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P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44

369

Focussed on: Fracture and Structural Integrity related Issues

Application of Digital Image Correlation (DIC) in
resonance machines for measuring fatigue crack growth

P. Lorenzino
Dpto. de Ingeniería de los Materiales. INSA de Lyon, Francia.
pablo.lorenzino@insa-lyon.fr

G. Beretta, A. Navarro
Escuela Técnica Superior de Ingeniería, Universidad de Sevilla.
navarro@us.es

ABSTRACT. This paper presents a simple experimental procedure that greatly facilitates the use of digital image
correlation (DIC) techniques in fatigue test conducted in resonant testing machines, without the need of test
interruptions. This is possible due to the implementation of USB interface optical microscopes of very small
dimensions, so that they can be mounted on the specimen as a contact extensometer. Thus, the microscope-
sample assembly oscillates at the resonance frequency of the test. This is how, although the resonant testing
machine is in motion, and although the specimen is subjected to fatigue, changing its dimensions according to
the applied load, the acquired image is completely static. This allows the evaluation of the plastic deformations
generated by the crack growth, avoiding the elastic ones. Preliminary results on monitoring cracks with this
technique on flat specimens with cylindrical notches of 1050 aluminium alloy are also presented.

KEYWORDS. Digital Image Correlation; Fatigue test; Resonance Machines; Crack growth.

INTRODUCTION

here is a growing interest in using digital image correlation (DIC) to characterize materials for mechanical
properties. DIC techniques allow the strain fields arising from application of a stress to a component to be
defined. The procedure involves producing a spot pattern on the surface of the target component and assessing

relative motion in the spots between the initial and final states in order to characterize the resulting strain fields. This
requires taking photographs of the target element before and after loading (see Fig. 1), and using an appropriate algorithm
to assess relative motion between spots to obtain detailed information about the resulting strain fields [1-3].
Successful implementation of DIC entails minimizing any relative motion between the image acquisition system and the
testing machine in order to avoid errors in calculating strain fields. This hinders use of the technique with resonance
machines because they are mounted on springs and hence in continuous motion.
Notwithstanding this difficulty, DIC has aroused much interest for fatigue mechanics studies as it provide the means for
analyzing plastic deformations arising during crack propagation.

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44

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In this work, we developed a simple, effective DIC technique for monitoring deformations throughout a fatigue test while
avoiding the typical errors due to relative displacements of the image acquisition system from the testing machine - even
in resonance machines.

Reference Image Deformed State

Figure 1: Changes in a spot pattern upon application of a specific strain.

EXPERIMENTAL DIFFICULTIES

Digital images correlation

s noted earlier, successful analysis of experimental deformations entails exercising some cautions. Thus,
• If a single camera is used (i.e., with 2D DIC), specimens should be planar.
• The camera should be positioned normal to the specimen surface.

• Any deformations should be planar (i.e., the distance between the camera and specimen should be constant throughout
the acquisition time).
• Even slight motion out of the plane can lead to substantial errors in calculated deformations (see Fig. 2). Such errors can
be minimized by maximizing the distance between the camera and specimen (L) or minimizing displacement of the
specimen from the plane (w).

Figure 2: Error resulting from changes in the camera–specimen distance.

 There is the additional experimental difficulty arising from high magnifications. As can be seen from Fig. 3, the
translation ΔY resulting from application of a load caused the target zone to fall outside the microscope acquisition field.

Figure 3: Using a high magnification can displace the target zone outside the microscope observation field.

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44

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Resonance machines
Resonance testing machines provide a simple means for tests involving a large number of fields. These machines can
operate at much higher frequencies than servohydraulic machines and have other salient advantages including reduced
size, servicing requirements and energy consumption (1/100)[4].
However, resonance machines pose some problems in implementing DIC on fatigue loaded specimens. Thus, the
resonance frequency cannot be accurately controlled by the user as it depends on the rigidity of the target specimen and
changes lengthwise and widthwise in it. Also, the specimen cross-section decreases as cracks grow, which alters its rigidity
and hence its resonance frequency.
The inability to accurately set and maintain the resonance frequency throughout a test hinders the use of stroboscopic
cameras, which are frequently used on servohydraulic machines. Stroboscopic cameras can take photographs at user-
defined frequencies. In fixed-frequency tests, this allows the camera to be set at the test frequency and a static video
filmed for subsequent DIC processing with a view to calculating deformations due to crack formation and growth [3].
One other technique used with servohydraulic machines involves periodic halting of the test to photograph the specimen.
This technique is difficult to implement with resonance machines because they are mounted on springs and hence in
continuous motion. The oscillations are insubstantial to the naked eye but can have a considerable impact on DIC results.
As stated above, in this work we developed a simple solution to the above-described problems.

NEW ACQUISITION SYSTEM

he difficulties encountered in applying DIC to a resonance testing machine were overcome by developing a new
method to acquire images involving the use of very small Digimicro microscopic cameras with magnifications of
10–100× and 40–200× (see Fig. 4).

Figure 4: Microscope with a USB interface used for in situ measurements of crack growth.

This type of camera had previously been successfully used to monitor crack growth uninterruptedly in planar specimens
[6–8]. With DIC, however, even a very small displacement can lead to error in calculating deformations. This required
altering the cameras for sticking to the specimen in roughly the same manner as a contact extensometer in order to
prevent relative motion between the specimen and acquisition system. Fig. 5 depicts the camera assembly used, which
included two cameras in order to allow both sides of the specimen to be photographed at once. The cameras were
mounted by means of elastic bands and fixed by means of two small metal pins along their equator.

Figure 5: Microscopes attached to the specimen surface.

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44

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Fig. 6 shows the target surface (both specimen sides). The inside of the notch was filled with plasticine in order to avoid
light from one microscope interfering with the photographs acquired by the other.

Figure 6: View of the notch during the test as acquired with the microscopes.

TEST

ests were conducted on a Rumul Testronic 100 KN testing machine using loads of the tensile–tensile type (R =
0.1). The target material was 2 mm thick aluminum alloy 1050 cut into 300×42 mm sheets. Notches were 1, 2 or 4
mm in size. Some specimens were subjected to a thermomechanical treatment described elsewhere [6] to obtain

abnormally large grains (8.22 mm) in order to detect microstructural interactions with the plastic zones around the cracks.

RESULTS

ig. 7 shows an event of crack growth as examined by DIC. The target specimen had a grain size of 0.07 mm and a
cylindrical notch 2 mm in diameter. The figure shows 6 photographs taken at different crack growth stages.
Propagation of the resulting plastic zone is clearly apparent. These results are quite promising as they confirm that

the DIC technique allows the above-described experimental difficulties to be overcome simply by altering the way the
cameras are arranged.

Figure 7: Crack growth as determined by digital image correlation (DIC). Number of cycles (N): (a) 100 000, (b) 130 000, (c) 160 000,
(d) 180 000, (e) 210 000, (f) 230 000.

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44

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Below are also discussed the results of other studies currently under way at the University of Seville [6–8] on the influence
of specimen microstructure on crack propagation. These studies are aimed at confirming the differences in propagation
between “short” cracks, which are comparable to the specimen grain size in length, and “long” cracks, which are much
longer. Below is commented on the experimental contribution of the proposed technique in this respect.

Figure 8: Image of the specimen immediately before plastic collapse. Grain size = 8.22 mm [8].

Fig. 8 shows the situation for a specimen of grain size 8.22 mm. The specimen was 2 mm thick and the grain size across
its thickness coincided with the specimen physical thickness. This allowed the microstructure of the material to be
assimilated to a two-dimensional arrangement of aluminium single crystals. We used DIC to examine one side only —the
other was used to extract microstructural information (viz., the position of grain boundaries). Thus, the notch, the crack
and the grain boundaries —which were outlined to facilitate analysis— are shown at the top, and the deformation fields
resulting from propagation of the crack at the bottom. Clearly, this example is one case of “short” cracks as the cracks
propagated through active sliding of the grains they spanned. In order to propagate from one grain to the next, the crack
must switch to a different propagation plane (specifically, to the active sliding plane in the adjacent grain). The active
sliding plane is dictated by Schmid’s factor. This leads to non-symmetric cracks that propagate at dissimilar rates by effect
of successive acceleration and deceleration [6-8].
Fig. 9 provides a detailed depiction of Fig. 7f. The deformation field shown is that occurring immediately before plastic
collapse. This example is one of “microstructurally long” cracks, where the specimen microstructure has no influence.
Thus, cracks grow symmetrically on both sides of the notch, in a plane at 90º with the loading axis.
Visual comparison of Figs 8 and 9 exposes the influence of microstructure on the way cracks and their associated plastic
zones propagate. Thus, the crack of Fig. 9, corresponding to the specimen with the smaller grain size, exhibited a typical
propagation pattern (viz., horizontal growth from the notch equator). On the other hand, growth in the crack of Fig. 8
was strongly dependent on microstructure. As can be seen, there was little symmetry. Thus, immediately before plastic
collapse, the crack on the left and its plastic zone stopped at the grain boundary. Also, the new plastic zone did not start at
the next grain, but rather at one below that was probably more favorably arranged with respect to the loading direction.
Finally, let us highlight the sturdiness of the microscopes, which were used without problem to acquire images over more
than 20 tests. Also, they are very inexpensive to replace (less than 50 € each) if they fail.

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44

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Figure 9: Analysis of the specimen immediately after plastic collapse. Grain size = 0.07 mm.

CONCLUSIONS

n this work, we developed a simple procedure for implementing digital image correlation (DIC) on resonance fatigue
testing machines. The experimental difficulties typically encountered with this type of machine were easily
circumvented and deformation resulting from crack growth was analyzed without the need to stop the tests in order

to acquire images. The DIC technique affords high magnifications with no image displacement errors.
Preliminary results for crack propagation and its plastic zone were obtained. Under special conditions, it is even possible
to study crack–microstructure interactions and observe how grain boundaries act as barriers to propagation of the crack
and its plastic zone. Our group is currently engaged in refining the surface spot pattern in order to increase the precision
of the technique, and in using microscopes of similar size and weight but higher resolution and magnification.

ACKNOWLEDGEMENTS

he authors are grateful to Spain’s Ministry of Education and Science for funding this work through Projects
DPI2008-01100 and DPI2011-27019, and to the Andalusian Regional Government for additional support through
Project P07-TEP-03045.

REFERENCES

[1] http://www.correlatedsolutions.com. (January 2014).
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Lasers in Engineering, 27 (1997) 409-428.
[3] Vanlanduit, S., Vanherzeele, J., Longo, R., Guillaume, P., A digital image correlation method for fatigue test

experiments Optics and Lasers in Engineering, 47 (2009) 371-378.
[4] http://www.rumul.ch Comparision of costs between Resonant Testing Machine and Servohydraulic testing machine.

(January 2014)
[5] Carroll, J. D., Abuzaid, W., Lambros, J., Sehitoglu, H., High resolution digital image correlation measurements of

strain accumulation in fatigue crack growth, International Journal of Fatigue, 57 (2013) 140-150.
[6] Lorenzino, P., Navarro, A., Krupp, U., Naked eye observations of microstructurally short fatigue cracks, International

Journal of Fatigue, 56 (2013) 8-16.
[7] Lorenzino, P., Fatiga en componentes con concentradores de tención bajo carga en modo I, Tesis doctoral, Sevilla,

España, (2012) 17.
[8] Lorenzino, P., Navarro, A, Initiation and growth behavior of very-long microstructurally short fatigue cracks, Frattura

ed Integrità Strutturale, 25 (2013) 138-144.