The term composite materials covers
a wide range of engineered materials comprising
several distinct constituent materials chosen to
give overall properties superior to anyone of the
parts
For example Carbon fibre cloth and plastic resin
are combined to make carbon fibre composite which
can be stronger, lighter and more rigid than an
equivalent metal structure. Not all composite materials
are modern – The structures built by primitive men
combining a wooden structure plastered with mud
or animal dung are an early example.
Modern high technology composites are often used
to minimise weight in applications such as aerospace,
motorsport or racing yachts. To achieve the necessary
design performance requires confidence in the materials
as overdesign would squander the precious gains
in weight.
By their nature composite materials are inhomogeneous
– It is therefore necessary to use inspection methods
which can distinguish between relevant defects and
inherent material variations- in contrast any variation
in a metal which is not in the design drawings can
be assumed to be a defect.
Because of their structure some composite materials
are difficult to get sound through – so the inspection
method selected must be tailored to the individual
circumstances – Even superficially similar structures
can behave quite differently, and application studies
are frequently needed before results can be predicted
with any level of confidence.
Monolithic composites such as Glass or Carbon reinforced
plastics often have low enough acoustic scattering
that a clear ‘back wall echo’ can be obtained, so
these materials can frequently be inspected using
‘standard’ pulse echo ultrasonic methods. For manual
inspection Ultrasonic Flaw Detectors such as the
Sitescan 250S or Masterscan 350M can be used. These
can also be used as the instrumentation package
within an automated scanning system, which is required
for most production applications. Typically this
would use an immersion tank.
The RapidScan 2 system is an extremely fast and
convenient way of producing a C-scan of flat or
nearly flat materials. This uses an array probe
and a polymer wheel assembly to manually scan the
part. Typically a 50mm wide sensitive area can scan
at around 3-400 mm per second, producing a scan
rate of around 1 square meter per minute. The RapidScan
3D enhances this by using a coordinate measurement
arm to assemble a large C-scan from Multiple passes
over a three dimensional surface.
More highly scattering materials such as Aramid-fibre
(Kevlar) based ballistic Materials, or sandwich
structures such as foam or honeycomb filled composites,
cannot be tested in pulse echo mode – The level
of scattered sound masks reflections from defects.
For these a through-transmission approach is needed.
This requires two aligned probes, one on each side
of the material. Disbonds or similar defects will
block the sound completely. In many cases a water
coupled squirter’ system can be used.
While water coupling is convenient, and acceptable
for many applications, some materials are not compatible
with water. In these situations we have some other
alternatives:
The Dryscan 410D system uses a pair of rubber faced
spot or roller probes to couple sound to the part.
The QMI Airscan uses the air itself as a couplant.
Atlantis can provide scanning systems to work with
all these technologies.
When testing cored materials from one side there
may be limits to what can be achieved but normally
it is possible to reliably detect disbands on at
least the accessible surface using the ZETEC ‘Sondicator’
Instruments MIZ-21SR and S-21R
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