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Topical Issues of Rational Use of Natural Resources
(International forum-competition, Saint Petersburg Mining University)
Experimental method for capturing dynamic crack propagation
in rock-like material
I Kukolj1, F Ouchterlony2, P Moser3
1. Junior researcher, Chair of Mining Engineering, Montanuniversitaet Leoben, Franz-Josef-Strasse
18, 8700 Leoben, Austria, e-mail: ivan.kukolj@unileoben.ac.at
2. Senior researcher, Chair of Mining Engineering, Montanuniversitaet Leoben, Franz-Josef-Strasse
18, 8700 Leoben, Austria, e-mail: finn.ouchterlony@unileoben.ac.at
3. Head of Chair, Chair of Mining Engineering, Montanuniversitaet Leoben, Franz-Josef-Strasse 18,
8700 Leoben, Austria, e-mail: peter.moser@unileoben.ac.at
Abstract
Blast-generated fines represent an economical and environmental liability. A possible
major source of blast-generated fine material is the dynamic mechanism of crack branching
and merging at the tip of a running crack. The research aim is to develop an experimental
method for capturing and observing this phenomenon in rock-like specimens. The method
includes capturing the dynamic crack propagation in blast-loaded confined mortar cylinders
by means of digital high-speed imaging. The research employed a special blast confinement
to control the effects that influence dynamic crack development and improve safety. The
paper discusses the thus far developed experimental set-up, test results, and points for further
improvement.
1 Introduction
In industrial blasting applications blast-induced fines result in increase of waste and
exploitation cost. A plausible dynamic mechanism behind blast fines generation is related to
crack branching and merging at the tip of a running crack. Although many researches
employed small-scale blasting tests to investigate the effects of blast loading in rock and
mortar samples on the resulting fragmentation and crack patterns (e.g., Moser et al. 2003,
Reichholf 2003, Johansson 2008, Schimek et al. 2013, etc.), none of them investigated
dynamic crack propagation in real-time with respect to dynamic crack propagation,
branching, and merging.
The FWF project – “Fines generated by dynamic crack propagation, as in blasting of
rock and rock-like materials”, which started in late 2015, will find out if branching-merging
of fast growing cracks is a major source of fines. A part of the project relates to developing
an experimental method for observing blast-induced dynamic crack propagation in cylindrical
rock and mortar samples in real-time.
The experimental method consists of small-scale blast tests that include detonating an
explosive charge inside the central borehole of the mortar cylinder positioned in a blasting
chamber. The method employs high-speed imaging (HSI) camera to observe the dynamic
crack development at the frontal face of the cylinder through a protective Polycarbonate
window on the chamber.
The paper presents the thus far developed experimental method that was tested at the
Montanuniversität blast site at the Erzberg mine (Styria, Austria) in 2015 and 2016.
2 Methodology
The experimental procedure includes blasting the PETN (pentaerythritoltetranitrate)
cord of certain charge concentration (6, 12, or 20 g/m) inside the hollow mortar cylinder,
which is radially surrounded by a damping layer inside the blasting chamber.
The PETN cord represents a reliable explosive appliance for inducing dynamic
loading of high strain rate. When initiated, the detonation propagates throughout the PETN
cord inside the cylinder, starting at the rear of the blasting chamber towards the frontal face
of the cylinder. Resulted detonation shock waves introduce initial radial cracks around the
borehole in the cylinder. Following, blast-induced gas forms inside the borehole, penetrates
already generated cracks, and further develops them. By this point, the firstly introduced
cracks usually propagate, branch and merge in a complex, seemingly hectic, manor, forming
new crack families and intersections, which lead to final fragmentation.
The cylinder production procedure follows the recipe from researches of Johansson &
Ouchterlony (2011) and Schimek (2013). The mortar cylinders are of size ø140x280 mm with
a centred axial borehole, of 10 mm in diameter, for placing the PETN cord. The weight of the
mortar cylinder is approx. 9 kg with average material density of 2 g/cm3 in dry state. The final
cylinder preparation procedure includes applying a thin layer of white paint and a rectangular
grid mesh onto the frontal cylinder-face. Additionally, a plug is fitted at the frontal cylinder-
face, 25-50 mm deep inside the borehole, to prevent the blast-generated gas to rush in-
between the window and the cylinder and to protect the camera from the detonation-caused
flash inside the borehole.
The blasting chamber (Figure 1) includes four concrete segments and employs the
"impulse trap" concept (Sun 2013). The damping layer, protective window and four segments
of the blasting chamber directly affect boundary conditions preventing the spalling effect and
reducing circumferential counter-directed cracks, which could interfere and obscure cracks
that propagate towards the cylinder's circumference (Rossmanith et al. 2005). The damping
layer and the window are designed to impede radial and axial blast-induced material
expanding and, hence, provide more uniform radial crack propagation along the cylinder's
axis.
Figure 1. The experimental set-up at the blast site on Erzberg (left) and the model of the
blasting chamber with the cylinder in a half-section view (right)
The blast tests employed the HSI camera - Imager HS 4M (LA Vision) to capture the
crack propagation at frame rates of 20,000-37,000 fps at the image resolution of 256x256
pixels and 336x336 pixels.
3 Results and discussion
Thus far, seven blast tests have employed the experimental procedure providing HSI
sequences of the crack development process with time steps of 26.3-41 µs in-between frames.
Figure 2 shows images of three frames from the same sequence.
As the research goal is to capture the blast-induced crack propagation of the frontal
cylinder-face, the imaging should provide a valid 2-D representation of what is apparently a
3-D event. The cylinder can consist of several axially coupled layers (discs) to impede the
axial crack propagation through the cylinder. Barriers in-between the discs can consist of
cardboard sheets to reduce the transfer of sheer failure from one mortar disc to another.
Figure 2. Example images from the same HSI sequence with lapsed time after the initiation
A possible solution for gas-induced flaking at the frontal cylinder face and dynamic
window-bending (see Figure 2) is to improve the window-to-cylinder coupling and apply an
aluminium half-filled pipe as the plug.
4 References
Johansson, D. & Ouchterlony, F. 2011.Fragmentation in small‐scale confined blasting. Int.
J. Mining and Mineral Eng. 3, pp 72‐94.
Moser, P., Olsson, M., Ouchterlony, F. & Grasedieck, A. 2003. Comparison of the blast
fragmentation from labscale and full-scale tests at Bårarp. In R. Holmberg (ed.),
Proc. EFEE 2nd World Conf. on Explosives & Blasting Techn, pp 449-458.
Balkema, Rotterdam.
Reichholf, G. 2003. Experimental investigation into the characteristic of particle size
distributions of blasted material; Doctoral Thesis, Montanuniversitaet Leoben, Chair
of Mining Engineering and Mineral Economics.
Rossmanith, H.P., Hochholdinger-Arsic, V. & Uenishi, K. 2005. Understanding size and
boundary effects in scaled model blasts – plane problems, Fragblast, 9:2, 93-125,
DOI: 10.1080/13855140500296671.
Schimek, P., Ouchterlony, F. & Moser, P. 2013. Experimental blast fragmentation research
in model-scale bench blasts. In Sanchidrián & Singh (Eds), Measurement and
Analysis of Blast Fragmentation, pp 51-60. Balkema, Rotterdam.
Sun, C. 2013. Damage zone prediction for rock blasting; Doctoral Thesis, The University of
Utah, Department of Mining Engineering, pp 136-138.
