Investigation of composite material destruction under lightning stroke effect

Destruction processes of composite materials under lightning stroke effect have been studied. The results of characteristic damages of samples have been presented upon a series of tests conducted with standard lightning currents and those ones whose shape is close to actual lightning stroke. There have been exhibited distinctions between the levels of sample damages: in the second case significant damages of composite matrix external layers have been shown, among which multiple destructions of bundles of carbon fibers at a depth of several layers, layer delamination and cavities formed inside sample. There have been described the results of generalized analysis of damages obtained by applying nondestructive inspection methods: stereoscopic microscope investigation and X-ray microtomography. The mechanism of damages has been presented on the basis of obtained data. The results of testing electrical, mechanical, thermal properties of composite samples have been given upon a set of experiments for lightning current resistance.


Introduction
Composite materials are widely used for manufacturing details and components in electric power engineering, automotive and aircraft industry, in space-based technology, bridge and high-rise construction. The use of composite materials is associated with the combination of their useful properties, i.e. lightness, elasticity, strength, environmental resistance. In electrical engineering industry composite materials are used to manufacture towers for overhead transmission lines, blades for wind-driven power plants of various capacities etc. [1]- [3]. The use of composite materials instead of traditional ones enables us to create environmentally compatible plants while reducing the weight of equipment and increasing the output of power-producing plants.
In the process of operation performance violations of wind-driven power plants have been detected up to total fading of their service characteristics which may be due to the action of lightning currents on composite-based blades. Blades of wind-driven plants have been partially or completely destroyed as a result of fire initiated by lightning stroke. Similar damages have been detected in composite materials whose component structure was different when using types of composite base and different reinforcing carbon or glass fibers. Conventionally, a standard lightning pulse is used to describe the process of object destruction under lightning impact [4]- [9]. In papers [10], [11] it has been shown that the application of standard lightning pulse for calculations or tests prevents from reproducing a  [12]- [14]. The energy of such a pulse is small and does not result in heating the point of lightning stroke impact. Actual pulse does not show the fall of current to zero values after its quick rise. The current may reach the value of kiloampere in final stage of pulse during a long time (milliseconds). This current transfers the main charge and causes high-temperature breakdown of composite materials [15], [16].

Model description
Dimensional data and physical properties of composite materials have been chosen on the basis of values got for manufactured samples. Major parameters used for simulation are given in Table 1.

Method of analysis
To analyze lightning resistance properties of composite materials there has been designed in Peter the Great St. Petersburg Polytechnic University (SPbPU) a bench simulating the impact of either standard or actual lightning current pulse [15]- [17]. Surge-current generator designed on the basis of two capacitor storages with distinct time and energy responses and interface developed on the basis of switching unit with exploding conductor make the base of test facility. The task of the optimal selection of the impulse source parameters is the task of the parametrical synthesis [18]- [20]. Standard pulse contains only "fast" component while actual pulse is simulated through combination of "fast" and "slow" components. Representative results of composite sample damages after test runs under standard lightning pulse impact as well as under the action of pulse whose shape is close to actual one are given in Table 2.  As for samples 1 and 2 whose test results are given in Table 1, the analysis of damages has been conducted by applying nondestructive inspection technique. External damages observed on samples differ greatly. As a result of current pulse passing of greater duration and consequently of greater charge composite samples get more marked damages.
At performing surface analysis with stereoscopic microscope insignificant damages have been detected on sample 1 matrix made of epoxide resin and partial damages of carbon fibers. Studies of sample 2 with the use of stereoscopic microscope have shown heavy damages not only on the surface but in inner layers of composite material. However, the performance of optical analysis was hampered due to upper layer destruction. That is why the both samples were subjected to nondestructive inspection method with the use of X-ray microtomography. The results for sample 1 are shown on Figure 1 where one can see fiber destruction inside sample with blind cracks between inner layers of composite material. Results for sample 2 are given on Figure 2.

Electrothermal and thermomechanical analysis
The purpose of electrothermal analysis was to get electrical potential distribution on sample volume. At that, the sample lower layer was grounded and the current was supplied to the sample in central part of upper layer. In the first layer carbon fibers are on the angle 45. The electrical conductivity along fibers is extremely high as compared to that one across fibers. This influences potential distribution, in the first layer it is of ellipsoidal shape. In the second layer carbon fibers are on the angle 0, so potential distribution is of another shape. Moreover, the potential amplitude is far less in the second layer as compared to the first one.
The goal of thermomechanical analysis is to define the ultimate breaking strength of a sample once upon lightning current passing through it. For this purpose one should take out of a sample under electrothermal analysis components in a part as wide as 25 mm and as long as 250 mm as recommended by ASTM 3039 standard for performing mechanical tests [21]-[23]. One can simulate mechanical test for a given 25x250 mm plate when one end is fixed and another one is extended. Temperature variations result in variations of mechanical properties of samples, their dependence on temperature is shown in Table 3 and on Figure 3.  Figure 3. Strength gain as a function of temperature.

Simulation results
The results of deformation simulation obtained for samples with layers 45/0/-45/90 are shown on Figure 4 for several amplitude values of lightning current pulse passed through sample on the first stage of simulation. The significance of lightning current pulse amplitude for limit strength and elastic modulus has been proven. The greater is the amplitude value of current pulse passed through sample the more marked is the degradation of its mechanical properties. Computed values of elastic modulus E and strength limit at extension f are given in Table 4. At current pulse component of 50 kA amplitude passing, the elastic modulus is reduced by 77 per cent as compared to undamaged sample and its strength limitby 59 per cent.  6 Similar analysis has been performed for a sample with unidirectionally oriented carbon fibers. Obtained performance is shown on Figure 5. 6. Conclusions 1. First investigations of damaged samples have been performed by applying non-destructive inspection method with the help of X-ray microtomogrtaphy. There have been demonstrated internal damages of composite samples under lightning current effect. The mechanism of damages has been described on the basis of obtained data. Electrical, mechanical, thermal properties of samples have been defined in experimental way by using modern techniques and devices.
2. Investigations have shown significant distinctions between damages of samples under the effect of lightning current of standard shape and those ones close to actual lightning currents. In the second case internal damages of carbon fibers and epoxide resin matrix cause the degradation of their mechanical properties. Such a comparison of standard and actual lightning current effect for composite materials was performed for the first time and it should be applied for lightning resistance testing of blades of wind-driven power plants, aircraft and helicopter components which may be subjected to lightning stroke effect.
3. Testing of pre-production units following the proposed technique will enable to carry out a correct assessment of their potential external and internal damages and to select an appropriate model of lightning protection so as to reduce their damages and prevent possible failures.