## 5 Comparison of Strain Data Yielded by the Regular and the New Running Reference Method  
### Strain Determined by the Regular Method  
Before any loads are applied (or the conditions are changed in any way), a reference record is taken to determine the pristine state of the fiber. The strain along the whole fiber is evaluated as zero for this measurement. In a regular OBR data analysis, all subsequent measurements are compared to this reference record. If the sample/fiber is, e.g., stretched a certain amount, a new record is taken. Comparing the new record with the reference record allows calculating the strain along the fiber for the stretched state with respect to the zero-strain state before the experiment started. When the sample is stretched more in the next step, the respective record will again be compared to the reference record, and higher strain values are calculated for this measurement. Thus, it is clear that the regular OBR data analysis determines absolute strains, because the strain values are always calculated with respect to the reference record taken before the mechanical measurement started. In the following, this method will be called the traditional or classical approach, and the reference used in this traditional technique will be denoted as \"absolute reference record\". Strain measurements from the epoxy curing experiments obtained by the traditional data analysis method are shown in Figure 2. The abscissa in this figure shows the position of the strain measurement along the fiber. In this case, the position is also the length of the epoxy block, as shown in Figure 1. The ordinate shows each local strain measurement as a point. Figure 2: Representative strain data for different states of epoxy cure, determined by utilizing the traditional analysis method (comparing each measurement to the absolute reference record taken before the experiment started). The numbers show the measurement number. For better visualization, the ordinate is cut off at 0.2% strain. Measurement times: #16 \(\rightarrow\) 200 min, #21 \(\rightarrow\) 226 min, #34 \(\rightarrow\) 267 min. Three strain-measurements (out of 250) are shown, taken at different times during the cure of the epoxy using the traditional analysis method. Whenever strain data are shown, the position along the length of the fiber is from now on related to the entry point of the fiber into the material. The most striking feature of this graph is the fact that the data for later measurements show much scatter: the above-described procedural noise (cf. Section 3). Measurement 16 (black dots in Figure 2) provides a very reliable curve. At this time, the fiber is still surrounded by liquid epoxy, and it measures basically the increase in temperature due to the exothermal reaction. The increasing temperature manifests itself as (apparent) positive strain readings. This rather experiment-specific feature is described in detail in [17]. It is important however to note that the fiber is not yet subject to a shrinking environment. Hence, no microbending occurs, and the signal is very clear. When Measurement Number 21 (red squares in Figure 2) was performed, the solidification had been at a stage at which the surrounding material already \"gripped\" the fiber. Volume shrinkage of the material clearly resulted in a compression of the fiber, leading to negative strain values. Between Measurements 16 and 21, the temperature increases from 63 to 161 degrees Celsius inside the epoxy. Thus, the negative strain is solely caused by the shrinkage of the sample. We refer to [17] regarding details about the temperature development during similar experiments. However, the measurement curve shows already many outliers. The outliers look like very local and random strain concentrations, which do not make any physical sense for a block of hardening epoxy. This scatter shows clearly that the traditional measurement method starts to fail to determine properly the strain exactly in all points.