# Dynamic Characterization of Cohesive Material Based on Wave Velocity Measurements
## 2 Methods and Materials  
### Test Equipment  
For the experiments, the authors employed a Stokoe fixed-free type of resonant column, manufactured in 2009 by the British company GDS Instruments Ltd, with its office in Hook, Hampshire, U.K. A detailed description of this device is contained in the authors' other publications [24; 25] and will not be discussed in this paper. The BEs, produced as well by GDS Instruments Ltd in 2009, were installed in the above-mentioned resonant column apparatus. The GDS bender elements are made from piezoelectric ceramic bimorphs. Two sheets are bonded together with a metal shim in between them. Excitation voltage is used to produce a displacement in the source transducer, resulting in a wave being sent through the sample. This wave generates a displacement in the receiver, which induces a voltage that can be measured.  
The GDS bender elements come in the form of an insert that can be mounted in a top cap or a pedestal (Figure 2). This makes them easy to replace and able to be used with a wide range of triaxial cells. The insert for the base pedestal is made of stainless steel, while the one for the top cap is made of titanium. This reduces the weight by half and minimizes the axial load caused by the top cap, which is being applied to the sample. The inserts are embedded in a modified Perspex top cap and stainless steel base pedestal. At the same time, the top cap and the base pedestal are mounted on the specimen in the same way, as during a conventional triaxial test [26].  
The S-wave source comprises two piezoceramic strips both polarized in the same direction. The application of an excitation voltage induces the extension of one strip and the contraction of the other, causing the strip to bend. Turning the excitation voltage in the opposite direction contributes to strips bending reversely. The two strips in the receiver are polarized in opposite directions. The P-wave source consists of two piezoceramic strips polarized conversely. When an excitation voltage is applied, strips extend simultaneously. Reversion of the excitation voltage causes the strips to contract [26].  
In Figure 3, the BE arrangement adopted for the presented experiments is shown. Elements are manufactured to allow both S- and P-wave testing to be performed (in opposite propagation directions) on the same sample. The software switches input gain levels (of the received signal), sets the level of the output signal voltage and controls switching between P- and S-wave modes for our combined wave type elements [26]. In the current research, bender elements were used to obtain \(\upsilon_{VH}\) velocities, propagating in the vertical plane (index \(V\)), polarized in the horizontal (index \(H\)). When performing a bender element test, one of its most important aspects is the phase orientation of the elements. The authors always checked the relationship between the received signals with respect to the source signal. The desirable orientation was \"in-phase\". If the orientation was correct, the source and received traces were exactly \"in-line\". An example of non-desirable output is presented in Figure 4. The source signal and the received one are not simultaneously upward.  
Figure 3: Frontal sample view with bender element (BE) arrangement and an example of the waves’ path.  
Figure 2: Bender element inserts mounted in a top cap and base pedestal.