Research and Review Journal of Nondestructive Testing (ReJNDT) - ISSN 2941-4989 - www.ndt.net
Using the DIC Technique in Damage Detection for a Cantilevered
Composite Beam
Anna RZEPKA1, Dominika ZIAJA1
More info about this article: https://www.ndt.net/?id=29389
1
Department of Structural Mechanics, The Faculty of Civil and Environmental Engineering and
Architecture, Rzeszow University of Technology, Poznanska 2, 35-084 Rzeszow, Poland, Phone 178651617,
e-mail: arzepka@prz.edu.pl (A. Rzepka corresponding author), dziaja@prz.edu.pl (D. Ziaja)
Abstract
The subject of the article is the use of the Digital Image Correlation (DIC) technique in tests of damage
detection in a composite cantilevered beam. Laboratory tests of the beam were conducted using the Q-400 system
from Dantec Dynamics GmbH. The research aimed to study the behaviour of the structure under external forces
and to analyse the influence of cross-section damage on the displacement field of the beam. The analysed quantities
were also the changes in strain fields of the structure. For the laboratory model, the finite element models were
prepared, and satisfactory compliance in terms of displacement fields between numerical and laboratory models
for undamaged structure was obtained. The research has shown, that it is possible to indicate the approximate
location of damage based on the information about changes in the displacement field.
Keywords: digital image correlation (DIC), non-destructive testing (NDT), damage detection, cantilever
composite beam
1.
Introduction
Non-contact methods of measuring displacements and strains, including optical-electronic
systems, are increasingly being used in the research of building structures. One of the visionbased measurement methods is Digital Image Correlation (DIC), which has been used in the
research of a composite cantilever beam presented in this article.
The popularity of DIC method continues to grow, and it is currently applied in various
scientific fields, including construction. DIC studies are successfully conducted on individual
structural elements such as beams or plates, as well as on multielement systems, such as steel
frames [1, 2, 3]. The continuously evolving civil engineering field demands increasingly
modern and efficient types of non-destructive testing, as well as the search for more efficient
and functional materials, examples of which are composites. Composite is a material formed
by combining at least two components, called phases, which possess different properties. One
type of composite is fibre-reinforced composite, with which the beam analysed in this article
was made. Among the diverse group of composites, fibre-reinforced polymers (FRP) are the
most efficient. They have low specific weight while exhibiting high-level mechanical and
physical properties. Various types of fibre-reinforced composites can be distinguished regarding
the fibre material, namely: aramid fibre-reinforced composites (AFRP), carbon fibre-reinforced
composites (CFRP), or glass fibre-reinforced composites (GFRP). The usage of selected fibre
material results in the specific properties of the whole composite - it gives the possibility to
tailor the material for special applications. For example the most important advantages of GFRP
composites are high tensile strength with relatively low longitudinal elasticity modulus, high
hardness, and excellent corrosion resistance [4, 5].
The combination of DIC and innovative structural materials, such as composites, represents
an interesting research area that can contribute to the development of both non-contact
measurement methods and the advancement of studies on composite structures, beyond
common composite plates. In [5], research on a composite laminate plate (GFRP) was
presented. The authors of this article identify and localize damages using different
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https://doi.org/10.58286/29389
non-destructive testing (NDT). Digital Image Correlation has been utilized there for quick
verification in the impact-deformed zones by applying a modified rapid thermal strain field
technique. Another example of using DIC for composites is presented in [6]. Researchers
identified structural defects in FRP composites. In this context, DIC to monitor the evolution
of strain fields was used. It allowed for the detection of damages before permanent signs of
destruction became evident. The authors of the paper [7] presented results for a bent sandwich
beam. In the study, a comparison of results for several theoretical models and a numerical model
was provided. The DIC to monitor the deformation state of the tested samples was used and the
displacement field distribution was obtained. The aim of the research described in [7] was to
compare experimental results with theoretical models to assess the accuracy of these models in
predicting the actual deformation of the beam under different loading conditions. Furthermore,
researchers in the study [3] highlight the use of DIC in measuring deformations and strains in
various structural elements, including those made of composites. They emphasize that the DIC
technique shows promise as an effective method for investigating composite deformations, it
provides a full-field measurement tool for strains and displacements.
The applications of DIC in non-destructive testing of composite materials, shown in the
literature, are limited to simple-shaped structures (plates) or specimens under standard tests.
There is a lack of examination of more complicated elements, like those used in civil
engineering. Checking the possibility of using DIC in real objects was the main motivation for
here-presented research. Therefore, an experiment on a cantilever I-beam made of GFRP was
conducted. Its description is presented in the subsequent part of this article. During the analysis
of the experiment's results, the focus was placed on aspects such as verifying the anchoring
zone, investigating the force-displacement relationship, and, most intriguingly, conducting
a trial to detect beam damages based on displacement analysis.
2.
DIC in displacement and strain field measurements
A pivotal factor in selecting suitable measurement methods lies in their versatility to perform
diagnostic procedures in both laboratory settings and during the operation of the structure. The
possibility of proficient execution of investigations on existing constructions or their elements,
as well as the assessment of material degradation degree, has to be taken into account. Digital
image correlation, as a vision-based method, facilitates non-contact measurements
encompassing geometry and variations in displacement and strain components of the specimen.
The process of conducting structural analyses using this method entails appropriate illumination
and monitoring the intensity of the light beam reflected from the surface, both in a zero state
(without any load) and in a state after the application of a load, resulting in deformation. The
DIC system operates based on algorithms that ascertain the positions of analysed measurement
points in successive image sequences. By exploiting the correlation between images, one can
obtain an abundance of measurement data concerning displacements and strains. Cameras are
utilized during the examination, and their quantity and selection depend on the complexity of
the problem [1, 3, 8]. The comprehensive guidelines for the proper execution of measurement
using digital image correlation are detailed in the international guide 'A Good Practices Guide
for Digital Image Correlation' [8], prepared by the International Digital Image Correlation
Society. Despite drawbacks such as the system's sensitivity to lighting conditions, the necessity
for precise sample preparation, or time-consuming measurements in specific configurations,
DIC offers numerous benefits. These include non-invasive operation, high precision, the ability
to study a broad range of materials, real-time monitoring capabilities, and compatibility for
integration with other measurement devices [3, 8].
The experiment was conducted on a composite beam with an I-beam cross-section. The
profile used in the study was made with a glass fibre-based composite, evident from its light
colour. It belongs to the group of fibre composites, as indicated by its shape and texture.
A laboratory model was designed and prepared as a cantilever beam. The beam was positioned
on a steel frame to stabilize the structure. Additional equipment included a weight, used to
measure the applied load, and a winch to introduce tension to a steel cable through which the
beam was loaded. The weight was suspended from a metal clamp to ensure the forces were
uniformly transmitted to the contact surface. Fixation was achieved using wooden beams, steel
C-sections, plates, and connectors. The sample surface preparation adhered to the guidelines
outlined in the reference [8]. Figure 1a illustrates the view of the laboratory model of the
cantilever composite beam. The tests were carried out on a beam in two states: an undamaged
beam and five incremental damages. The research scenario involved inducing damage to one
selected cross-section of the beam by gradually weakening it using a device oscillating.
Weakening started from the bottom flange (in two steps) then the top flange was cut (also in
two steps), and at the end, the notch in the web was made (one-fifth of height from the bottom
and the top). Selected damages are shown in Figures 1b and 1c. In all cases, the behaviour of
the structure under the influence of increasing external force was checked.
a)
b)
c)
.
Figure 1 a) Laboratory model of a cantilevered composite beam b) exemplary damage (Damage 4 see Figure
6.) of the cross-section of a beam c) exemplary damage (Damage 5 see Figure 6.) of the cross-section of a beam
During the research, two high-resolution cameras of the Q-400 system developed by Dantec
Dynamics GmbH were used. This enabled three-dimensional analysis. The measurement setup
also included a portable computer with Istra4D software. This software was used for recording
images and their evaluation to visualize measurement results. The cameras were attached to
a beam on a tripod, allowing for easy adjustment of their positions relative to each other. The
tripod was positioned 140 cm in front of the structure, with a spacing of 100 cm between the
cameras. The cameras were connected to the laptop and each other using cables for
synchronization. The calibration process utilized two boards: Pl-35-WBM_9x9 (calibration for
the entire beam view) and Al-08-BMB_9x9 (calibration for the selected part of the beam view).
Stable lighting conditions and equipment stability were ensured at the measurement location.
The first analysis focused on the unaltered model. Twenty series of tests (S01-S10 and
S11-S20) subjected the beam to a defined range of forces (the range of applied force was set at
0 - about 300 N; due to the method of applying the load it was impossible to obtain the exact
magnitude of force). Each measurement series consisted of several distinct images (about 10
for each camera: the first one - without loading and each subsequent one after the next increase
in load). These investigations aimed to analyse the anchoring zone, determine the extent of the
load influence zone and provide an overview of the entire bracket's behaviour under external
forces. The accessible format of the presented results, including colour maps from the Istra4D
software allowed for initial verification after completing and post-processing of the image
series. The exemplary maps of displacement field are shown in Figure 2. Both pictures
presented there show the same moment in examination, but they are recorded by two cameras:
left (Figure 2a) and right (Figure 2b). Additionally, in Figure 2 a points selected for further
analysis were depicted.
b)
a)
Figure 2 Vertical displacement for undamaged beam a) view from the left and b) from the right camera for
force 312 N
To conduct a thorough verification of the anchoring zone, the decision was made to minimize
the observed area to a fragment containing the anchorage. In the analysed region, four main
points were strategically chosen (Figure 3a) and their displacements were analysed. During the
analysis of the captured images, displacements were analysed in three directions: vertical,
horizontal, and in-plane (Figures 3a-3c). Interpreting the outcomes led to the conclusion that
the anchoring zone does not exhibit perfect rigidity, as evidenced by the points situated in
proximity to this area shifting under the applied load. However, the magnitudes of these
displacements were sufficiently small to be deemed non-significant in influencing the overall
behaviour of the structure. Figure 3d presents a sample graph illustrating vertical displacement
values corresponding to the applied force for one of the points located at the fastening. In each
series, the forces increased in steps to the maximal magnitude in the previously specified range.
After every load increase, new photos were registered.
a)
b)
c)
d)
Figure 3 a) X- displacement with selected test points in the anchorage zone b) Y- displacement
c) Z- displacement d) Force- displacement diagram for force 325 N (view from the left camera)
Further investigations aimed to determine the extent of the influence zone of the applied
load. Similar to verifying the anchoring zone, the research area was limited to the location
where the weight was attached. However, the obtained strain fields were too small to conclude
them.
After verifying the support and loading zones, studies on the entire cantilever were
conducted. The changing positions of selected points on the beam (see Figure 2) were observed,
and the relationship between force and displacement was examined (see Figure 4a). The study
aimed to verify whether increasing deflection of the beam would be directly proportional to the
increment of the applied load. Displacements in three directions were visible on the
displacement maps (exemplary maps are shown in Figure 2 and Figure 5). In the analysis of
displacements in three directions for selected measurement points, it is crucial to observe the
repeatability of results, independent of the examined area of interest. All measurements
conducted within the scope of this task demonstrated a consistent trend, irrespective of the
registered area of the beam. Figure 4a illustrates that the displacement values for selected point
was highly consistent or sometimes identical across ten conducted (S11-S20) measurement
series. The consistent results suggest that the analysed structure operates within the elastic
range. The appearance of horizontal displacements indicates that the beam was not perfectly
anchored, while the occurrence of out-of-plane displacements along the axis indicates that the
bent model twisted during load increments.
a)
b)
0
0
Figure 4 a) Point 4 (see Figure 2) at an undamaged beam b) Comparison of force-displacement relationships
for numerical models and laboratory model
a)
0
b)
0
c)
0
Figure 5 Displacement a) horizontal b) vertical c) out-plane (left camera view)
For a quick verification of the measurements, a numerical model was created to check if the
results from DIC correspond to numerical calculations. The numerical models were created
using finite element method (FEM) software- Autodesk Robot Structural Analysis (ARSA).
Two models have been prepared: 1) a simple beam model and 2) a shell model. In the first case,
a completely clamped and linearly straight beam was built using standard 2-noded elements.
Elements connected nodes located at the beginning, consecutive points 1 to 4 (Figure 2), and at
the end of the beam. The shell model (second one) has been built to obtain a more accurate
reflection of reality in comparison to the beam model. The system had over four thousand nodes
and over one thousand one hundred finite elements. In the shell model, two support conditions
(fully rigid and hinged) and two loading methods (surface and point) were considered. For both
numerical models, the displacement of the system was determined according to the rules of
global elastic analysis based on the theory of the first order. Due to the lack of precise
parameters, material characteristics were adopted based on the literature [4]: density –
2000 kg/m3, longitudinal elastic modulus - 30,90 GPa, Poisson's ratio - 0,33. As shown in Figure
4b, the results from the considered models are similar. However, comparing the beam and shell
models, greater compliance can be observed for the second one - a difference in vertical
displacement values is about 3% only, while for the beam model, it reaches 16%.
3.
Detecting damage on a beam through displacement field analysis
Based on the collected data, the possibility of damage detection in the analysed beam was
tested. The research scenario involved weakening the cross-sectional area of the beam in several
phases (Figure 6b). After each notching of the flanges and web, a new series of loading was
made, and displacements of the beam were recorded. In the next step, the feasibility of
identifying the precise location where a disruption in beam continuity occurred was conducted,
relying solely on displacement analysis.
Initially, it was assumed that when the cross-section became weaker, the displacement values
would increase. Comparing Figure 2 and Figure 5b, it is evident that vertical displacements
have noticeably increased. The comparison of displacement at selected points (see Figure 2)
regarding the damage scenario (Figure 6b) is shown in Figure 6a. The blue horizontal line shows
the reference beam axis (without any load). On this line, the location of four selected points is
depicted, in which displacement values under a force magnitude equal to 300 N were calculated.
The measurement for the points exactly at the beginning, and at the end of the beam was not
made. Using information about the displacements in points (1 to 4), the approximate course of
the axis's trajectory was determined. The lines, shown in Figure 6b, should not be treated as an
exact image of the structure's axis.
The influence of successive damages is visible. It is worth noting that the lines
approximating the shape of the beam axis for damages 1-4 have a very similar shape to the axis
of the undamaged beam. A noticeable difference only appears for damage 5. Between points
2-3, there is a change in the slope of the line compared to the previous cases. Therefore, it can
be inferred from the graph alone that weakening of the cross-section occurred at this sector.
This deduction finds confirmation in reality, as the damage was intentionally inflicted midway
along the entire length of the cantilever.
a)
b)
Point 1
Point 2
Point 3
Point 4
Figure 6 a) Comparison of beam axis slopes regarding different damage scenarios b) Considered damage
scenarios
4.
Conclusion
The digital image correlation system has enormous potential for use and further
development, thereby expanding the boundaries of knowledge in the field of static and dynamic
testing, among others, in civil engineering. The laboratory research presented in the article
shows the potential of the DIC for non-contact displacement and strain measurement methods
on structural elements with cross-sections such as I-beams. As shown in this publication, the
analysis of displacement field changes enabled the approximate localization of beam damage.
Furthermore, it provides insight into the behaviour of profiles made from composites, which
could offer a viable alternative to conventional materials such as steel in the future. The analysis
in the study primarily focused on changes in displacement fields. Many issues arose during the
measurements and result analysis will be addressed in further scientific research. One of the
concern areas involves the application of genetic algorithms or artificial neural networks for
damage detection of the analysed model. Additionally, the exploration of the behaviour of
structures made from various materials (including the increasingly common composites ) under
the influence of external forces is planned.
Statement
The presented work is based on the Master thesis of Anna Rzepka entitled “Experimental
verification and validation of the numerical model of the cantilever composite beam”, which
was submitted at Rzeszow University of Technology in July 2023.
Acknowledgements
This publication used measurement data obtained as part of grant Preludium, project no.
2019/35/N/ST8/01086 founded by National Science Centre (NCN), Poland.
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