Composites: Part A 36 (2005) 1073–1078
www.elsevier.com/locate/compositesa
Visualisation of barely visible impact damage in polymer matrix
composites using an optical deformation and strain
measurement system (ODSMS)
Z.Y. Zhang*, M.O.W. Richardson
Centre of Excellence—Manufacturing (RCMI), Department of Mechanical and Design Engineering, University of Portsmouth,
Anglesea Building, Portsmouth, Hampshire PO1 3DJ, UK
Received 16 August 2004; accepted 25 October 2004
Abstract
This study deals with damage evaluation using an Optical Deformation and Strain Measurement System (ODSMS). Damage was
introduced using an Instrumented Falling Weight Impact Test (IFWIT) machine. The incident impact energy was controlled to low
magnitude so that the specimen was not penetrated and only internal or barely visible damage was introduced. This damage can have an
adverse effect on structural integrity, and potentially lead to catastrophic failure. It is important to identify the damage and assess the
structural integrity of components made of composites non-destructively. This paper presents the theoretical aspects involved in ODSMS and
its capability of evaluating the impact induced internal damage. It has been found that ODSMS can successfully identify the internal damage
in terms the strain concentrations at different loading levels. The damage location and geometrical characteristics are in good agreement with
those evaluated by conventional NDT techniques.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer-matrix composites (PMCs); B. Defects; B. Impact behaviour; D. Non-destructive testing
1. Introduction
Polymer matrix composites (PMCs) exhibit distinct
properties and have found ever-increasing applications as
engineering components and structures in land transportation, aviation, aerospace, military, marine, sports and
recreational industries [1,2]. In these fields it is critically
important to evaluate the integrity and reliability of the
components and structures in a non-destructive way because
routinely employed mechanical property testing is destructive in nature. This is not practically and economically
viable in most circumstances. Although conventional NDT
techniques have been widely practised and showed varying
degrees of success, they invariably suffer from inherent
limitations and shortcomings [3–6]. Implementation of
reliable and effective assessment of the structural integrity
* Corresponding author. Fax: C44 23 92842351.
E-mail address: zhong.zhang@port.ac.uk (Z.Y. Zhang).
1359-835X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesa.2004.10.035
of PMCs has long been regarded as an ongoing challenge to
NDT community. This requirement has led to the necessity
for research and development of more reliable and effective
non-destructive testing and evaluation techniques.
ODSMS is an optical technique used to measure the
deformation and strain of the surface of an object before and
after loading. It has the advantage of simple specimen
preparation, large measuring area, non-contact and full field
measurement, non-laser illumination, material independent
determination, full field and graphical results, threedimensional (3D) presentations and good mobility. Its
capabilities include structural stability estimation, components dimensioning, non-linear behaviour examination,
creep and ageing processes characterisation [7–9]. It is well
known that the deformation and strain of an object under
mechanical loading is associated with its structure integrity.
The abnormalities and irregularities in deformation and
strain profiles are indicative of the damage presence. Thus
ODSMS has the potential to be adapted to detect PMC
damage. This paper describes the application of ODSMS
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Fig. 1. Intensity distribution in the non-deformed (1) and the deformed (2) state.
in impact induced damage evaluation of glass fibre
reinforced unsaturated polyester composites.
process, it is required that the calibration object is
recognized by the two cameras from several views.
2.1. Displacement determination
2. Theory
ODSMS is an optical deformation and strain measurement system. The fundamental principle is based upon the
fact that the distribution of grey scale values of a rectangular
area (facet) in the un-deformed state corresponds to the
distribution of grey scale values of the same area in the
deformed state as illustrated in Fig. 1.
ODSMS combines the advantages of photogrammetry
and the object grating method. Photogrammetry is one of the
optical methods that identify 3D-coordinates of surface
points. The displacement vectors, local strain values and
contour difference can be computed from the data when the
object is deformed. When the object points on the surface of
the specimen are arranged like a grating, this is commonly
known as the ‘grating method’ in experimental mechanics.
Instead of an expandable line mesh, a random pattern is
applied to the surface using AKEMI black spray paint that
allows high local resolution. The technique possesses many
unique features including simple preparation of the specimen, large measurement area, non-contact and full field
measurement, material independent determination, full field
and graphical results and 3D representations.
The optical arrangement photogrammetry consists of two
CCD cameras and a loading device where the specimen is
located as illustrated in Fig. 2. If the position of the two
cameras and two homologous image points P1 (x1, y1) and
P2 (x2, y2) are known, the corresponding object point P (X,
Y, Z) can be calculated. This procedure is known as space
intersection. A geometric model must then be established,
which transforms from image points to object points. The
parameters of the camera are necessary to establish the
model that is calculated by a calibration procedure.
As far as calibration is concerned, a special calibration
object is needed that has small circular targets with known
diameters attached on the surface. During the calibration
The displacement determination is the calculation of the
object coordinates for each deformation state for point P. It
is important that point P with the coordinates (X, Y, Z) in the
non-deformed state can be found in each deformed state
with its new coordinates (Xt, Yt, Zt). Only in this case, the
displacement and strain can be calculated. The calculation
of homologous image points of two deformation states
recognized from two cameras can be achieved by a
combination of three 2D displacement calculation processes. Here it is important that one image is defined as the
reference state. After determining all 2D displacement
fields, the homologous image points can be easily
calculated. With this information, the object coordinates
for each state can be calculated by space intersection.
2.2. Strain determination
After the determination of the displacement field, the
strain distribution can be numerically calculated. The surface
Fig. 2. Optical arrangement of ODSMS.
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Fig. 5. Schematic of ODSMS.
Fig. 3. Determination of the tangential plane.
strain is calculated by the transformation of the 3D displacement distribution into a 2D displacement distribution. After
this, the strain can be calculated in 2D space.
The strain calculation can be split into two steps. First a
tangential plane is calculated for each of the object points
Pu and Pv. For this purpose, the object points in the
neighborhood are used as illustrated in Fig. 3. This
rectangular area is referred to as the object facet. Second
the object points in these object facets are projected onto the
tangential plane. This must be done in the direction of the
normal vector of the tangential plane. The problem can be
defined as 2D as shown in Fig. 4. The calculation of the
deformation gradient can be implemented according to
appropriate algorithms.
to introduce damage using a semi-spherical shape striker
with a diameter of 20 mm. Incident energy can be systematically varied by using the different strike masses and /or
releasing the striker from different heights [10–11]. Incident
impact energy of 11.5 J was deliberately selected in an
attempt to introduce invisible or barely visible impact
damage. ODSMS, also known as ARAMIS, from
Gesellschaft fur Opticsche Mestechnik (GOM) was used
for the deformation and strain measurement for the
impacted specimen, which is schematically shown in
Fig. 5. The principle and theory were discussed in last
section. Electronic Speckle Pattern Interferometry (ESPI)
and Electronic Shearography (ES) from Ettemeyer AG
Germany and Ultrasonic C-scan from Physical Acoustics
Corporation haven been used in conjunction with ODSMS
for impact induced damage evaluation.
4. Results and discussion
3. Experimentation
4.1. Visual inspection
The pultruded glass fibre reinforced unsaturated polyester composite panel was provided by Euro-Projects (LTTC)
Ltd. Its lay-up was found to be symmetrical by deplying the
specimen. It consisted of unidirectional roving in the centre,
sandwiched with random glass fibre mat, woven fibre fabric
and surface veil symmetrically on both sides. Its fibre
volume fraction was approximately 0.5. The panel was cut
into rectangular specimens with overall dimensions of
150!40!4 mm.
An instrumented low velocity falling weight impact
machine Model IFW413 from Zwick/Roell was employed
Visual inspection is one of the oldest and easiest NDT
techniques and it continues to be a useful due to the fact that
it is simple, speedy and cost effective compared to other
more sophisticated NDT approaches. The representative
surface features of impacted specimen on both their front
and backsides are shown in Fig. 6. There is usually no
indication of a damage presence on the front surface,
implying no defects. By contrast, crack can be visible on
back surfaces, indicating the damage presence. Visual
inspection can provide a certain indication of any damage
presence but is incomplete due to the possibility of hidden
internal damage.
4.2. Damage evaluation using ODSMS
Fig. 4. Projection onto tangential plane.
As already indicated, ODSMS is an optical technique
for measuring the deformation and strain of the surface of
an object before and after loading. The random speckles
created by spraying the black paint on a white background
function as optical tracers. Left and right cameras capture
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Fig. 6. Pictures of impacted specimen on front and back surfaces.
Fig. 7. Speckle images from left and right cameras.
the speckled images prior to loading and again after each
loading step. The representative speckle images are shown
in Fig. 7.
Deformation determination involves the calculation of
the object coordinates for each deformation state. The
global deformation can then be visualised in X, Y, and Z
directions. It can be seen that 3D deformations have been
introduced due to the loading. The magnitude of this
deformations is illustrated in Fig. 8. There were some
differences in deformation profiles and magnitudes in X, Y
and Z-axes. This is inherently associated with the complex
loading schemes that can involve tension, compression,
bending, twisting or their combination at different magnitudes. In this context, measurement of absolute deformation
and strain values is not of primary concern and interest. The
objective of this current study was to identify structural
weaknesses associated with invisible defect and damage due
to impact loading. The perturbation on the deformation
profile can reveal idea of the damage but it can sometimes be
difficult to visualise and quantify. However, abnormalities
Fig. 8. Deformation profile in X, Y, and Z directions.
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Fig. 9. Damage visualisation by strain profiles.
Fig. 10. Damage evaluated by ESPI, ES and ultrasonic C-scan.
and irregularities in deformation profiles can be of great help
in determining whether further strain analysis and other NDT
techniques should be employed as far as the structural
integrity evaluation is concerned. It is similar to damage
evaluation using intensity fringe patterns in ESPI and
ES [12–14]. Damage is identified in terms of irregularities
or degradations in deformation or strain profiles. Damaged
deform more that un-damaged ones because of stiffness
differences. Although these differences can be very small in
magnitude, these techniques can pick them up due to their
high sensitivity. ODSMS in particular can be operated in very
relaxed stability environments without any safety and
security precautions due to the fact that no laser and coherent
requirements are involved.
After the determination of the deformation field, the
strain distribution can be numerically calculated. The strain
profiles at three loading levels are shown in Fig. 9. It can be
seen that there are pronounced strain concentrations.
Damage magnitudes increase with increasing loading levels
with consistent locations and geometrical features. When
the specimen is subjected to mechanical loading, strain
concentrations are introduced because the damaged area is
mechanically weaker and more vulnerable compared with
surrounding areas. These mechanical differences can be
picked up by ODSMS enabling the visualisation and
quantification of barely visible damage. The findings from
ODSMS show good agreement with those from ESPI, ES
and ultrasonic C-scan in terms of damage features as shown
in Fig. 10. It should be noted that although NDT can be
implemented in visual, mechanical, acoustic, electrical,
thermal or radiation mode, mechanical excitation is
preferred because it is directly associated with the structural
integrity. In this context, ODSMS, ESPI and ES are
particularly appropriate because they detect damage via
changes in deformation and strain between the damaged and
undamaged areas [15,16].
Another capability pertaining to ODSMS is sectional
analysis on strain distribution profiles that can further assist
in structural integrity analysis and damage evaluation. It
can ‘virtually’ implement singular and multiple sectioning
at any location vertically, horizontally and diagonally. As
can be seen in Fig. 11, the specimen under study was
Fig. 11. Sectional analysis of strain distributions.
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vertically sectioned in centre. The negative strain values
are due to the fact that the specimen is mechanically loaded
opposite to the Cartesian coordinates. The strains in
undamaged areas are incrementally increased from K0.1,
K0.22 and then K0.4%. They result from a global
response to the mechanical loading. If there were no
damage, the three lines would be parallel with no change as
a function of the surface distance. In this case, the strains
change considerably. They first increase, reach a peak and
then finally decrease. The onset points where the strain
starts to change are very close together indicating
consistent damage geometrical damage features. Different
strain values at three loading levels in the damaged area
were expected because of the damage presence. It can be
assumed that horizontal or diagonal sections will generate
similar results.
5. Conclusions
Low velocity impact loading can lead to invisible or
barely visible damage in polymer composites and it is
important to b able to determine it. Visual inspection can
only provide superficial information on internal damage.
Global deformation and strain can be here readily visualised
by ODSMS, which is able to indicate the damage location
and its geometrical features in terms of strain concentrations. With advantages of simple specimen preparation,
non-contact and full field measurement, material independent determination, 3D representation and good mobility, it
is clearly a viable NDT technique enabling the evaluation of
the structural integrity of polymer composites in conjunction with other NDT techniques.
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