# Automatic Transmission Fluids in Electrified Transmissions: Compatibility with Elastomers
## 3 Results and Discussion
### Changes in Tensile Strength and Elongation at Break  
Table 4 shows the values of tensile strength of all the aged elastomers as well as the fresh samples (0 h aging). These values of Table 4 allow us to analyse and compare the absolute tensile strength of all studied elastomers. Figures 3 and 4 show the percentage variation in the tensile strength and the elongation at break of the different materials after the aging process and allows us to analyse the relative evolution of these characteristics. The values of the fresh material are taken as a reference to establish the percentage variation. Horizontal discontinuous black lines indicate the acceptance variations reported in the literature for the FKM and silicone (upper and lower limits) [9]. EPDM acceptance limits are not reported in the literature. The complete stress-strain curves of the three elastomers (fresh and aged) for different periods of time are reported in Supplementary Materials Figures S1-S3. These curves were the basis from which the tensile strength and the elongation at break data (Table 4), and tensile strength variation curves (Figures 3 and 4) were generated. The reduction in tensile strength was noticeable for the EPDM and silicone after 168 h of aging, and moderate for the FKM. In addition, the FKM and silicone showed different degrees of degradation, depending on the ATF used (Figure 3). From 168 h onwards (long-term tensile strength behaviour), all the materials maintained relatively constant values of tensile strength, except for silicone in oil C. According to the limits established by the specifications used, the materials with compatibility problems are the EPDM and the silicone in oil C. In the case of the silicone, oils A, B, and D had a similar compatibility to that of the oil used by [PERSON] et al. [21], who reported just 2.5% tensile strength reduction when it was aged in synthetic oil at room temperature for 670 h. [PERSON] et al. [18] obtained similar values, but recognised that it could be degraded at relatively low temperatures due to the ionicity of Si-O bonds, causing compatibility problems with engine oils. This would correspond with the results with oil C, which shows a much more important reduction in the tensile strength. \begin{table}
\begin{tabular}{c c c c c} \hline
**ATF** & **Aging (h)** & **FKM** & **EPDM** & **Silicone** \\ \hline Fresh & 0 & 10.57 (1.62) & 9.57 (0.31) & 10.03 (0.47) \\ \hline \multirow{4}{*}{Oil A} & 168 & 10.40 (0.26) & 3.00 (0.20) & 7.23 (1.34) \\  & 336 & 8.73 (0.40)  
Figure 4: Changes in elongation at break (expressed as the ratio of the elongation at break variation to the initial elongation at break) of FKM, silicone, and EPDM, after aging in oils A, B, C, and D.  
Figure 3: Changes in tensile strength (expressed as the ratio of the tensile strength variation to the initial tensile strength) of FKM, silicone, and EPDM after aging in oils A, B, C, and D.  
Regarding the elongation at break, all the elastomers showed similar behaviour, with a certain reduction in the measured values (Figure 4). Among these materials, the EPDM was the most affected by aging after 168 h (except in oil D), while the silicone showed the lowest reduction in this property, despite the higher variability of the measurements. After 168 h, the elongation at break was more affected by the aging process than the tensile strength was, except in the case of silicone (Figure 3), where there was also more variability in the results. The elongation at break of the FKM followed a pattern similar to that of the tensile strength, which coincides with that reported by [PERSON] et al. [23]. On the contrary, [PERSON] [13] reported a decrease of 50% in both the tensile strength and the elongation at break after 504 h, and no sign of stabilization was detected during the whole test. The results in the present study were similar to those reported by [PERSON] [13] up to 336 h, reaching a 45% decrease in the elongation at break with oil B, but remaining constant from that time until the end of the test.