material underneath the conductive top layer.