## 6 Conclusions--Identification of the KST-DFH Model Based on Static Experiments  
In a percussive drilling process, tensile stresses and highly confined stress states are induced beneath the indenter. In the present work, three types of static experiments were conducted in order to identify the parameters of the KST-DFH model in view of modelling a sandstone rock subjected to impact loading or indentation operation. The results can be summarised as follows:  
On the one hand, bending tests have been conducted to characterise, respectively, the quasistatic tensile strength of the sandstone rock. These tests showed an elastic-brittle response with a strong scatter on the failure stresses, confirming the relevance of the DFH constitutive model to represent this behaviour. The experimental data demonstrates the major influence played by structural defects leading to low strength and high scatter, obviously complicating material characterisation in view of identifying a given constitutive model. In the same way, uniaxial compression tests show a nonlinear response in the curve-foot part along with highly scattered Young's modulus and ultimate strength, again illustrating the influence of structural defects. On the other hand, quasioedometric compression tests conducted to characterise the deviatoric and volumetric responses of sandstone at hydrostatic pressure up to 350 MPa show a continuous increase of deviatoric strength and a quasilinear volumetric response with a slope close to the expected elastic bulk modulus once the curve-foot is exceeded. In addition, the results of the five tests are remarkably reproducible. In conclusion, under confinement, the influence of structural defects seems to be small. This is in sharp contrast to the situation at uniaxial compressive testing and bending testing where the strength of the material varies considerably due to structural defects. Finally, the observed pressure-dependent response is well caught by the KST constitutive model. In conclusion, according to static experimental results, it is concluded that in unconfined situation pre-existing structural defects play a major role in the response of Lingulid sandstone. On the other hand, under confinement, these structural defects have a much smaller influence. Therefore, these defects have to be accounted for in the model identification strategy prior to simulating numerically any industrial process such as percussive drilling. The influence of structural defects at high loading rates is discussed in a parallel paper [1]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Tests**} & **Max. Axial** & \multirow{2}{*}{\begin{tabular}{c} **Max. Radial** \\ **Strain (\(\times\) 1000)** \\ \end{tabular} } & **Max. Axial** & \multirow{2}{*}{\begin{tabular}{c} **Max. Radial** \\ **Stress (MPa)** \\ **(abs. val.)** \\ \end{tabular} } & **Max. Radial** & \multirow{2}{*}{\
\begin{tabular}{c} **Max.** \\ **Stress (MPa)** \\ **(abs. val.)** \\ \end{tabular} } & **Max. Deviatoric** & \multirow{2}{*}{
\begin{tabular}{c} **Deviatoric** \\ **Stress (MPa)** \\ \end{tabular} } & **Pressure** & **(abs. val.)** \\ \hline \#1 & 21.6 & 0.915 & 716.3 & 110.2 & 607.0 & 312.0 & 19.9 \\ \hline \#2 & 18.8 & 0.676 & 775.8 & 101.0 & 677.4 & 325.7 & 17.6 \\ \hline \#3 & 19.2 & 0.787 & 774.0 & 107.5 & 668.2 & 329.6 & 17.7 \\ \hline \#4 & 19.5 & 0.882 & 774.9 & 129.0 & 647.6 & 344.0 & 17.8 \\ \hline \#5 & 19.5 & 0.879 & 774.0 & 107.5 & 668.2 & 329.6 & 17.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Maximum values of sample radial \(\mu\)-strain, axial stress, radial stress, deviatoric stress, hydrostatic pressure, and volumetric strain in quasioedometric compression tests.