# Experimental Study on Properties of Ultrasonic Coupling Agent with Graphene in NDT
## 4 Experimental Results
### Performance Evaluation Indexes of the Glycerol-Graphene Composite Coupling Agent  
The glycerol-graphene composite coupling agent was used to perform a corresponding experiment; the results are shown in Figure 6a-c. Peak-to-peak values, attenuation coefficient, and energy value of the received signal was calculated. It can be seen that the change of the three indexes of the glycerol-graphene composite coupling agent is similar that of the hydrogel-graphene composite coupling agent. Figure 6d shows acoustic impedance values and ultrasonic wave velocity with the increase of graphene mass fraction in the glycerol-graphene composite coupling agent.  
Figure 6a shows that when the mass fraction of graphene in the glycerol-graphene coupling agent is 0.75%, the peak-to-peak value is the largest. When the mass fraction of graphene in the glycerol-graphene coupling agent is 1%, the peak-to-peak value is worse than that without graphene. As is shown in Figure 6b, when the mass fraction of graphene is 0.75%, the attenuation coefficient of the glycerol-graphene composite coupling agent is minimum. As is shown in Figure 6c, when the mass fraction of graphene is 0.75%, energy value of the glycerol-graphene composite coupling agent is the best. The glycerol-graphene composite coupling agent performed best at a mass fraction of 0.75% of graphene at all test frequencies. It can be seen from Figure 6d that the ultrasonic velocity values changed little with the increase of graphene mass fraction while the impedance value gradually increased after the addition of graphene.  
The experimental results indicated that the appropriate amount of graphene added to glycerin has a positive coupling effect on the glycerin coupling agent, and the best effect is when the mass fraction of graphene is 0.75%. Firstly, the viscosity of pure glycerol coupling agent is not high, thus flowing freely. In the coupling process, it is easy to cause excessive loss due to the coupling force, and the coupling effect cannot be achieved, and even excessive friction between the sensor and the tested specimens may occur. Therefore,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mass Fraction of Graphene** & **0.25\%** & **0.5\%** & **0.75\%** & **1\%** \\ \hline Maximum particle diameter (\(\mu\)m) & 6.9 & 6.5 & 6.4 & 11.2 \\ Minimum particle diameter (\(\mu\)m) & 1.0 & 1.0 & 1.0 & 1.0 \\ Average particle diameter (\(\mu\)m) & 3.1 & 3.2 & 2.7 & 5.1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Graphene-agglomerated particle diameter in micrograph of a glycerol–graphene coupling agent.  
Figure 6: (**a**) Peak-to-peak values, (**b**) Attenuation coefficient, (**c**) Energy value of the received signal with the glycerol–graphene composite coupling agent. (**d**) Acoustic impedance values and ultrasonic wave velocity of the glycerol–graphene composite coupling agent with the mass fraction of graphene.  
appropriately increasing the viscosity of glycerol coupling agent can optimize ultrasonic detection and protect the tested specimens. It has been pointed out that as the mass fraction of graphene increases, the viscosity of graphene-glycerol dispersion increases [7]. With the increase in viscosity, the mobility of the graphene-glycerol composite coupling agent decreases accordingly, which reduces the micro-gap between the filling of joint surfaces of the coupling agent. It is not conducive to the propagation of ultrasonic waves. Therefore, when the mass fraction is 0.75%, three indexes are the maximum. Secondly, the addition of graphene changes the acoustic impedance of the composite coupling agent. The matching of acoustic impedance is enhanced, which reduces the energy attenuation of ultrasonic wave in the workpiece.