# Non-Contact and Real-Time Measurement of Kolsky Bar with Temporal Speckle Interferometry
## 3 Experiment and Results
### Verification Experiment of In-Plane Displacement Measurement Capability  
According to Equations (12)-(14), the dynamic mechanical properties of the specimen depend on the velocity of the incident and transmitter bars, and the velocity is obtained by the displacement of the two bars after numerical differentiation, therefore, it is necessary to verify the displacement measurement capability of the constructed optical path. In other words, if the measurement accuracy of displacement can be guaranteed, then the dynamic mechanical properties of the sample can be considered accurate. The verification experiment setup is shown in Figure 6. A single longitudinal mode green laser with a wavelength of 532 nm was used as the light source. The coherence length of the laser is greater than 50 m, and the beam diameter at the aperture is less than 1.5 mm. The detector (MER-030-120 UC) is a color GigE Vision CCD camera (IMAVISION Company, Beijing, China) with a resolution of \(656\times 492\) pixels. The pixel size is \(5.6\)\(\upmu\)m \(\times\)\(5.6\)\(\upmu\)m, and the numerical aperture NA of camera lens is 0.052 in the image plane, which gives a speckle size in the image plane of 6.1 \(\upmu\)m according to Formula (23) [16],  
\[\sigma=\frac{1.2\lambda}{2 NA} \tag{23}\] \begin{table}
\begin{tabular}{c c c} \hline \hline
**Part Name** & **Diameter (mm)** & **Length (mm)** \\ \hline Striker bar & 2 & 30 \\ Incident bar & 2 & 200 \\ Transmitter bar & 2 & 200 \\ Sample & 1.5 & 0.75 \\ Guide & 3 & 50 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The size of each component in Kolsky bar. Figure 5: Physical map of each component in Kolsky bar. The translation stage (Thorlabs, NJ, USA) consists of the MAX302(/M) 3-Axis NanoMax Flexure Stage and three DRV3 Differential Micrometers, with a calculated fine resolution of 100 nm. The accuracy of the grating ruler is 0.1 \(\upmu\)m.  
What needs to be emphasized here is that since the entire measurement is based on the speckle, it is necessary to determine whether the optical path that we have built can form speckle interference fringes. Since the diameter of the Kolsky bar we used here is very small, the speckle interference fringes of both the incident and transmitter bar cannot be observed directly even if it does exist, two iron plates of dimensions 40 mm \(\times\) 30 mm \(\times\) 2 mm were installed in the incident and transmitter bars to verify the optical path. In Figure 7, the left part is the original structure of the experimental setup, the incident bar and the transmitter bar are placed on the support base. The right part of the figure is the structure using two iron plates to replace the incident and the transmitter bars to verify the existence of speckle interference fringes. The iron plate is fixed to the support base by two screws. In the measurement process, applying a force along the \(x\)-direction to the iron plate, the surface of the iron plate will have a slight deformation in the \(x\)-direction. The CCD camera will record the entire process of the iron plate before and after the deformation. If clear speckle interference fringes can be seen from the processed image, it is proved that the optical path to be debugged meets the measurement conditions. It is noteworthy that the position of the spot on the iron plate is the same as the position on the incident and transmitter bars; this condition has been achieved when designing the support base structure of the Kolsky bar. This ensures that the replacement between the iron plate and the bar is only for more convenient observation of the existence of speckle interference fringes, it is not related to the accuracy of the system displacement measurement capability, nor the robust position of the measuring beams. The observation result of the speckle interference fringes is shown in Figure 8. The grey-scale maps of fringes in the incident and the transmitter bars can be observed by real-time subtraction. From this we can prove that the measurement light path meets the speckle interference measurement conditions.