ELECTRICAL RESISTANCE MEASUREMENTS OF CARBON FIBRE REINFORCED POLYMER (CFRP) MATERIALS. MEASUREMENT TECHNIQUES, METHODS AND PITFALLS TO AVOID: A CASE STUDY

Lightning strikes are a relatively common event to aircraft
and can cause considerable damage due to high energy
electrical current passing through the aircraft structure.
Traditionally composite aircraft are protected from lightning
strike by expanded metal foil mesh embedded in the
composite. Alternatively, metallic lightning strike protection
(LSP) coatings have also been investigated [1, 2]. Current
aircraft design incorporates a range of mechanically fastened
structural joint assemblies, which are often located in the
electrical current path and these joints may reduce electrical
conductivity performance. On post lightning strike inspection,
these joint assemblies are sometimes subject to damage and
require repair [3, 4].
Maintaining electrical continuity across joints in composite
panels can be quite difficult to achieve. Electrical continuity
is important for lightning strike protection (LSP) and for
protection against High Intensity Radiated Fields (HIRF).
Innovative jointing techniques were explored and developed
under the Clean Sky 2 project “C-JOINTS”. A follow-on
project named “D-JOINTS”, currently in progress, is
examining
mathematical
modelling
and
software
development of a design tool to aid in the sizing of certain
components associated with these joints.
As part of the mathematical modelling of the design tools for
predicting the lightning and HIRF behaviour of these
composite aircraft panels and components, it is clear that
electrical resistivity, or its reciprocal, conductivity, is an
essential parameter to quantify. However, electrical resistivity
values for CFRP materials are very difficult to find in the
literature. One the major reasons for this apparent lack of data
is perhaps that the electrical properties of CFRP materials are
enormously variable and are very dependent upon both the
composition and fabrication methods of the material itself.
Moreover, it is well known that the electrical properties of

CFRP panels are anisotropic, i.e., the lengthwise properties
are quite different from the through-thickness properties, but
this difference is not often quantified.
Given the apparent lack of data in the literature, but, more
importantly, the inherent material and process dependability
of the parameters, it becomes apparent that these parameters
need to be determined individually for each material
composition and its manufacturing process.
This case study describes the development of relatively
simple-to-implement measurement tools and procedures.
Perhaps more importantly, it highlights some of the pitfalls
encountered in evolving these test methods and describes how
the problems were resolved. Once developed, these test
methods gave encouragingly consistent results over a range of
panel thicknesses. The electrical resistivity data for the
composite material employed in the project is presented with
the caveat that this data is particular to this material and its
manufacturing methods. Both lengthwise and through-
thickness measurements are presented.
A brief description of the D-JOINTS project and its progress
to date will be included to provide some background
information as to the requirement for these measurements.

This project has received funding from the Clean Sky 2 Joint
Undertaking under the European Union’s Horizon 2020
research and innovation programme under grant agreement
No 887042. The authors acknowledge the support and
guidance provided by Evektor, the Topic Manager of the D-
JOINTS project.