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On the mechanical behavior of granite: Constitutive modeling and application to percussive drilling
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0002-8645-4892
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The mechanical behavior and fragmentation response of rock materials is investigated in this work. In particular, Bohus granite is selected with application to percussive drilling. It is well known that rock behaves totally different in compression and tension and dynamic loading conditions and high strain rates under the percussive drilling process makes the material behavior even more complicated. The KST-DFH material model is shown in this work to be appropriate in order to constitutively describe granite at dynamic fragmentation. It consists of a plasticity model in compression and a damage model in tension. The yield surface locus is a quadratic function of the mean pressure in the principal stress space and the damage model is anisotropic.

Several experiments are performed in order to define the mechanical behavior and dynamic response of granite and calibrate the KST-DFH model parameters for this material. The material model is implemented as in a commercial finite element program and validated based on dynamic tests such as Edge-on Impact (EOI) and spalling test using Hopkinson bar. The numerical tool is then used to model the rock response during the percussive drilling process. In doing so, only one spherical tool button and just the first impact are considered for simplicity. The anticipated fracture mechanism in percussive drilling is captured and the penetration stiffness obtained is in agreement with practical drilling experiments.

In paper A, the experimental work is described and the granite mechanical response is explained. In particular, the influence from pre-existing cracks and defects is examined in great detail. In paper B, the experimental results are used to calibrate the material model parameters. The numerical tool discussed earlier is employed to investigate the rock fracture mechanisms at percussive drilling. In paper C, the effect of pre-existing, or structural, cracks on dynamic fragmentation of granite is investigated in detail. These cracks may be the result from former impact of the drill bit, or by means of other unconventional methods such as microwave and laser that are used to increase the effectiveness of the percussive drilling process. In paper D, the dynamic tensile behavior of granite samples is investigated. Spalling tests using a Hopkinson bar are performed and a strain rate of order 102 1/s is obtained. This experimental technique involves the same order of strain rate as present in rock materials during percussive drilling. A dynamic tensile strength of 18.9 MPa is obtained at a strain rate of 70 1/s. This is more than twice the tensile strength of the specimen (with the same size) at quasi-static conditions, which is 8 MPa.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , 28 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0567
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-159911ISBN: 978-91-7595-435-6 (print)OAI: oai:DiVA.org:kth-159911DiVA: diva2:787683
Public defence
2015-03-06, Sal K2, Teknikringen 28, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150212

Available from: 2015-02-12 Created: 2015-02-11 Last updated: 2015-02-12Bibliographically approved
List of papers
1. On the mechanical behavior of granite material with particular emphasis on the influence from pre-existing cracks and defects
Open this publication in new window or tab >>On the mechanical behavior of granite material with particular emphasis on the influence from pre-existing cracks and defects
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2013 (English)Report (Other academic)
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 545
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-159909 (URN)
Note

QC 20150212

Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2016-05-12Bibliographically approved
2. Granite rock fragmentation at percussive drilling - experimental and numerical investigation
Open this publication in new window or tab >>Granite rock fragmentation at percussive drilling - experimental and numerical investigation
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2014 (English)In: International journal for numerical and analytical methods in geomechanics (Print), ISSN 0363-9061, E-ISSN 1096-9853, Vol. 38, no 8, 828-843 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this study is to numerically model the fracture system at percussive drilling. Because of the complex behavior of rock materials, a continuum approach is employed relying upon a plasticity model with yield surface locus as a quadratic function of the mean pressure in the principal stress space coupled with an anisotropic damage model. In particular, Bohus granite rock is investigated, and the material parameters are defined based on previous experiments. This includes different tests such as direct tension and compression, three-point bending, and quasi-oedometric tests to investigate the material behavior at both tension and confined compression stress states. The equation of motion is discretized using a finite element approach, and the explicit time integration method is employed. Edge-on impact tests are performed, and the results are used to validate the numerical model. The percussive drilling problem is then modeled in 3D, and the bit-rock interaction is considered using contact mechanics. The fracture mechanism in the rock and the bit penetration- resisting force response are realistically captured by the numerical model.

Keyword
granite, fragmentation, KST-DFH model, percussive drilling
National Category
Materials Engineering Geology
Identifiers
urn:nbn:se:kth:diva-145798 (URN)10.1002/nag.2235 (DOI)000335362400004 ()2-s2.0-84899994313 (Scopus ID)
Note

QC 20150626

Available from: 2014-06-03 Created: 2014-06-02 Last updated: 2017-12-05Bibliographically approved
3. A numerical study of the influence from pre-existing cracks on granite rock fragmentation at percussive drilling
Open this publication in new window or tab >>A numerical study of the influence from pre-existing cracks on granite rock fragmentation at percussive drilling
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2015 (English)In: International journal for numerical and analytical methods in geomechanics (Print), ISSN 0363-9061, E-ISSN 1096-9853, Vol. 39, no 5, 558-570 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this study is to investigate the effect of pre-existing, or structural, cracks on dynamic fragmentation of granite. Because of the complex behavior of rock materials, a continuum approach is employed relying upon a plasticity model with yield surface locus as a quadratic function of the mean pressure in the principal stress space coupled with an anisotropic damage model. In particular, Bohus granite rock is investigated, and the material parameters are chosen based on previous experiments. The equation of motion is discretized using a finite element approach, and the explicit time integration method is employed. The pre-existing cracks are introduced in the model by considering sets of elements with negligible tensile strength that leads to their immediate failure when loaded in tension even though they still carry compressive loads as crack closure occurs because of compressive stresses. Previously performed edge-on impact tests are reconsidered here to validate the numerical model. Percussive drilling is simulated, and the influence of the presence of pre-existing cracks is studied. The results from the analysis with different crack lengths and orientations are compared in terms of penetration stiffness and fracture pattern. It is shown that pre-existing cracks in all investigated cases facilitate the drilling process.

Keyword
granite, percussive drilling, KST-DFH model, pre-existing cracks, fragmentation
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-159908 (URN)10.1002/nag.2331 (DOI)000351068800006 ()2-s2.0-84924022663 (Scopus ID)
Note

QC 20150414

Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2017-12-04Bibliographically approved
4. On the tensile strength of granite at high strain-rates considering the influence from pre-existing cracks
Open this publication in new window or tab >>On the tensile strength of granite at high strain-rates considering the influence from pre-existing cracks
2014 (English)Report (Other academic)
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 566
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-159910 (URN)
Note

QC 20150212

Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2015-02-12Bibliographically approved

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