## 1 Introduction  
When casting components with polymers, cure shrinkage and thermal contraction need to be considered when making precision parts. The behavior becomes especially critical when the parts are large. Examples of components using large volumes of epoxy are transformers, electrical motors or fiber reinforced parts of wind turbines. This paper investigates how the curing behavior of large components made of epoxy can be measured throughout the component with good local resolution. It is shown that large components show unexpected curing behavior compared to the well known characteristics of small volumes of epoxy. Volume shrinkage of thermosetting resins during the hardening process is a well known phenomenon. Volume shrinkage may lead to the development of residual stresses; these may subsequently lead to early failure of the final product. The knowledge of the actual cure shrinkage at given regions of the product is essential for finding remedies against failure of the part during manufacturing. A multitude of techniques exist to measure volume shrinkage of polymers during cure, but they either measure the average shrinkage of the entire part or they measure local shrinkage at one or a few points. Measuring the average shrinkage can be quite simple by measuring the outer dimensions or measuring the buoyancy of the sample in water [1]. Other more complicated methods are based on rheometry, pycnometry, dilatometry or thermo-mechanical analysis [2; 3; 4; 5]. Point strain measurements (as a measure for the volume shrinkage) are performed during the curing process,by utilizing strain gauges on the outside of (thin) metal tubes [6; 7]. Another method to measure the strain in one location is optical fibers with Bragg gratings. The strain of the epoxy is transferred to the Bragg grating in the fiber. The fibers with gratings have good strain resolution at the small location of the grating. In general, optical fibers are somewhat brittle but they offer advantages that justify their use. For example, the coating of the fiber can be made chemical resistant; they do not contain metal parts; and they do not require electricity to work. Thus, such fibers can be employed in environments which are difficult to monitor by other means, e.g., areas with strong electromagnetic fields or explosive or corrosive atmospheres. Once an optical fiber is applied or embedded in a system, all the advantages apply while the disadvantages are usually no longer of concern [8; 9; 10; 11; 12]. Lastly, it should be mentioned that digital image correlation allows assessing the shrinkage of a samples surface in two dimensions [13]. The main disadvantage of all these methods is that the strain information is limited. Either an average volume shrinkage value for the whole sample is measured, or the shrinkage in just one point is determined. This does not allow mapping strain gradients inside the sample. Besides that most of these techniques require small sample dimension, the resin cures in a very constraining environment or the shrinkage is not measured directly. In the work by [PERSON] and [PERSON] [14], these issues are avoided by placing the strain gauges directly in a large volume of resin. However, their experimental setup seems rather complicated and not applicable to general shapes, volumes or types of epoxy. A restricted possibility to measure strain gradients is employed by, e.g., [PERSON] et al. [8] and [PERSON] and [PERSON] [10]. They usd a modified optical low-coherence reflectometry method [15] to determine non-constant strain over the length of a fiber Bragg grating. However, the length of such gratings is very limited, less than 25 mm in the cited works. As the above cited works show, these methods are usually performed under either isothermal conditions or the size/geometry of the sample or the properties of the resin allow controlled temperature programs with known temperature throughout the sample.