## 6 Discussion
### A Possible Explanation for the Formation of a Strain-Plateau in Large Samples  
One possibility to explain the formation of the strain plateau would be if the epoxy in the strain-plateau area is a different material than the one in the strain-valley area. This is not expected, but, to exclude this possibility, the 40 cm sample was cut into pieces and the density and glass transition temperature from material of either the strain-valley or -plateau area was measured. No significant differences in these two properties were found. Thus, it can be concluded that the fully cured epoxy block is indeed of one and the same material. Stage VI of the curing process is governed almost exclusively by thermal shrinkage. This allows for the determination of the coefficient of thermal expansion (CTE) if the strain development of fully cured epoxy is plotted in dependency of the temperature. This is done in Figure 9 for the middle of the 10 cm and 60 cm samples. The measurements at the end of Stage V, as shown in the upper graphs in Figure 5, were used as the reference to calculate the strain. For the 60 cm sample, the actual temperature yielded by the sensor at this position and, for the 10 cm sample, the average temperature were used as the abscissa. For the 10 cm sample, it was chosen to already use values at 90% conversion to have a significant amount of values above the glass transition temperature. For the 60 cm sample, the conversion for the first values was 97%. As mentioned above, the cure shrinkage plays a minor role during Stage VI and the results are not to be expected to be very different, even if this would (or could) be taken into account. For the epoxy in the middle of the 10 cm mold, the CTE is determined to be \(107\cdot 10^{-6}\) K\({}^{-1}\) below and \(288\cdot 10^{-6}\) K\({}^{-1}\) above the glass transition temperature. This is in the same range as reported for other DGEBA-epoxies [34; 35; 36; 37; 38]. In the strain-valley of the 60 cm sample, coefficients of thermal expansion in the same range were determined with \(87\cdot 10^{-6}\) K\({}^{-1}\) below and \(177\cdot 10^{-6}\) K\({}^{-1}\) above the glass transition temperature. With a CTE of \(75\cdot 10^{-6}\) K\({}^{-1}\), the same can be said for the middle of the 60 cm sample in the glassy state. However, in the rubbery state, the CTE in the middle of the 60 cm sample is with \(16\cdot 10^{-6}\) K\({}^{-1}\), one order of magnitude lower than expected. Below, a possible mechanism that explains this discrepancy is given. A CTE of \(16\cdot 10^{-6}\) K\({}^{-1}\) is well within the the range of CTEs steel exhibits [39; 40; 41]. Thus, we conclude the following. Once the epoxy gelates, volumetric changes due to thermal expansion can no longer be balanced by a rising liquid level in the vessel. Hence, the epoxy starts to press against the side walls and bottom of the mold and forms a mechanical bond. After solidification, the epoxy block is a rather rigid body, even above the glass transition temperature. However, epoxy is still a softer material than steel. Hence, one could reasonably expect that the thermal shrinkage in fiber direction is governed by the steel and thus constrained for the epoxy. Once a low enough temperature is reached, the epoxy detaches from the vessels walls because it's natural volume becomes smaller then the molds width. Without this constraint, the epoxy is free standing with a CTE approximately as expected. This mechanism would also explain the development of a \"M\"-shape during Stage IV. While the thermal expansion is restricted for the core of large samples during Stage IV, the cure shrinkage may not be affected by this effect. Hence, just negligible thermal expansion of the core takes place but a significant cure shrinkage. Thus, the OBR values are below the average temperature values for the core of the sample (cf. blue curve in the right graph of Figure 4).