# Assessment of Coating Properties in Car Body by Ultrasonic Method
## 4 Discussion
This is of course a consequence of obtaining a larger contact area between the putty and steel and, thus, provides better conditions for the transmission of the mechanical wave at the putty \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **R\({}_{\text{a}}\)** & **R\({}_{\text{z}}\)** & **Reflection Coefficient** & **Force [N]** & **Stress [MPa]** \\ \hline
60.8 & 1.043 & 6.860 & 0.325 & 3812 & 18.97 \\
60.9 & 0.771 & 5.305 & 0.325 & 3449 & 17.16 \\
60.10 & 0.872 & 6.735 & 0.316 & 4410 & 21.94 \\ Average & 0.846 & 5.730 & 0.341 & 2764.4 & 13.756 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._mass-steel interface for higher temperatures \(T\). At the same time, it can be noted that there is no clear dependence between the reflection coefficient \(\left|\,\right|\) and the roughness parameters; however, this assessment is not complete, because the samples included in individual groups corresponding to different temperatures do not represent the same ranges of \(R_{\mathrm{a}}\) and \(R_{\mathrm{z}}\) parameters, which currently make the comparison of results difficult. Nevertheless, the roughness parameters are dependent on each other, which is confirmed by the plot presented in Figure 7c. The last factor analyzed was the drying temperature level of the putty. The obtained results allow for a clear observation of a decrease in the value of the reflection coefficient \(\left|\,\right|\) and, thus, an increase in the quality of the joint, along with the increase in temperature \(T\). To sum up, based on observations, it can be stated that two parameters in the NDT assessment of the quality of the putty mass-steel connection can be clearly determined as decisive ones, i.e., the reflection coefficient \(\left|\,\right|\) and drying temperature \(T\). To confirm the observations and to quantify the importance of the information carried by all four parameters, a procedure based on the F-test feature selection algorithm was used. It is a statistical method used to assess the importance of features in a data set in which the test statistic adopts the Snedecor \(F\) distribution [31]. The purpose of this method is to select such variables that have the greatest power to differentiate between different groups (categories) of the target variable. For the test, the key concept is the ratio of the estimates of the intergroup and intragroup variance \(F\). The higher the value of the coefficient \(F\), the greater the power of differentiation between states/categories and the lower the probability that this difference arose by chance. The method used is to examine each feature individually and build a ranking of features taking into account the \(F\) values. The results of the analysis are presented in Figure 8. They clearly confirm the previous observations. Reflection coefficient \(\left|\,\right|\) and the drying temperature of the putty \(T\) are distinguished from the set by their differentiation power, which in turn will allow for greater efficiency in assessing the quality of the tested joint. Figure 6: Relationship between the amplitude of the pulse \(A\) for a different putty width \(W\), expressed in reference to the no coating pulse height reading; results given in %. Based on the results obtained, at a later stage, only the first two parameters (\(|\)r\(|\) and \(T\)) from the above ranking were considered, and two models of the joint quality assessment procedure were proposed: the first taking into account only the value of the reflection coefficient \(|\)r\(|\) and the second taking into account both selected parameters. Both models were built using the curve fitting procedure, and their results are shown in Figures 9 and 10. In both cases, the coefficient of determination R\({}^{2}\) as well as adjusted R\({}^{2}\) were close to 0.9. Such models allow for clear interpretation, as they directly show the relationship between the analyzed quantities.