# Non-Destructive Testing of Joints Used in Refrigerated Vehicle Bodies
## 2 Research
### Materials and Methods
#### 2.2.2 Ultrasonic Method  
In ultrasonic testing, ultrasonic probes with frequencies above 4 MHz were pre-selected for assessing the condition of the joint, due to the attenuation and length of the ultrasonic wave. It was verified which probes were able to obtain pulses on the screen of the ultrasonic defectoscope that are useful for the purpose of testing the systems. The choice of ultrasonic probes with relatively high frequencies is due to the relationship between wavelength, frequency, and speed. For a steel sheet, a wavelength comparable to the thickness of the sheet to be tested (1 mm) is obtained at a frequency of 6 MHz. Increasing frequency increases measurement accuracy by shortening the ultrasonic wavelength (for a frequency of 20 MHz, the wavelength will be about 0.3 mm), while increasing attenuation, which is unfavorable. The research was carried out both for the side plating and for the profile, which can be used for joining the walls and doors of the cargo space. It was decided to use ultrasonic probes such as P10-10L (frequency 6.37 MHz), DS6 MB 4-12 (frequency 12.21 MHz), SB3 PB 6-16 (frequency 13.70 MHz), S6 WB10 WM (frequency 7.68 MHz), and G20 MNX (frequency 20 MHz). The choice of ultrasonic probes with a relatively high frequency is due to the relationship between wavelength, frequency, and speed. For a steel sheet, a wavelength comparable to the thickness of the sheet to be tested (1 mm) is obtained at a frequency of 6 MHz. Increasing frequency increases measurement accuracy by shortening the ultrasonic wavelength (for a frequency of 20 MHz, the wavelength will be about 0.3 mm), while increasing attenuation, which is unfavorable. Two digital ultrasonic defectoscopes--USM35 XS and CUD--were used in the first stage of the study. Due to the relatively high frequencies and bandwidth range of the defectoscope, the USM35 XS was selected for alkaline testing, which allowed the desired signals to be obtained for both the SBS3 PB 6-16 and G20 MNX ultrasonic probes. A view of the test stand for the first stage of testing is shown in Figure 8. Figure 8a shows the test attempt for the CUD defectoscope and the S6 WB10 WM ultrasonic probe, while Figure 8b shows the attempt to obtain the desired signal for the same ultrasonic probe (S6 WB10 WM) and the USM35 XS defectoscope. In practice, the authors have used various measurement systems [33], while temperature-controlled transfer vehicles are not feasible for immersion testing due to their dimensions. Attempts have been made to model adhesive joints in terms of nondestructive testing [34], but the work did not include the types of joints studied in this paper. Figure 7: Sample used in the study: (**a**) dimensions, (**b**) model. Figure 8c,d shows that at a frequency of 10 MHz, it is possible to obtain a string of pulses on the screen of the defectoscope, and that it is possible to make measurements of the thickness of 1 mm thick sheet metal; however, the decreases and increases in the height of the pulses on the screen of the defectoscope are indicative of testing conducted in the near field of the ultrasonic probe. Working in the near field makes it impossible to use known ultrasonic measures to evaluate adhesive joints, and the values obtained may be random. The ultrasonic transducer G20 MNX was selected to test the adhesive joint of the sheet metal. In the case of testing the adhesive joint of an aluminum profile (with a thickness of about 2.5 mm) with a hybrid adhesive, the SBS3 PB 6-16 transducer was selected for use. It allowed us to obtain useful signals for testing. In the aluminum profile, the attenuation of the 20 MHz ultrasonic wave from the G20 MNX ultrasonic probe was too high. The decibel drop in the height of the first two pulses was used as a measure of the quality of the adhesive bond. The choice of such a measure is dictated by limiting the influence of the force and stability of pressing the probe against the surface to be tested, which is particularly important when testing objects in road or field conditions.