INVESTIGATION OF THE FRACTURE CRACKING BEHAVIOR OF
SELF-HEALING SYSTEMS BY USE OF OPTICAL AND ACOUSTIC
EXPERIMENTAL METHODS
E. Tsangouri 1 , K . Van Tittelboom 2, X. K. D. Hillewaere3, D. Van Hemelrijck 1 N. De Belie 2 and F E. Du Prez 3
1 Department of Mechanics of Materials and Constructions, Faculty of Engineering Sciences, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium & SIM vzw - program SHE, Technologiepark 935, B - 9052 Zwijnaarde, Belgium– e-mail: etsangou@vub.ac.be;
dvhemelr@vub.ac.be
2 Magnel Laboratory for Concrete Research, Ghent University, Technologiepark-Zwijnaarde 904, 9052 Ghent, Belgium & SIM vzw - program SHE, Technologiepark 935, B - 9052 Zwijnaarde, Belgium – e-mail: kim.vantittelboom@ugent.be; nele.debelie@ugent.be
& SIM program SHE vzw, Technologiepark 935, B - 9052 Zwijnaarde, Belgium
3 Department of Organic Chemistry, Polymer Chemistry Research Group, Ghent University,
Krijgslaan 281 S4-bis, B-9000 Ghent, Belgium & SIM vzw - program SHE, Technologiepark
935, B - 9052 Zwijnaarde, Belgium – e-mail: xander.hillewaere@ugent.be;
filip.duprez@ugent.be
Keywords: self-healing, mechanical characterization, Digital Image Correlation, Acoustic Emission, NDT methods, cracking deformations, fracture parameters
ABSTRACT
Nowadays the self-healing process efficiency in loaded structural materials is evaluated by studying the damage mechanisms. Based on fracture mechanics theories, the resistance to damage and the cracking recovery can be an indication of healing performance.
Experimentally, the cracking behavior is quantified by measuring the fracture energy of the material during cracking and the fracture process zone area at which the damage is expanded. In literature, damage detection at loading stage of testing and damage recovery due to healing mechanisms at the reloading stage is monitored by application of several experimental (Non-) Destructive Methods.
In this study, the Fracture Process Zone (FPZ) in different heterogeneous materials (polymer and cementitious composites) is visualized in strain and deformation (crack opening-close-reopening) profiles of the crack tip area by application of Digital Image Correlation (DIC) and the fracture energy released in different stages of cracking is quantified and located by Acoustic Emission (AE). The combination of the aforementioned optical and acoustic techniques can confirm the recovery of cracked specimens in which healing mechanisms are applied.
1. INTRODUCTION
Cracking phenomena in structural materials are liable for degradation of the mechanical properties in local areas and generally decrease of service life of the construction. Healing approach upgrading the mechanical response of these materials, pursues to reclose the cracked region and restrain damage. As a matter of fact, the supervision and evaluation of the healing response conforms to study the
cracking stages (loading- crack formation, curing- crack closure, reloading- crack re-opening differently due to healing) of an experimental configuration.
Two different experimental healing system arrangements are examined by the use of advanced optical and acoustic methods and a critical review of their potentials at healing efficiency evaluation is done. In more details, tubular capsules filled with healing agent are embedded into concrete beams and tested to form cracks under three and four point bending [2]. Additionally, manually injected healing agent is applied on polymer resin beams and tested under pin loading [1]. The response of both material systems under two stages of loading (loading- crack formation, reloading- crack reopening after healing) is monitored by Digital Image Correlation (DIC), Acoustic Emission (AE) and a microscope device.
2. METHODS OF TESTING
Digital Image Correlation is an optical deformation and strain measuring method well- established in fracture mechanics experiments in recent years. The DIC system consists of 2 cameras (-CCD) and a data acquisition system and requires a random speckle pattern attached to the material. A number of images are captured by cameras and via a digitizing board, the analog signal is converted into an integer gray level intensity value for each pixel of the image. The grey scale images of an object during loading are compared by image correlation software which calculates the displacement of regions in each image relatively to the reference image. Then, the strains are derived from the displacement gradients [3].
Acoustic Emission is an acoustic Non- Destructive technique applied to monitor the fracture process when samples undergo deformation. The AE system consists of sensors attached to the surface of the material, a pre-amplifier that receives the dynamic motion caused by an AE event and a display and analysis software that reports the AE characteristics and locates the AE events position at the tested specimen [4].
Microscope device is the crucial tool to visualize fracture surfaces and quantify crack opening and closure phenomena. The length, width and orientation of cracking and the volume of fracture process zone is scanned by Leica MZ 125 while a digital
camera Leica DFC295 fixed to the microscope captured and stored the images. Crack Mouth Opening Device calibrated to 10 mm width, and attached on the specimen at the two sides of the pre-crack groove measures accurately the crack opening of the crack for mode I of fracture (3 point bending and TDCB pin loading). 3. EXPRERIMENTAL SET-UP
A series of six pre- notched concrete beams prepared, cured for 14 days, and tested and re-tested after 1 day of curing according to the RILEM TC 50-FMC protocol in order to determinate the fracture characteristics during cracking. CMOD device is attached at the two sides of the notch groove to measure the crack opening during the two stages of loading. DIC cameras watching one of the side surfaces of the beam are visualizing the displacement and strain profiles around the pre- crack notch and across the height of the beam. A group of eight AE sensors are attached to the surfaces of the beam surrounding the pre-crack region and monitor the AE activity. Another series of six concrete beams, are reinforced by two steel bars, prepared, cured for 14 days, and tested under four point bending to determinate the healing response in the case of multiple cracking fracture. DIC cameras watching
perpendicular the side of the beam visualize the profiles of deformations as described above and AE sensors, on a configuration similar to the of the previous tests, monitor fracture events during the two stages of loading (loading- curing 1 day- reloading). Finally, pin loading applied on TDCB resin samples, prepared following White’s protocol and tested under displacement control evaluate the healing efficiency, by measuring during testing the load response, the mode I crack opening by the use of CMOD attached to the edge of clampers of the specimen, and the strain concentration around the crack tip indicated by area of increased strain at the DIC profiles. The varied maximum load, crack opening and mode of crack fracture are analyzed at loading (formation and propagation of crack) and reloading stage (reopen of the sample after 5 days of curing).
4. RESULTS
The figures that follow present the different testing set-ups and the analysis obtained by the use of DIC, AE and microscope for unreinforced concrete beams, reinforced concrete beams and resin polymer beams respectively. It is proved that the characterization of the healing efficiency of embedded healing agent into concrete systems can be done only if tests of notched beams under three point bending are combined with tests on beams with more realistic cracking formation under four point bending. Furthermore, the loading response of TDCB specimens can be critically reviewed by visualizing the strain profiles during testing in order to correlate the final mechanical behavior with the fracture mode of the cracking.
In any case, the combination of an optical and acoustic advanced technique forms a powerful, accurate and promising experimental set-up that can fully characterize the healing efficiency of encapsulated healing systems.
(a) (b) (c)
Figure 1: (a) experimental set-up of 3-point bending test, (b) fracture process around cracking visualized by DIC, (c) AE events of crack formation and capsule breakage.
exx ( um/m) 9375 5000 0 ICSHM2013_________________________________________________________________________________ 728
(a) (b)
Figure 2: (a) Crack opening of beams tested under 4- point bending at loading and reloading stage in which crack healing is observed, (b) profiles of deformation measuring crack opening.
(a) (b) (c)
Figure 3: (a) TDCB strain profile pointing out the tip of cracking during propagation, (b) Microscope view of the side groove indicating thickness increase anomalies, (c)
loading response of TDCB specimen in case of manually healed systems. ACKNOWLEDGEMENTS
Financial support from the SIM research program (Strategic Initiative Materials) for this study is gratefully acknowledged.
REFERENCES
[1] S. Billiet, W. Van Camp,X. Hillewaere,H. Rahier, F. Du Prez, Development of optimized autonomous self-healing systems for epoxy materials based on maleimide chemistry, Polymer 53 (2012) 2320- 2326
[2] K. Van Tittelboom, N. De Belie, D. Van Loo, and P. Jacobs, Self-healing efficiency of cementitious materials containing tubular capsules filled with healing agent, Cement and Concrete Composites 33 (2011) 497-505, 2011
[3] M.A. Sutton, J. Orteu, H.W. Schreier, Image Correlation for Shape, Motion and Deformation Measurements (2009) USA
[4] D. Soulioti, N.M. Barkoula, N.M. Barkoula, A. Paipetis, T.E. Matikas, T. Shiotani, D.G. Aggelis, Acoustic emission behavior of steel fibre reinforced concrete under bending, Construction and Building Materials (2009) 23 3532–3536
0 0,5 1 1,5 2 0 50 100 150 200 250 U Dis pla ce m ent ( m m ) X position (mm) 0 0.2 0.4 0.6 0.8 0 20 40 60 80 100 110 opening displacement (mm) load (N) virgin healed -1.12 U (mm) -0.16 ICSHM2013_________________________________________________________________________________ 729
