Lightning strike protection: novel nonwoven technology

(Published on April 2006 – JEC Magazine #24)

BY KAAREN K. HAYNES,
PH. D.,
MICHAEL CLARK
HOLLINGSWORTH AND VOSE COMPANY,
ADVANCED FIBER NONWOVENS
DIVISION
EAST WALPOLE, USA

Background

As the use of carbon fibre composites increases in aircraft manufacturing, so does the need for EMI shielding and lightning strike protection [1-4]. Lightning strike protection usually takes the form of a conductive sacrificial layer, which minimizes the damage due to the initial attachment of lightning strike [5]. The outer layer is designed to prevent damage in the form of punctures and cracks and to provide a sufficient lightning current pathway to prevent damage to joints [3].

Expanded aluminium foil is highly conductive and has been the traditional choice for lightning strike protection. However, it can be difficult to handle and requires a second manufacturing step for proper application. Furthermore, it may also require an isolation layer to prevent galvanic corrosion [2, 5].

Nickel- or copper-coated carbon nonwoven materials are both highly conductive with surface resistivity below one ohm. These materials provide the conductivity necessary to transfer the energy of the lightning strike away from the strike zone [3, 5], as well as a rough texture that promotes arc root dispersion [3]. The rough texture of both the expanded aluminium foil and the nickel-coated carbon nonwoven promotes breakdown streamers, which propagate towards the approaching lightning channel. The streamers dissipate a single attachment out over many smaller attachments, rather than one concentrated attachment [3].

Conductive nonwovens have been used as surfacing veils in aerospace applications for decades. Traditionally, the high cost of nickel-coated carbon fibre nonwoven materials has limited their use in military applications. These materials have not yet been widely utilized for commercial applications either [5]. However, the use of nickel-coated carbon nonwovens for lightning strike protection eliminates the concern of corrosion between the metallic mesh and carbon materials and eliminates the need for a glass isolation layer [6]. Furthermore, this technology simplifies the manufacturing process [7] and provides weight and cost advantages over the expanded mesh technology.

Zone 1A lightning strike tests were conducted on composite panels for the purpose of comparing the performance of nonwoven materials with the accepted baseline configuration. Reviewed here are the main features of the new nonwoven technology and its use for lightning strike protection, including the results of visual and ultrasound inspections of the panels following the simulated lightning strikes.

Test results

Zone 1A testing was done in accordance with the requirements of RTCA/DO-160E. Testing consisted of Zone 1A lightning strike tests and included the following components:

– Component A – First return stroke current
– Component B – Intermediate current
– Component C – Continuing current

Following the lightning strike tests, a visual inspection and ultrasound tests were done for each panel. The extent of the damage to each panel is detailed below. Pictures of the panels following the lightning strike tests are included in figure 1. The picture of the paint only panel clearly shows the lightning strike did severe damage to the panel. However, there is a marked difference between the damage patterns for the baseline panel (protected with expanded aluminium foil) and the nickel carbon nonwoven protected panel. The nickel carbon nonwoven protected panel distributed the energy of the strike over the entire surface of the panel.

Paint only

The strike area suffered severe damage to all layers of the panel including a hole blown completely through the panel. Subsequent ultrasonic testing revealed that 80% of the surface of this panel was subject to delamination.

Baseline expanded aluminium foil

The strike area of the panel was lightly burned, including damage to the paint, woven glass, metal mesh and aramid layers. There was delamination at the interface between the glass and the foil layers. However, there was no visible structural damage to the panel, and the back of the panel was not damaged. Ultrasonic testing revealed that there was no structural damage or delamination to the panel itself.

Nickel-coated carbon fibre nonwoven

Paint was vaporized from the majority of the surface of the panel, while the nickel-coated carbon nonwoven layer was burned and partially removed. There was no apparent damage to the aramid layer. The energy of the strike appears to have been spread over the entire surface of the panel. Ultrasonic testing revealed that there was no structural damage or delamination of the panel itself.

Main features

The novel nickel-coated carbon nonwoven technology provides composite panels with the protection necessary to pass strict Zone 1A lightning strike testing without structural damage to the base layers of the panels. Based upon the testing presented here, it is likely that this highly conductive nonwoven would allow a finished product to pass certification.

New technology in producing nickel-coated carbon nonwoven materials has enabled the production of a highly conductive veil material. The nickel-coated carbon fibre nonwoven material provides several advantages over the use of expanded mesh:

– The concerns with galvanic corrosion between carbon and aluminium are eliminated and the material does not require an isolation layer.
– The nonwoven material can be incorporated into the normal manufacturing process and does not require an additional processing step.
– Using this material reduces costs by reducing the weight of the finished product and streamlining the manufacturing process.

References

1. M. A. Council and G. B. Park, SAMPE International Symposium, 34, 1644-1655 (1989).
2. P. K. Ackerman, International Conference on Lightning and Static Electricity, MAT-54 (2005).
3. F. Fisher, J. A. Plumer, and R. Perala, Lightning Protection of Aircraft, Second Edition, Lightning Technologies Inc., Pittsfield, MA, 2004, pp183-192.
4. S. Birch, Aerospace Engineering, 10, (2), 51, (1990).
5. M. Brown, International Conference on Lightning and Static Electricity, MAT-53 (2005).
6. J. A. E. Bell and G. Hansen, International SAMPE Technical Conference, 24, T902 (1992).
7. J. J. Toon, IEEE International Symposium on Electromagnetic Compatibility, 5-7 (1990).