**1. Introduction**

Tensile strength tests are widely applied in rock mechanics to obtain input parameters for planning of hydraulic fracturing on all scales. In literature only few experimental data sets are published dealing with samples size effects on tensile strength tests [1,2] or the comparison of different tensile tests in general [1,3]. Usually, results of laboratory tensile

© 2013 Molenda et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

tests are taken to be size independent when used as input parameter for numerical studies at different spatial sizes.

Rock type (location)

Marble (Carrara)

Limestone (Treuchtlingen)

Rhyolite (Flechtingen)

Andesite (Dönstedt)

Sandstone (Bebertal)

tions see Table 2.

Ruhrsandstone (Ruhr area)

ρd [g/cm3]

2.71 ±0.002

2.56 ±0.008

2.57 ±0.006

2.63 ±0.015

2.72 ±0.023

2.66 ±0.061

Axial displ. transducer (Scheavitz) MHR 250 LVDT 1 & 2

Displ. transducer at pressure intensifier (HBM) WA 100 mm LVDT 3

Load cell for Hoek Cell Load cell for pressure intensifier (Burster) 8219R-3000

**Table 2.** Technical specifications of the measurement system.

ρs [g/cm3]

2.721 ±0.003

2.713 ±0.002

2.688 ±0.008

2.657 ±0.011

2.734 ±0.006

2.44 ±0.059

compressional wave velocity, Φ porosity, k permeability, c cohesion, φ friction angle.

**2.3. Testing procedure of the tensile strength tests**

vp [m/s]

5.67 ±0.06

5.59 ±0.05

4.61 ±0.13

5.39 ±0.34

5.26 ±0.28

3.61 ±0.61

All experiments are performed in a stiff servo-hydraulic loading frame from Material Testing Systems (MTS) with a load capacity of 4000 kN. For further details on the technical specifica‐

Acoustic Emission (AE) signals are acquired with an AMSY-5 Acoustic Emission Measurement System (Vallen Systeme GmbH, Germany) equipped with up to 6 Sensors of type VS150-M. The Sensors are sensitive in a frequency range of 100-450 kHz with a resonance frequency of 150 kHz and a preamplification of 34 dBAE. Due to machine noise in the range below 100 kHz incoming signals are filtered by a digital bandpass-filter in a frequency range of 95-850 kHz.

**Device (manufacturer) name max. capacity accuracy BDT MTT MF** Axial load cell (Althen) CPA-50 500 KN ± 100 N x x x

**Table 1.** Averaged values of petrophysical properties of the rock samples. ρd dry bulk density, ρs grain density, vp

Φ [%] k [m²] c [MPa]

0.40 1E-19 29 22

Comparison of Hydraulic and Conventional Tensile Strength Tests

5.64 1E-18 27 53

4.39 8E-18 36 50

1.02 9E-19 20-36 55

0.51 - 20-41 50

8.27 11E-15 15 45

6.3 mm ± 1∙10-4 mm x x x

100 mm ± 1∙10-3 mm x

300 MPa ± 0,03 MPa x

φ [°] 983

http://dx.doi.org/10.5772/56300

We compare the results of 3 different, easily applicable laboratory tests for tensile strength of rocks. The sample set comprises a micritic limestone, a coarse-grained marble, a fine-grained Ruhr-Sandstone, a medium-grained rhyolite, a medium- /coarse-grained andesite and a medium grained sandstone. All tested rocks were characterized petrographically as well as by ultrasonic velocities, density, porosity, permeability, static, dynamic elastic moduli and compressive strength. In order to determine the effects of specimen size on test results, we carried out BDT according to ISRM [4] with disc diameters of 30, 40, 50, 62, 75 and 84 mm, respectively. The recently presented MTT [5] was used as a tensile strength test with an approximately uniform tensile stress distribution. Hydraulic tensile strength was evaluated by MF experiments (core diameter 40 and 62 mm; borehole/diameter ratio 1:10) under uniaxial compression [6]. MF pressurization was performed with a constant fluid volume rate of 0.1 ml/s representing a stress rate of 0.3 MPa/s. In all tests relevant acoustic emission (AE) values have been evaluated to get additional information on the failure processes.
