·Table of Contents
·Materials Characterization and
Inspection of Fibre Based Technical Composites by Radioscopy and
Computed TomographyC. Sauerwein, M. Simon
GmbH, Schießstattweg 16, D- 88677 Markdorf
J. T. Rheinländer
National Laboratory, Materials Research Department, PO Box 49, DK-4000
Project No BE97-5129, funded by the European
Polymeric composites reinforced by glass fibres have been replacing metals
in a variety of applications in mechanical and civil engineering in the past
years. Besides the conventional fibre composites there is a growing interest
in plant fibre composites. For both materials there are very few NDT
Within the framework of the FIBRINS project that is
being funded by the European Community a NDT technique based on low energy
radioscopy and computed tomography is being developed. The aim of the
radioscopy technique is to introduce a fast inspection method that can be
integrated in the production chain. The computed tomography technique was
developed to investigate bodies with a complex structure that cannot be
inspected by ordinary radioscopy.
The system comprises of a microfocus
x-ray tube, a high precision positioning system and a newly developed x-ray
detector based on a large fibre optic taper bonded to a large area CCD chip
with a resolution of 2048*2048 pixel. The spatial resolution of the x-ray
detector is 25 microns with a density resolution of 16 bit.
material manufacturers provided different test bodies and materials to test
the inspection techniques. The technological challenge is to inspect materials
with very similar density between fibres and matrix. As an approach to achieve
an optimum contrast between fibre and matrix a dual energy imaging method was
Both methods were used to inspect glass fibre and plant fibre
reinforced test samples with the aim to verify the relationship between
manufacturing parameters, sample structure and the mechanical performance. The
results of the project will help fibre material manufacturers to improve the
quality and reliability of their products.
The introduction of plant fibre based composites as a new group of
materials and the steadily increasing application of conventional fibre
composites has lead to an industrial demand for new, fast and reliable
nondestructive testing techniques. Glass fibre composites are used for high
strength engineering applications for pipes, storage tanks and other high risk
areas. A new, environmentally attractive supplement to traditional fibre
composite materials are offered by composites made of plant fibres. They are
made on the basis of natural cellulose fibres. Therefore they are combustible,
compostable, renewable and CO2 neutral. Additional attractive
properties are their availability and their low weight. At present, the
industrial interests are particularly related to the applications in the
transportation sector, e.g. for panels. However, it is expected that R&D
will lead to an improvement of the mechanical properties that may cause a
widening of the application area also for structural (load bearing)
applications. Plant fibre composites are currently gaining significant
interests (e.g. [1-3]), mainly on assessing mechanical properties, the
reproducibility on different manufacturing routes, the influence of
variability of the raw material and to prevent moisture causing damage to the
Plant fibre composites may have similar mechanical properties as
conventional (e.g. glass fibre) composites, although with a lower impact
strength. Since plant fibre composites constitute a relatively new group of
materials, the limitations imposed on the mechanical performance due to flaws
are not well known. In these respects, numerous destructive and
non-destructive characterisation techniques may be applied in order to obtain
a maximum amount of information on the processing, the structure, the
microstructure and the mechanical performance.
The work reported in this
paper has been executed in the framework of an European research and
development project by the FIBRINS consortium, funded by the European
Community under project No BE97-5129. The focus of this paper is on x-ray
based NDT-techniques for both glass fibre composites as well as plant fibre
composites. For a more detailed report about nondestructive characterisation
of plant fibre composites refer to .
In this paragraph the investigated plant fibre samples are shortly
described, because they are not as commonly known as glass fibre composites.
The plant fibre materials considered are mostly based on jute nonwoven fibres
in a polypropylene matrix. Jute is a natural fibre obtained from the bark of
the jute plant, which grows in the south-east Asia. The fibres are about 15 -
25 mm in diameter, usually grouped in bundles of
about 0.1 - 0.15 mm in diameter. The bundles may extend for meters. The basis
for the structure and strength of the fibres are the cellulose molecules which
form a sometimes crystalline structure, where the cellulose chains are
stabilised by hydrogen-bonds between hydroxyl groups and oxygen atoms. The
stiffness of jute fibres varies from 10 to 70 GPa, with a density around 1.46
g/cm3. In comparison, glass fibres have a stiffness of about 70
GPa, and a density about 2.54 g/cm3. The density of the
polypropylene matrix is 0.906 g/cm3. Most of the presently
considered samples are of rectangular shape, measuring 40 by 30
cm2, 4.0 to 4.6 mm thick. Different manufacturing variables are
considered: type of reinforcement, varying fibre orientation, low and high
volume fraction of fibres and in the case of plant fibres varying moisture
content of the fibres.
X-ray imaging of fibres and fibre bundles combined with strength tests are
important, because the consequences of different defects on the performance of
fibre reinforced composites are still little known. The aim is to predict the
influence of the population and location of defects, of different fibre
orientation in the reinforcement or of the manufacturing processes on product
performance for conventional fibre composites and plant fibre composites. The
materials are difficult to inspect, owing to very similar response of fibres
and matrix material on x-rays, especially in the case of plant fibres. In
order to define the potential and the limits of x-ray related real time
techniques, a series of samples of different fibre geometry have been
Industrial x-ray sources with focus sizes of 0.2 mm and microfocus tubes
have been applied in combination with different x-ray detector types.
Radioscopic images taken by means of scanning the fibre reinforced samples
with a 200 mm linear diode array with 2048 pixels have been compared to those
taken directly by a high quality radioscopic system with image intensifier
type (GAMMASCOPE GS300) as well as to those taken with a flat panel detector.
The flat panel detector is based on amorphous silicon with 1024 x 1024 pixels
and a pixel size of 400 µm.
Due to the combination with the microfocus
x-ray source with a focal spot size of about 10 microns all detectors could
have been used to visualise not only the fibre bundles but also the individual
fibres of the samples.
As examples microfocus radioscopies of a glass fibre reinforcement taken
with the amorphous silicon detector are shown in figures 1 and 2. The fibre
bundles are orientated as unidirectional woven roving, which is normally used
for the so called hand-lay-up process. Figure 1 shows the fibre bundles. The
x-ray voltage has been chosen in order to visualise only the glass fibres, but
not the thread, which is orientated perpendicular to the fibre bundles. Figure
2 shows a radioscopy of the same sample, but with a higher magnification by
means of a lower source to object distance. Therefore the individual glass
fibres inside the bundles are visible
| Fig 1: Radioscopy of a glass fibre reinforcement. The
unidirectional orientated fibre bundles are visible.
|| Fig 2: Enlarged segment of the sample of figure1. Individual
fibres of the fibre bundles are visible.
The FIBRINS inspection system
Based on the systematic study of samples, described in the last paragraph
the FIBRINS consortium develops a prototype of an x-ray inspection system for
fibre reinforced composites.
One of the industrial partners (Photonic
Sciences) is developing a new radioscopy detector, which is optimised for
x-rays of low voltage, like they are used for the inspection of fibre
reinforced composites (e.g. 30 - 80 keV). The features of the detector are
high resolution and a wide dynamic range. The directionally optimized
scintillator with a diameter of 150 mm is coupled to CCD by means of well
bonded directionally aligned fibre optics. The peltier cooled CCD with a
dynamic range of 16 bit has 2048 x 2048 pixels. By this an active pixel size
of less than 50 microns is achieved.
One of the research institutes
involved in the FIBRINS consortium (Risø National Laboratory) is developing a
software package for dual energy imaging. The main idea is to enhance the
possibility of distinction between fibre and matrix materials in the x-ray
image. This is particularly relevant for plant fibre composites, since the
fibres and the matrix have similar radiation attenuation characteristics.
However also in case of conventional fibre reinforced composites the dual
energy imaging technique permits a fast and effective "background correction"
in order to facilitate the detection of defective structures.
industrial partner (CBI&M) is developing and manufacturing a prototype of
a manipulating system for the handling of various objects, mounting devices
for the detector and the x-ray source as well as the computer control.
WÄLISCHMILLER company will integrate the detector, a microfocus x-ray-tube,
the manipulating system and the dual energy software together with own
software into a combined system for high resolution radioscopy and cone beam
computed tomography (3D-CT). The latter technique is important for the
inspection of bodies with a complex structure, where radioscopy would lead to
ambiguities. For further details about 3D-CT refer to .
A combined system for high resolution radioscopy and computed tomography
is being developed based on a new detector for low energy x-rays. The new
system is an important tool for quality control of polymeric composites
reinforced by glass fibres or plant fibres. A new software for dual energy
imaging is aimed to achieve an optimum contrast between fibre and matrix
- Lilholt, H. & Bjerre, A. B., "Composites based on jute-fibres and
polypropylene matrix, their fabrication and characterisation", in
Proceedings of the 18th Risø International Symposium on
Materials Science: Polymeric Composites - Expanding the Limits, Risø
National Laboratory, Denmark, 1997, pp. 411-423.
- Mohanty, A.K. and Misra, M., "Studies on Jute composites - a literature
review", Polymer Plastics Technology and Engineering, 34(5) pp. 729
- 792, 1995.
- Thomsen, A. B., Schmidt, A.S., Toftegaard, H., Pedersen, W.B., Woidemann,
A. and Lilholt, H., "Natural plant fibre composites based on wet-oxidised
wheat straw and polypropylene", presented
- Rheinländer, J.T., "Quantitative Non-Destructive Characterisation (NDC)
of plant fibre composites", 9th Annual Symposium on
Non-Destructive Characterisation of materials, Sydney (Australia) June, July
- Simon, M., Sauerwein, C., "Quality control of light metal castings by
3D computed tomography ", presented at the 15th World
Conference on Non-Destructive Testing, Rome (Italy), October 2000.
CT_filmless_radiography, CT_microfocus_x-ray, CT_composite_testing,
CN_Germany, CN_Denmark, AU_Sauerwein_C., , AU_Simon_M., ,