Non-destructive Evaluation of Lightning Strike Induced Damages in Sandwich

Composites for Unmanned Air Vehicle Structures

Hema K S1, a

, Samudra Dasgupta1, b

1 Aeronautical Development Establishment, Defence R& D Organisation

New Thippasandra Post, Bengaluru -560075, India

abhat.hema@gmail.com,

bsamudradg@ade.drdo.in

Abstract. Sandwich composites are commonly used as weight efficient solutions in primary

structures of Unmanned Aerial Vehicles (UAV). The present study was aimed to evaluate the

lighting strike immunities of similar structures by assessing the lightning induced damages in

representative panels through Non-destructive testing / evaluation (NDT/E).

Several sandwich panels representative of the fuselage of a Medium Altitude Long Endurance

(MALE) UAV were fabricated using Carbon Fibre Reinforced Polymer (CFRP) face sheets and

Rohacell foam core, with and without additional lightning protection layers (viz., copper mesh,

aluminium mesh and carbon-aluminium interwoven fabric) on the top. All the panels were

subsequently subjected to lightning strikes through high current injections as per MIL-STD-1757A

guidelines. The panels were evaluated for their responses to direct effects of lightning strikes for

both Zone 1/2 (i.e. Waveform A, B and C) and Zone 3 (i.e. Waveform C only) conditions.

NDT/E of the sandwich panels are carried out before and after the lightning strikes by Air-Coupled

Ultrasonic C-scanning Testing (AC-UT), with an objective to assess the extent of external/internal

damages. Two different ultrasonic frequencies, viz., 120 kHz and 225 kHz were used for the study.

Based on the C-scan images, it was concluded that, while unprotected CFRP skin of the present

thickness may be sufficient for lightning protection in Zone 3 of the UAV, it is unsuitable for use in

Zone 1/2. For the latter, all the above additional lightning protection layers were found to be

suitable. But, given the significantly reduced weight penalty associated with the carbon-aluminium

interwoven fabric as compared to meshes, the same appears to be the most optimal solution for

lightning protection of the UAV structures.

Keywords: NDT, ACUT, Sandwich, Lightning Test, Interwoven fabric

Introduction

Fibre reinforced polymer matrix composites are known to have an edge over conventional materials

because of their superior specific strength and stiffness properties. Foam core sandwich variants of

such composites are commonly used as weight efficient solutions in primary structures of

Unmanned Aerial Vehicles (UAV). Traditionally, UAVs have not been designed for lightning

protection. But given the increasing weight, cost and complexities of the recent and next generation

UAVs, demand for lightning protection of these UAVs are growing among the users and regulatory

agencies. Hence, this preliminary study was aimed to evaluate the lighting strike immunities of

panels representing possible structural materials to be used in a MALE UAV, through NDT/E.

NDT/E of the sandwich panels were carried out before and after the simulated lightning strikes by

visual inspection and Air-Coupled Ultrasonic C-scanning Testing (AC-UT), with an objective to

assess the extent of external/internal damages. AC-UT, a relatively newer and emerging NDT

technique, uses ambient air itself as the couplant between probe and the sample, which is a major

advantage especially for foam core sandwich structures. Further, the lower frequencies employed in

this new air scan method ensures improved transmission characteristics of the ultrasound through all

the materials in general, foams and sandwich composites in particular [1-4].

National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014)

Vol.20 No.6 (June 2015) - The e-Journal of Nondestructive Testing - ISSN 1435-4934www.ndt.net/?id=17886

Experimental

Materials and Processes: Several sandwich panels representative of the fuselage of a MALE UAV

(2.5 ton all up weight class) were fabricated by vacuum bagging method using epoxy based

Carbon Fibre Reinforced Polymer (CFRP) face sheets and Rohacell foam core with and without

additional lightning protection layers on the top (viz., copper mesh, aluminium mesh and carbon-

aluminium interwoven fabric). Accordingly, seven variants of sandwich composite panels (refer

Table 1 for details) of size approximately 650mm x 650 mm x 7mm (thickness) were chosen for the

study.

Table 1. Material configurations for the lightning test panels

Sample

No.

Skin material Core

material

Protective layer Lay-up sequence

Material Orientation

1/1& 1/2 Carbon fabric-

epoxy (C)

Rohacell

Foam (RF)

Nil C + (0/90)

C X (+45)

C UD (0)

C +(0/90)

RF NA

C +(0/90)

C UD (0)

C X (+45)

C + (0/90)

2/1& 2/2 Carbon fabric-

epoxy (C)

Rohacell

Foam (RF)

Copper mesh (Cu)

- 1 layer

Cu + (0/90)

Sample 1sequence

3/1&3/2 Carbon fabric-

epoxy (C)

Rohacell

Foam (RF)

Copper mesh (Cu)

- 2 layers

Cu + (0/90)

Cu + (0/90)

Sample 1sequence

4/1&4/2 Carbon fabric-

epoxy (C)

Rohacell

Foam (RF)

Aluminium mesh

(Al) - 1 layer

Al + (0/90)

Sample 1sequence

5/1&5/2 Carbon fabric-

epoxy (C)

Rohacell

Foam (RF)

Aluminium mesh

(Al) - 2 layers

Al + (0/90)

Al + (0/90)

Sample 1sequence

6/1&6/2 Carbon fabric-

epoxy (C) &

Aluminium

carbon

interwoven(IW)

fabric-Epoxy

(IW)

Rohacell

Foam (RF)

Aluminium carbon

interwoven (IW) -

1 layer

IW + (0/90)

C X (+45)

C UD (0)

C +(0/90)

RF NA

C +(0/90)

C UD (0)

C X (+45)

C + (0/90)

7/1 Carbon fabric-

epoxy (C) &

Aluminium

carbon

interwoven(IW)

fabric-Epoxy

(IW)

Rohacell

Foam (RF)

Aluminium carbon

interwoven (IW) -

2 layers

IW + (0/90)

C X (+45)

C UD (0)

IW +(0/90)

RF NA

C +(0/90)

C UD (0)

C X (+45)

C + (0/90)

Details of the materials and processes used for fabrication of the above panels are given below.

Raw materials

o Resin system: Novolac based epoxy (Araldite LY 5052 from M/s Huntsman Advanced

Polymers, USA) and cyclo-aliphatic amine hardener (Aradur 5052CH from the same

source)

o Carbon Fabric: 3K fibre, 7 mil plain weave, balanced bidirectional and weighing

approximately 204 gsm (from M/s SAATI, Italy)

o Carbon UD Tape: 3K fibre, 140 gsm tape ( from M/s Hyfil, UK)

o Rohacell Foam: 51WF grade (from M/s Evonik Degussa, Germany)

o Copper Mesh: 125 gsm, 500 µ sq. mesh

o Aluminium Mesh: 90 gsm, 100 µ sq. mesh

o Interwoven fabric: 3K carbon & Aludraht AL MG 5 Fibres, 200 + 23 tex, 220 gsm

(from M/s ECC GmbH, Germany)

Process: Vacuum bag moulding process

Cure cycle: Curing at room temperature for > 24hours, followed by post curing for 3 hours at

120°C in a hot air circulated oven.

Lightning Tests: Lightning strike zone locations for any aircraft are defined depending upon the

likelihood of a strike to the location and the possibility of the initial attachment hanging on in the

same location [5-7]. The zones depend upon the vehicle's configuration and orientation and will

vary between vehicles. The lightning strike zones to be defined for any aircraft / UAV are as

follows:

Zone 1A: Initial attachment point with low possibility of lightning channel hang-on

Zone 1B: Initial attachment point with high possibility of lightning channel hang-on

Zone 2A: Swept stroke zone with low possibility of lightning channel hang-on

Zone 2B: Swept stroke zone with high possibility of lightning channel hang-on

Zone 3: Portions of the vehicle between the other zones that may carry substantial amounts of current

due to lightning strike to one of the other zones

All the sandwich panels described in above section were subjected to simulated lightning strikes

through high current injections as per MIL-STD-1757A / MIL-STD-464A guidelines. As per these

standards, the lightning environment consists of a combination of current waveforms consisting of

components „A‟, „B‟, „C‟ and „D‟ as defined in the Figure 1.

Figure 1. Lightning current environment (Source: MIL-STD-464)

The present test plan included the following waveforms:

1. Waveforms „A‟, „B‟ and „C‟ was used in series for one specimen of each configuration. This was

to assess the performance of the materials in case of a direct attachment of lightning (Zone 1 & 2

applications). The high currents were injected to one end/edge of the panels through a carbon fibre

contact and collected near the other end/edge through metallic collector plate.

2. Waveform „C‟ was used for the other specimen of all the samples (except Sample No. 7, for

which only one specimen was available). This was to assess the performance of the materials for

Zone 3 applications.

Non-destructive Evaluation (NDE): The test samples were subjected to air-coupled ultrasonic C-

scanning both before and after the simulated lightning strikes (instrument procured from M/s

Quality Material Inspection Inc., USA).The unit consists of SONDA 007CX ultrasonic module with

transducers of 120 kHz and 225 kHz frequencies coupled with an industrial workstation (loaded

with WINSPECT SOFTWARE) and a 2-axis motion controller. The C-scanning helped to assess

the quality of the composite panels before the lightning testing as well as for assessing the damages

in the panels after lightning strikes. The gain and attenuation settings used were 60 dB & 71 dB for

120 KHz and 40 dB &77 dB for 225 KHz respectively.

Results and Discussions

Lightning Test Results: As mentioned before, the sandwich panels were subjected to high current

strikes as per the above mentioned standards. Samples 1/1 to 7/1 were subjected to Zone 1/2

conditions (A, B and C waveforms) and sample 1/2 to 6/2 were subjected to Zone 3 conditions (C

waveform only). It may be noted that in some of the samples for Zone 1/2 applications, Waveform

C could not be fired totally. However, Waveform A is generally considered to be the most damaging

component of the lightning test and hence, the results obtained from the test even without

Waveform C or with incomplete Waveform C component are expected to be adequate for initial

screening / usability of the material configurations in such zones of the UAV. Moreover, effects of

Waveform C were subsequently captured in the Zone 3 tests on other panels of same configurations,

although the effects were not cumulative to prior Waveform A and B strikes. Following are the

detail parameters captured during the lightning tests.

Sample 1/1: Waveform A (211 kA), Waveform B (8.13 coulombs) and Waveform C (incomplete)

Sample 2/1: Waveform A (200 kA), Waveform B (8.86 coulombs) and Waveform C (incomplete)

Sample 3/1: Waveform A (215 kA), Waveform B (8.14 coulombs) and Waveform C (incomplete)

Sample 4/1: Waveform A (219 kA), Waveform B (8.85 coulombs) and Waveform C (190.66

coulombs)

Sample 5/1: Waveform A (196 kA), Waveform B (8.56 coulombs) and Waveform C (144.3 C

coulombs)

Sample 6/1: Waveform A (192 kA), Waveform B (8.71 coulombs) and Waveform C (incomplete)

Sample 7/1: Waveform A (200 kA), Waveform B (8.86 coulombs) and Waveform C (74.2

coulombs)

Sample 1/2: Waveform C (249 coulombs)

Sample 2/2: Waveform C (234 coulombs)

Sample 3/2: Waveform C (210.5 coulombs)

Sample 4/2: Waveform C (192.4 coulombs)

Sample 5/2: Waveform C (225.86 coulombs)

Sample 6/2: Waveform C (229.6 coulombs)

NDT/E Results: As mentioned before, the NDT for the panels before and after lighting testing was

carried out using AC-UT at different frequencies viz. 120 KHz and 225 KHz and the C-scan images

of the same are given in Table 2. As can be seen from the images, upper middle portions of all the

panels, which were the locations of lightning current injections, revealed zones of lower

transmissions (i.e. darker patches) after the lightning tests. The distinct patches in the lower middle

extremities of some of the panels may be ignored as those were the signatures of the test fixture

used during scanning. When compared with the corresponding NDT images of the panels before

lightning strikes, the damages induced by the lightning current seem to be localized in nature, being

mostly around the area of strike and do not look to have spread to other areas of the panels. As

expected, the damages occurred after Zone 1/2 strikes (A, B and C waveforms in series) were found

to be more severe compared to the Zone 3 strikes (only C waveform).

Sample 1/1 i.e. the panel without any protection layer, was severely damaged by lightning strike of

A, B and C waveforms, and hence NDT for the same was not necessary. NDT of all other

configurations after Zone 1/2 tests revealed only minimal extents of damages (few inches) which

were of repairable nature and didn‟t pose any threat of catastrophic failure to the structures. For

Zone 3 lightning tests, all the configurations, including the unprotected one were found to be

acceptable, with very minimal localized damages (< inch) recorded by AC-UT.

Table 2. NDT C-scan images of test panels before and after lightning tests

Sample

No.

Transmission Index

Before lightning test After lightning test

@120 KHz @225 KHz @120 KHz @225 KHz

1/1 Sample 1/1 was severely damaged by lightning strike of A, B and C waveforms, and

hence NDT for the same was not necessary.

1/2

2/1

2/2

3/1

Sample

No.

Transmission Index

Before lightning test After lightning test

@120 KHz @225 KHz @120 KHz @225 KHz

3/2

4/1

4/2

5/1

5/2

6/1

Sample

No.

Transmission Index

Before lightning test After lightning test

@120 KHz @225 KHz @120 KHz @225 KHz

6/2

7/1

Conclusion

The following were the conclusions drawn from the above studies.

Unprotected CFRP sandwich of the configuration discussed in the paper may be sufficient

for lightning protection in Zone 3 of an UAV, however, it will not be suitable for use in

Zone 1 and 2.

For Zones 1 & 2, all the additional lightning protection layers were found to be suitable.

Given the significantly reduced weight penalty associated with the carbon-aluminium IW

fabric as compared to meshes (less than 5 grams / sq.m for IW fabric as compared to more

than 100 grams / sq.m for the meshes), the same appears to be the most optimal solution for

lightning protection of the UAV structures.

The above preliminary studies and conclusions were suggestive in nature and more detailed

study may be required before confirming the same.

Acknowledgement

The authors acknowledge the supports of Mr. P Srikumar, Director, ADE, Mr. N Radhakrishna,

Head-ASM Division, ADE, Mr. V Prabhakaran, Group Director, ADE and Mr. APVS Prasad,

Program Director, ADE for their constant supports and encouragement. The technical support of

Dr. Y Purushottam and his team in Lightning Test Facilitis of CABS, Bangalore is also sincerely

acknowledged. The authors are also grateful to Prof. GR Nagabhushna, IISc. (Retd.) and his team

for all techical guidance. The help from Mr. Ravishankar BN and Mr. K Ravi Sekhar, scientists

from the composite technologies group in ADE for fabrication of the test lminates are also

appreciated.

References

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