# Experimental Study on Properties of Ultrasonic Coupling Agent with Graphene in NDT
## 4 Experimental Results
### Performance Evaluation Indexes of the Hydrogel-Graphene Composite Coupling Agent  
The hydrogel-graphene composite coupling agent was used to perform a corresponding experiment; the results are shown in Figure 4a-c. Peak-to-peak values, attenuation coefficient, and energy value of the received signal were calculated. With the increase in mass fraction of graphene in the coupling agent, the variation trend of the three indexes was found to be similar. Figure 4d shows acoustic impedance and ultrasonic wave velocity with the increase of graphene mass fraction in the composite coupling agent.  
In particular, Figure 4a shows that when the hydrogel-graphene composite coupling agent has mass fraction of 0.75%, the peak-to-peak values of the received signal is the highest. In this situation, a three-dimensional graphene structured powder disperses well. As can be seen from Figure 4b, within a certain range, with the mass fraction of  
Figure 4: (**a**) Peak-to-peak values, (**b**) Attenuation coefficient, (**c**) Energy value of the received signal with hydrogel-graphene composite coupling agent. (**d**) Acoustic impedance values and ultrasonic wave velocity of the hydrogel–graphene composite coupling agent with the mass fraction of graphene.  
Figure 3: Micrographs of the hydrogel–graphene coupling agent. (**a**) graphene mass fraction is 0.25%, (**b**) graphene mass fraction is 0.5%, (**c**) graphene mass fraction is 0.75%, (**d**) graphene mass fraction is 1%.  
graphene increasing, the attenuation coefficient gradually decreases and the coupling effect of composite coupling agent on ultrasonic wave is positive. When the mass fraction of graphene is 0.75%, the attenuation coefficient is the lowest. However, the attenuation is greatest when the mass fraction of graphene is 1%. Figure 4c describes that when the mass fraction of graphene is 0.75%, the energy value of the gel-graphene composite coupling agent is the highest. As can be seen from the figure, of all the test frequencies, the compound coupling agent has the best effect when the mass fraction is 0.75%. As can be seen from Figure 4d, with the increase in mass fraction of graphene, the ultrasonic wave velocity of the composite coupling agent is basically unchanged, but its ultrasonic impedance value is gradually increased.  
In other words, a small amount of graphene can significantly optimize the effect of the coupling agent, while excessive addition of three-dimensional graphene will increase the absorption of three-dimensional graphene to ultrasonic energy. As a dopant, graphene has special properties. When the composite material is prepared by combining with a polymer, it will exhibit a certain degree of absorption. [PERSON] et al. [22] reported the sound absorption properties of three-dimensional graphene-based composites. Due to sound absorption properties of graphene, the energy propagated into specimens will decrease. Furthermore, when the mass fraction of graphene is 0.75%, the energy value is much larger than that of other graphene contents' counterpart. In this situation, the acoustic impedance of the hydrogel-graphene composite coupling agent is more compatible with the specimen, thus the energy value of ultrasonic echo is effectively improved.