# Effectiveness of Bonding Steel Elements with Polyester-Coated Paint
## 3 Results
### Failure Analysis  
The types of failure patterns among the adhesive joints containing the three types of adherends are presented in Table 6 and Figure 5.  
Table 6 shows the dominant type of failure in a given adhesive joint depending on the type of joined steel sample, i.e., reference, with a paint coating applied, and with a primer  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Adhered** & \multicolumn{3}{c}{**Failure Patterns of Adhesive Layer ***} \\ \cline{2-4}
**and Adhesive** & **CF** & **SCF** & **AF** & **ACF (p)** \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c} \hline \hline
**Reference** & \multicolumn{3}{c}{**Epoxy adhesive**} \\ \cline{2-4}
**Paint coating** & \multicolumn{3}{c}{\(\bullet\)} \\
**Primer and paint coating** & \multicolumn{3}{c}{\(\bullet\)} \\
**Polyurethane adhesive** & \multicolumn{3}{c}{\(\bullet\)} \\
**Reference** & \multicolumn{3}{c}{\(\bullet\)} \\
**Paint coating** & \multicolumn{3}{c}{\(\bullet\)} \\
**Primer and paint coating** & \multicolumn{3}{c}{\(\bullet\)} \\ \hline \hline \end{tabular}  
*CF—cohesion failure, SCF—special cohesion failure, AF—adhesion failure, ACF (p)—adhesion and cohesion failure with peel.  
\end{table}
Table 6: Adhesive layer failure patterns of adhesive joints (according to EN ISO 10365).  
Figure 5: Examples of the failure of the adhesive joint of the polyurethane adhesive. (**a**) SCF—special cohesion failure, primer and paint coating samples; (**b**) AF—adhesion failure, reference sample.  
and paint coating applied (Figure 5). Since there was no damage to the binder, results pertaining to the type of damage to the adhesive layer are presented. Figure 5a shows an example of a polyurethane adhesive joint failure (specimens with primer and coating) that corresponds to the special cohesion failure (SCF) that was most common during the strength tests.  
By visually assessing the failures of the adhesive joint samples (according to EN ISO 10365) after the strength test, the following types of failure were noted: cohesion failure (CF), special cohesion failure (SCF), and adhesion failure (AF). However, adhesion and cohesion failure with peel failure (ACF (p)) was not observed. According to the EN ISO 10365 standard, cohesion failure (CF) is defined as a failure along the middle part of the adhesive layer, while special cohesion failure (SCF) also applies to the failure of the adhesive layer, but the course of this failure is heterogeneous in the area of the adhesive layer. In the case of the adhesive joints to which reference samples were joined, adhesion failure was observed (partial separation of the adhesive layer from the substrate), which may indicate less favorable surface characteristics compared to the samples with a paint coating.  
In the case of the adhesive joints made with the epoxy adhesive, the following results were observed in relation to the size of the failure area:  
* For reference samples, the estimated area of adhesive failure (AF) was approximately 75%;
* For samples containing the paint coating, the estimated area of special cohesion failure (SCF) was approximately 65%;
* For samples containing the primer and paint coating, the estimated cohesion failure area (CF) was approximately 60%.  
In the case of the adhesive joints made with the polyurethane adhesive, the following results were observed in relation to the size of the failure area:  
* For reference samples, the estimated area of adhesive failure (AF) was approximately 95%;
* For samples containing the paint coating, the estimated area of special cohesion failure (SCF) was approximately 15%;
* For samples containing the primer and paint coating, the estimated area of special cohesion failure (SCF) was approximately 15%.