# Assessment of Coating Properties in Car Body by Ultrasonic Method
## 2 Materials and Methods
### Correlational Research Methodology  
The main goal of the correlation study was to determine the reflection coefficient \(|\)r\(|\) values and mechanical strength considering the curing temperature of the putty coating used in automotive body repairs. To achieve this goal, the surface of all specimens was prepared equally. The samples used in the study were cylindrical in shape and 48 mm in diameter with a pin in the center of the sample (Figure 4). Evaluation of the quality of the joint between the putty coating and the steel substrate was carried out by two different methods: ultrasonic, which is nondestructive, and destructive, which is the strength test. Since the implemented tests were to determine the relationship between the reflection coefficient of the longitudinal wave and the mechanical adhesion of the coating to the substrate, it was necessary to maintain an appropriate regime during the performance of various activities such as:  
- Preparation of the surface to which the car putty was applied (the same for all samples);  
- Preparation of the regeneration material--car putty (maintaining appropriate, i.e., identical proportions when mixing the components of the regeneration material for all samples);  
- Curing the coating in an oven (maintaining the appropriate time and temperature);  
- Performing the ultrasonic testing according to the specified procedure using the same test equipment and the same ultrasonic flaw detector settings for each sample;  
- Conducting strength tests with constant settings of the apparatus. Figure 3: Ultrasonic measurement by Rayleigh wave; 1—transmitting transducer, 2—holder, 3—receiving transducer, 4—car putty coating, 5—surface wave, 6—car body sheet. The coating was applied not only to the part of the specimen that was peeled off but to the entire surface of the specimen (all specimens were identical in diameter). The thickness of the coating was 2 mm. When ultrasonic measurements were carried out, the change in the gain of the ultrasonic longitudinal wave pulse (amplitude of pulse verification) was determined. From these measurements the reflection coefficient was calculated as a nondestructive measure of adhesion. The study used a series of cylindrical samples, which were divided into 4 measurement series. Samples from the first series were characterized by the fact that the applied coating was cured at 20 \({}^{\circ}\)C. For the second series, the applied coating was annealed at 40 \({}^{\circ}\)C. The remaining measurement series, the third and fourth, contained samples on which the applied coating was cured at 50 and 60 \({}^{\circ}\)C, respectively. In the first stage of the tests, the surfaces of all samples were prepared according to the recommendations of the manufacturer of the car putty coating. Degreasing was performed, the surfaces were hand sanded, and then the surface was degreased again. The samples prepared in this way were subjected to a surface roughness profile test on a Taylor Hobson Surtronic 3+ profilometer (Sutronic, Wegalaan, The Netherlands) to determine its basic parameters such as R\({}_{\text{a}}\) (average surface roughness for the length of the measurement) and R\({}_{\text{z}}\) (an average of the five largest total deviations recorded for length). In the next stage of the study, ultrasonic measurements were performed on samples without an applied coating. The parameter measured was the pulse amplification of the longitudinal ultrasonic wave that propagates in the material (amplitude of the pulse). A UMT 12 (Ultramed, Radom, Poland) digital ultrasonic flaw detector and a [PERSON] ([PERSON], Wuppertal, Germany) DS-12 HB 1-6 ultrasonic transducer were used in the study. The ultrasonic head was mounted from the side of the pin to a surface to which no coating was applied. This allowed for the performance of an identical test when the coating has already been applied. The next step in the testing was to prepare the samples, i.e., apply the coating material to the previously prepared surface and anneal it accordingly.